1. Cutting principle
In NC programming, the programmer must determine the cutting amount for each process, including the spindle speed, the amount of backing tool, the feed rate, etc., and enter it into the program in the format specified by the CNC system. Cutting amount For different processing methods, different cutting amounts are required. Reasonable choice of cutting amount has a great influence on the surface quality, precision, and processing efficiency of parts. This is difficult to master in practice. It requires a lot of practical experience to be able to determine the appropriate cutting amount. In the numerical control programming can only be determined by the experience of the programmer and the recommended cutting value of the tool, and the final cutting amount will be determined according to the debugging results of the numerical control program of the part and the actual processing conditions.
The principle of cutting amount selection is: to improve productivity mainly during rough machining, taking into account both economical efficiency and processing cost considerations; when semi-finishing and finishing are performed, both the cutting efficiency and processing cost should be taken into account to ensure the processing of parts. quality. It is worth noting that the cutting amount (spindle speed, depth of cut, and feed rate) is an organic whole, and only the three are adapted to each other to achieve the most reasonable matching value in order to obtain the best cutting amount.
When determining the amount of cutting, specific requirements in terms of processing properties, processing requirements, workpiece material and tool dimensions, and material properties, etc. shall be determined by consulting the cutting manual and combining experience to determine the amount of cutting in addition to following general principles and methods. The following factors should also be considered:
(1) Influence of tool difference - The tool quality produced by different tool manufacturers varies greatly, so the cutting amount needs to be corrected based on the actual tool and field experience.
(2) Impact of machine tool characteristics - The cutting performance is limited by the power of the CNC machine tool and the rigidity of the machine tool and must be selected within the range specified in the machine tool specification. To avoid boring car phenomenon due to insufficient power of the machine tool, or to produce large machine tool vibration phenomenon due to insufficient rigidity, affecting the machining quality, accuracy, and surface roughness of the parts.
(3) The influence of the productivity of numerically-controlled machine tools: The cost of the man-hours for numerically-controlled machine tools is relatively high. Relatively speaking, the proportion of tool wear and tear costs is relatively low. High cutting rates should be used as far as possible, and CNC machine tools should be improved by appropriately reducing the tool life. Productivity.
2. Choice of cutting amount
(1) determine the amount of back ap (mm)
The size of the back knife is mainly determined by the rigidity of the process system consisting of the machine tool, fixture, tool and workpiece. When the system stiffness allows, the blank allowance is removed to minimize the number of feeds. The remaining part of the part determines the depth of the layered cutting and selects a larger amount of the back knife to increase the production efficiency. In the numerical control processing, in order to guarantee the necessary processing precision and surface roughness of the parts, it is recommended to leave a small amount of margin (0.2-0.5mm), and go along the contour in the final finishing process. During roughing, except for the necessary semi-finishing and finishing allowances, rough machining should be performed a minimum number of times with the rigidity of the process system allowed. The margin left for finishing should be greater than the amount of deformation of the part and ensure the integrity of the surface of the part.
(2) Determine the spindle speed n (r/min)
Spindle speed n is mainly determined by the cutting speed VC (m/min) of the tool:
Among them: VC - cutting speed.
d - The diameter of the part or tool (mm).
Cutting speed VC is closely related to the tool's durability. With the increase of VC, the tool life will drop sharply. Therefore, the choice of VC depends mainly on tool life.
After the spindle speed n is determined, it must be written in the NC program according to the format specified by the CNC machine control system. In actual operation, the operator can control the spindle rotation speed to determine the optimal spindle speed by appropriately adjusting the spindle speed override switch on the control panel of the numerical control machine tool according to the actual machining conditions.
(3) Selection of feed rate or feed rate F (mm/r, mm/min)
The feed speed F is the relative displacement of the part and the milling cutter along the feeding direction in unit of cutting, in mm/r or mm/min.
The feed rate or feed rate is represented on the CNC machine tool using the feed function word F. F is an important parameter in the cutting amount of the CNC machine tool. It is mainly based on the machining accuracy and surface roughness requirements of the part, and the tool used. The workpiece material is determined. The higher the machining accuracy of the part, the lower the surface roughness requirement, the smaller the value of the selected feed. In practice, comprehensive consideration should be given to the accuracy of machine tools, tools, fixtures, and machined parts, mechanical properties of materials, curvature changes, structural rigidity, rigidity of the process system, and chip breaking conditions, and appropriate feed rates should be selected.
The feed rate is a special feed rate representation, that is, the reciprocal time of the feed rate - FRN (abbreviation of Feed Rate Number). The feed rate for linear interpolation is:
Where F - feed (m/min).
L - The machining length of the block is the effective distance in mm from the tool.
The feed rate FRN is programmed in the program segment and actually specifies the time T for executing the program segment. The relationship between them is:
When the feed F is selected during programming, the tool center movement speed is fixed. In straight-line cutting, the speed of the cutting point (cutting point of the tool and the machining surface) is the given amount of feed during programming. However, in the arc cutting, the actual feed amount of the cutting point is not equal to the feed amount of the tool center selected during programming.
When using FRN programming, the actual feed rate is different from the programmed FRN of the program because the distance between the tool center movement and the length of the linear machining in the program is often different. In arc cutting, the feed angular velocity of the tool is fixed, so the cutting point feed is consistent with the programmed FRN value. It can be seen that when a numerical control device can be programmed with F, FRN can also be used when programming, linear cutting is suitable for the use of feed F programming, arc cutting should adopt FRN programming.
When selecting the feed amount F in the contour processing, attention should be paid to the “overtravel†problem at the corner of the contour, especially when the corner is larger and the feed amount is larger, the speed is appropriately reduced near the corner. After the corner, the speed is gradually increased to ensure the machining accuracy.
In NC programming, the programmer must determine the amount of cutting for each process and write it into the program in the form of instructions. The cutting amount includes the spindle speed, the back knife amount and the feed speed. For different processing methods, different cutting amounts need to be selected. In order to obtain the highest productivity and the highest resection rate per unit time, the cutting parameters should be reasonably determined under the premise of ensuring the quality of the parts and the durability of the tools.
With the wide application of CNC machine tools in production, NC programming has become one of the key issues in CNC machining. In the process of NC programming, it is necessary to select the tool and determine the cutting amount in real time under the condition of human-machine interaction. Therefore, programmers must be familiar with the selection method of cutting tools and the principle of determining the cutting amount, so as to ensure the processing quality and processing efficiency of parts, give full play to the advantages of CNC machine tools, and improve the economic efficiency and production level of enterprises.
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