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The following issues should be paid attention to in the design of CNC machining process route
Added:2018-07-23     Views:    

1. Division of process


According to the characteristics of numerical control processing, the division of numerical control processing procedures can generally be carried out according to the following methods:


(1) Take one-time installation and processing as one process.


(2) Divide processes according to the processing content of the same tool


(3) Divide the process by the processing part


(4) Divide processes by roughing and finishing


2. Order arrangement


The sequence arrangement should generally be carried out according to the following principles:


(1) The processing of the previous process cannot affect the positioning and clamping of the next process, and the general machine tool processing processes should also be considered comprehensively;


(2) Carry out inner cavity processing first, and then shape processing;


(3) The process of processing with the same positioning and clamping method or processing with the same tool is best to be processed continuously to reduce the number of repeated positioning, tool changes and the number of times of moving the platen.


3. The connection between CNC machining process and ordinary process


Normally, other ordinary processing procedures are interspersed before and after the numerical control processing procedures. If the connection is not good, it is easy to cause contradictions. Therefore, while being familiar with the content of the entire processing process, it is necessary to understand the respective technical requirements, processing purposes, and processing characteristics of the CNC processing procedures and the ordinary processing procedures, such as whether or not to leave a machining allowance, how much; the accuracy requirements and shape of the positioning surface and the hole Position tolerance; technical requirements for the calibration process; heat treatment status of the blank, etc.


2.2 Design method of CNC machining process


After selecting the content of the numerical control processing technology and determining the part processing route, the design of the numerical control processing procedure can be carried out. The main task of CNC machining process design is to further the processing content, cutting amount, process equipment, positioning and clamping of this process


The mode and tool motion path are determined, ready for the compilation of the processing program.


2.2.1 Determine the path of the knife and arrange the processing sequence


The path of the tool is the path of the tool in the entire machining process. It not only includes the content of the work steps, but also reflects the order of the work steps. The path of the knife is one of the basis for programming. Pay attention to the following points when determining the path of the knife:


1. Seek the shortest processing route


Such as processing the hole system on the part as shown in the figure below. The cutting path in the middle figure is to process the inner ring hole after the outer ring hole is processed first. If you switch to the tool path shown in the figure on the right to reduce the empty tool time, you can save nearly double the positioning time and improve the processing efficiency.


2. The final contour is completed in one pass


In order to ensure the roughness requirements of the workpiece contour surface after processing, the final contour should be arranged to be continuously processed in the last pass.


The following figure shows the cutting route for machining the inner cavity with the line cutting method. This kind of cutting can remove all the margin in the inner cavity without leaving dead corners and not hurting the contour. However, the line cutting method will leave a residual height between the start and end points of the two passes, and the required surface roughness will not be achieved. I want to learn UG programming, machining center CNC programming, and learn to add QQ group: 45867470 to improve my income and salary. Therefore, if you use the knife path in the middle map, first use the line cutting method, and finally make a circular cut in the circumferential direction to smooth the contour surface, and you can get better results. The picture on the right is also a better way to move the knife.


Choose the cut-in and cut-out direction


When considering the entry and exit (cut-in, cut-out) route of the tool, the cut-out or cut-in point of the tool should be on the tangent line along the contour of the part to ensure the smooth contour of the workpiece; avoid vertical cutting up and down on the contour surface of the workpiece The surface of the workpiece is scratched; try to reduce the pause in the contour machining cutting process (the sudden change of cutting force causes elastic deformation), so as to avoid leaving tool marks, as shown in the figure.


Choose a route that makes the workpiece less deformed after processing


For slender parts or thin-plate parts with small cross-sectional area, the cutting path should be arranged in several passes to the final size or the symmetrical removal of margin method. When arranging the work steps, first arrange the work steps with less rigid damage to the workpiece.


Determine positioning and clamping plan


The following issues should be paid attention to when determining the positioning and clamping plan:


(1) As far as possible to achieve the unity of design benchmarks, process benchmarks and programming calculation benchmarks;


(2) Try to concentrate the process, reduce the number of clamping, and try to process all the surfaces to be processed after one clamping;


(3) Avoid using a clamping plan that takes a long time for manual adjustment of the machine;


(4) The point of action of the clamping force should fall on the part with better rigidity of the workpiece.


