The main types of blanks and their characteristics. Types of blanks and their characteristics. Production capabilities of the enterprise

31.10.2019

Technological characteristics of typical harvesting processes

5.1 Types of blanks and their characteristics

5.2 Preparation methods

5.3 Workpiece selection and design

5.4 Machining allowances

5.5 Factors affecting the size of allowances

5.5 Determination of intermediate sizes according to the processing route

A workpiece is an object of production from which, by changing the size, shape, and surface quality, a finished part is obtained. The overall labor intensity and cost of manufacturing the part largely depend on the correct choice of the workpiece.

The following types of blanks are used in the automotive and tractor industries:

– castings from cast iron, steel and non-ferrous metals;

– forgings and stampings from steel and some non-ferrous alloys;

– long products from steel and non-ferrous metals (circle, square, hexagon, profile, sheet);

– stamp-welded blanks from rolled steel and other metals (they are the most expedient and economical);

– stampings and castings from plastics and other non-metallic materials;

– ceramic-metal billets obtained by powder metallurgy.

The mechanical properties of castings, on the one hand, and forgings and stampings, on the other, differ significantly from each other, therefore, already when designing machines, the type of workpiece for each of its parts is usually determined by the designer. However, he must do this in agreement with the technologists of the mechanical and procurement shops. In some cases, when different types of workpieces can be used (for example, forgings, stampings or bar metal), the most advantageous solution is obtained by comparing competing options.

Cast blanks. Various casting methods are used. Castings serve as blanks for shaped parts. Crankcases, boxes, bearing housings, flywheel brackets, pulleys, flanges, etc. are cast from cast iron. With higher requirements for the mechanical properties of parts, similar castings are made of steel. Cylinder blocks, crankcases, boxes, pistons are cast from aluminum alloys.

The main methods for obtaining castings:

- casting in sand molds (manual or machine molding), casting accuracy 15-17 quality, surface roughness R Z 320-160 microns;

- casting into shell molds - a method for obtaining accurate and high-quality small and medium-sized castings from iron and steel, the accuracy of castings is 14 quality, this method is advisable to use in serial and mass production;

- investment casting is used to obtain small castings of complex configuration, provides high accuracy of 11-12 quality and surface roughness R Z 40-10 microns, the surfaces of parts are either not processed at all, or only polished;

- mold casting (metal molds) provides castings with an accuracy of 12-15 quality and surface roughness R Z 160-80 microns;

- injection molding is used to obtain small castings of complex shape from non-ferrous alloys in large-scale production, castings are performed with an accuracy of 9-11 quality and roughness R Z 80-20 microns;

- centrifugal casting is mainly used to obtain blanks having the shape of bodies of revolution (cylinders, glasses, rings), accuracy 12-14 quality and roughness R Z 40-20 microns.

Preforms obtained by pressure treatment. The methods of obtaining initial blanks by pressure treatment include free forging, hot and cold stamping. The mechanical properties of forged and stamped blanks are higher than the properties of blanks obtained by casting. This is the main type of blanks for the manufacture of critical parts from steel and some non-ferrous alloys.

Obtaining blanks by forging is used mainly in the conditions of individual or small-scale production, when it is not economically feasible to manufacture expensive dies.

To reduce metal consumption during forging blanks, rings and backing dies are used.

In the conditions of serial and mass production, small and medium-sized steel blanks are obtained by stamping. Advantages of this method: significant productivity, a sharp decrease in the size of the allowances compared to free forging.

Depending on the equipment used, stamping is divided into stamping on hammers, presses, horizontal forging machines and special machines. Stamping is carried out both hot and cold.

Cold stamping makes it possible to obtain a workpiece with high physical and mechanical properties, but this method is very energy intensive and is used very rarely.

Rolled stock. Rolled products are used in cases where the configuration of the part closely matches any type of sectional material (round, hexagonal, square, rectangular). Widely used are also hot-rolled seamless pipes of various thicknesses and diameters, as well as profiled products (angle steel, channels, beams).

Rolled products are produced hot-rolled and calibrated cold-drawn. When choosing the size of a rolled material, material standards should be used, taking into account the configuration of the part, the accuracy of the dimensions performed and the need to save metal. Round hot-rolled sectional material of increased and normal accuracy is produced in accordance with GOST 2590-2006, round calibrated - in accordance with GOST 7417-75. In order to approximate the shape of the workpiece to the configuration of parts such as shafts and axles, it is advisable to use rolled products with a variable cross section (periodic rolling) in the conditions of large-scale and mass production.

Combined blanks. In the manufacture of workpieces of complex configuration, a significant economic effect is provided by the manufacture of individual elements of the workpiece by advanced methods (stamping, casting, sectional and shaped steel) with the subsequent connection of these elements by welding or other methods. In agricultural machines, welding is used: in the manufacture of frames, wheels, etc.

Metal-ceramic blanks. Metal-ceramic materials obtained by pressing a powder mixture with subsequent sintering are porous, so their use is effective in the manufacture of bearing bushings. Ceramic-metal linings are also made for brake pads and other friction parts with a high coefficient of friction (0.26-0.32 for dry steel and 0.10-0.12 for oil operation).

Powder metallurgy includes the following steps:

– preparation of raw material powders (copper, tungsten, graphite, etc.);

– pressing blanks in special molds. If it is necessary to obtain the most dense part, then compaction is carried out with preheating to the sintering temperature, but below the melting point of the main component.

The powder is sintered in gas or electric furnaces in hydrogen or other protective gases. If the part operates under conditions of significant friction, then it is impregnated with oil or graphite powder is added to the composition. To obtain accurate workpieces after sintering, they are calibrated.

Workpiece selection and design. An important task in the manufacture of blanks is their approximation in shape to finished parts.

The choice of the type of blank and the method of its production is influenced by the material of the part, its dimensions and structural forms, the annual production of parts, and other factors.

When developing processes for manufacturing parts, two main areas are used:

- obtaining blanks that are closest in shape to the dimensions of the finished part, when the procurement processes account for the main labor intensity;

- obtaining blanks with large allowances, i.e. the main labor input falls on the machining shop.

The design of blanks is carried out in the following sequence:

- the type of the original workpiece is determined (rolled, stamping, casting);

– a technological route for the machining of the workpiece is being developed;

- the operating and total allowances for all machined surfaces are determined (calculated);

- on the drawing of the part, general allowances for processing each surface are drawn;

- preliminary dimensions of blanks and tolerances for them are assigned;

- the dimensions of the workpiece are adjusted taking into account the method of its manufacture, overlaps, molding slopes, radii, etc. are set.

Tolerances and allowances for machining for cast iron and steel blanks cast in sand molds are regulated by GOST 26645-89 "Castings from metals and alloys".

For the selected casting method, the tables determine the class of dimensional accuracy, the class of mass accuracy and the series of allowances.

Tolerances for the main dimensions of the casting and the main allowances are determined. To determine the additional allowance, the degree of warping is determined (the ratio of the smallest overall dimension of the casting to the largest). The sketch of the casting is shown in Figure 6.

Figure 6

For diametrical dimensions, the dimensions of the workpiece are determined by the formulas:

d= d N + (Z 1 + Z 2) 2 ± T (5.1)

D \u003d D N - (Z 1 + Z 2) 2 ± T (5.2)

where Z 1 - the main allowance

Z 2 - additional allowance;

T - size tolerance (symmetrical).

