What is an allowance and how to calculate it?

Before getting the finished part, a lot of operations are performed with the workpiece. One of the most important is determining the allowance for processing. Its size is the difference between the size of the workpiece and the size of the part according to the drawing. In this article, we will learn what an allowance is and how to calculate it correctly.

Types of allowances

The allowance is a layer of metal that is removed from the surface of the workpiece to obtain the finished part. This is necessary in order to ensure the accuracy of the required dimensions and the required surface quality of the part. We have already found out what an allowance is, and now we will find out what kinds of it exist. Depending on the method of manufacturing the parts, the allowances can be intermediate, interoperative and general. As a rule, the latter are calculated by adding the interoperational ones. The allowance can be calculated as a certain size, and on the side.

On what does the value depend?

We have already found out earlier what an allowance is, and now we learn from what factors its size may depend. First of all, of course, from the technical conditions of the quality of the surface and the accuracy of the workpiece. In addition, the size of the allowance depends on the configuration of the part and its dimensions. Another important factor that influences the chosen value is the type of production. We should not forget about the error in installing the part in the device.

Calculation of allowance in two ways

We have already found out what an allowance is and what factors affect its size, and now we learn how to calculate it correctly. There are two ways: statistical (tabular) and analytical (calculated). The first is determined by the corresponding GOSTs and depends on the type of workpiece and processing route. First of all, in order to calculate the size of the allowance for processing by a statistical method, it is necessary to develop a route for processing the workpiece. Further, technological tolerances for technological transitions are assigned. Now, according to the tables, the allowances R _z and h are assigned with the corresponding GOSTs. In addition, it is necessary to calculate the magnitude of the total spatial errors. The technological allowance calculated by the tabular method must be calculated from the last transition, that is, in the order of the reverse process.

The following formulas are used to calculate the allowance by the analytical method.

For cylindrical surfaces:

For flat surfaces:

Where:

• R _z is the microroughness value;
• H is the depth of the defective layer;
• Ρ _i-1 - the value of the total spatial deviations;
• Ε _i - error of workpiece setting;
• I - allowance for this operation.

Intermediate allowances for machining (minimum) are calculated with very high accuracy - up to a micrometer, and rounding must be made upwards.

In spite of what kind of allowance is calculated, it must also be taken into account that its value should exceed the minimum chip thickness that the cutting tool removes in the calculated process operation. Of course, in the various situations and with a variety of processing blanks, the above formulas can vary slightly. For example, the formula removes the depth of the defective layer and its allowance (h _i-1 ) from grinding after heat treatment, since in this particular case the surface layer must be retained.