As shown in the figure on the left, the axial rigidity of the thin-walled sleeve is better than the radial rigidity. When the clamping jaws are clamped radially, the deformation of the workpiece is large. If the clamping force is applied in the axial direction, the deformation will be much smaller. When clamping the thin-walled box shown in the figure, the clamping force should not act on the top surface of the box, but on the convex edge with better rigidity, or change to three-point clamping on the top surface , Change the position of the force point to reduce the clamping deformation, as shown in the figure.


Determine the relative position of the tool and the workpiece


For CNC machine tools, at the beginning of processing, it is important to determine the relative position of the tool and the workpiece. This relative position is achieved by confirming the tool setting point. The tool setting point refers to the basis for determining the relative position of the tool and the workpiece through the tool setting


point. The tool setting point can be set on the part to be processed, or it can be set at a certain position on the fixture where the part positioning datum has a certain size connection. The tool setting point is often selected at the processing origin of the part.


The selection principle of the tool setting point is as follows:


(1) The selected tool setting point should make the programming simple;


(2) The tool setting point should be selected at a position that is easy to align and to determine the origin of the part processing;


(3) The tool setting point should be selected at a location that is convenient and reliable for inspection during processing;


(5) The selection of the tool point should be conducive to improving the machining accuracy.


For example, when processing the parts shown in the drawing, when compiling the NC machining program according to the illustrated route, the intersection of the center line of the cylindrical pin of the fixture positioning element and the positioning plane A is selected as the tool setting point for machining. Obviously, the tool setting point here also happens to be processing.


When using the tool setting point to determine the machining origin, it is necessary to perform "tool setting".


The so-called tool setting refers to the operation of making the "tool position point" coincide with the "tool setting point". The radius and length of each tool are different. After the tool is installed on the machine tool, the basic position of the tool should be set in the control system.


"Tool location point" refers to the positioning datum point of the tool. As shown in the figure below, the tool location point of the cylindrical milling cutter is the intersection of the tool center line and the bottom surface of the tool; the tool location point of the ball end mill is the center point or the apex of the ball end; the tool location point of the turning tool is the tool The center of the tip or tip arc; the tool position point of the drill is the apex of the drill.


Tool change points are set for machining centers, CNC lathes and other machine tools that use multi-tools for processing, because these machine tools need to automatically change tools during processing. For CNC milling machines with manual tool change, the corresponding tool change position should also be determined. In order to prevent damage to parts, tools or fixtures during tool change, the tool change point is often set outside the contour of the processed part, and a certain amount of safety is left.


Determine cutting amount


For high-efficiency metal cutting machine processing, the processed material, cutting tool, and cutting amount are the three major elements. Economical and effective processing methods require reasonable selection of cutting conditions.


When determining the cutting amount for each process, the programmer should choose according to the durability of the tool and the specifications in the machine tool manual. When selecting the cutting amount, it is necessary to fully ensure that the tool can finish a part, or ensure that the tool durability is not less than one work shift, and at least not less than half the working time of the work shift.


The amount of back-grabbing is mainly limited by the rigidity of the machine tool. When the stiffness of the machine tool allows, the amount of back-grabbing should be equal to the machining allowance of the process as much as possible, which can reduce the number of passes and improve the processing efficiency. For parts with high surface roughness and precision requirements, sufficient finishing allowance should be left. The finishing allowance of CNC machining can be smaller than that of general-purpose machine tools.


Fill in the technical documents of CNC machining


Filling in special technical documents for CNC machining is one of the content of CNC machining process design. These technical documents are not only the basis of numerical control processing, the basis of product acceptance, but also the procedures that operators follow and execute. The technical document is a specific description of the numerical control processing, the purpose is to let the operator more clearly the content of the processing program, the clamping method, the tool selected for each processing part and other technical issues.


 
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