An example of recording the casting accuracy 9-9-5-3 GOST 26645-85, where 9 is the size accuracy, 9 is the mass accuracy, 5 is the degree of warpage, 3 is a number of allowances.

For the manufacture of shafts, hot-rolled round steel is used according to GOST 2590-2006 with a diameter of 5 to 270 mm, three degrees of accuracy: A - high accuracy; B - increased accuracy; B - normal accuracy (Figure 7).

Figure 7

Rolled steel calibrated round in accordance with GOST 7417-75, with a diameter of 3 to 100 mm with a tolerance field h9, h10, h11 and h12 (Figure 8):

Figure 8

If the shaft has large differences in steps, the workpiece is obtained by forging or stamping. Forging according to GOST 7829-70 from carbon alloy steel, manufactured by free forging on hammers (Figure 9):

Figure 9

The dimensions of the workpiece are determined by the formula:

d 1 \u003d d N + Z 1 +,

where Z 1 - size allowance;

T 1 - size tolerance (symmetrical tolerance).

Forgings according to GOST 7062-90 are applicable for large-sized blanks made by forging on presses.

When forging blanks, it is desirable that it has a simple symmetrical shape, and the intersection of cylindrical elements with each other should be avoided.

Stamped blanks are made in accordance with GOST 7505-89 "Stamped steel forgings". The standard establishes allowances, dimensional tolerances, shape deviations and the smallest corner radii.

Allowances and tolerances are set depending on the mass and dimensions of the forging, the steel group, the degree of complexity, the accuracy class of the forging, the roughness of the machined surface of the part (Figure 10).

The surface roughness of the forgings is R Z 320-80 µm. If, after stamping, chasing is carried out, then it is possible to maintain the accuracy of individual dimensions up to 0.02 ... 0.05 mm.

Figure 10

The geometric shape of the workpiece must allow free removal from the die. For this purpose, surface slopes are provided.

Recesses and recesses in the workpiece can only be made in the direction of movement of the stamp. Narrow and long protrusions in the die parting plane or perpendicular to them are not allowed. The side surfaces must have stamping slopes. Transitions from one surface to another must have roundings, the dimensions of the corners and the radii of roundings are established by the standards. Shanks with a conical shape make stamping difficult, so it is recommended to make them cylindrical.

Allowances for machining. Any workpiece intended for further machining is made with allowance to the size of the finished part. The allowance is an excess of material necessary to obtain the final dimensions and a given class of surface roughness of the parts; it is removed on machines with cutting tools. The surfaces of the part that are not subjected to processing do not have allowances.

The difference between the dimensions of the workpiece and the finished part determines the amount of the allowance, i.e. layer to be removed during machining.

Allowances are divided into general and interoperational.

Total allowance for processing- a layer of metal to be removed during the machining of the workpiece to obtain the shape, dimensions and quality of the machined surface specified by the drawing and specifications. mejo operating allowance- a layer of metal removed during one technological operation. The amount of allowance is usually given "per side", i.e. indicates the thickness of the layer to be removed on the given surface.

The total processing allowance is the sum of all operating allowances.

Allowances can be symmetrical and asymmetric, i.e. located in relation to the axis of the workpiece symmetrically and asymmetrically. Symmetrical allowances can be on the outer and inner surfaces of the bodies of revolution; they can also be at opposite flat surfaces processed in parallel, at the same time.

The allowance must have dimensions that ensure the performance of the machining required for a given part while meeting the established requirements for roughness and quality of the metal surface and the accuracy of the dimensions of the parts at the lowest consumption of material and the lowest cost of the part. This allowance is optimal. It is advisable to assign an allowance that can be removed in one pass. On machines of medium power in one pass, you can remove the allowance up to 6 mm per side. With excessive allowances, the machines must work with high voltage, their wear and tear and repair costs increase; the cost of cutting tools increases, because the operating time of the tool increases, and, therefore, its consumption increases; increasing the depth of cut requires increasing the power of the machine, which as a result leads to an increase in energy consumption.

Factors affecting the amount of allowances. The values of allowances for processing and tolerances for the dimensions of the workpiece depend on a number of factors, the degree of influence of which is different. The main factors include the following:

- workpiece material;

- configuration and dimensions of the workpiece;

- type of workpiece and method of its manufacture;

– requirements for machining;

– specifications regarding quality and surface roughness class and dimensional accuracy.

Workpiece material. For billets produced by casting, the surface layer has a hard crust. For normal operation of the tool, it is necessary that the depth of cut be greater than the thickness of the casting skin. The thickness of the crust is different, it depends on the material, dimensions of the casting and casting methods; for cast iron castings - from 1 to 2 mm; for steel castings - from 1 to 3 mm.

Forgings and stampings can be of alloy or carbon steel; forgings are made from ingot or rolled products. During the manufacture of forgings, scale is formed on them. To remove this layer when machining carbon steels, a depth of cut of 1.5 mm is often sufficient; for alloy steels, the depth of cut should be 2–4 mm.

The surface layer of forgings is decarburized and must be removed during processing. The thickness of this layer for stampings made of alloy steels is up to 0.5 mm; for stampings made of carbon steels 0.5–1.0 mm, depending on the configuration and dimensions of the part and other factors.

Workpiece configuration and dimensions. It is difficult to obtain workpieces of complex configuration by free forging, therefore, in order to simplify the shape of the workpiece, it is sometimes necessary to increase the processing allowances.

In stampings of complex configuration, the flow of material is difficult, therefore, for such stampings, it is also necessary to increase allowances.

In castings of complex configuration, in order to more uniformly cool the metal, it is necessary to make smooth, gradual transitions from thin walls to thick ones, which also necessitates an increase in the allowance. In the manufacture of large castings, shrinkage must be taken into account.

Type of workpiece and method of its manufacture. Billets, as mentioned, are in the form of castings, forgings, stampings and rolled products. Depending on the type of workpiece and the method of its manufacture, the allowances and tolerances for the dimensions of the workpiece are different. So, for a cast part made by hand molding, the allowance is larger than for metal molds. The most accurate, therefore, with the smallest allowances, are obtained when casting into shell and metal molds, when casting under pressure, according to investment models. If we compare the allowances of forgings and stampings for the same parts, we can see that the allowances of forgings are greater than those of stampings. In rolled blanks, the allowances are smaller than in blanks obtained by casting, forging or stamping.

Machining Requirements. In accordance with the requirements for surface roughness and dimensional accuracy of the part, one or another method of machining is used. For each intermediate machining operation, it is necessary to leave an allowance removed by the cutting tool in one or more passes. Therefore, the total allowance is dependent on the machining methods required to produce the part to specification.

Specifications for the quality and accuracy of surfaces. The higher the requirements for the part in accordance with the technical requirements, the greater the allowance should be. If the surface must be smooth, then it is necessary to give an allowance that allows, after roughing, to produce a finish one. If the dimensions must be made exactly within the established tolerances, then the allowance must ensure the ability to achieve the required accuracy and surface roughness class, which must be taken into account when determining the allowance value. In this case, it is necessary to provide a metal layer that compensates for shape errors resulting from previous processing (especially thermal), as well as the installation error of the part in this operation.

