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Discussion on the Process Problem of NC Turning

Posted by: Chris Na 2021-05-26 Comments Off on Discussion on the Process Problem of NC Turning

The ideal processing program should not only ensure that qualified workpieces conforming to the drawings are processed, but also should enable the functions of the cnc machine tool to be reasonably applied and fully utilized. cnc machine tools are a kind of high-efficiency automation equipment. Its efficiency is 2 to 3 times higher than that of ordinary machine tools. To make full use of this feature of CNC machine tools, it is necessary to perform process analysis on the workpiece before programming. According to specific conditions, choose economical , Reasonable process plan. Inadequate consideration of the Cnc Machining process is an important factor that affects the machining quality, production efficiency and processing cost of CNC machine tools. Starting from production practice, this article discusses and summarizes some technological issues in the CNC turning process.
1 Division of CNC machining processes
Processing parts on CNC machine tools is relatively concentrated in processes, and all processes should be completed as much as possible in one clamping. There are two common process division principles as follows.
Guaranteed accuracy principle
Numerical control machining has the conditions of concentration of processes. Roughing and finishing are often completed in one clamping to ensure the machining accuracy of the parts. When thermal deformation and cutting force deformation have a greater impact on the machining accuracy of the parts, rough and finishing should be processed Separately.
Principles of improving production efficiency
In CNC machining, in order to reduce the number of tool changes and save tool change time, after all the parts that need to be processed with the same tool are completed, another tool should be used to process other parts. At the same time, the idle stroke should be minimized. When processing multiple parts of the workpiece with the same tool, the shortest route should be used to reach each processing part.
In actual production, CNC machining often divides processes according to tools or machined surfaces.
2 Selection of tool location points for turning tools
In NC machining, the NC program should describe the motion path of the tool relative to the workpiece. In CNC turning, the formation of the workpiece surface depends on the position and shape of the envelope of the moving blade, but in programming, only the trajectory of a selected point on the tool system needs to be described. The tool location point of the tool is the point selected on the tool that represents the location of the tool during programming, and the processing path described by the program is the motion path of this point.
In CNC turning, theoretically any point on the tool can be selected as the tool location point, but in order to facilitate programming and ensure machining accuracy, the selection of the tool location point has certain requirements and skills. In CNC machining, the selection of tool location points generally follows the following rules: the end mill should be the intersection of the tool axis and the bottom of the tool: the ball end mill is the center of the ball: the drill should be the drill tip: the turning tool should be imaginary Attention should be paid to the tool nose or the arc center of the tool nose and the tool position point when selecting:
·Choose the point that can be directly measured on the tool, and the tool position point should be as consistent as possible with the measurement point when the tool length is pre-adjusted;
·When possible, the tool position point should be directly related to the size with higher accuracy requirements or the size that is difficult to measure;
·The selected tool position point can make the limit position of the tool directly reflected in the motion instruction of the program;
·Programmers should have a habitual selection method of tool positions, which should not be changeable;
·The selected tool location point should be marked with graphics in the tool adjustment diagram.
3 End position of the tool during layered cutting
When the machining allowance of a certain outer surface is too large and it is necessary to cut multiple passes in layers, it is necessary to prevent the sudden increase in the amount of back-grabbing from the second pass to the end point. As shown in Figure 2, suppose a cutting tool with a 90° entering angle is used to turn the outer circle in layers. A reasonable arrangement should be that the cutting end point of each tool is sequentially advanced by a small distance e (e=0.05). If e=0, that is, every tool ends at the same axial position, the main cutting edge of the turning tool may be subject to instantaneous heavy load impact. For example, the termination position of the layered cutting is arranged to be regressive layer by layer, which is beneficial to prolong the service life of the rough machining tool.
4 Determining the tool compensation value when “giving the knife”
