With the continuous improvement of quality requirements for mechanically processed products, people have invested a lot of time and energy in exploring methods and measures to improve product quality, but they have ignored the impact of machining allowance on product quality during the machining process. As long as there is a margin during processing, it will not have much impact on the quality of the product. 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 will be difficult to eliminate the residual shape errors and surface defects in the previous process; if the allowance is too large, it will not only increase the machining workload, but also increase material consumption; , tools and energy. What is more serious is that the heat generated by removing a large amount of machining allowance during processing will deform the parts, increase the processing difficulty of the parts, and affect the quality of the product. strictly control the machining allowance of the parts.
1 The concept of machining allowance
Machining allowance refers to the thickness of the metal layer that is removed from the machined surface during the machining process. The machining allowance can be divided into process machining allowance and total machining allowance. Process machining allowance refers to the thickness of the metal layer removed from a certain surface during a process. It depends on the difference between the dimensions of the process before and after adjacent processes. Total machining allowance refers to the total thickness of the metal layer removed from a certain surface during the entire processing of the part from blank to finished product, that is, the difference between the size of the blank and the size of the part on the same surface on the part. The total machining allowance is equal to the sum of the machining allowances for each process.
Since there are inevitable errors in the blank manufacturing and dimensions of each process, the total machining allowance and the process machining allowance are variable values, and there are minimum machining allowances and maximum machining allowances. The 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 this process. The maximum machining allowance refers to the difference between the maximum size of the previous process and the minimum size of the process; size of this process. The variation range of the machining allowance of the process (the difference between the maximum machining volume and the minimum machining allowance) is equal to the sum of the dimensional tolerances of the previous process and this process. The process dimension tolerance zone is usually specified in the part entry direction. For shaft parts, the base 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 precision
2.1 The impact of excessive machining allowance on machining precision
Parts must generate cutting heat during machining. Part of this cutting heat is carried away by the iron shavings and cutting fluid, part is transferred to the tool, and part is transferred to the workpiece, causing the workpiece temperature to increase. The temperature has a great relationship with the machining allowance. If the machining allowance is large, the roughing time will inevitably become longer, and the cutting quantity will also increase appropriately, which will cause the cutting heat to increase and the workpiece temperature to increase. The biggest damage caused by increasing part temperature is the deformation of parts, especially materials sensitive to temperature changes (such as stainless steel), and this thermal deformation propagates throughout the processing process , making processing more difficult and product quality affected. . Influence.
For example, when processing thin shaft parts such as screw rods, due to the one-clamp, one-top processing method, the lengthwise degree of freedom is limited at this time , if the room temperature is too high. , thermal expansion will occur. When the lengthwise extension is blocked, the workpiece will inevitably bend and deform under the influence of stress, causing great problems in subsequent processing. The bending deformation diagram of the workpiece after heating is shown in Figure 2. If the processing continues at this time, the protruding part will be processed to the finished product. After cooling to normal temperature, the parts will undergo reverse deformation under the action of stress, causing shape and position errors and affecting quality. The bending deformation diagram of the workpiece after normal temperature is shown in Figure 3. After expansion in the diameter direction, the expanded part will be removed, and cylindrical and dimensional errors will occur after the workpiece is cooled. When grinding precision screws, thermal deformation of the workpiece will also cause pitch errors.
