Calculation of allowances in the manufacture of rolled parts

Determining the amount of allowances

As can be seen from the consideration of the factors influencing the size of the allowance, the value of the normal allowance must be such that it is possible to perform mechanical processing and achieve the required surface finish and dimensional accuracy of the part in ways that meet these requirements.
It follows from this that the amount of allowance depends on the thickness of the surface layer that is to be removed, and the allowances required for all intermediate machining operations.

Since workpieces have tolerances that give deviations from the size of the workpiece in positive and negative directions, when determining the total amount of allowance, the value of the negative deviation (if allowed) should be added to the size of the workpiece.

Thus, the normal allowance will consist of the following values:

1) the thickness of the surface layer to be removed during the first rough pass of the cutting tool;

2) the amount of allowances for all intermediate machining operations

3) the magnitude of the negative deviation from the nominal size of the workpiece.

The allowance for the thickness of the surface layer to be removed during the first rough pass of the cutting tool, and the total allowance can be determined from the data in table. 9.

The allowance values ​​for intermediate machining operations can be taken according to the data in Table. 10-21.

Table 10. Allowances after rough turning of shafts for finishing turning in mm. per diameter.

Using Table 11, you need to take larger values ​​for preliminary rough surface treatment and smaller values ​​for fine processing.

Table 11. Limit allowances after turning for cylindrical grinding at the centers of hardened products in mm. per diameter.

Tolerances for pre-processing are accepted:

during rough turning according to the 5th accuracy class (OST),

during finishing turning according to the 4th accuracy class (OST),

during finishing turning with increased accuracy according to the 3rd accuracy class (OST).

If rough grinding is performed before heat treatment, then the total allowance for rough and finish grinding must be taken 1.6 times more than the values ​​​​indicated in the table. 11. Of this total allowance, approximately 40% is removed before heat treatment and 60% after heat treatment.

For thin products with a sharp transition in diameters, which are subject to warping during heat treatment, it is necessary to take the largest allowance values.

Table 12. Limit allowances after turning for cylindrical grinding in the centers of non-ground products from all materials in mm. per diameter.

In table 12 it is also necessary to take larger values ​​for preliminary rough processing and smaller values ​​for precise processing. During pre-processing, tolerances are accepted as indicated for the previous table.

Table 13. Limit allowances for centerless external grinding of hardened products in mm per diameter.

In table 13 and 14, the largest allowance values ​​are accepted for preliminary rough processing, the smallest for fine processing. During pre-processing, tolerances are accepted: during rough turning according to the 5th accuracy class (OST), during finishing turning according to the 4th accuracy class (OST), during finishing turning of increased accuracy according to the 3rd accuracy class (OST).

When rough grinding before heat treatment, the total allowance for rough and finish grinding must be increased by 1.6 times compared to the values ​​​​indicated in the table. 13.

Table 14. Limit allowances for centerless external grinding of non-hardened products from all materials in mm per diameter.

For thin products with sharp transitions in diameter, which are subject to warping during heat treatment, it is necessary to take the largest allowance values ​​indicated in the table. 13.

Below are tables of allowance values ​​for fine boring, grinding, countersinking and reaming of holes, for fine milling and grinding of planes and broaching (Table 15-21a).

Table 15. Average allowance values ​​after rough boring for finishing boring of holes with a cutter in mm per diameter.

It is very convenient for clarity to graphically depict the location of allowances at different stages of processing in the form of diagrams shown in Fig. 15a (for the shaft) and in FIG. 15b (for the hole).

The dimensions of the workpieces are called free if they relate to unprocessed surfaces or surfaces that are processed but not mating with other surfaces. Permissible deviations in the dimensions of workpieces in unprocessed areas depend on the type of workpieces and methods of their manufacture.

Figure 15a. Layout of allowances at various stages of shaft processing.

Figure 15b. Layout of allowances at various stages of hole processing.

