Allowances for heat treatment of shafts for general purposes. Properly selected allowance ensures stable operation of the equipment while achieving high product quality, as well as the minimum cost of production. So, when turning a cylindrical surface

The concept of allowances, operating dimensions and permissible deviations on them. Influence of allowances on profitability technological process. Factors affecting the amount of allowance

The original workpiece differs from the part in that allowances are provided on all machined surfaces - layers of material to be removed from the surface of the workpiece during processing to obtain the specified accuracy and roughness. The material left in the recesses, grooves and holes of castings and forgings forms an overlap, which is also removed during processing. An overlap is also a layer of rolled material that exceeds the dimensions of the workpiece, taking into account the processing allowance. The overlap is removed, as a rule, in two passes (60 ... 70% - the first; 40 ... 30% - the second).

Allowances are divided into general (operational) - removed during the entire processing process and interoperational (intermediate), removed during individual operations. The interoperational allowance is determined by the difference in the dimensions of the workpiece obtained on the adjacent previous and performed transitions.

General allowance is equal to the sum interoperational allowances for all technological operations.

Allowances can be symmetrical (for bodies of revolution) and asymmetric - (prismatic parts).

There are nominal, minimum and maximum allowance.

Minimum allowance, i.e. the smallest layer of metal removed during processing is the difference between smallest size workpiece and the smallest size after performing this transition.

The maximum allowance is equal to the nominal allowance minus the tolerance for this transition.

Nominal allowance - the difference between the nominal dimensions of the surface after the previous and after this transition.

The maximum allowance is the difference between the smallest size of the surface after the execution of the previous transition and its smallest size after the execution of this transition.

There are regulatory data, summing which you can get the value of the minimum allowance.

There are also GOST for the values ​​of general allowances for the processing of castings and forgings. When assessing the value of the total allowance, factors are taken into account:

1) size and structural forms;

2) material and method of obtaining the workpiece;

3) the size of the defective layer;

4) installation error;

5) the degree of deformation.

It is important that the machining allowances are as small as possible in order to save metal, time, etc. To do this, in order to limit the values ​​​​of intermediate allowances, technological tolerances are assigned to individual transitions.

Usually, technological intermediate tolerances for the male surface (shaft journal) are assigned as a minus, and for the female (holes) - as a plus. Anyway intermediate tolerance directed into the metal body.

Minimum allowance - the minimum required thickness of the material layer to perform this operation. It is the initial value when calculating allowances.

A tolerance is set on the allowance, which is the difference between the largest and the smallest values allowance. The values ​​of allowances and tolerances determine the intermediate (operational) dimensions. Determination of allowances for machining consists of two main stages - the determination of the processing allowances in accordance with the technological transitions of the technological process and the determination of the dimensions of the workpiece, in accordance with the technical requirements of the working drawing. In this case, the dimensions of the workpiece (or the dimensions of the semi-finished product made from the starting material) are determined by summing up the processing allowances assigned for individual operations and transitions in the technological process.

Machining allowances are determined by two methods:

1) experimental-statistical- in which the values ​​of general and intermediate allowances are determined from reference tables compiled on the basis of a generalization of production experience. The disadvantage of the method is that it does not take into account the specific conditions for constructing the technological process. The resulting allowances, as a rule, are overestimated, as they are guided by the complete absence of marriage;

2) calculation and analytical method (Professor V.M. Kovan), according to which intermediate allowance should be such that when it is removed, processing errors and surface layer defects obtained at the previous transition, as well as installation errors at this transition, are eliminated. The basis of the method is the determination of min Z.

The influence of the size of the allowance on the efficiency of the machining process is very large, since the larger the allowance, the more working strokes are required to remove the corresponding layer of metal, which leads to an increase in the complexity of the process, energy consumption, wear cutting tool and increases the waste of metal turned into shavings. Overestimated allowances lead to an increase in the fleet of equipment and production areas required for its placement. The size of the allowance is ensured by the accuracy of manufacturing blanks, however, increasing the requirements for accuracy in some cases also increases the cost of their manufacture in blank shops, so the allowance should be chosen optimal, i.e., ensuring the quality of the machined surface at the lowest cost of processing in mechanical and blank shops.

