If the machining allowance is too small, it is difficult to eliminate the residual form and position errors and surface defects in the previous process. And the allowance is too large, not only to increase the workload of mechanical processing, but also increase the consumption of materials, tools, energy, more serious is in the process of processing due to the removal of a large number of processing allowance generated by the heat will make parts deformation, increase the difficulty of parts processing, affect the quality of the product, so it is necessary to strictly control the processing allowance of parts.
Machining allowance refers to the thickness of the metal layer cut from the machined surface during machining. Machining allowance can be divided into process machining allowance and total machining allowance. Process allowance refers to the thickness of metal layer removed from a surface in one process, which depends on the difference between the dimensions before and after the adjacent process. Total machining allowance refers to the total thickness of metal layer excised on a surface of a part during the whole processing process from blank to finished product, that is, the difference between blank size and part size on the same surface of the part. The total machining allowance is equal to the sum of the machining allowance of each process.
The influence of excessive machining allowance on machining accuracy
Parts in the process of machining is bound to produce cutting heat, these cutting heat is part of the iron filings and cutting fluid away, part of the tool, and part of the workpiece, so that the temperature of the parts. The temperature has much to do with the processing allowance. Machining allowance is large, rough machining time will inevitably become longer, cutting dosage will be appropriate to increase, resulting in increasing cutting heat, parts temperature rising. The greatest harm caused by the rising temperature of parts is to make parts deformation, especially the more sensitive to the temperature change of materials (such as: stainless steel), and the thermal deformation throughout the whole process, the processing difficulty increases, product quality is affected.
The influence of too small machining allowance on machining accuracy
The machining allowance of parts should not be too large but not too small. If the machining allowance is too small, the residual form and position tolerances and surface defects in the previous process cannot be eliminated, thus affecting the product quality. In order to ensure the machining quality of the parts, the minimum machining allowance of each process should be able to guarantee the basic requirements of the minimum machining allowance of the previous process. The schematic diagram of the components of the minimum machining allowance of the inner hole of a part is shown in Figure 4. Figure 4a) is the part of the inner hole to be machined. If the position error n exists when the axis O1-O1 deviates from the reference axis O-O in the previous process, and the cylindricity error p(such as taper, ellipse, etc.) and surface roughness error h exist in the inner hole (as shown in FIG. 4b), then in order to eliminate the form and position tolerance before boring, the minimum machining allowance on one side of the boring process should include the values of the above errors and defects. Considering the inevitable installation error of the workpiece when boring in this process, that is, the error e between the original hole axis O-O and the rotating axis O '-O' after installation of the workpiece (as shown in Figure 4c), as well as the dimensional tolerance T when boring in this process, the minimum machining allowance z of this process can be expressed as follows: