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NC lathe programming processing process processing

1 determine the processing position and specific content of the workpiece

determine the process content of the workpiece to be processed on this machine tool and its connection with the previous and subsequent processes

· the condition of the workpiece before processing in this process. Such as castings, forgings or bars, shape, size, machining allowance, etc

· the shape and size of the machined part in the previous process or the datum plane and datum hole processed in the previous process in this process

· parts and specific contents to be processed in this process

· in order to facilitate the preparation of processes and procedures, the blank drawing before the processing of this process and the processing drawing of this process should be drawn

2 determine the clamping mode of the workpiece and design the fixture

select or design the fixture according to the determined workpiece processing position, positioning benchmark and clamping requirements. CNC lathes mostly use three jaw self centering chucks to hold workpieces; Shaft workpieces can also use tailstock center to support workpieces. Because the spindle speed of CNC lathe is very high, in order to facilitate workpiece clamping, hydraulic high-speed power chuck is often used. Because it has passed strict balance in the production plant, it has the advantages of high speed (the limit speed can reach 4000~6000r/min), high clamping force (the maximum pushing force is 2000~8000n), high precision, convenient jaw adjustment, through-hole, long service life and so on. 4. Experimental space (excluding piston stroke): 7m; Soft claws can also be used to clamp the workpiece, and the arc surface of the soft claws is randomly prepared by the operator, which can obtain ideal clamping accuracy. By adjusting the pressure of the oil cylinder, the clamping force of the chuck can be changed to meet the special needs of clamping various thin-walled and easily deformed workpieces. In order to reduce the stress and deformation of slender shaft during machining and improve the machining accuracy, and when machining the inner hole of shaft workpiece with hole, the hydraulic automatic centering center frame can be used, and the centering accuracy can reach 0.03mm

3 determine the processing scheme

1) the principle of determining the processing scheme

the processing scheme is also known as the process scheme. The processing scheme of the NC machine tool includes the formulation of working procedures, working steps and tool paths

in the process of machining with CNC machine tools, due to the complexity and diversity of machining objects, especially the ever-changing shape and position of contour curves, and the influence of various factors such as different materials and batches, when formulating machining schemes for specific parts, we should make specific analysis, treat them differently, and deal with them flexibly. Only in this way can the processing plan be reasonable, so as to achieve the purpose of high quality, high efficiency and low cost

the general principles for formulating the processing plan are: rough first and then fine, close first and then far, inside first and then outside, minimum program segments, shortest cutting route, and special treatment under special circumstances

a. rough first and then finish

in order to improve production efficiency and ensure the finishing quality of parts, the rough machining process should be arranged first, and a large amount of machining allowance (as shown in the dotted line in the figure) before finishing should be removed in a short time, while trying to meet the uniformity requirements of finishing allowance

after the rough machining process is arranged, the semi finish machining and finish machining after tool change should be arranged. Among them, the purpose of arranging semi finish machining is to arrange semi finish machining as a transitional process when the uniformity of the allowance left after rough machining cannot meet the requirements of finish machining, so as to make the finish machining allowance small and uniform

when arranging the finishing process that can be carried out with one or more knives, the final contour of the part shall be formed by continuous machining with the last knife. At this time, the forward and backward position of the processing tool should be considered properly, and try not to arrange cutting and cutting out or tool change and pause in the continuous contour, so as to avoid elastic deformation caused by the sudden change of cutting force, resulting in surface scratches, sudden changes in shape or residual knife marks and other defects on the smooth connecting contour

b. close first and then far

the distance between the machining part and the tool setting point is used here. In general, especially in rough machining, the parts close to the tool setting point are usually processed first, and the parts far from the tool setting point are processed later, so as to shorten the moving distance of the tool and reduce the empty travel time. For turning, it is beneficial to maintain the rigidity of blank parts or semi-finished parts and improve their cutting conditions

c. inside first and then outside

for parts that need to process both the inner surface (inner mold and cavity) and the outer surface, when formulating their processing plan, it is usually arranged to process the inner mold and cavity first and then the outer surface. This is because it is difficult to control the size and shape of the inner surface, the rigidity of the tool is relatively poor, the durability of the tool tip (edge) is easily affected by cutting heat, and it is difficult to remove chips in processing

d. the shortest tool path

the focus of determining the tool path is mainly used to determine the tool path of rough machining and empty travel, because the tool path of finishing machining and cutting process is basically carried out along the contour of its parts

the tool path generally refers to the path that the tool moves from the tool setting point (or the fixed origin of the machine tool) until it returns to the point and ends the machining program, including the cutting path 2 The load sensor wiring is disconnected or the socket is unreliable, and the diameter and tool lead-in, cut-out and other non cutting empty strokes

on the premise of ensuring the machining quality, making the machining program have the shortest tool path can not only save the execution time of the whole machining process, but also reduce some unnecessary tool consumption and the wear of the sliding parts of the machine tool feed mechanism

in addition to relying on a lot of practical experience, the optimized process plan should also be good at analysis, which can be supplemented by some simple calculations when necessary

the above principles are not immutable. For some special cases, flexible and variable schemes need to be adopted. For example, some workpieces must be finished first and then rough machined to ensure their machining accuracy and quality. These all depend on the continuous accumulation and learning of the actual processing experience of programmers