For thin-walled workpieces, especially thin-walled workpieces with difficult-to-cut materials, the phenomenon of “giving the knife” is serious during cutting, which leads to changes in the size of the turned workpieces. Generally, the outer circle becomes larger and the inner hole becomes smaller. “Give a knife” is mainly caused by the elastic deformation of the workpiece during processing, and the degree of “give a knife” is closely related to the amount of back-grabbing during cutting. Adopt the “equal back-grabbing depth method” and adjust the compensation value in a small range to reduce the influence of “giving the knife” on the machining accuracy. As shown in Figure 3, suppose the size of the outer circle to be processed is A, and the double-sided margin is 2t. During trial cutting, half of the value of t is taken as the amount of back cutting during cutting. Trial cutting is carried out on the full length of the surface. After the trial cutting, the program arranges to stop, and measure whether the outer circle size is equal to A+t, and press Appear. Adjust the tool compensation value for the error size of the tool, and then continue to run the program to complete the finishing pass. Since the finishing process and the trial cutting process use the same back-cutting amount and the same cutting speed and feed rate, the cutting resistance is the same, and the corresponding elastic deformation of the workpiece is the same. The resulting deformation ensures the dimensional accuracy of the turned workpiece. Which brand of L-carnitine is good for removing wrinkle eye cream after sun? Which brand of hand cream is good for breast enhancement, which weight loss exercise is good for eye cream, which is good for body moisturizing
5 Chip breaking problem during turning
CNC turning is automated processing. If the chip breaking performance of the tool is too poor, it will seriously hinder the normal processing. In order to solve this problem, the chip breaking performance of the tool itself should be improved as much as possible, and the cutting amount of the tool should be selected reasonably to avoid the generation of strip-shaped chips that hinder the normal processing. In CNC turning, the most ideal chips are spiral-shaped chips with a length of 50-150mm and a small diameter, or pagoda-shaped chips, which can be regularly removed in a certain direction for easy collection and removal. If chip breaking is not ideal, you can arrange a pause in the program if necessary to force chip breaking: a chip breaking table can also be used to enhance the chip breaking effect. When using up-pressing machine-clamped indexable inserts, the pressure plate can be used to simultaneously compress the chip breaker and the inserts together: for the inner hole of the car, the cutting method with the tool rake face down can be used to improve chip removal.
6 Choice of insert shape of indexable tool
Compared with ordinary machine tool processing methods, CNC machining puts forward higher requirements on cutting tools. It not only requires good rigidity, high precision, but also requires dimensional stability, high durability, and good chip breaking and chip removal performance: at the same time, it requires easy installation and adjustment. In this way, it meets the high-efficiency requirements of CNC machine tools. The selected tools on CNC machine tools often use tool materials suitable for high-speed cutting (such as high-speed steel, ultra-fine-grained cemented carbide) and use indexable inserts.
Machine clamp indexable tools are widely used in CNC turning, which is an important means to improve the productivity of CNC machining and ensure product quality. There are many types of indexable turning tool blades, the most widely used is diamond blades, followed by triangular blades, round blades and grooving blades. Rhombus blades are divided into three categories: 80°, 55° and 35° according to their diamond acute angles.
The 80° rhombus blade has a moderate cutting edge angle. The blade not only has good strength, heat dissipation and durability, but also can be assembled into a tool with an entering angle slightly greater than 90° for end faces, outer circles, inner holes, and steps. Processing. At the same time, this kind of insert has good clamping ability. It can be positioned on the bottom surface of the insert and the adjacent two sides of the 80° nose angle at the non-cutting position. The positioning method is reliable, and the accuracy of the position of the nose is only the accuracy of the outer dimension of the insert itself. Related, high indexing accuracy, suitable for CNC turning.
The 35° diamond blade is mostly used for turning complex profiles or digging grooves of workpieces because of its small nose angle and less interference.
7 The path of the grooving knife
Deeper grooves are commonly processed with grooving tools on CNC lathes. If the width of the tool is equal to the required groove width, the grooving tool will cut the slot at one time. If the narrower grooving tool is used to process a wider groove , You should cut in multiple times. The reasonable cutting route is: first cut the middle, then cut left and right. Because the fillet radius on both sides of the cutting edge is usually smaller than the transition fillet radius of the workpiece groove bottom and the side wall, when the left and right cuts are cut, when the tool approaches the groove bottom, each needs to go through a circular arc. If the middle cut is not cut in advance, it will not create the necessary conditions for the two arcs. Even if the fillet radius on both sides of the blade is the same as the fillet radius on both sides of the groove bottom of the workpiece, it is better to cut in the middle first, because when this blade is cut, the load on both sides of the blade is equal. The left side has a heavy load, and a knife is a heavy load on the right side, and the tool wear is still uniform. The machine-clamp-type grooving knife is not suitable for horizontal cutting, but for straight cutting.
8 Summary
The NC machining program is a command file of the NC machine tool. The NC machine tool is controlled by the program instructions. The whole process of machining is carried out automatically according to the program instructions. The NC machining program includes not only the technological process of the part, but also the cutting amount and the path of the tool. , Tool size and the movement process of the machine tool, the process of CNC machining is more detailed. The quality of the process plan directly affects the efficiency of the machine tool and the quality of parts processing. Special attention should be paid in actual production.

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