Determination of intermediate sizes in accordance with the processing route. Regulatory allowances are established by the relevant standards. Under production conditions, the dimensions of the allowances are set on the basis of experience, using practical data depending on the weight (mass) and overall dimensions of the parts, structural shapes and dimensions, the required accuracy and class of processing cleanliness. Many factories, research and design institutes have their own standard allowance tables, developed by them on the basis of long experience in relation to the nature of their production.

In mechanical engineering, the experimental-statistical method for establishing processing allowances is widely used. At the same time, general and intermediate allowances are taken according to the tables, which are compiled on the basis of a generalization of the production data of advanced factories. The disadvantage of this method is that allowances are assigned without taking into account the specific conditions for constructing technological processes.

The calculation and analytical method for determining allowances consists in analyzing various processing conditions and establishing the main factors that determine the intermediate allowance (factors affecting the allowances of the previous and completed transitions) of the surface treatment process. The value of the allowance is determined by the method of differentiated calculation for the elements that make up the allowance, taking into account the processing error at the previous and given technological transitions. This method was proposed by Professor V.M. forged,

The symmetrical allowance for diametrical dimensions is determined by the formula:

2Z b min = 2[(H a + T a) +].

Symmetrical allowance for two opposite parallel flat surfaces:

2Z b min = 2[(H a + T a) + ()].

Asymmetrical allowance on one of the opposite parallel flat surfaces:

Z b min \u003d (H a + T a) + (),

where Z b min is the minimum allowance for the transition to the side;

H a - the value of microroughness from the previous processing;

T a is the value of the defective surface layer remaining from the previous treatment;

ρ a is the total value of spatial deviations from the previous processing;

ε b - workpiece installation error during operation

The calculation method, due to its complexity, has not received wide distribution, although it is of some interest from a methodological point of view.

For ease of calculation, operating allowances and tolerances are located at various stages of processing in the form of diagrams.

When the sequence and method of processing each surface is established, it is necessary to determine the values of intermediate allowances and intermediate dimensions of the workpiece as it is processed from transition to transition. As a result, the dimensions of the workpiece are determined more reasonably, that is, taking into account the processing to which it will be subjected.

For processing the outer surface (shaft machining accuracy - 7th grade, roughness R a 1.25 μm), the arrangement of intermediate sizes is shown in Figure 10.

The layout of intermediate dimensions when machining a hole (machining accuracy - 7th grade) is shown in Figure 11.

The arrangement of intermediate dimensions in the processing of the end surface (processing accuracy - 11th grade, roughness R a 2.5 μm) is shown in Figure 12.

T 3 - tolerance after finishing turning;

z 3 - allowance for finishing turning;

T 4 - tolerance after rough turning;

T 5 - workpiece tolerance

Figure 10 - Scheme of arrangement of intermediate dimensions when processing external surfaces

T 1 - size tolerance specified by the drawing;

z 1 - allowance for fine grinding;

T 2 - tolerance after preliminary grinding;

z 2 - allowance for preliminary grinding;

T 3 - tolerance after pulling;

z 3 - broaching allowance;

T 4 - tolerance of the boring field;

z 4 - allowance for boring;

T 5 - workpiece tolerance

Figure 11 - Layout of intermediate dimensions when processing internal surfaces

T 1 - tolerance specified by the drawing;

z 1 - allowance for preliminary grinding;

T 2 - tolerance after finishing turning;

z 2 - allowance for finishing turning;

T 3 - tolerance after rough turning;

z 3 - allowance for rough turning;

T 4 - workpiece tolerance

Figure 12 - Layout of intermediate dimensions when processing end surfaces

blank, according to GOST 3.1109--82, the subject of labor is called, from which a part is made by changing the shape, size, surface properties and (or) material.

There are three main types of blanks: machine-building profiles, piece and combined. Machine-building profiles are made of a constant section (for example, round, hexagonal or pipe) or periodic. In large-scale and mass production, special rolled products are also used. Piece blanks are obtained by casting, forging, stamping or welding. Combined blanks are complex blanks obtained by joining (for example, welding) separate, simpler elements. In this case, it is possible to reduce the mass of the workpiece, and use the most suitable materials for more loaded elements.

Workpieces are characterized by their configuration and dimensions, the accuracy of the obtained dimensions, the state of the surface, etc.

Forms and dimensions of the workpiece largely determine the technology of both its manufacture and subsequent processing. Dimensional accuracy workpiece is the most important factor affecting the cost of manufacturing a part. In this case, it is desirable to ensure the stability of the dimensions of the workpiece over time and within the limits of the manufactured batch. The shape and dimensions of the workpiece, as well as the condition of its surfaces (for example, the chill of iron castings, the layer of scale on forgings) can significantly affect subsequent machining. Therefore, for most workpieces, preliminary preparation is necessary, which consists in the fact that they are given such a state or appearance in which they can be machined on metal-cutting machines. This work is especially carefully carried out if further processing is carried out on automatic lines or flexible automated complexes. Pre-processing operations include cleaning, straightening, peeling, cutting, centering, and sometimes processing of technological bases.

Allowances, laps and dimensions

Machining allowance- this is a layer of metal removed from the surface of the workpiece in order to obtain the shape and dimensions of the part required according to the drawing. Allowances are assigned only to those surfaces whose required shape and dimensional accuracy cannot be achieved by the accepted method of obtaining a workpiece.

Allowances are divided into general and operational. Total allowance for processing- this is a layer of metal necessary to perform all the necessary technological operations performed on a given surface. Operating allowance- this is a layer of metal removed during one technological operation. The allowance is measured along the normal to the surface in question. The total allowance is equal to the sum of the operating ones.

The size of the allowance significantly affects the cost of manufacturing the part. An overestimated allowance increases the cost of labor, the consumption of material, cutting tools and electricity. An underestimated allowance requires the use of more expensive methods for obtaining a workpiece, complicates the installation of the workpiece on the machine, and requires a higher qualification of the worker. In addition, it is often the cause of marriage during machining. Therefore, the assigned allowance should be optimal for given production conditions.

The optimal allowance depends on the material, dimensions and configuration of the workpiece, the type of workpiece, the deformation of the workpiece during its manufacture, the thickness of the defective surface layer, and other factors. It is known, for example, that iron castings have a defective surface layer containing shells, sand inclusions; forgings obtained by forging have scale; forgings obtained by hot forging have a decarburized surface layer.

The optimal allowance can be determined by the calculation and analytical method, which is considered in the course "Mechanical Engineering Technology". In some cases (for example, when machining technology has not yet been developed), allowances for processing various types of workpieces are selected according to standards and reference books.

Rice. 2.1. Allowances, laps and dimensions of the bearing housing (a), plugs (b) and shaft (in): BUT eag, B zag, AT zag, D zag, D? zag - the original dimensions of the workpiece; A det, B det, AT det, D" det, D"det, - dimensions of the finished part; D 1 , D 2 , O" 1 , O" 1 , -- workpiece operating dimensions

The actual layer of metal removed in the first operation can vary widely, since in addition to the operating allowance, it is often necessary to remove the overlay.

lap- this is an excess of metal on the surface of the workpiece (in excess of the allowance), due to technological requirements to simplify the configuration of the workpiece to facilitate the conditions for its production. In most cases, the overlap is removed by machining, less often it remains in the product (forging slopes, increased radii of curvature, etc.).

In the process of converting a workpiece into a finished part, its dimensions acquire a number of intermediate values, which are called operating dimensions. On fig. 2.1 details of various classes show allowances, laps and operating dimensions. Operating dimensions are usually affixed with deviations: for shafts - minus, for holes - plus.