2) relationship between processing route and machining allowance

under the condition that CNC lathes have not reached universal use, generally, excessive allowance on blank parts, especially the allowance containing forged and cast hard skin, should be arranged to be processed on ordinary lathes. If it is necessary to process with a numerical control lathe, pay attention to the flexible arrangement of the program. Arrange some subroutines to cut the parts with excessive allowance first

1) processing route for step cutting of large allowance blanks

2) end position of the tool during layered cutting

3) spindle speed during thread turning

when the CNC lathe processes threads, due to the change of its transmission chain, in principle, its speed is as long as it can ensure that the tool moves one pitch along the main feed axis (mostly Z axis) direction every revolution of the spindle, and it should not be limited. However, when NC lathes process threads, they will be affected by the following aspects:

a) the pitch (lead) value of the instruction in the thread processing program section is equivalent to the feed speed f expressed in the feed rate (mm/r). If the spindle speed of the machine tool is selected too high, the converted feed speed (mm/min) must greatly exceed the normal value

b) at the beginning/end of its displacement, the tool will be constrained by the frequency rise/fall of the servo drive system and the interpolation calculation speed of the numerical control device. Because the frequency rise/fall characteristics can not meet the processing needs and other reasons, it may be that the sheet metal produced by the main feed movement is the coat of the universal experimental machine "leading" and "lagging", resulting in the pitch of some screw teeth not meeting the requirements

c) turning thread must be realized through the synchronous operation function of the spindle, that is, turning thread requires a spindle pulse generator (encoder). When the spindle speed is selected too high, the positioning pulse sent by the encoder (i.e. a reference pulse signal sent by the spindle every revolution) may cause the workpiece thread to buckle disorderly due to "overshoot" (especially when the quality of the encoder is unstable)

therefore, when threading, the determination of spindle speed should follow the following principles:

a) under the condition of ensuring production efficiency and normal cutting, it is appropriate to choose a lower spindle speed

b) when the lead-in length D1 and the cut-out length D2 (as shown in the figure) in the thread processing program section are considered to be sufficient, that is, when the thread feed distance is larger than the length of the thread specified in the drawing, an appropriately higher spindle speed can be selected

c) when the allowable working speed specified by the encoder exceeds the maximum speed of the spindle specified by the machine tool, the spindle speed as high as possible can be selected

d) generally, the spindle speed (n-screw) during thread turning should be determined according to the calculation formula specified in the machine tool or CNC system manual. The calculation formula is:

n-screw ≤ n allowable/l (r/min)

where: n allowable - the maximum allowable working speed of the encoder (R/min)

l - thread pitch (or lead, mm) of workpiece thread

4 determine the cutting amount and feed rate

when programming, the programmer must determine the cutting amount of each process. When selecting cutting parameters, we must fully consider the various factors that affect the impact height of the cutting number display, correctly select the cutting conditions, and reasonably determine the cutting parameters, which can effectively improve the machining quality and output. The factors that affect the cutting conditions are: the rigidity of machine tools, tools, cutters and workpieces; Cutting speed, cutting depth, cutting feed rate; Workpiece accuracy and surface roughness; Tool life expectancy and maximum productivity; Type and cooling mode of cutting fluid; Hardness and heat treatment condition of workpiece material; Number of workpieces; Service life of machine tool

among the above factors, cutting speed, cutting depth and cutting feed rate are the main factors

cutting speed directly affects cutting efficiency. If the cutting speed is too small, the cutting time will be longer, and the tool cannot play its function; If the cutting speed is too fast, although the cutting time can be shortened, the tool is easy to produce high heat, which affects the service life of the tool. There are many factors that determine the cutting speed, which can be summarized as follows:

1) tool material. The maximum allowable cutting speed varies with the tool material. The high-temperature cutting speed of high-speed steel tools is less than 50m/min, the high-temperature cutting speed of carbide tools can be more than 100m/min, and the high-temperature cutting speed of ceramic tools can be as high as 1000m/min

2) workpiece material. The hardness of the workpiece material will affect the cutting speed of the tool. When the same tool is used to process hard materials, the cutting speed should be reduced, while when processing softer materials, the cutting speed can be increased

3) tool life. If the service time (service life) of the tool is required to be long, a lower cutting speed should be adopted. On the contrary, a higher cutting speed can be adopted

4) cutting depth and feed. The cutting depth and feed amount are large, the cutting resistance is also large, and the cutting heat will increase, so the cutting speed should be reduced

5) shape of the tool. The shape of the cutting tool, the size of the angle and the sharpness of the cutting edge will all affect the selection of the cutting speed

6) use of coolant. The machine tool has good rigidity and high precision, which can improve the cutting speed; On the contrary, it is necessary to reduce the cutting speed

among the above factors affecting the cutting speed, the influence of tool material is the most important

the cutting depth is mainly restricted by the rigidity of the machine tool. If the rigidity of the machine tool allows, the cutting depth should be as large as possible. If it is not limited by the machining accuracy, the cutting depth can be equal to the machining allowance of the part. This can reduce the number of knife walks

the spindle speed should be determined according to the allowable cutting speed of the machine tool and the tool. It can be selected by calculation method or look-up table method<

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