2.2 The impact of too small a machining allowance on machining precision
The machining allowance of the parts must not be too large or too small. If the machining allowance is too small, the residual geometric tolerances and surface defects from the previous process cannot be eliminated, thereby affecting product quality. In order to ensure the processing quality of parts, the minimum machining allowance left in each process must be able to meet the basic requirements of the minimum machining allowance of the previous process. The schematic diagram of the components of the minimum machining allowance for the inner hole of a certain workpiece is shown in Figure 4. Figure 4a) shows the workpieces with inner holes. If the O1-O1 axis deviates from the OO reference axis when the hole is processed in the previous process, there will be a position error n and the inner hole will have a cylindrical error p (such as taper, ellipse , etc.) and surface roughness error h (as shown in Figure 4b) ), then in order to eliminate the geometric tolerance before boring, the minimum machining allowance on one side of the process reaming must include the values of the above errors and defects. Considering that there is inevitably an installation error in the workpiece when reaming in this process, that is, the error e between the original hole axis OO and the axis of O’-O’ rotation after installing the part (as shown in Figure 4c). , and the bore in this process The dimensional tolerance T of the hole, therefore the minimum machining allowance z for this process can be expressed by the following formula:
z≥T/2+h+p+n+e (margin on one side)
Figure 4 Illustration of the components of the minimum machining allowance
For different parts and different processes, the values and manifestations of the above errors are also different. They must be treated differently when determining machining allowances. For example, the thin shaft is prone to bending and deformation, and the linear error of the bus bar exceeds the diameter tolerance range. The machining allowance of the process should be enlarged appropriately for the process of using tools such as floating reamers for positioning. the processing surface itself, the installation error does not need to be considered. e, the process machining allowance can be reduced accordingly for some finishing processes which are mainly used to reduce surface roughness, the size of the process machining allowance is only related to; surface roughness h.
3 Reasonable selection of machining allowance
3.1 Principles of part machining allowances
The choice of parts processing allocation is closely related to the materials used, size, precision level and processing method of parts, and depends on the specific situation. The following principles must be respected when determining the machining allowance of parts:
(1) The minimum machining allowance should be used to shorten the processing time and reduce the processing cost of parts.
(2) Sufficient processing margin must be left, especially in the final process. The machining allowance must be able to guarantee the precision and surface roughness specified in the drawing.
(3) When determining the machining allowance, the deformation caused by heat treatment of parts must be taken into account, otherwise waste may be produced.
(4) When determining the machining allowance, the processing method and equipment, as well as possible deformations during processing, should be considered.
(5) The size of the workpieces to be processed should be taken into account when determining the machining allowance. The larger the part, the greater the machining allowance. Because as the part size increases, the possibility of deformation caused by cutting forces, internal stresses, etc. increases. will also increase.
3.2 Method for determining machining allowances
3.2.1 Empirical estimation method
The empirical estimation method is commonly used in production practice. This is a method of determining machining allowance based on the craftsman’s design experience or by comparison with parts of the same type. For example, the machining allowances for the rudder stock, rudder pin, intermediate shaft and stern shaft of the vessel under construction are determined based on the craftsmen’s many years of design experience. Considering the importance of the workpiece, coupled with the influence of factors such as its large size and high stress on the forging blank, a semi-finishing allowance of 6 mm is left after rough turning of the circle exterior, a fine turning allowance of 3 mm is left after semi-finishing, and a fine turning allowance of 3 mm is left after fine turning of 1 mm. In order to avoid insufficient machining allowance and producing scrap, the machining allowance estimated by the empirical estimation method is generally too large. This method is often used for the production of unique pieces and small batches.
3.2.2 Table search correction method
The look-up table correction method is a method that is compiled into a table based on the machining allowance data accumulated in production practice and experimental research, and revised according to actual processing conditions to determine the machining allowance. This method is widely used. . The machining allowances for the finely turned outer circle and the ground outer circle of the rolling parts after rough turning are shown in Table 1 and Table 2, respectively.
3.2.3 Method of analysis and calculation
The analytical calculation method is a method for comprehensively analyzing and calculating various factors that affect the machining allowance based on test data and calculation formulas to determine the machining allowance. The machining allowance determined by this method is accurate, economical and reasonable, but it requires the accumulation of relatively complete data. It is not as simple and intuitive as the above two methods, so this method is currently rarely used.
4 Conclusion
In actual production, the production methods of many part blanks are temporarily determined. For example, the spin-cast stainless steel sleeve is made of rolled and welded steel plates; the cooler end cover, engine base and gearbox sand castings are replaced with sand castings. welded parts, wait. There are many uncertain factors in the production process of these parts, and their shape errors are difficult to predict. Therefore, the three methods for determining the machining allowance of parts presented in this article are not suitable for determining the machining allowance of these parts. They can only be used in practice. Be flexible during the production process.
Table 1 Machining allowance in mm of the outer circle of shaft parts after rough turning after rough turning
Table 2 Machining allowance mm for cylindrical grinding of shaft parts
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