1. Purpose and trend of development of procurement production. 2

1.1. An approximate structure of the production of blanks in mechanical engineering. 2

2. Basic concepts of blanks and their characteristics. 3

2.1. Procurement, basic concepts and definitions. 3

2.2. Allowances, laps and dimensions .. 4

3. The choice of the method of obtaining blanks. 6

3.1. Technological capabilities of the main methods for obtaining blanks 6

3.2. Basic principles for choosing a method for obtaining blanks. eight

3.3. Factors determining the choice of method for obtaining blanks. 9

3.3.1 The shape and dimensions of the workpiece. 9

3.3.2 Required accuracy and quality of the surface layer of blanks. ten

3.3.3 Technological properties of the workpiece material. eleven

3.3.4. Production program. 12

3.3.5 Production capabilities of the enterprise. fourteen

3.3.6. Duration of technological preparation of production. fifteen

3.4. Method for choosing a method for obtaining blanks. 16

3.5. The metal consumption rate and the mass of the workpiece. 17

3.6. Requirements for blanks in terms of subsequent processing. eighteen

3.7. Influence of the accuracy and quality of the surface layer of the workpiece on the structure of its machining. twenty

1. Purpose and development trend of procurement production

The main purpose of blank production is to provide machine shops with high-quality blanks.

In mechanical engineering, blanks are used that are obtained by casting, forming, welding, as well as from plastics and powder materials (Table 1.2). Modern blank production has the ability to form blanks of the most complex configuration and the most diverse sizes and accuracy.

1.1. An approximate structure for the production of blanks in mechanical engineering

At present, the average labor intensity of procurement work in mechanical engineering is 40 ... 45% of the total labor intensity of machine production. The main trend in the development of blank production is to reduce the labor intensity of mechanical processing in the manufacture of machine parts by increasing the accuracy of their shape and size.

2. Basic concepts about blanks and their characteristics

2.1. Procurement, basic concepts and definitions

A blank, according to GOST 3.1109-82, is an object of labor, from which a part is made by changing the shape, size, surface properties and (or) material.

There are three main types of blanks: machine-building profiles, piece and combined. Machine-building profiles are made of a constant section (for example, round, hexagonal or pipe) or periodic. In large-scale and mass production, special rolled products are also used. Piece blanks are obtained by casting, forging, stamping or welding. Combined workpieces are complex workpieces obtained by joining (for example, welding) separate, simpler elements. In this case, it is possible to reduce the mass of the workpiece, and use the most suitable materials for more loaded elements.

Workpieces are characterized by their configuration and dimensions, the accuracy of the obtained dimensions, the state of the surface, etc.

The shapes and dimensions of the workpiece largely determine the technology of both its manufacture and subsequent processing. The dimensional accuracy of the workpiece is the most important factor influencing the cost of manufacturing a part. In this case, it is desirable to ensure the stability of the dimensions of the workpiece over time and within the limits of the manufactured batch. The shape and dimensions of the workpiece, as well as the condition of its surfaces (for example, the chill of iron castings, the layer of scale on forgings) can significantly affect subsequent machining. Therefore, for most blanks, preliminary preparation is necessary, which consists in that they are given such a state or form in which they can be machined on metal-cutting machines. This work is carried out especially carefully if further processing is carried out on automatic lines or flexible "automated complexes. The pre-processing operations include cleaning, straightening, peeling, cutting, centering, and sometimes processing of technological bases.

2.2. Allowances, laps and dimensions

The machining allowance is a layer of metal removed from the surface of the workpiece in order to obtain the shape and dimensions of the part required by the drawing. Allowances are assigned only to those surfaces whose required shape and dimensional accuracy cannot be achieved by the accepted method of obtaining a workpiece.

Allowances are divided into general and operational. The total allowance for processing is a layer of metal necessary to perform all the necessary technological operations performed on a given surface. The operating allowance is a layer of metal removed during one technological operation. The allowance is measured along the normal to the surface in question. The total allowance is equal to the sum of the operating ones.

The optimal allowance depends on the material, dimensions and configuration of the workpiece, the type of workpiece, the deformation of the workpiece during its manufacture, the thickness of the defective surface layer, and other factors. It is known, for example, that iron castings have a defective surface layer containing shells, sandy inclusions; forgings obtained by forging have scale; forgings obtained by hot forging have a decarburized surface layer.

The actual layer of metal removed in the first operation can vary widely, because in addition to the operating allowance, it is often necessary to remove the overlap.

An overlap is an excess of metal on the surface of the workpiece (in excess of the allowance), due to technological requirements to simplify the configuration of the workpiece in order to facilitate the conditions for its production. In most cases, the overlap is removed by machining, less often it remains in the product (forging slopes, increased radii of curvature, etc.).

In the process of converting a workpiece into a finished part, its dimensions acquire a number of intermediate values, which are called operational dimensions. In Fig.2.1. on parts of various classes, allowances, laps and operational dimensions are shown. Operating dimensions are usually affixed with deviations: for shafts - minus, for holes - plus.

3. Choosing a method for obtaining blanks

3.1. Technological capabilities of the main methods for obtaining blanks

The main ways of producing blanks are casting, forming, welding. The method of obtaining a particular workpiece depends on the service purpose of the part and the requirements for it, on its configuration and dimensions, type of structural material, type of production, and other factors.

Casting produces billets of almost any size, both simple and very complex configurations. In this case, castings can have complex internal cavities with curved surfaces intersecting at different angles. Dimensional accuracy and surface quality depend on the casting method. Some special casting methods (die casting, investment casting) can produce blanks that require minimal machining.

Castings can be made from almost all metals and. alloys. The mechanical properties of the casting largely depend on the conditions of crystallization of the metal in the mold. In some cases, defects may form inside the walls (shrinkage looseness, porosity, hot and cold cracks), which are detected only after rough machining when the casting skin is removed. .

Machining of metals by pressure is used to obtain machine-building profiles, forged and stamped blanks.

Machine-building profiles are made by rolling, pressing, drawing. These. methods make it possible to obtain blanks that are close to the finished part in cross section (round, hexagonal, square rolled products; welded and seamless pipes). Rolled products are produced hot-rolled and calibrated. The profile required for the manufacture of the workpiece can be calibrated by drawing. In the manufacture of parts from calibrated profiles, processing without the use of a blade tool is possible.

Forging is used for the manufacture of blanks in a single production. In the production of very large and unique blanks (weighing up to 200 ... 300 tons), forging is the only possible way of pressure treatment. Stamping allows you to get blanks that are closer in configuration to the finished part (weighing up to 350...500 kg). The internal cavities of forgings have a simpler configuration than castings, and are located only along the direction of movement of the working body of the hammer (press). The accuracy and quality of blanks produced by cold forging is not inferior to the accuracy and quality of castings obtained by special casting methods.

In mechanical engineering, from the point of view of the sequence of the technological process, there are two types of products: parts and blanks:

DETAIL - a finished product that goes directly to the assembly;

BLANK - a semi-finished product intended for further processing in order to obtain a finished part.

The “Z” allowance is a layer of metal on the surface of the workpiece, intended to be removed during subsequent machining in order to obtain the desired properties of the machined surface of the part. . The smaller the allowance, the smaller the amount of workpiece metal is converted into chips.

