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Influence of machining allowance on machining accuracy

With the continuous improvement of the quality requirements of machined products, people have invested a lot of time and energy in exploring methods and measures to improve product quality, but they ignore the impact of machining allowance on product quality in the machining process, and believe that only allowance in the machining process will not have much impact on product quality. In the actual machining process of mechanical products, it is found that the machining allowance of parts directly affects the product quality.
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 machining, but also increase the consumption of materials, tools and energy. More seriously, the heat generated by cutting a large amount of machining allowance in the machining process will deform the parts, increase the machining difficulty of the parts and affect the product quality. Therefore, it is necessary to strictly control the machining allowance of the parts.
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Concept of machining allowance

Machining allowance refers to the thickness of metal layer cut from the machined 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 removed from a surface in one process, which depends on the difference between the dimensions of the adjacent processes before and after the process. The total machining allowance refers to the total thickness of the metal layer removed from a certain surface during the whole machining process of the part from blank to finished product, that is, the difference between the blank size on the same surface and the part size. The total machining allowance is equal to the sum of machining allowance of each process.
Because there are inevitable errors in blank manufacturing and process dimensions, both the total machining allowance and process machining allowance are variable values, resulting in the minimum machining allowance and the maximum machining allowance. 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 variation range of process machining allowance (the difference between the maximum machining quantity and the minimum machining allowance) is equal to the sum of the dimensional tolerances of the previous process and the current process. The tolerance zone of process dimension is generally specified in the entry direction of parts. For shaft parts, the basic size is the maximum process size, while for holes, it is the minimum process size.

Analysis of the influence of machining allowance on machining accuracy

Influence of excessive machining allowance on machining accuracy

Parts in the machining process will inevitably produce cutting heat. Part of these cutting heat is taken away by iron filings and cutting fluid, part is transmitted to the tool, and part is transmitted to the workpiece to raise the temperature of the parts. The temperature is closely related to the machining allowance. If the machining allowance is large, the rough machining time will inevitably become longer, and the cutting parameters will be appropriately increased, resulting in the continuous increase of cutting heat and the continuous increase of part temperature. The biggest harm brought by the temperature rise of parts is to deform the parts, especially the materials sensitive to temperature change (such as stainless steel). Moreover, this thermal deformation runs through the whole processing process, increasing the processing difficulty and affecting the product quality.
For example, when machining screw rod slender shaft parts, the degree of freedom in the length direction is limited due to the one-to-one machining method. At this time, if the workpiece temperature is too high, thermal expansion will occur. When the extension in the length direction is blocked, the workpiece will inevitably produce bending deformation under the influence of stress, which will bring great trouble to the later processing. The bending deformation of the workpiece after heating is shown in Figure 2. At this time, if you continue processing, process the protruding part until the finished product. After cooling to normal temperature, the part will produce reverse deformation under the action of stress, resulting in form and position error and affecting the quality. The bending deformation of the workpiece after normal temperature is shown in Figure 3. After expanding in the diameter direction, the increased part will be cut off, and cylindricity and dimensional error will occur after the workpiece is cooled. During precision screw grinding, the thermal deformation of the workpiece will also cause pitch error.
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Influence of too small machining allowance on machining accuracy

The machining allowance of parts shall not be too large but not too small. If the machining allowance is too small, the residual geometric tolerance 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 shall meet the basic requirements of the minimum machining allowance in the previous process. The composition factors of the minimum machining allowance of the inner hole of a part are shown in Figure 4. Figure 4a) shows the parts of the inner hole to be machined. If the axis o1-o1 deviates from the reference axis O-O in the previous process, there is a position error n, and the inner hole has cylindricity error P (such as taper, ellipse, etc.) and surface roughness error H (as shown in Fig. 4b), in order to eliminate the geometric tolerance before boring, the minimum machining allowance on one side of the boring process shall include the values of the above errors and defects. Considering the inevitable installation error of the workpiece during boring in this process, that is, the error e between the original hole axis O-O and the rotation axis O ‘- o’ after workpiece installation (as shown in Fig. 4C) and 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)

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Fig. 4 diagram of components of minimum machining allowance

For different parts and different processes, the values and manifestations of the above errors are also different. When determining the process machining allowance, it shall be treated differently. For example, the slender shaft is easy to bend and deform, the linear error of the bus has exceeded the diameter tolerance, and the process machining allowance should be appropriately enlarged; For the process using floating reamer and other tools to locate the machining surface itself, the influence of installation error E can be ignored, and the process machining allowance can be reduced accordingly; For some finishing processes mainly used to reduce surface roughness, the size of process machining allowance is only related to surface roughness H.

Reasonable selection of machining allowance

Principles for machining allowance of parts

The selection of machining allowance is closely related to the material, size, accuracy grade and machining method used by the part, which shall be determined according to the specific situation. The following principles must be followed when determining the machining allowance of parts:

  • (1) The minimum machining allowance shall be adopted in order to shorten the machining time and reduce the machining cost of parts.
  • (2) Sufficient machining allowance shall be reserved, especially for the final process. The machining allowance shall ensure the accuracy and surface roughness specified on the drawing.
  • (3) When determining the machining allowance, the deformation caused by part heat treatment shall be taken into account, otherwise scrap may be generated.
  • (4) When determining the machining allowance, the machining method and equipment as well as the possible deformation during machining shall be considered.
  • (5) The size of machined parts shall be taken into account when determining machining allowance. The larger the part, the larger the machining allowance. Because when the size of the part increases, the possibility of deformation caused by cutting force and internal stress will also increase.

Method for determining machining allowance

Empirical estimation method

Experience estimation method is commonly used in production practice. It is a method to determine the machining allowance according to the design experience of technicians or compared with the same type of parts. For example, the machining allowance of rudder stock, rudder pin, intermediate shaft and stern shaft of ships under construction is determined according to the design experience of technicians for many years. Considering the importance of the workpiece and the influence of factors such as large volume and large forging blank stress, 6 mm semi finishing allowance is reserved after rough turning, 3 mm finishing allowance is reserved after semi finishing, and 1 mm grinding allowance is reserved for finishing. In order to prevent scrap due to insufficient machining allowance, the machining allowance estimated by empirical estimation method is generally too large. This method is often used in single piece and small batch production.

Look up table correction method

The look-up table correction method is a method to prepare a table based on the data related to machining allowance accumulated in production practice and experimental research, and revise it in combination with the actual machining situation to determine the machining allowance. This method is widely used. See Table 1 and table 2 for machining allowance of bearing parts after rough turning, fine turning and grinding.

Analysis and calculation method

The analysis and calculation method is a method to determine the machining allowance by analyzing and comprehensively calculating various factors affecting the machining allowance according to the test data and calculation formula. The machining allowance determined by this method is accurate, economical and reasonable, but it needs to accumulate more comprehensive data. It is not as simple and intuitive as the above two methods, so this method is rarely used at present.

Conclusion

In actual production, because the manufacturing methods of many parts are temporarily determined, for example, the centrifugal cast stainless steel sleeve is welded after being rolled with steel plate; The cooler end cover, motor base and gear box sanding parts shall be replaced with welding parts, etc. There are many uncertain factors in the manufacturing process of these parts, and its shape error is difficult to predict. Therefore, the three methods to determine the machining allowance introduced in this paper are not applicable to the determination of the machining allowance of these parts, and can only be flexibly mastered in the actual manufacturing process.
Table 1 machining allowance of outer circle of shaft parts after rough turning and fine turning/mm
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Table 2 machining allowance for grinding outer circle of shaft parts/mm

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Source: Network Arrangement – China Pipe Sleeve Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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