During machining, the layer of metal that is removed to change the size and shape of the workpiece is called machining allowance. The size of the machining allowance has a great influence on the processing speed, quality and processing cost. If the machining allowance is too large, it will not only increase the labor of machining, reduce production efficiency, but also increase the consumption of materials, tools and electricity, and increase the cost of processing; if the machining allowance is too small, it will not eliminate the previous Various surface defects and errors in the process, and can not compensate for the installation error of the workpiece during machining in this process, which is easy to scrap the workpiece. So how should the machining allowance be determined? Yuntuo large-scale machining factory has summarized the following aspects for your reference after years of experience.

1. Look-up table method

According to the machining allowance data provided in the relevant manuals of machining, and combined with the actual production situation of the factory, the machining allowance is determined after correction. This is a method commonly used by large machining factories.

2. Empirical estimation method

Determine the machining allowance according to the machining experience of the machining technicians. What are the main bases for machining technicians to determine machining allowance standards?

(1) The minimum machining allowance is adopted to shorten the machining time and reduce the cost of workpiece manufacturing.

(2) There is sufficient machining allowance, especially in the final process. The machining allowance should be able to ensure the surface roughness, size, shape and position accuracy specified in the machining drawings.

(3) There is sufficient machining allowance to eliminate workpiece deformation caused by heat treatment.

(4) When formulating machining allowance, the machining method, machining equipment and possible changes of the workpiece during machining should be considered.

(5) Factors such as the size and shape of the workpiece to be machined should be considered in formulating the machining allowance.

3. Analytical calculation method

According to the theoretical formula and relevant test data, the factors affecting the machining allowance are analyzed and calculated to determine the machining allowance. What are the formulas for calculating machining allowance?

(1) The calculation formula of the minimum margin:

     External surface processing: Zbmin=aamin-abmax

     External machining: 2Zbmin=damin-dbmax

     Internal surface processing: Zbmin=abmin-aamax

     Inner hole processing: 2Zbmin=dbmin-damax

Note: Zbmin (minimum allowance on one side of this process), 2Zbmin (minimum allowance for diameter on both sides of this process), aamin, damin, aamax, damax (minimum and maximum limit size of the previous process), abmin, dbmin, abmax, dbmax (The minimum and maximum limit dimensions of this process (after machining))

(2) The calculation formula of the maximum margin:

     Zbmax=Zbmin+Ta+Tb

     2Zbmax=2Zbmin+Tda+Tdb

Notes: Ta, Tda (dimensional tolerance and diameter tolerance of the previous process), Tb, Tdb (dimensional tolerance and diameter tolerance of this process)

(3) Calculation formula for basic allowance of machining inner and outer surfaces:

     Unilateral margin: Zb basic=Zbmin+Ta

     Outer circle allowance: 2Zb basic=2Zbmin+Tdo

     Inner hole allowance: 2Zb basic=2Zbmin+TDa

The above are the most commonly used methods for confirming machining allowances in large machining plants. Among them, the table look-up method is widely used in large machining plants. The empirical estimation method is often used in single-piece and small batch production. Analysis and calculation rules Generally, it is only used in large-scale mechanical processing plants where the materials are relatively expensive or a small number of large-scale and mass-produced.