There are TWO WAYS OF DETERMINING THE ALLOWANCE:

1. TABLE METHOD. Used in small scale production.

The allowance is assigned according to the reference tables of GOSTs, regardless of the route of the technological process of machining the part.

2. CALCULATION AND ANALYTICAL. The total value of the allowance on the workpiece is determined by sequential "layering" on the size of the finished part of the operational allowances for machining.

LAP is called the ADDITIONAL VOLUME OF THE METAL OF THE BLANK (Fig. 1.3), which simplifies its configuration (filled holes I, local recesses 2, transitions and ledges 3), associated with the technological features of its manufacture (casting and stamping slopes 4, fillet radii 5) or caused by it not a multiplicity of 6 when cutting.

INITIAL BLANK is a product of metallurgical processing (rolled ingot, melt) entering the first technological operation of the procurement process.

The blanks of machine parts are mainly obtained in two ways: CASTING and PRESSURE TREATMENT.

Blanks obtained by casting

In the case of obtaining blanks by casting (Fig. 1.4), liquid metal, MELTING, IS FILLED into a pre-prepared CASTING MOLD corresponding to the configuration and dimensions of the finished part, but taking into account allowances and overlaps. After solidification of the metal, a product is obtained, called a CASTING.

The ADVANTAGES of foundry production over other methods of producing blanks are: the possibility of obtaining products of COMPLEX CONFIGURATION and ANY WEIGHT, as well as the RELATIVELY LOW COST of castings.

DEFECTIVE - RELATIVELY LOW STRENGTH OF CAST PRODUCTS due to the cast granular structure, in contrast to the fibrous structure that forged and stamped products have.

MODEL KIT (Fig. 1.8, see p. 11) - a set of fixtures, CASTING MODEL, CORE BOX, GATE SYSTEM MODELS, MODEL PLATE, MODEL PLATES).

Methods for obtaining machine-building profiles and shaped blanks by metal forming

In the case of obtaining workpieces by pressure treatment, the ORIGINAL BLANK, heated or cold, but necessarily hard, is DEFORMED with a special tool, in the form of BREAKERS or DAMPS and given to it a NEW FOMA, corresponding in configuration and dimensions to the finished part, but taking into account allowances and overlaps. The resulting products are called FORGINGS or STAMPED BLANKS.

OMD processes are based on the use of the PLASTIC PROPERTIES of metals, i.e. their ability under the action of external forces to change their shape without destruction.

ADVANTAGES of OMD processes are:

SAVING metal due to small allowances and small technological waste in operations;

HIGH PRODUCTIVITY due to high processing speeds;

GREAT PRECISION products;

IMPROVEMENT OF PERFORMANCE PROPERTIES of products due to the creation of a FINE-GRAINED and FIBER purposeful metal STRUCTURE during deformation.

DISADVANTAGE - relatively HIGH COST of products.

There are six main methods of OMD: rolling, pressing, drawing, forging, volumetric and sheet stamping.

First three under the general name ROLLING AND DRAWING PRODUCTION are used in the metallurgical industry to obtain MACHINE-BUILDING PROFILES.

Second three- under the general name FORGING AND STAMPING PRODUCTION, they are used in mechanical engineering for the production of SHAPED PRODUCTS.

A number of processes are carried out with metal heating above the RECRYSTALLIZATION THRESHOLD (0.4 of the melting temperature on an absolute scale) - HOT DEFORMATION, a row without heating - COLD DEFORMATION.

1. ROLLING- the process of obtaining machine-building profiles and shaped products by plastic deformation of metal between rotating rolls of a rolling mill. The accuracy of obtaining products from rolled products is shown in Appendix 3 (see p. 90).

There are three main rolling schemes:

LONGITUDINAL rolling in smooth (a) and grooved (b) rolls produces sheets and strips, rods, beams, rails and pipes;

CROSS ROLL (c, d) - solid-rolled rings, wagon and gear wheels;

CROSS-SCREW PRODUCTS - seamless sleeves, and periodic profiles.

ROLLING RANGE includes four product groups:

SHEET - sheets and tapes;

GRADE - bars, beams and rails;

PIPES - seamless and welded;

SPECIAL STEEL PRODUCTS - wagon and gear wheels, bimetals, periodic and bent profiles;

2. PRESSING(Fig. 1.10, a) - the process of obtaining machine-building profiles by extruding metal from a closed cavity through a PROFILING hole.

Three pressing schemes are used: direct, reverse and combined.

PRESSING PRODUCTS - bars of various cross sections, smooth and ribbed pipes made of hard-to-deform high-alloy steels and alloys based on aluminum, magnesium and tungsten;

3. DRAWING- the process of finishing processing of machine-building profiles by PULLING the metal through the CALIBRATED hole. Always no heat.

DRAWING PRODUCTS - bars of various cross-sections, pipes and wires made of non-ferrous alloys and steel.

4. FORGING- the process of obtaining shaped products by purposeful repeated and sequential deformation of a heated initial workpiece using a universal backing tool (piercing, crimping, mandrels, axes) between the heads of a hammer or press.

Forging is carried out (see Fig. 1.12, p. 14) manually, on pneumatic and steam-air hammers and hydraulic forging presses and is used in small-scale production, as well as to obtain heavy forgings weighing more than 200 kg. The main forging operations are (see Fig. 1.13, p. 15): DISCHARGE (a), BROADING (b), BREAKING (c), CUT (d), BENDING (e)

5. HOT FORGING- the process of obtaining shaped products by deforming a heated initial workpiece in a STREAM - a closed cavity of the tool - STAMP (see Fig. 1.14, p. 15). The configuration and dimensions of the strand completely predetermine the configuration and dimensions of the resulting forging. Stamping is carried out on hammers, presses and horizontal forging machines, used in mass and large-scale production, where the manufacture of dies is economically viable. Products are: shafts, levers, connecting rods, rods, gears. Three types of stamp designs are used:

OPEN STAMP (a);

CLOSED STAMP WITH ONE PART PLANE (b);

CLOSED STAMP WITH TWO PART PLANES (c).

Rice. 1.14. Scheme of hot forging: 1 and 2 - upper

and lower stamps; 3 - forging; 4 - flash; 5 - punch;

6 - matrix; 7 - ejector; 8 - detachable matrix

6. SHEET STAMPING- the process of obtaining flat and voluminous thin-walled products from sheet material on presses using stamps (see Fig. 1.15, p. 16). Basic operations: CUTTING, PLUGING, BENDING, DRAWING, FENDING, COMPRESSING and FORMING. All without heating.

The main types of blanks for parts are blanks obtained:

pressure treatment;

Cutting long and profile rolled products;

Combined methods;

Special methods.

Receiving blanks by casting .

Compared to other methods of producing blanks, casting has a number of advantages:

High utilization of metal and weight accuracy;

Practically unlimited dimensions and mass of castings;

The possibility of using alloys that are not amenable to plastic deformation and difficult to machine.

The method of obtaining blanks by casting into sandy-clayey forms due to their versatility, they are used in all types of production. About 80…85% of cast billets are produced by this method. The most complex castings, of virtually unlimited dimensions, can be produced. Castings have a uniform structure and are characterized by good machinability. Casting slopes are 1-3˚ for wooden models, 1-2˚ for metal models with manual molding, with machine -0.5-1˚.

