Cutting speed selection for turning stepped parts

Introduction The best goal of metal cutting is to maximize economic efficiency. There are a lot of variable parameters to achieve this goal, and a reasonable choice of cutting amount is particularly important. The economics of turning cylindrical cylinders of equal diameter have been studied, but trapezoidal parts of unequal diameters are often encountered in factories, and turning such parts can have two methods of selecting cutting speeds, ie constant speed method and constant. Cutting speed method. This paper analyzes the economics of the machining of stepped parts by these two methods and provides the basis for selecting cutting speeds for turning stepped parts. 1 Machining costs for turning stepped parts In the turning stepped parts, the machining of a cutting stroke at the i-th step is: ti=pDiLi/(1000fivi)=Li/Nif (1) where: i— - indicates the i-th step surface of the part; Di - the diameter of the i-th step surface (mm); Li - the axial length of the i-th step surface (mm); vi - the i-th step surface of the turning Cutting speed (m/min); f = feed rate (mm/r); Ni - Spindle speed (r/min) of the i-th step surface. For parts with m steps, the process cost per part is: C=pm/(1000f) m DiLi/vi+Mtvt+(MIct+Ci)/P ∑ i=1 (2) Where: M— Cost of the whole plant shared by the process per unit time (yuan/min); tvt-hours of auxiliary work in addition to tool change (min/piece); tct—tool change time (min); Ct—tool cost (yuan/ P)—The number of parts to be machined before each cutting edge of the tool reaches the tool's durability standard. 2 Constant spindle speed method Turning the machining cost of the stepped parts and the determination of the optimal cutting conditions The constant spindle speed method turns the stepped parts, that is, the spindle speed is unchanged during turning, and the cutting speed of each step is changed. of. Let the spindle speed be N, then the cutting speed of the i-th step surface is: vi=pDiN/1000 (3) For a step-like part with m steps, m ti/T=1/P ∑ i=1 (4 In the formula, T is the tool life. Then, 1/P=m ti/T=pxNx-1fy-1apz(m DixLi)/1000xCT ∑ ∑ i=1 i=1 (5) CN=M/(fN) m Li+Mtct+(Mtct+Ct)pxNx -1fy-1apz(m DixLi)/1000xCt ∑ ∑ i=1 i=1 (6) Similarly, the spindle speed that can export the lowest machining cost is: Nmin*C=[1000xCtM m Li/[(x-1)(Mtct) +Ct)pxfy] m DixLi]1/x ∑ ∑ i=1 i=1 (7)

Figure 1 Parts diagram

3 The machining cost of the stepped parts and the determination of the optimal cutting conditions by the constant cutting speed method The constant speed cutting method is used to turn the stepped parts and the spindle speed needs to be changed during the machining process. The processing cost should be based on equation (2) plus the cost of the time required to change the speed. Let tg be the time required to change the spindle speed, then the cost of increasing the speed change is Mmtg. Thus, the machining cost of each step type part for constant cutting speed turning is: Cp=pM/(1000Fv) m Di*Li+Mtct+ Mmtg+(Mtct+Ct)/P ∑ i=1 (8) where: 1/P=pNx-1fy-1apz(m DixLi)/(1000xCT ∑ i=1 (9) The lowest cost cutting speed is derived: vmin* C=[MCt/[(x-1)(Mtct+Ct)fyapz]1/x (10) 4 Comparison of processing costs for the two methods Using the data listed in the table below, the constant speed method and the constant cutting speed method are used for turning Comparing the machining costs of 1 parts, the calculation results are shown in Fig. 2.

Fig. 2 Diameter ratio and processing cost

Table Calculation of machining costs Data used Definition symbols and data Tools Tool life equation T=8.3*1011/(V5f1.75ap0.75)min Feedrate f=0.6mm/r Depth of cut ap=1mm Other auxiliary hours except for tool change Tot=3min Tool change time tct=0.8min Spindle rotation speed change time tg=0.1min Tool cost Ct=4yuan/blade The whole plant expenses shared by the process unit time M=1yuan/min 5 Conclusion It can be seen from the calculation results. (1) When the stepped part has a small diameter ratio, the machining cost of the constant rotation method turning is lower than that of the constant cutting speed method, so the constant rotation method should be used for cutting. (2) Which method is more advantageous for a stepped part with a larger diameter ratio depends on the processing lengths L1 and L2 of the part. The smaller the L1 and Lt, the greater the favorable diameter ratio range for constant-speed turning and vice versa. Of course. (3) For large-diameter ratio stepped parts, the constant cutting speed method has a low processing cost, so a constant cutting speed method should be used.

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