Choice of processing route

Selection of installation bases

The choice of mounting bases is carried out in order to outline the procedure for changing the bases (if necessary) when performing the technological process of machining the part. The initial data when choosing bases are:

Working drawing of a part with predetermined dimensions;

Specifications for its manufacture;

Type of workpiece;

Desired degree of process automation.

The main provisions for the choice of bases and requirements for base surfaces are discussed in Section 1. However, when choosing installation bases, it is useful to keep in mind the two principles discussed earlier, i.e. the principle of alignment of bases to avoid the error of basing and the principle of constancy of bases, which helps to increase the accuracy of the relative position of the surfaces of the part.

The purpose of establishing a machining route and machining method is to provide the most rational machining process for a part. The route indicates the sequence of technological operations, and for each operation, the processing method, the equipment used, the device used, the working and measuring tools, processing modes, time standards, work qualifications are established.

The plan should provide for the division of the technological process of processing into its constituent parts: operations, installations, positions, transitions, passages, and, if necessary, techniques.

The calculation of processing allowances includes:

Factors determining the amount of the minimum allowance;

Methods for determining allowances.

Any workpiece, if it is further subjected to machining, is made with an allowance. What is meant by machining allowance? A machining allowance is understood as a layer of material that must be removed during machining in order to obtain a given dimensional accuracy, shape and surface roughness of the finished part. Based on the definition, it follows that surfaces that are not processed do not have allowances. The value of the total allowance for the machined surface is determined by the difference between the dimensions of the workpiece and the finished part.

There are general allowances and interoperational allowances. The general allowance is a layer of material removed during the entire processing, and the interoperation allowance is in one operation. It is easy to find out that the total processing allowance will be determined by the sum of the interoperational allowances.

According to the location of the allowances, symmetrical and asymmetrical ones are distinguished. Symmetrical allowances can be on the outer and inner surfaces of the bodies of revolution, as well as on opposite flat surfaces while simultaneously processing them. An asymmetric arrangement of allowances is observed with one-sided surface treatment, however, the possibility is not ruled out when processing the above-mentioned surfaces.

The machining allowance must be optimal, i.e. it must provide the specified accuracy of machining and at the same time have the lowest material consumption, i.e. Excessive allowances cause measurement costs in the manufacture of the part, and underestimated allowances, on the contrary, do not meet the established requirements for roughness, surface layer quality, material and dimensional accuracy.

Thus, the following number of factors influence the value of machining allowances and tolerances on the dimensions of workpieces:

workpiece material;

The configuration and dimensions of the workpiece;

Type of workpiece and method of its manufacture;

Machining requirements;

Specifications regarding the quality and class of surface roughness and dimensional accuracy of the part.

Currently, there are two methods for determining allowances for machining parts: experimental-statistical, or more often it is called tabular and calculation-analytical.

The essence of the tabular method for determining allowances lies in the fact that under production conditions, the allowances are set on the basis of experience, using practical data depending on the mass and overall dimensions of the parts, structural shapes and dimensions, the required accuracy and the roughness class of the treated surface. Based on these statistics, normative tables of allowances are compiled, which are used for their production or industry. The value of the tabular allowance for the work surface of the same name is greater than the allowance determined by the calculation and analytical method, i.e. a certain percentage of margin is given to satisfy all requirements for the surface to be machined.

The calculation and analytical method for determining allowances was proposed by prof. V.M. Forged. Its value is determined by calculation according to the formulas:

For a symmetrical allowance - for the diameter of the outer and inner surface bodies of revolution:

Symmetrical allowance for both opposite parallel

flat surfaces:

Asymmetric allowance - on one of the opposite parallels

molded flat surfaces:

where Z is the minimum allowance for the transition to be performed

R a is the height of microroughnesses;

T a is the thickness of the defective surface layer remaining

from previous processing;

ρ a - the total value of spatial deviations;

ε in - the error in the installation of workpieces when performing

operations.

The index "a" at the terms of the allowance shows that its value must be taken from the previous transition or processing, and "c" - from the transition being performed. The coefficient "2" in the formulas means that the allowance is accepted for the diameter or for both sides.

The maximum allowance is determined by the formula

where δ a is the size tolerance obtained on the previous

transition;

δ in - tolerance on the size obtained on the performed

transition, and the accuracy class that the processing gives

at the technological transition.

Practical work No. 3

Topic: Analytical calculation of allowances

Purpose of work: Acquisition of practical skills in determining allowances in a calculation and analytical way.