The disadvantages of this method are:

High consumption of metal and molding materials;

Large allowances for m / o;

Large production areas;

Large capital costs to create normal working conditions;

A significant number of marriages.

Casting in permanent metal molds allows you to increase productivity and removal from production areas, increase accuracy and reduce the roughness of the pov-tey, reduce the consumption of metal and molding materials, allowances for m / o, improve the mechanical properties of the material, reduce the cost of castings and the number of defects.

Molds of molds are made of cast iron or steel by casting with subsequent m / o. Applies also casting in lined mold.

Non-ferrous alloys, which have a lower melting point and, consequently, a higher form stability, have received the greatest application for die casting.

The resistance of molds is: when casting non-ferrous alloys - up to 150 thousand castings, when casting cast iron - up to 1-5 thousand fillings, steel - no more than 100-500 fillings.

The disadvantages of die casting are:

The need to simplify the configuration of castings and increase the wall thickness of hollow castings;

Difficulty in the release of gases from the mold, and as a result - the possibility of the formation of gas shells;

The possibility of the appearance of a bleached layer on the surface of cast iron blanks.

centrifugal casting it is used to obtain castings such as bodies of revolution (pipes, disks, bushings, cylinders, spindles) and shaped castings from steel, cast iron, non-ferrous metals and alloys.

The method of centrifugal casting has several varieties: with a vertical axis of rotation, horizontal, inclined, vertical, not coinciding with the axis of the casting. It allows to obtain, in comparison with previous methods, a higher quality of the structure due to a more organized arrangement of metal atoms, less metal consumption (there are no profits, gating systems), to reduce the number of defects - the yield of good casting reaches 95% (20-60% more, than when casting in sand-clay molds), reducing the cost of manufacturing castings by 20-40%.

The disadvantages are limited configuration and size of castings, the complexity of the form for castings of complex configuration.

Injection molding allows to obtain accurate castings from non-ferrous alloys with low roughness and small wall thickness, increased strength of castings by 25-40% compared to casting in sand-clay molds, reduce or completely eliminate machining allowances, implement high automation of the process, improve working conditions, shorten the production cycle. This method is used to cast billets of parts: carburetor bodies, electromagnets, shields of small electric motors, etc.

Injection molding is carried out on special injection molding machines with horizontal or vertical compression chambers; a type of injection molding is casting using a vacuum.

The downside of the way yavl-Xia need to use complex forms and special equipment.

Investment casting makes it possible to obtain high accuracy and low surface roughness of castings, reduce internal stresses in castings or eliminate them altogether, obtain minimal allowances and improve working conditions.

Varieties of the method are: casting on soluble salt models, casting on burnt models.

Lack of data methods is a complex technological process for producing castings, requiring special equipment and special tooling, a long production cycle.

Casting in shell molds compared to casting in sand-clay molds, it gives higher accuracy and lower surface roughness, small machining allowances, reduced labor intensity for all elements of the process, high productivity, several times smaller number of molding sands, improved working conditions, the possibility of introducing an integrated automation.

Shell forms can be: sand-resin, chemically hardening and liquid glass.

Disadvantages of Shell Mold Casting- expensive and complex equipment, expensive molding sands, the need to manufacture accurate metal models.

Billets obtained by stamping liquid metal , have a high density structure. The method allows to reduce metal consumption by 1.5-3 times compared to casting in sand-clay molds, does not require expensive equipment and tooling.

Liquid metal stamping has several varieties:

With crystallization under piston pressure;

Squeezing;

Vacuum suction;

Continuous casting, etc.

In addition to the above casting methods, there are others, for example, casting into molds: gypsum, sand-cement, brick, fireclay-quartz, clay, stone, ceramic, etc.

In 1988, a unified GOST 26645-85 "Castings from metals and alloys" was put into effect for castings obtained by any method from ferrous and non-ferrous metals and alloys. This standard establishes tolerances for dimensions, shape, location and surface irregularities, mass tolerances and machining allowances. According to GOST 26645-85, casting accuracy is characterized by four indicators:

Dimensional accuracy class (22 classes);

The degree of warping (11 degrees);

The degree of accuracy of surfaces (22 degrees);

Mass accuracy class (22 classes).

Classes of dimensional accuracy and mass accuracy of castings are subject to mandatory application.

The standard provides for 18 rows of casting allowance.

In the technical requirements of the casting drawing, the casting accuracy standards must be indicated in the following order:

Dimensional accuracy class;

The degree of warping;

The degree of accuracy of surfaces;

Mass accuracy class;

Casting offset tolerance.

An example of a symbol for casting accuracy of the 8th class of dimensional accuracy, 5th degree of warping, 4th degree of surface accuracy, 7th class of mass accuracy with a displacement tolerance of 0.8 mm: Casting accuracy 8-5-4-7 cm 0.8 GOST 26645-85.

In the technical requirements of the casting drawing, the values \u200b\u200bof the nominal masses of the part, machining allowances must be indicated in the following order. Technological laps and mass of the casting.

An example of a symbol for nominal masses equal to -20.35 kg for a part, -3.15 kg for processing allowances, 1.35 kg for technological allowances, and 24.85 kg for casting.

Weight 20.35-3.15-1.35-24.85 GOST 26645-85.

For unmachined castings or in the absence of laps, the corresponding values \u200b\u200bare denoted by "0". For example: Weight 20.35-0-0-20.35 GOST 26645-85.

Preforms obtained by pressure treatment .

There are the following methods of obtaining workpieces by pressure treatment:

Stamping (hot and cold);

special ways.

All metal forming processes are based on the ability of metals in the solid state to stably change shape and size under the action of external forces, i.e., plastically deform. In the process of plastic deformation, the metal acquires not only the required shape, but also changes its structure and physical and mechanical properties.

Methods for producing workpieces by pressure are mainly high-performance processes, provide small allowances and an improved metal structure.

The material from which pressure blanks are obtained must have malleability: strength and ductility at high temperatures. The ductility mainly depends on the chemical composition of the material and its components. For example, elements such as chromium, silicon, carbon and manganese reduce, and nickel increases ductility. The presence of sulfur (at a temperature of 800-900 degrees) causes the phenomenon of red brittleness, phosphorus (more than 0.03%) cold brittleness.

Forging .

During forging, shaping occurs due to the free flow of metal to the sides perpendicular to the movement of the shaping tool - the striker.

By forging blanks on hammers and presses, forgings of a simple configuration with a large mass (up to 250 tons) are obtained. Forgings have a good metal structure over the entire section, because the metal flow is not limited by the tool, and it is well forged. Forging does not require special tools and equipment.

disadvantage yavl-Xia low productivity, high labor intensity, large allowances and allowances for processing, low accuracy. To obtain forgings of a more complex configuration, backing rings and dies are used. To reduce allowances for processing and reduce labor intensity allows the use of radial forging machines. However, their scope is limited only to bodies of revolution.

Depending on the mass of the forgings, forging is used: pneumatic hammers, steam-air hammers, hydraulic presses.

hot stamping .

Compared to forging, hot forging has a number of advantages:

More complex forging shape and better surface quality;

Reduced processing allowances;

Metal saving;

Improving the accuracy of manufacturing blanks;

Reducing stamping slopes due to the presence of ejectors in the design of stamping equipment;

Increasing labor productivity;

Reducing labor intensity;

Improvement of working conditions.

