Technological characteristics 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 where it is possible to use different kinds workpieces (for example, forgings, stampings or bars), the best 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. From aluminum alloys cast cylinder blocks, crankcases, boxes, pistons.
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 way it is advisable to apply 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 in the form 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, significant economical effect allows the manufacture of individual elements of the workpiece by progressive 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. Cermet 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. Important task in the manufacture of blanks, 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 the 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 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 during processing 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 sometimes turns out to be 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 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 on the quality and accuracy of surfaces. The higher the requirements for a part according to technical requirements, the larger 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) technological process surface treatment. 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 on 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
Test
in the discipline "Technologies and equipment of procurement production"
Completed: student gr. TAMP-12bzu
HE. Strelnikova
Checked by: teacher
T.R. Ablyaz
Perm, 2015
Exercise 1.
Appointment and trends in the development of procurement production.
The level of development of mechanical engineering is one of the most significant factors of technical progress. The production of blanks is one of the main stages of machine-building production, which directly affects the consumption of materials, the quality of products, the laboriousness of their manufacture and the cost.
The share of labor intensity of procurement production in mechanical engineering is about 45% of the total labor intensity.
Thus, the purpose of blank production is the production of blanks for machining and assembly shops. The main trends in the development of blank production can be called a decrease in the metal consumption of blanks (due to a decrease in allowances) in combination with an increase in surface quality and manufacturing accuracy; application of new methods of product manufacturing; the use of high technologies in the study of the manufacture of machine parts.
What blanks are used in mechanical engineering.
Blanks, depending on their type and type of production, are obtained in foundries, forges, stamping, and other shops. Welded blanks, blanks from composite materials, plastics are also made. Modern blank production has the ability to supply blanks of the most complex configuration, various sizes and accuracy. An approximate structure for the production of blanks in mechanical engineering is shown in Table 1.
The concept of preparation. Classification of blanks.
A blank is an object of labor, from which a part is obtained by changing the shape, dimensions, surface properties and (or) material (the definition is given in accordance with GOST 3.1109-82 ESTD. Terms and definitions of basic concepts.
Workpieces can be classified into three main types:
1. Engineering profiles.
Constant section (round, hexagonal, pipes) and periodic section are made. In large-scale and mass production, special rolled products are also used.
2. Piece blanks.
Piece blanks include castings, forgings, stampings, welded blanks. Piece blanks are used in all types of production.
3. Combined blanks.
These are complex blanks obtained by joining (for example, welding) individual simple elements. This type of blanks is used when it is necessary to manufacture large massive structures. This allows you to reduce the weight of workpieces, and for the most loaded elements to use suitable materials.
Workpieces are characterized by configuration, dimensions, accuracy, surface condition. The shape and dimensions of the workpiece, the state of its surface to a large extent affects the further machining of the part. The dimensional accuracy of the workpiece is one of the critical factors affecting the cost of manufacturing the part. Considering all of the above factors, most workpieces require preliminary mechanical processing: cleaning, straightening, peeling, centering, processing of technological bases.
Methodical instructions for implementation practical work and sections in course and diploma projects for students of the specialty 151001 "Mechanical Engineering Technology" Sarov 2009 Ministry of Education Nizhny Novgorod region GOU SPO "Sarov Polytechnic College"
Design of stamped forgings
Guidelines for the implementation of practical work and sections in course and diploma projects for students of the specialty 151001 "Mechanical Engineering Technology"
Compiled by: Sunyaykina N.N.- teacher the highest category special disciplines GOU SPO SPT
Reviewer: Khaldeev V.N.- Ph.D., deputy. head Department of "Mechanical Engineering Technology" FGOU VPO
"Sarov State Institute of Physics and Technology"
These guidelines summarize the theoretical and practical issues on the topic "Choice of blanks", give characteristics to the main methods for obtaining blanks, in particular, blanks obtained by stamping, consider the basic requirements for performing practical work and sections of course and graduation projects to determine the size of blanks obtained by stamping, purpose allowances and tolerances on the surface of stamped blanks, drawing up a stamping drawing. Provided reference material on the topic. The procedure for performing calculations is comprehensively stated.
The manual is intended for students of the specialty 151001 "Technology of mechanical engineering" of primary, secondary and higher vocational education, as well as for the heads of course and diploma projects.
