Determination of process parameters in CAM program

2022-08-14
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As we all know, in the process of NC programming with the help of CAM software, the selection of process parameters is very important. It not only has a great impact on the quality of machined parts, but also determines the effectiveness of machine tools and the smooth progress of safe production. According to the characteristics of mold parts, the author analyzes the influence of the selection of process parameters in the NC Milling Programming of mold parts on the machining quality, and introduces the setting methods and principles of process parameters in CAM programming in mold NC machining

I. Introduction

NC machining technology has been widely used in mold manufacturing industry, such as NC milling, boring, turning, wire cutting, EDM, etc., among which NC milling is the main processing method of complex mold parts. Numerical control equipment provides the basic conditions for the processing of precision and complex parts, but in order to achieve the expected processing effect, it is essential to compile high-quality numerical control programs, because numerical control processing programs not only include the process of parts, but also include the shape and size of tools, cutting parameters, tool path and other process information. For simple mold parts, manual programming is usually used. For complex mold parts, it is often necessary to compile processing programs with the help of CAM software, such as pro/engineer, UG, Cimatron, Mastercam, etc. Whether it is manual programming or computer-aided programming, selecting reasonable process parameters is the premise of compiling high-quality processing programs

second, the choice of tools

in the NC milling of mold cavity, the choice of tools directly affects the processing quality, processing efficiency and processing cost of mold parts, so the correct choice of tools is of great significance. In mold milling, the commonly used tools include flat end milling cutter, fillet end milling cutter, ball head cutter and taper milling cutter, as shown in Figure 1

Figure 1 tool definition

when the talents of state-owned enterprises of molds begin to be out of touch, the selection of tools for cavity processing should follow the following principles:

1 Select the tool type according to the shape of the machined surface

for concave surfaces, ball end cutters should be selected in semi finishing and finishing to obtain good surface quality, but flat end end mills or fillet end mills should be selected in rough machining, because the cutting conditions of ball end cutters are poor; For convex surfaces, flat end mills or fillet end mills are generally selected for rough machining, but fillet end mills should be selected for finish machining, because the geometric conditions of fillet mills are better than flat end mills; For the side with demoulding slope, taper milling cutter should be selected. Although the inclined plane can also be processed by using flat end end milling cutter through interpolation, it will lengthen the processing path and affect the processing efficiency. At the same time, it will increase the wear of the cutter and affect the processing accuracy

2. Select the tool according to the principle from large to small

the mold cavity generally contains multiple types of surfaces, so you can't select a tool to complete the processing of the whole part during processing

whether it is rough machining or finish machining, we should choose tools with large diameter as much as possible, because the smaller the tool diameter is, the longer the machining path is, which will reduce the machining efficiency. At the same time, the wear of tools will cause obvious differences in machining quality

3. Select the tool according to the curvature of the profile

during finish machining, the radius of the smallest tool used should be less than or equal to the fillet radius of the inner contour on the machined part, especially in corner machining, the tool with a radius less than the fillet radius at the corner should be selected and processed in the way of arc interpolation, so as to avoid overcut caused by linear interpolation; In rough machining, considering the principle of using large-diameter tools as much as possible, the tool radius generally selected is large. At this time, it is necessary to consider whether the allowance left after rough machining will cause excessive cutting load to semi finishing or finishing tools, because larger diameter tools will leave more allowance at the corner of the part contour, This is often the direct cause of tool damage or tool planting due to the sharp change of cutting force in the finishing process

4. When rough machining, try to choose the fillet milling cutter

on the one hand, the fillet milling cutter can give a relatively continuous change of cutting force within the range of 0~90 ° between the blade and the workpiece during cutting, which is not only beneficial to the machining quality, but also greatly prolong the tool life; On the other hand, when selecting fillet milling cutter in rough machining, it has good cutting conditions compared with ball head cutter. Compared with flat end milling cutter, it can leave more uniform finishing allowance. As shown in Figure 2, it is the stress at the tangent point between the stress elastic hysteresis strain curve and the inclined straight line, which is very beneficial to subsequent processing

Figure 2 Comparison of allowance after rough machining between fillet milling cutter and flat end milling cutter

III. determination of tool path and cutting mode

tool path refers to the distribution form of tool path in the machining process. Cutting mode refers to the movement mode of the tool relative to the workpiece during machining. In NC machining, the choice of cutting mode and tool feeding mode directly affects the machining quality and efficiency of mold parts. The selection principle is to make the cutting time as short as possible and the force on the tool is stable in the cutting process according to the geometric characteristics of the machined part surface and on the premise of ensuring the machining accuracy

1. Cutting mode

in mold processing, the commonly used cutting modes include one-way cutting, reciprocating cutting and circular cutting, as shown in Figure 3. Among them, figure 3A shows the one-way cutting mode. During the processing, our training lasts for 1 week, and the cutting mode remains unchanged, which can ensure that it is difficult to ensure full coverage; In addition, the consistency of forward milling or reverse milling after the institutional reform, but due to the increase of tool lifting and empty tool walking, the cutting efficiency is low. In rough machining, due to the large amount of cutting, one-way tool walking is generally selected to ensure the uniformity of tool force and the stability of cutting process. Fig. 3b is a reciprocating tool walking mode. Continuous cutting without lifting the tool in the processing process has high processing efficiency, but the reverse milling and forward milling are carried out alternately, and the processing quality is poor. In general, due to the large cutting amount, it is not suitable to use the reciprocating cutter in rough machining, but it can be used in semi finishing and finishing with low surface quality requirements. Figure 3C shows the circular cutting tool path, which is composed of a group of closed circular curves. The tool is not lifted during the machining process, and the forward milling or reverse milling cutting method is adopted. It is a common tool path for cavity machining