Disadvantages of hot forging applies to:

Expensive equipment (tool - stamp), which allows the use of stamping only with a large volume of production of parts;

Restrictions on the mass of forgings obtained;

Additional waste of metal in the burr (10-30% of the weight of the forging);

Greater deformation forces than forging.

The use of standardized die blocks with interchangeable inserts and the unification of other equipment make it possible to use stamps even in small-scale production. A good effect is given by combined methods of manufacturing blanks: forging and subsequent stamping, etc.

Hot forging is divided into different types depending on the types of dies, equipment, initial workpiece, method of installing the workpiece in the die, etc.

Depending on the equipment, the following types of forging are available:

On stamping steam-air hammers of double action;

On crank hot stamping presses;

On horizontal forging machines (HCM);

On hydraulic presses;

On high speed hammers;

On special machines (forging rolls, horizontal bending machines, rotary swaging and radial swaging machines, electric upsetting machines, rolling machines).

Depending on the type of stamp, stamping is divided into the following types:

In open stamps;

In closed stamps;

In extrusion stamps.

Forging in open dies is characterized by the fact that the die remains open during the deformation process. The gap between the movable and fixed parts of the stamp is variable, metal flows (is squeezed out) into it during deformation, forming a burr. The main purpose of this burr is to compensate for fluctuations in the initial workpieces by weight. This type of die can be used for parts of any configuration. However, the presence of a burr increases the consumption of metal, and for trimming the burr, it is necessary to use special trimming presses and dies.

When stamping in closed dies (flashless stamping), the stamp remains closed during the deformation process, i.e. the metal is deformed in a closed space. The absence of burr reduces metal consumption, there is no need for trimming presses and tools. The macrostructure of the forgings is of higher quality, since there is no fiber damage that occurs when cutting the burr. However, this type of die is used for simple parts, mainly solids of revolution.

Stamping in extrusion dies- the most progressive. At the same time, metal consumption is reduced (up to 30%), the coefficient of weight accuracy is increased, forging accuracy and surface cleanliness are increased, labor productivity is increased by 1.5-2.0 times.

Flaws- high specific deformation forces, high energy consumption and low durability of die tooling. Used for workpieces with high ductility.

Hammer stamping improves the accuracy of workpieces, but is a laborious process. The big difficulty is the centering of the halves of the stamp relative to each other. The process is difficult to automate.

Press stamping ( crank, hydraulic, friction) due to the use of ejectors allows to reduce processing allowances, stamping slopes by 1.5-2.0 times compared to stamping on hammers, improve working conditions, and increase productivity. The absence of impacts during operation reduces vibrations, increases the durability of the dies, improves the centering of the halves of the dies.

Stamping on horizontal forging machines (HCM), compared to stamping on presses and hammers. Provides the possibility of obtaining complex forgings with deep cavities and holes, obtaining high-quality blanks without flash and stamping burrs with small machining allowances.

GCM is a mechanical press located in a horizontal plane. Unlike hammer and press dies, GCM dies have two mutually perpendicular slots and can be open or closed. The presence of two connectors in the stamp creates the best conditions for landing work and allows you to significantly reduce stamping slopes (outer 15´-1 degrees, internal 30´-2 degrees), up to their absence.

Forgings produced at GCM are usually in the form of bodies of revolution.

disadvantage yavl-Xia need to use a bar (rolled products) of increased accuracy.

When developing a forging drawing, GOST 7505-89 is used, the data of which apply to stamped parts weighing up to 250 kg, manufactured by hot forging from ferrous metals on various types of stamping equipment.

When determining allowances and permissible deviations of dimensions, it is necessary about determine the original index.

The initial index is a conditional indicator that takes into account the design characteristics (accuracy class, steel group, degree of complexity, parting surface configuration) and the weight of the forging. The standard establishes 23 initial indexes. The initial data for determining the initial index is:

- forging weight;

Group of steel;

The degree of complexity of the forging;

Forging accuracy class.

M1 - carbon and alloy steel with a carbon content of up to 0.35% and alloying elements up to 2%;

M2 - carbon steel with a carbon content of more than 0.35 to 0.65% and alloyed, with the exception of that specified in group M1.

The degree of complexity of the forging (4 in total) is determined by calculating the ratio of the mass (volume) of the forging to the mass (volume) of the geometric figure into which the shape of the forging fits.

The standard provides for five accuracy classes for forgings.

The forging drawing must indicate: the initial index, accuracy class, steel group and the degree of complexity of the forging.

Cold stamping.

Volumetric cold stamping;

Sheet stamping;

Stamping on horizontal bending machines;

Rolling;

knurling;

Calibration.

Volumetric cold stamping is divided into a number of types:

extrusion;

disembarkation;

Radial compression;

reduction, etc.

This shaping method eliminates metal loss and dross that occurs when metal is heated, provides more accurate workpiece dimensions and surface quality. As a result of cold deformation in the metal, some internal defects are eliminated, the uniformity of its structure is ensured, and the surface layer is strengthened.

Plastic blanks .

Plastics are non-metallic materials that are obtained on the basis of high-molecular compounds - polymers.

plastics, obtained from artificial and natural resins and their mixtures with various substances, can be formed by pressing, casting and extrusion. They have valuable physical and mechanical properties (resistance to aggressive environments, electrical and thermal insulation, antifriction, etc.), it is easy to make complex design parts from them.

Plastics are used: for the manufacture of small parts (plugs, plugs, gaskets, liners, gears, impellers, etc.). However, plastics are characterized by low impact strength, insufficient strength, low heat resistance, and aging.

Basic provisions for choosing the optimal workpiece .

The chosen method of obtaining a workpiece must be economical, providing the required quality of the part, productive, and not labor-intensive process.

The main thing when choosing a workpiece is to ensure the specified quality of the finished part at its minimum cost.

It is advisable to transfer the solution of the problems of forming parts to the procurement stage and thereby reduce material costs, reduce the share of machining costs in the cost of the finished part.

First of all, when choosing a workpiece, it is necessary to determine which method is most appropriate to obtain a workpiece for a given part. In this case, it is necessary to focus on the material and the requirements for it in terms of ensuring the service properties of the part. Next, using a qualitative assessment, outline a preliminary method for obtaining it.

Pre-selection of the material and method of obtaining the workpiece on the basis of economic indicators can be made according to the tables or graphs given in the literature. The graphs show the dependence of the cost of obtaining a workpiece on the program for the production of parts and the accuracy of manufacturing.

The final choice of the workpiece is made on the basis of economic calculations of the cost of obtaining the workpiece and the cost of its further m / o.

As the configuration of the workpiece becomes more complex, the allowances decrease, and the dimensional accuracy increases, the technological equipment of the blank shop becomes more complicated and more expensive, and the cost of the workpiece increases, but at the same time, the labor intensity and cost of the subsequent m/o workpiece decrease, and the material utilization rate increases. Blanks of a simple configuration are cheaper, because they do not require subsequent labor-intensive processing and increased material consumption.

As blanks for machine parts are used:

1.rental . Calibrated bars and hot-rolled steel of increased and ordinary accuracy are used. According to GOST 7417, calibrated bars are made with a diameter of 3-30 mm according to accuracy class 2, a diameter of 3-65 mm according to the 3rd accuracy class and 3-100 mm according to the 4-5th accuracy class.

When fastening in collets, calibrated bars of the 5th accuracy class are used. Workpieces from calibrated bars of the 4th and higher accuracy classes are usually not processed with a blade tool, but are ground.