Agreed by the meeting of the graduation PCC GOU SPO SPT
approved by the meeting methodological council GOU SPO SPT
Protocol No. ___ dated “____” _____________20 g
1. Types of blanks and their characteristics………………......................................……. four
2. The choice of the type and method of obtaining the workpiece………………………………..... 6
3. Stamped forgings……..………………………………………………. eight
4. GOST 7505 - 89 “Stamped steel forgings. Tolerances, allowances
and blacksmith laps”………………………………………………………….. 15
5. GOST 3.1126 - 88 "Rules for the execution of drawings of forgings"……………. 24
6. An example of calculating the workpiece obtained by hot forging ... 25
7. Laboratory work on the course "Technology of Mechanical Engineering"………….. 32
List of used literature………………………………………….. 34
TYPES OF BLANKS AND THEIR CHARACTERISTICS
blank- an object of production from which a part or an integral assembly unit is made by changing the shape, size, surface roughness and material properties.
Preparation before the first technological operation called the original workpiece.
The choice of a workpiece consists in establishing a method for its manufacture, calculating or choosing allowances for machining and determining the dimensions of the original workpiece.
The method of manufacturing the workpiece is determined by the shape and dimensions of the part, the technological properties of the material, its melting point, structural characteristic(fiber direction and grain size). When choosing a workpiece, the assortment of material (pro-cut), available equipment, production program, type of production, degree of mechanization and automation are taken into account. The best option workpiece manufacturing is established on the basis of technical and economic calculations. Increasing the accuracy of blanks (reducing allowances) allows you to save metal, reduce the cost and labor intensity of machining, but this may increase the cost of manufacturing the original blanks. With a small production program, the use of some technological processes for manufacturing blanks (hot stamping, etc.) may not be economically feasible due to the high cost technological equipment and rigging.
The most common types of blanks are:
Blanks from rolled products and special profiles;
Cast blanks;
Forged and stamped blanks;
Combined blanks;
Billets obtained by powder metallurgy.
Rolled blanks
From high-quality round hot-rolled steel, optimal blanks are obtained for the manufacture of stepped shafts with a small difference in diameters, axes, lead screws, rods and other similar parts of an extended cylindrical shape for any type of production.
Round, square, hexagonal, strip and sheet products are widely used in one-off production for the manufacture of parts of any configuration. Even with a low metal utilization rate, this often proves to be more profitable than using special methods for obtaining precise workpieces that require complex, expensive tooling. Naturally, with a small volume of production, such equipment cannot pay for itself.
Rolled tubulars are beneficial for the manufacture of hollow shafts, rings, cylinders, sleeves, etc.
Profile rolled products in the form of angles, channels, etc. used for welded metal structures, frames, beds, housings, etc.
In the conditions of large-scale and mass production, rolling of a periodic profile obtained by cross-helical rolling is used. After cutting such a rolled product, step blanks are obtained that are close in shape to the finished part.
Cast blanks
Cast blanks are used in cases where:
The material does not allow the workpiece to be obtained in another way;
With large dimensions of the workpiece, which cannot be obtained in other ways;
If a cast billet is more profitable for economic reasons.
Casting in sand-clay molds It is used in all types of production, as it is technologically versatile. This method produces ~80% of all castings, and only 20% is accounted for by all other casting methods. In mass production, more accurate blanks are used, obtained by machine molding on metal models, in single production - with low accuracy, with manual molding on wooden models.
In serial and mass production, in addition to casting in sand-clay molds, the following special casting methods are used.
Casting in shell molds receive workpieces of complex configuration. They are much more accurate than castings obtained in sand-clay molds, but require more complex equipment and are therefore more expensive.
Investment casting beneficial for the manufacture of complex and precise workpieces from difficult-to-machine materials. This method is the most time-consuming among the casting methods, but can pay off due to a significant reduction in material consumption and labor-intensive machining.
Casting in metal molds (in a chill mold) has two distinctive features:
Metal molds can be used repeatedly;
Metal forms provide intensive heat dissipation and high speed cooling of molten metal.
The latter circumstance reduces the fluidity of the metal and does not allow to obtain thin-walled workpieces. But the same property plays a positive role, contributing to the formation of a stronger fine-grained metal structure.