Nominal machining allowances

Table E.1 - Allowances for machining of shafts (external surfaces of rotation)

In millimeters

Continuation of Table E.1

Continuation of Table E.1

Continuation of Table E.1

Continuation of Table E.1

Table E.2 - Allowances for centerless grinding of shafts after finishing turning

In millimeters

Table E.3 - Allowances for honing holes depending on the initial shape error and surface roughness

Table E.4 - Allowances for fine (diamond) boring of holes

In millimeters

Table E.5 - Allowances for grinding holes (per diameter)

In millimeters

Table E.6 - Allowances for honing holes

In millimeters

Table E.7 - Allowances for lapping holes

Table E.8 - Allowances for scraping holes

In millimeters

Table E.9 - Allowances for machining holes by broaching

In millimeters

Table E.10 - Allowances for processing holes in solid material according to 7 - 8 qualifications

In millimeters

Continuation of Table E.10

Table E.11 - Allowances for processing stitched or cast holes according to 7 - 8 grades

In millimeters

Continuation of Table E.11

Table E.12 - Allowances for processing holes in solid material according to the 9th and 11th qualifications

In millimeters

Continuation of Table E.12

Table E.13 - Allowances for processing stitched or cast holes according to the 9th and 11th qualifications

In millimeters

Continuation of Table E.13

Calculation of allowances in the manufacture of rolled parts

Smooth, stepped shafts and other similar parts are made from rolled steel of increased or normal precision. Surface treatment methods are determined depending on the degree of accuracy of the rolled product adopted. For stepped shafts, the calculation of allowances and maximum dimensions is carried out according to the step with the largest diameter, and if they are equal, according to the step, which is subject to high technical requirements for accuracy, surface quality and the magnitude of spatial deviations of the surface.

Spatial deviations of the surface of the rolled product location are determined by the formula:

(18)

where is the deviation of the shape of the workpiece surface (local or general), μm.; – the amount of displacement and rotation of the surface relative to the technological bases during alignment, µm

The value is equal to: with cantilever mounting; when installed in centers; where is the specific deviation of the surface shape, µm/mm; - the distance from the section for which the amount of shape deviation to the workpiece attachment point is determined (for cantilever processing of the workpiece £, and for processing in centers £0.5, where L is the total length of the workpiece, mm).

The value is determined as follows:

where dз is the tolerance on the diameter of the base surface of the workpiece used for alignment, mm.

The magnitude of the residual spatial deviation of the surface after performing the transition (operation) is equal to:

where Ku is the refinement coefficient (Table 25); – spatial deviation of the workpiece surface, µm.

When processing holes with a floating tool, spatial deviations of the surface are not corrected, therefore they are not involved in further calculations.

The installation error of the workpiece is determined by the formula:

(19)

where is the basing error, µm; – error of workpiece fixing, µm; when combining technological and measuring bases, basing error = 0.

When based at centers, the installation error in the radial direction is equal to:

where is the tolerance on the diametrical size of the workpiece, mm.

The deformation of the workpieces with a clamping force directed perpendicular to the surface being processed is assumed to be 100-150 μm for rough workpieces, 50-65 μm after roughing, and 10-15 μm after finishing.

When securing workpieces in vice-type devices, the clamping errors are found from the relationship:

where is a coefficient depending on the characteristics of the surfaces that perceive the clamping force; for surfaces before treatment Kz = 17.5x10-4; after roughing Kz = 5.8; b – width of the workpiece surface, mm.

Intermediate dimensions of the processed surface for technological operations (transitions) are calculated using the formula:

Dpi = Dnom i + 2zmini,

where Dnomi is the nominal size of the machined surface of the part in the previous operation (transition), mm.

Table 25

The value of the refinement coefficient Ku

Calculation of intermediate surface dimensions is carried out by successively approximating the nominal dimensions of the part according to the drawing performed in the final operation or transition to the dimensions of the workpieces in accordance with the technological sequence of surface treatment. For example, if it is necessary to determine the intermediate design size for a rough turning operation, add the minimum design allowance for finishing, etc.

The minimum (maximum) intermediate dimensions of technological operations (transitions) are determined by adding (for shafts) or subtracting (for holes) the minimum (maximum) values ​​of intermediate allowances:

Dmin = Dmini + 2zmini;

Dmax = Dmaxi + 2zmaxi;

where Dmaxi, Dmini – maximum surface dimensions for operations (transitions), mm; 2zmini, 2zmaxi - maximum (rounded) upward according to the degree of accuracy of the allowance for the operation (transition), mm.

Based on the maximum maximum dimensions of the workpiece, the nearest dimensions of rolled products are selected according to the standard for the product range (see Table 27).

The actual total allowance for processing the part is equal to:

2z0min = Dpr + Dдmin,

where Dpr is the diameter of the rolled product according to the standard, mm.

It is allowed not to develop a working drawing for a rolled product.

Example. Determine intermediate allowances and intermediate dimensions when processing the surface of a shaft with a diameter of 80f7 according to the working drawing (see Fig. 57). The initial workpiece is hot-rolled steel of ordinary precision B, GOST 2590-71. After cutting, the workpiece is straightened and centered. Type of production: mass production.

