Before you get the finished part, with the workpiece do a lot of various operations. One of the most important is the determination of machining allowance. 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
An allowance is a layer of metal that is removed from the surface of a workpiece to produce a 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 figured out what an allowance is, and now we will find out what its varieties exist. Depending on the manufacturing method of the part, allowances can be intermediate, interoperational and general. As a rule, the latter are calculated by adding the interoperational ones. The allowance can be calculated both on a certain size, and on the side.
What does the quantity depend on?
We already found out earlier what an allowance is, and now we learn what factors its size may depend on. First of all, of course, from the technical conditions of surface quality and accuracy of the workpiece. In addition, the size of the allowance depends on the configuration of the part and its dimensions. Another important factor affecting the selected value is the type of production. We must not forget about the error in installing the part in the device.
Calculation of stock in two ways
We have already found out what an allowance is and what factors influence its size, and now we will learn how to calculate it correctly. There are two ways: statistical (tabular) and analytical (calculated). The first is determined by the relevant GOSTs and depends on the type of workpiece and the 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 processing route for the workpiece. Next, technological tolerances for technological transitions are assigned. Now, according to the tables, with the relevant GOSTs, stock elements R z and h are assigned. In addition, it is necessary to calculate the magnitude of the total spatial errors. The technological allowance calculated by the tabular method, you need to start calculating from the last transition, that is, in the reverse order of the process.
To calculate the allowance by the analytical method, the following formulas are used.
For cylindrical surfaces:
For flat surfaces:
Where:
- R z is the value of microroughness;
- h is the depth of the defective layer;
- ρ i-1 - the value of the total spatial deviations;
- ε i - the error in the installation of the workpiece;
- i is the allowance for this operation.
Intermediate machining allowances (minimum) are calculated with very high accuracy - up to a micrometer, and rounding must be done upwards.
Despite what kind of allowance is calculated, it must also be taken into account that its value must exceed the minimum chip thickness that the cutting tool removes in the calculated technological operation. Of course, in various situations and with various types of processing of the workpiece, the above formulas may vary slightly. For example, the depth of the defective layer and its allowance (h i-1 ) are removed from the formula when grinding after heat treatment, since in this particular case the surface layer must be preserved.