Initial data: Detail drawing (d/z No. 1).

1. Assign a processing route for a given surface;

2. Fill in the table of initial data by selecting the values ​​of the min allowance elements from the reference literature.

5. Build a layout of allowances and tolerances on a given surface;

6. Assign tabular values ​​of allowances and tolerances to the remaining surfaces of the part;

7. Make a conclusion about the work done.

Completing of the work:

1. As a machined surface, on which allowances will be calculated analytically, I accept the outer cylindrical surface Ø 141 h9. The processing route for this surface is indicated in Table 3.1.

Table 3.1 Surface treatment route

Name of the operation (transition)	Achieved accuracy class IT	Roughness parameter Ra µm.
1. Preparation stage
2. Rough turning
3. Turning semi-finishing
4. Fine turning

2. I select elements of allowances for transitions:

2.1 Height of microroughness Rz and depth of defective layer T

a) for casting blanks Rz + T = 500 µm.

b) by transitions:

After rough turning Rz=100 µm, T=100 µm;

After semi-finishing Rz =50 µm, T =50 µm.

2.2 The value of spatial deviations of the form ρ:

a) for casting blanks:

ρ zag = ρ cor =Δ to (3.1)

where ρcor is the amount of warpage;

Δ to - warping of body parts.

ρ zag \u003d ρ cor \u003d Δ k \u003d 86.5 1.5 \u003d 129.8 ≈ 130 (μm).

b) by transitions:

ρ i = ρ cor K Y (3.2)

where: K U - refinement coefficient (K U cher \u003d 0.06, K U p / h \u003d 0.05, K U h \u003d 0.04) :

ρ cher =130 0.06 ≈8 (μm);

ρ p / h \u003d 129.8 0.05 ≈ 7 (μm);

ρ h \u003d 129.8 0.04 ≈ 5 (μm).

2.3 Installation error Σ Y:

a) for the workpiece: Σ Y \u003d 140 microns;

b) by transitions, the installation error Σ Y = 0, since the processing takes place on one installation.

The data obtained are summarized in Table 3.2.

Table 3.2 Initial and calculated data for given size

Technological operations on transitions	allowance elements,				Calculation of allowances, microns			Size calculation, mm
Harvesting stage
Rough turning			1 30
Turning semi-finishing
Finishing turning						Finishing turning: dminh= dLF - eih=141-0.1 =140.9 (mm); (3.4) dmax h = dLF=141 (mm); (3.5) 2 zmin h=214 (μm); 2 zmax h=2 zmin h + ei h +ei p/h=214 +100 +400 = 714 (μm); (3.6) 2 znom h =2 zminh + ei p/h =214 +400 = 614 (μm); (3.7) Semi-finish turning: dmin p/h= dmax h +2 zmin h=141+0.214 =141.214 (mm) (3.8) dmax p/h = dn.p.h. = dmin p/h + eip/h=141.214 +0.4=141.614 (mm); (3.9) 2 zmin p/h=416 (μm); 2 zmax p/h=2 zmin p/h + ei p/h +eiblack=416 + 400 + 1000 = 1816 (µm); (3.10) 2 zrated p/h =2 zmin p/h + eicher =416 + 1000 = 1416 (µm). (3.11) Rough turning: dmin black= dmax p/h +2 zmin p/h=141.614+0.416 =142.03 (mm); (3.12) dmax black = dn.cher = dmin black + eiblack=142.03 +1=143.03 (mm); (3.13) 2 zmin black=1382 (μm); 2 zmax black=2 zmin black + ei black +eizag=1382 + 1000 + 800 = 3182 (μm); (3.14) 2 znom cher =2 zminblack + eizag =1382 + 800 = 2182 (µm). (3.15) Workpiece dimensions: dmin zag= dmax black +2 zmin black=143.03 +1.382 =144.412(mm); (3.16)dn.zag = dmin zag + eizag=144.412 +0.8=145.212 (mm); (3.17) dmax.zag = d n zag + eszag=145.212 +0.8=146.012 (mm). (3.18) General allowance for surface treatment: 2 z n.total =2 znom cher+2 zrated p/h +2 znom h=2182 +1416 +614 =4212 (µm). (3.19) Conclusion: in the course of practical work, I acquired practical skills in determining allowances in a computational and analytical way.