Figure 3. Tool feeding mode

2 Milling mode

the selection of milling mode directly affects the quality of machined surface, tool durability and the stability of machining process. When using circular milling, according to the size of machining allowance and the requirements of surface quality, forward milling and reverse milling should be reasonably selected. Generally, the allowance is large in the rough machining process, and the reverse milling processing method should be selected to reduce the vibration of the machine tool; When finishing, in order to meet the requirements of accuracy and surface roughness, the forward milling method should be selected. When using end milling, different milling methods should be selected according to the different processed materials. Generally, symmetrical milling should be selected when processing high hardness materials; When processing ordinary carbon steel and high-strength low alloy steel, asymmetric reverse milling should be selected, which can prolong the service life of the tool and obtain better surface quality of the workpiece; Asymmetric forward milling should be selected when processing high plastic materials to improve the durability of cutters

IV. cutting in and cutting out of tools

in the NC milling of mold cavity, due to the complexity of mold cavity, it is often necessary to replace different tools for many times to complete the processing of mold parts. During rough machining, the geometric shape formed by the residual allowance after each machining is changing. If the cutting mode is improperly selected during the next feeding, it is easy to cause a knife planting accident. In finish machining, the change of cutting conditions during cutting in and cutting out often leads to the difference of machined surface quality. Therefore, it is of great significance to reasonably select the cutting in and cutting out methods of cutters. The cutting in and cutting out methods provided by the general cam software include the vertical cutting in and cutting out of the workpiece (plunge), the cutting tool cutting into the workpiece (ramp), the cutting tool cutting into the workpiece (spiral) with a spiral path descent, the cutting tool cutting into the workpiece (entry hole) through the pre machining process hole, and the arc_tangent

among them, the tool vertical cutting in and cutting out the workpiece is the simplest and most commonly used way, which is applicable to the rough machining and finishing of punch workpieces that can be cut in from the outside of the workpiece and the finishing of the side wall of the mold cavity; The cutting tool cuts into the workpiece with oblique line or spiral line, which is often used for rough machining of softer materials; Cutting the workpiece through the pre-processing hole is a common cutting method for rough machining of female dies; Arc cutting in and cutting out workpiece is often used in surface finishing because it can eliminate tool marks. It should be noted that when roughing the cavity, if the unidirectional tool walking (Zig) mode is adopted, the cutting mode provided by the general cad/cam system is the cutting mode at the beginning of the processing operation, and the cutting mode of each time in the processing process is not defined. This problem is sometimes the main cause of tool or workpiece damage. One way to solve this problem is to adopt the circular cutting tool walking mode or bidirectional tool walking mode, Another method is to reduce the step over of machining so that the back draft is less than the milling cutter radius

v. control of cutting parameters

the selection of cutting parameters has a direct impact on machining quality, machining efficiency and tool durability. In CAM software, the parameters related to cutting mainly include spindlespeed, cut feed, lead in feed rate, step over and step depth

1. Spindle speed

spindle speed is generally calculated according to the cutting speed, and its calculation formula is: n = 1000 V C/π D, where D is the tool diameter (mm), VC is the cutting speed (m/min). The choice of cutting speed is closely related to the tool durability. When the workpiece material, tool material and structure are determined, the cutting speed becomes the most important factor affecting the tool durability. Too low or too high cutting speed will sharply reduce the tool durability. In mold processing, especially in mold finishing, we should try to avoid changing tools in the middle of the process to obtain higher processing quality. Therefore, we should carefully select the cutting speed in combination with the tool durability

2. The selection of feed speed and cutter cut in feed speed

feed speed directly affects the machining accuracy and surface roughness of die parts. Its calculation formula is f=nzf, where n is the spindle speed (r/min), Z is the number of milling cutter teeth, and F is the feed per tooth (mm/tooth). The selection of feed rate per tooth depends on the mechanical properties of workpiece materials, tool materials and milling cutter structure. The higher the hardness and strength of the workpiece, the smaller the feed per tooth; The feed rate per tooth of carbide milling cutter is higher than that of similar high-speed steel milling cutter; When the requirements of machining accuracy and surface roughness are high, a lower feed rate should be selected; The cutting feed speed should be less than the cutting feed speed

3. The size of the cutting amount

is mainly limited by the rigidity of the machine tool, workpiece and cutting tool. Its selection principle is to select the cutting amount as large as possible under the condition that the process requirements and the rigidity of the process system are met, so as to improve the processing efficiency. In order to ensure the machining accuracy and surface roughness, a finishing allowance of 0.2~0.5mm should be left. In rough machining, the cutting of allowance often adopts layer cutting

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