The technological route for surface treatment Æ80f7 provides for the following sequence of operations:

— Operation 005. Turning (roughing).

— Operation 010. Turning (finishing).

— Operation 015. Grinding.

Solution. In this type of production, shaft turning can be performed on a multi-cutting semi-automatic lathe. The blanks are installed in the centers. Grinding is carried out on a cylindrical grinding machine. The workpiece is mounted on rigid centers.

For clarity, the calculation of intermediate allowances and intermediate surface treatment dimensions is presented in the table (see Table 24).

The accuracy and quality of the surface after machining is established according to table 27.

Tolerances for the manufacture of parts are selected according to the tables: for billets from rolled products in accordance with GOST 2590-71, for blanks obtained by hot stamping, in accordance with GOST 7505-74, for cast blanks in accordance with GOST 1855-55, GOST 2009-55 and for dimensions, subject to processing according to GOST 25347-82.

Allowance elements (Rz and h) are assigned according to the table. 26 and 27 depending on the method of processing the surfaces of the workpiece and the condition of the rolled product.

The calculated minimum allowance for roughing the surface is determined by formula (15), and the parameters of the allowances are selected according to the table. 12.

Having chosen from the tables the value of the specific shape deviation equal to Ku = 0.07 µm/mm and having determined the distance Lk from the section for which the curvature is determined to the place of support when installed in the centers, it is determined from the ratio Lk £ 0.5 = 0.5 × 300 = 150 mm, where L is the total length of the workpiece, L = 300 mm, we calculate the deviation of the surface shape depending on the installation of the workpiece.

µm

Table 26

The quality of the outer surface of hot-rolled steel.

Table 27

Accuracy and surface quality of rolled stock after machining

The amount of displacement and rotation of the surface during the centering of the workpiece will be:

mm

where d3 is the tolerance for the diameter of the base surface, mm dз = 1.8 mm.

Table 28

Specific deviation of hot-rolled steel

The surface deviation for roughing the surface of the workpiece is determined by formula (18):

µm

Let's calculate the error in installing the workpiece when basing it on the centers:

µm

Based on the value, we determine the minimum allowance for roughing using formula 15:

Then, the maximum allowance for roughing the surface of the part will be equal to:

µm,

The value of the residual surface deviation of the workpiece surface after roughing is determined as:

µm,

where (see table 26)

The installation error during finishing of the workpiece surface will be

µm

During subsequent processing of the surface of the part, installation errors due to their small size are not taken into account.

The calculated minimum and maximum allowances for finishing the surface of the part will be determined using formulas (15) and (17):

2zmin = 2(60 + 60 + ) = 322.5 µm;

2zmax = 2(322.5 + 460 + 70) = 1705 µm.

The calculated minimum and maximum allowances for surface grinding will be:

2zmin = 2(6 + 12) = 36 µm;

2zmax = 2 (36 + 70 + 30) = 272 µm.

Intermediate design dimensions for the processed surfaces along technological transitions will be equal to:

— for finishing turning:

Dmin =Dmin + 2zmin shl = 80 + 0.036 = 80.036 mm;

— for rough turning:

Dр.black = Dр.clean + 2zmin clean = 80.036 + 0.323 = 80.359 mm;

- for preparing a part:

Dр.з = Dр.black + 2zmin = 80.359 + 2.4 = 82.759 mm.

Intermediate dimensions for technological transitions are determined by adding (for shafts) and subtracting (for holes) allowance values ​​according to the maximum and minimum values, starting from the part size established in the drawing.

Dclean = Dd + 2zmin shl = 79.94 + 0.04 = 79.98 mm;

Dmin clean = Dclean + 2zmin clean = 79.98 + 0.30 = 80.28 mm;

Dmin 3 = Dblack + 2zmin black = 80.28 + 2.4 = 82.68 mm.

Maximum intermediate dimensions:

Dmax net = Dmax + 2z max shl = 79.97 + 0.27 = 80.24 mm;

Dmax black = Dmax + 2zmax net = 80.12 + 1.7 = 81.82 mm;

Dmax 3. = Dmax + 2zmax black = 81.28 + 4.60 = 85.88 mm.

Based on the maximum dimensions of the workpiece, we select the diameter of the rolled product according to GOST 2590-71. Rolled diameter – Æ90 mm.

Calculation of intermediate allowances, intermediate dimensions and setting tolerances on dimensions allows you to draw up a diagram of the location of allowance and tolerance fields for the surface being processed. (see Fig. 24).