The influence of machining allowance on machining accuracy!

With the continuous improvement of the quality requirements for mechanical processing products, people have invested a lot of time and energy in exploring methods and measures to improve product quality, but have ignored the impact of machining allowance on product quality during the processing process, and believed that as long as there is a margin during the processing, it will not have much impact on product quality. In the actual processing of mechanical products, it is found that the size of the machining allowance of parts directly affects the quality of the product. If the machining allowance is too small, it is difficult to eliminate the residual shape and position errors and surface defects in the previous process; if the allowance is too large, it will not only increase the workload of mechanical processing, but also increase the consumption of materials, tools, and energy. What is more serious is that the heat generated by cutting off a large amount of machining allowance during the processing will cause the parts to deform, increase the difficulty of processing parts, and affect product quality. Therefore, it is necessary to strictly control the machining allowance of parts.

1. Concept of machining allowance

Machining allowance refers to the thickness of the metal layer cut from the machining surface during machining. Machining allowance can be divided into process machining allowance and total machining allowance. Process machining allowance refers to the thickness of the metal layer cut from a certain surface in a process, which depends on the difference between the process sizes before and after the adjacent processes. Total machining allowance refers to the total thickness of the metal layer cut from a certain surface during the entire machining process from the blank to the finished product, that is, the difference between the blank size and the part size on the same surface of the part. The total machining allowance is equal to the sum of the machining allowances of each process. Since there are inevitably errors in the manufacturing of the blank and the size of each process, both the total machining allowance and the process machining allowance are variable values, and the minimum machining allowance and the maximum machining allowance appear. Machining allowance and tolerance are shown in Figure 1. In the figure, the minimum machining allowance is the difference between the minimum process size of the previous process and the maximum process size of the current process; the maximum machining allowance refers to the difference between the maximum process size of the previous process and the minimum process size of the current process. The range of variation of the process machining allowance (the difference between the maximum machining allowance and the minimum machining allowance) is equal to the sum of the dimension tolerances of the previous process and the current process. The tolerance zone of the process dimension is generally specified in the direction of the part entering the body. For shaft parts, the basic size is the maximum process size, while for holes it is the minimum process size.

2. Analysis of the influence of machining allowance on machining accuracy

2.1 The influence of excessive machining allowance on machining accuracy

Parts will inevitably generate cutting heat during the machining process. Part of this cutting heat is taken away by iron chips and cutting fluid, part is transferred to the tool, and part is transferred to the workpiece, causing the temperature of the part to rise. The temperature is closely related to the size of the machining allowance. If the machining allowance is large, the rough machining time will inevitably become longer, and the cutting amount will also be appropriately increased, resulting in increasing cutting heat and increasing part temperature. The biggest harm caused by the increase in part temperature is that the part will be deformed, especially for materials that are sensitive to temperature changes (such as stainless steel). Moreover, this thermal deformation runs through the entire machining process, increasing the difficulty of machining and affecting product quality. For example, when machining slender shaft parts such as screw rods, due to the use of a one-clamp-one-top machining method, the degree of freedom in the length direction is limited. At this time, if the temperature of the workpiece is too high, thermal expansion will occur. When the extension in the length direction is blocked, the workpiece will inevitably bend and deform due to the influence of stress, which will cause great trouble for later processing. The workpiece bends and deforms after being heated. If the processing continues at this time, the protruding part will be processed until the finished product is obtained. After cooling to room temperature, the part will be deformed under stress, causing shape and position errors and affecting the quality. The workpiece will bend and deform at room temperature. After the diameter expands, the enlarged part will be cut off, and the cylindricality and size errors will occur after the workpiece cools down. When grinding precision screws, the thermal deformation of the workpiece will also cause pitch errors.

2.2 The influence of too small machining allowance on machining accuracy

The machining allowance of parts should not be too large or too small. If the machining allowance is too small, the residual geometric tolerances and surface defects in the previous process cannot be eliminated, thus affecting the product quality. In order to ensure the machining quality of parts, the minimum machining allowance left in each process should be able to meet the basic requirements of the minimum machining allowance of the previous process. The minimum machining allowance of the inner hole of a part is composed of the part whose inner hole is to be machined. If the axis of the hole deviates from the reference axis and there is a position error n during the previous process, and the inner hole has a cylindrical error p (such as taper, ellipse, etc.) and a surface roughness error h, then in order to eliminate the geometric tolerance before boring, the single-sided minimum machining allowance of the boring process should include the values ​​of the above errors and defects. Considering that the workpiece inevitably has installation errors during boring in this process, that is, the error e between the original hole axis and the rotation axis of the workpiece after installation, as well as the dimensional tolerance T during boring in this process, the minimum machining allowance z of this process can be expressed by the following formula: z ≥ T/2 + h + p + n + e (single-side allowance)

For different parts and different processes, the values and manifestations of the above errors are also different. Different treatments should be applied when determining the process machining allowance. For example, slender shafts are prone to bending and deformation, and the generatrix straight line error has exceeded the diameter size tolerance range, so the process machining allowance should be appropriately enlarged; for processes that use tools such as floating cutters to position the machining surface itself, the influence of the installation error e can be ignored, and the process machining allowance can be reduced accordingly; for some finishing processes that are mainly used to reduce surface roughness, the size of the process machining allowance is only related to the surface roughness h.

3. Reasonable selection of machining allowance

The selection of machining allowance for parts is closely related to the material, size, accuracy level and machining method used for the parts, and should be determined according to the specific situation. When determining the machining allowance for parts, the following principles must be followed:

(1) The minimum machining allowance should be used to shorten the machining time and reduce the machining cost of the parts.

(2) Sufficient machining allowance should be left, especially for the last process. The machining allowance should be able to ensure the accuracy and surface roughness specified on the drawing.

(3) When determining the machining allowance, the deformation caused by the heat treatment of the parts should be taken into account, otherwise scrap may be produced.

(4) When determining the machining allowance, the machining method and equipment, as well as the deformation that may occur during the machining process, should be taken into account.

(5) When determining the machining allowance, the size of the part to be processed should be taken into account. The larger the part, the larger the machining allowance. Because as the size of the part increases, the possibility of deformation caused by cutting force, internal stress, etc. will also increase.