In the conditions of large-scale and mass production, it is advisable to use the rental of special profiles; at the same time, m / o is almost completely eliminated or significantly reduced. Profile cold drawing provides the 4th class of accuracy and the 6th class of cleanliness. It is most expedient to use profile drawing for parts with the same profile along the entire length.

The machining of rolled blanks is preceded by straightening and cutting.

The cutting of workpieces is carried out on turning and turning-cutting machines, circular, band and hacksaws, crank and eccentric presses.

The method of cutting on presses provides high productivity, but it does not achieve perpendicularity of the cut to the axis of the bar and the end of the workpiece is crushed.

When cutting on hacksaws and band saws, metal consumption is reduced, but the productivity of these methods is low.

When choosing a method for cutting off a workpiece, the economic feasibility of one or another method is taken into account.

Blanks from sheet metal are cut off from a sheet or strip on guillotine shears, press shears, using gas cutting for marking on special machines that work on copiers and allow you to simultaneously cut several blanks with sufficiently high accuracy.

Blanks of sheet metal parts are produced by punching(flat parts of various configurations), bending, drawing and combining these methods. It is advisable to use stamping in the manufacture of a significant number of parts; at the same time, the cost of manufacturing stamps is offset by a decrease in the cost of manufacturing parts. Stamping of parts made of sheet material is carried out on mechanical (crank and eccentric) hydraulic presses.

2. Forgings. They are used for parts of complex configuration with a large cross section or parts with a large difference in sections along the length (gears, disks, stepped and flanged shafts). Forgings are produced on pneumatic and steam-air hammers and hydraulic presses from rolled bars or ingots.

The accuracy of blanks made by free forging is not high, so they have significant machining allowances. Tolerances on the dimensions of forgings made by free forging on presses are 12-72 mm, depending on the configuration and dimensions of the forging.

Free forging is difficult to obtain workpieces of complex configuration with projections, ribs, recesses.

Free forging is used to obtain blanks in individual and small-scale production in cases where a large amount of metal is consumed per chip during the use of rolled products, as well as to increase the mechanical properties of the material.

3.Punching. Stamped blanks are used for manufacturing parts of complex configuration. When stamping in closed dies, the f-ma and sizes of blanks are determined by the f-m and sizes of the stamp streams. In closed dies, you can get details of a complex configuration - with ribs, protrusions, bends. At the same time, labor productivity is high.

For example, labor productivity when stamping complex small parts in several streams is 200-400 parts per hour, and when stamping larger parts weighing about 100 kg, up to 100 parts per hour. The high accuracy of workpieces can significantly reduce processing allowances and, in some cases, using embossing. Completely refuse allowance.

But forging in closed dies is used only with a significant number of parts in the series. This is due to the high cost of forging and cutting dies.

Forgings are made on steam-air and friction hammers, on friction, crank and hydraulic presses, and on horizontal forging and rotary machines.

With small series of stampings, they can be made in backing dies on forging hammers.

Horizontal forging machines produce parts such as valves, shafts with flanges, gear shafts, bushings, levers. In this case, it is possible to obtain a workpiece without stamping slopes or with very small stamping slopes, with stitched blind or through holes, as well as workpieces with a large difference in cross-section along the length.

Allowances on stamped blanks are accepted in the range of 0.5-5 mm and depend on the method of manufacture and the dimensions of the part; manufacturing tolerances usually do not exceed half the size of the allowance.

Recently, new methods have appeared for producing stamped blanks from rolled bars and sheets;

Stamping with the use of explosives, in which. by a blast wave acting on the workpiece through an aqueous or air medium, it is given the form of a matrix made of metal, concrete, and other materials;

Stamping in an electromagnetic field, at which. under the action of a powerful short-term electromagnetic pulse, the f-ma of the matrix is given to the workpiece.

The advantages of these methods are the possibility of obtaining large blanks in the absence of powerful equipment, the simplicity of equipment and its low cost, the possibility of stamping blanks from materials that are difficult to stamp in other ways.

4. Castings from steel, cast iron and non-ferrous metals. They are used as blanks for parts of complex configuration.

Methods for obtaining castings:

1) casting in earthen molds, which. serve for the manufacture of only one part and are destroyed when the workpiece is removed;

2) casting into shell molds made of sand clad with bakelite or other polymerizing binders. In shell f-maxes, it is possible to obtain high-precision castings (class 4-5) with a surface finish of class 4-5 and small slopes, which makes it possible to reduce allowances for m / o;

Small casting slopes, which can significantly reduce allowances for m / o, and in some. cases turn out to be from processing;

3) investment casting. It is applied to details from steel and non-ferrous metals. Investment models can be used to obtain parts of a very complex configuration, with holes, channels, thin ribs and protrusions, with an accuracy of 4-7 classes and a purity of class 3-4. The use of this expensive method of obtaining blanks is advisable in cases where precision casting allows you to abandon the m / o. Precision casting manufacture parts (weights of regulators, pushers of fuel pumps, impellers of water pumps). This method can be used to obtain holes f up to 2.5 mm and walls up to 0.3 mm thick;

4).centrifugal casting method. In this way, blanks are obtained for parts having the form of bodies of revolution (bushings, pipes, sleeves) and blanks for shaped profile parts having an axis of symmetry (levers, forks, etc.);

5) casting by vacuum suction. In this way, bushings and other blanks of a simple shape are made;

6) injection molding. It is used for the manufacture of thin-walled large-sized parts such as covers, thin-walled plates, etc.

5. Liquid metal stampings. They are used for the manufacture of blanks from non-ferrous metals. Blanks are obtained by pouring liquid metal into a heated stamp. when cooled to a semi-liquid state under the pressure of the punch, it fills the mold and crystallizes. Crystallization under pressure ensures the density of the structure, high precision and cleanliness of the surface. This method is used for the manufacture of critical blanks.

6. Metal-ceramic blanks. They are obtained by pressing blanks from a mixture of metal powders in molds, followed by sintering and calibration. This method can be used to obtain parts with special properties: heat-resistant (valve seat inserts)

Anti-friction (bushings, bearings), friction, as well as parts that do not require additional processing.

Forgings, stampings, castings made of iron, steel and light alloys are often subjected to thermal treatment before m / o: normalization, annealing, improvement, aging, hardening, etc. This allows you to give the material of the workpieces increased fur-e properties, improve machinability or eliminate internal stresses that have arisen during the cooling of the workpiece and cause warping of parts during processing and operation.

The type of workpiece has a significant impact on the character of the TP, the laboriousness and efficiency of processing.

When choosing a workpiece, it is desirable that its shape should be as close as possible to the shape of the finished part. This allows better use of the material and reduces the cost of removing the allowance.

However, with the complication of the form and the increase in the accuracy of the workpieces, the cost of manufacturing increases, because requires the use of more complex and expensive tooling and equipment. Therefore, different workpieces are selected for the same parts of different series.

If issued several tens of crankshafts of engines, then a blank is used - forging;

If it is necessary to produce several thousand of such crankshafts, the workpiece is performed - by stamping.

When determining the f-we and solutions of the workpiece, it is necessary. provide for an allowance sufficient to obtain the required purity of the processed pov-tey, taking into account the compensation of errors caused by inaccuracies in the manufacture of the workpiece and its deformation, as well as errors in the installation of the workpiece during processing.