Injection molding allows you to speed up the filling of a metal mold and obtain complex precision castings with thin walls (up to 1 mm) from non-ferrous alloys.
centrifugal casting used to produce workpieces such as bodies of revolution: pipes, sleeves, cylinders, etc. Like injection molding, it provides a quick filling of a metal mold and a dense (without cavities and pores) casting, but this is created due to the “weighting” of the metal by centrifugal forces. The negative quality of centrifugal casting is an increase in segregation of alloys under the action of centrifugal forces: heavier alloy components move to the peripheral layers of the workpiece.
Forgings and stamped blanks
Such blanks are used in the following cases:
1) For the manufacture of workpieces with a large difference in sections (stepped and crankshafts, levers, etc.
2) With large dimensions of the workpiece, exceeding the dimensions of the rolled products.
3) To impart high mechanical properties to critical parts.
Forging is a universal method for the production of blanks weighing from 10 g to 350 tons. During forging, shaping is carried out by successive deformation of individual sections of the blank, which makes it possible to obtain large-sized blanks. It is mainly used in single-piece production due to low productivity and low accuracy of workpieces.
To improve the accuracy and quality of the surfaces of forgings, forging in backing dies is used.
In serial and mass production, hot forging is used. Stamping is much more productive than free forging. Stamped blanks are much more accurate, have better surfaces, but complex expensive dies are required for their manufacture. Stamping is performed on hammers, presses, horizontal forging machines (HCM) and other equipment. The mass of stamped blanks is from 0.5 to 30 kg. Stamping happens in open and closed stamps. Forging by extrusion and cold forging are promising.
Combined Methods
Combined methods are used for the manufacture of large and complex workpieces. The design of such blanks is divided into simple elements that are cast, stamped, cut out of rolled products, and then welded into a single blank. Sometimes blank elements are pre-treated before welding. Instead of welding, partial pouring of pre-treated elements obtained by other methods can be used. In combined blanks, you can use various materials to obtain individual elements, providing their special qualities.
Method of powder metallurgy.
A semi-finished product for obtaining blanks are finely dispersed powders of raw materials. The workpiece is pressed from powder in a mold and sintered into a monolith by heat treatment. The composition of the charge for sintering can include powders of hard refractory materials and obtain pseudo-alloys with unique properties, for example, copper-tungsten, tungsten carbide - cobalt (tool carbide), etc. The powder metallurgy method also makes it possible to obtain porous materials for bearings. By this method, it is possible to obtain workpieces with an accuracy of 7 quality without heat treatment. However, the high cost of tooling makes the method effective only for very large production volumes.
Before entering the cutting process, the original blanks are subjected to cleaning, straightening and heat treatment, depending on the methods of their manufacture and the requirements. Castings are cleaned of molding earth and cores, then sprues, bulges are removed, profits are cut off, burrs and random tides are cleaned. Cleaning is carried out on stationary and portable grinding and peeling machines, chisels, steel brushes. To mechanize the cleaning process, shot blasting machines and rotating (tumbling) drums are used. A workpiece obtained by hot stamping usually has a flash at the place of the die split, which is cut off or cut out in dies on cutting crank presses. After trimming, heat treatment and straightening are carried out in a hot or cold state. Heat treatment in order to obtain the desired microstructure and mechanical properties includes normalization, improvement and other processes.
Stampings are cleaned of scale and burrs by shot blasting, pickling, tumbling in rotating drums. To obtain accurate dimensions, some stamped blanks are calibrated and minted in a cold or hot state. Before this operation, annealing or normalization and descaling are performed. An allowance of 0.2 to 0.8 mm per side is given for chasing, depending on the area of chasing. Long rolled blanks are straightened manually, on presses or on special multi-roller straightening and sizing machines in 1-2 moves.
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:
SAVE 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 air-steam 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.
- castings received various methods, are used for the manufacture of parts of complex shape from cast iron, non-ferrous metals and special cast steel (the index L is added to the designation of the steel grade). Holes of various shapes can be obtained by casting methods in the workpiece. Casting blanks are characterized by increased surface roughness, increased hardness of the surface layer (peel), large machining allowances and high cost; forgings are used for the manufacture of plastic metal parts with a less complex configuration than castings, but with large differences in size (for example, diameters). Holes, as a rule, are not obtained by forging methods. The exception is cases when obtaining a hole in other ways is not economically feasible.
