Abstract: This paper introduces the three elements of cutting parameters, and expounds in detail the reasonable selection of cutting parameters in NC machine tool processing, so as to provide reference for NC machine tool programming and operators.

Cutting parameters are important parameters that represent the main motion and feed motion of machine tools. The determination of cutting parameters is an important content in NC machining technology. Cutting parameters have an important influence on machining efficiency, machining quality, tool wear and machining cost. Now, with the development of CAD/CAM technology, many CAD/CAM softwares provide automatic programming functions. These softwares will generally prompt related problems of process planning in the programming interface, such as tool selection, machining path planning, cutting parameter setting and so on. As long as the programmer sets the relevant parameters, the NC program can be automatically generated and transmitted to the NC machine tool to complete the machining. Therefore, the determination of cutting parameters in NC machining is completed in the state of man-machine interaction, which is in sharp contrast with ordinary machine tool machining. At the same time, programmers are also required to master the basic principles of determining cutting parameters, and fully consider the characteristics of NC machining and reasonably select cutting parameters when programming. This paper discusses the determination of cutting parameters in NC programming, gives some principles and suggestions, and discusses the problems that should be paid attention to.

1. Selection principle of cutting parameters in NC machining

Cutting parameters include cutting speed (spindle speed), back feed and feed, which are usually called cutting parameters. The selection of cutting parameters in NC machining is to give full play to the performance of machine tools and tools, maximize cutting efficiency and minimize machining cost on the premise of ensuring machining quality and tool durability. The selection principles of cutting parameters in rough machining and finish machining are as follows.

Roughing is generally to improve production efficiency, but economy and processing cost should also be considered. The selection principles of cutting parameters are as follows: firstly, choose the maximum undercut; Secondly, according to the power and stiffness constraints of the machine tool, the maximum feed speed is selected; Finally, the optimal cutting speed is determined according to the tool durability.

When semi-finishing and finishing, cutting efficiency, economy and processing cost should be taken into account on the premise of ensuring processing quality. Selection principle of cutting parameters Firstly, the back cutting amount is determined according to the allowance after rough machining; Secondly, according to the roughness requirements of the machined surface, a smaller feed rate is selected; Finally, on the premise of ensuring the tool durability, choose a higher cutting speed as much as possible. The specific values should be based on the machine tool manual, cutting parameter manual and practical experience.

(1) Selection of back feed ap (mm)

Determine the reverse feed ap according to the machining allowance and the rigidity of the process system. When the rigidity of machine tool, workpiece and cutter allows, ap is equal to machining allowance, which is an effective measure to improve productivity. In order to ensure the machining accuracy and surface roughness of parts, a certain margin should be left in finishing. The finishing allowance of CNC machine tools can be slightly smaller than that of ordinary machine tools. Specific selection

During rough machining, after finishing and semi-finishing allowance, cut off the remaining allowance as soon as possible; If the process system is not rigid enough or the allowance is too large to be removed at one time, it should also be processed according to the unequal allowance method of more first and less later. The ap of the first blade should be as large as possible, so that the blade can cut into the inner layer to avoid uneven surface of the workpiece and hard skin casting and forging.

When the impact load is large (such as intermittent surface) or the rigidity of the process system is poor (such as slender shaft, boring tool bar and old machine tool), the ap can be appropriately reduced to reduce the cutting force.

When finishing, ap should be determined according to the allowance left by rough machining, and the method of gradually reducing ap should be adopted to gradually improve machining accuracy and surface quality. In general finishing, AP = 0.05 ~ 0.8mm；; Semi-finishing, AP =1.0 ~ 3.0 mm.

(2) Cutting width L (mm)

Generally, l is directly proportional to the tool diameter d and inversely proportional to the cutting depth. In NC machining, the general value range of l is: l = (0.6 ~ 0.9) d.

(3) Selection of feed rate (feed speed) f (mm/min or mm/r)

Feed rate (feed speed) is an important parameter in cutting parameters of CNC machine tools. According to the surface roughness of parts, machining accuracy requirements, cutting tools and workpiece materials, refer to the cutting parameter manual for selection. For multi-tooth cutter, the relationship among feed speed vf, cutter speed N, cutter tooth number Z and feed per tooth fz is VF = FN = FZN.

In rough machining, due to the low requirements on the surface quality of the workpiece, F is mainly limited by the cutting force borne by the strength and stiffness of the tool rest, blade, machine tool and workpiece, and is generally selected according to the stiffness. When the stiffness of the process system is good, a larger f can be used; On the contrary, lower f appropriately.

When finishing or semi-finishing, f should be selected according to the surface roughness Ra of the workpiece. Ra requires F to be small, but not too small, because if F is too small, the cutting thickness hD will be too thin, but Ra will increase, which will aggravate tool wear. The larger the auxiliary deflection angle of the tool is, the larger the radius of the tool tip arc is, and the larger the f value can be selected. Generally, 20 ~ 25mm/min should be used for finish milling and 0. 10 ~ 0.20mm/r for finish turning. We should also pay attention to some special factors in parts processing. For example, in contour machining, when selecting the feed, the overtravel problem at the corner of contour should be considered. Especially when the corner is large and the feed speed is high, the feed speed should be appropriately reduced near the corner and gradually increased after the corner to ensure the machining accuracy.

(4) Selection of cutting speed Vc (m/min)

Select the cutting speed according to the selected reverse feed, feed and tool durability. It can be calculated by empirical formula, or it can be selected by looking up the table or consulting the relevant cutting parameter manual according to the production experience and within the cutting speed range allowed by the machine tool manual. When choosing cutting speed, we should also consider: try to avoid the area where chips are generated; When cutting intermittently, in order to reduce the impact and thermal stress, the cutting speed should be reduced appropriately; In the case of easy vibration, the cutting speed should avoid the critical speed of self-excited vibration; When machining large slender thin-walled workpieces, lower cutting speed should be selected; When machining workpieces with skins, the cutting speed should be reduced appropriately; If the rigidity of the process system is poor, the cutting speed should be reduced.

(5) Spindle speed n (rpm)

Spindle speed is generally selected according to cutting speed VC.

The formula is n =1000 VC/π d.

Where d is the diameter of the workpiece or tool (mm).

The control panel of CNC machine tools is generally equipped with a spindle speed adjustment (amplification) switch, which can adjust the spindle speed by integer times during machining.

conclusion

With the wide application of NC machine tools in production practice, NC programming has become one of the key issues in NC machining. In the process of compiling NC machining program, cutting parameters should be reasonably determined in the state of man-machine interaction. Therefore, programmers must be familiar with the principle of determining cutting parameters in NC machining, and choose reasonable cutting parameters in combination with the field production situation to ensure the machining quality and efficiency of parts, give full play to the advantages of NC machine tools, and improve the economic benefits and production level of enterprises.

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