- Forgings characterized by less surface roughness than castings, but greater waviness; increased hardness of the surface layer (crust), large processing allowances and low cost;
- stampings are used for the manufacture of parts from plastic metals of a more complex configuration than that of castings. When stamping, it is possible to obtain holes of any shape and configuration. The stamping blank is characterized by low surface roughness, high precision, small machining allowances and the highest cost. Stamping blanks are used in cases where there are surfaces that cannot be processed mechanically, but their high quality is required;
- Long products. Its main advantage is cheapness. It is made of steel and non-ferrous metals in the form of bars with various cross-sectional shapes (circle, square, hexagon, pipe, square, Taurus, etc.). Rolled blanks have found the widest application due to their simplicity and low cost. A significant disadvantage is the low utilization rate of the material.
The very first criterion when choosing the type of workpiece, the material from which the part is made is used: steel - rolled, forged, stamped, less often - casting; cast iron - various casting methods; col. metals - rolling, casting, less often - stamping. The second criterion are technological capabilities each type:
for details of a simple form, rolling is preferable; for parts of medium and large sizes of simple shape with large differences in size - forging; less preferred, due to high cost, casting or stamping; for parts of complex shape - casting or stamping.
Feasibility study for the correct choice of workpiece
The choice of workpiece type according to these criteria is approximate. They can satisfy several options for blanks at once. For example - flange ( see pic.).
For a more accurate determination, it is necessary to perform economic calculation- calculation of the technological cost of manufacturing the part. This calculation is quite complex and requires the use of a large amount of economic data of a real enterprise. For educational purposes, instead of calculating the technological cost, it is allowed to determine the cost of the workpiece and add to it the cost of distinctive operations. If in this case the selected methods for obtaining the workpiece are equivalent, preference should be given to the option with a higher material utilization factor g. It shows how much % of the workpiece material is used for its intended purpose, and how much goes to waste, to chips. 
where q- mass of the finished part, g Q- mass of the initial workpiece, g.
where r- density of the workpiece material, g/mm 3 ; V- the volume of the workpiece, mm 3. Before calculating the volume of the workpiece, it must be designed: according to the drawing of the part, the processing allowances are calculated, the dimensions of the workpiece are determined, and its drawing is developed. Based on the drawing, the workpiece is divided into elementary figures (cylinder, parallelepiped, ball, etc.), the volume of which can be calculated using known formulas. Separately, the volumes of bodies are considered, separately - the volumes of voids. The volume of the workpiece is determined as
If the part is made from rolled metal or forging, then the cost of the workpiece is determined by the weight of the material required for the manufacture of the part and the weight of the delivered chips, rub., 
where S- price of 1 kg of billet material (rolled products; forgings), rub.; S out- price of 1 ton of waste, rub. obtained by other methods, with sufficient accuracy for course design is determined by the formula:
Where C i- base cost of 1 ton of blanks, rub.;
k t, k s, k c, k m, k p- coefficients depending on the accuracy class, complexity group, weight of the workpiece, grade of material and volume of production of parts.
In the case when the choice of the type of workpiece affects the content technological process, determine the cost of distinguishing operations:
rub.,
Where T st- tariff rate worker - machine operator, rub./hour;
k=1.15 - coefficient taking into account the salary of the machine adjuster;
Tsh.k - piece-calculation time required to perform this operation, min. from comparing methods of obtaining blanks
N - annual program, pcs. 
| OPTION 1 - forging | OPTION 2 - stamping |
| Finished part weight q = 3.058 kg | |
| Billet weight Q=10.409 kg | Billet weight Q=5.794 kg |
| Material utilization factor g = 0.39 | Material utilization factor g = 0.53 |
| = 11.6 rubles. | S zag \u003d 18.02 rubles. |
 \u003d 1.25 rubles. |
Cost of distinguishing operations = 0 |
| Finally we get: | |
| S zag \u003d 11.6 + 1.25 \u003d 12.85 rubles. | S zag \u003d 18.02 rubles. |
| Annual economic effect E g \u003d (18.02 - 12.85) 10,000 \u003d 51,700 rubles. |
|
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