How to improve the machining accuracy of thin-walled parts
Because of the characteristics of light weight, material saving, compact structure, thin-walled parts have been increasingly widely used in various industrial sectors. But the processing of thin-walled parts is more difficult, because the thin-walled parts are poorly rigid, weak, easy to deformation in the processing, not easy to ensure the quality of parts processing. How to improve the processing accuracy of thin-walled parts will be a topic of increasing interest in the industry.
The processing of thin-walled parts has been more difficult to solve the problem. Thin-walled parts are now generally processed by means of CNC turning, for which the workpiece clamping, tool geometry parameters, the preparation of the program and other aspects of the test, so as to effectively overcome the deformation that occurs during the processing of thin-walled parts, to ensure machining accuracy. There are many factors that affect the machining accuracy of thin-walled parts, but in summary, there are three main aspects as follows.
Force
Due to the thin wall of the workpiece, it is easy to produce deformation under the action of clamping force, which affects the dimensional accuracy and shape accuracy of the workpiece, as shown in Figure 1.
Figure 1
Heat deformation
Because the workpiece is thin, the cutting heat will cause thermal deformation of the workpiece, making the size of the workpiece difficult to control.
Vibration deformation
Under the action of cutting force (especially radial cutting force), it is easy to produce vibration and deformation, which affects the dimensional accuracy, shape, position accuracy and surface roughness of the workpiece.
Since the factors affecting the processing of thin-walled parts fine found, how will we improve the processing accuracy of thin-walled parts? Next, I will introduce measures to improve the accuracy and efficiency of thin-walled parts processing through specific examples.
Figure 2 shows the thin-walled parts, is our school with CNC lathe external processing products in the more difficult parts. The equipment used is the CNC lathe equipped with Guangzhou CNC system GSK980T. In order to improve the qualified rate of the product, we make comprehensive consideration from the clamping of the workpiece, geometric parameters of the tool and the preparation of the program, etc. It is proved that the precision of the part is effectively improved and the quality of the product is guaranteed.
Figure 2
Analysis of workpiece characteristics
From the part drawing requirements and materials, the difficulty of machining this part is mainly two points.
Because it is a thin-walled part, the thickness of the threaded part is only 4mm, the material is 45 steel, and the batch is large, it is necessary to consider both how to ensure the positioning accuracy of the workpiece during processing and how to consider easy and reliable clamping. The usual turning method is to use a three-jaw chuck to clamp the outer circle or support the inner hole, but this part is thin and the turning force point is relatively far from the tightening force point. The thickness of the thread processing part is only 4mm, and the accuracy requirement is high.
The current Guangzhou CNC system GSK980T thread programming instructions are G32, G92 and G76. G32 is a simple thread cutting, which is obviously not suitable. G92 thread cutting cycle adopts the straight-in feed method, as shown in Figure 3, both edges of the tool cut the workpiece at the same time, the cutting force is larger, and it is difficult to discharge the cutting, and the two cutting edges are easy to wear when cutting, and when cutting threads with larger pitch, the cutting edge wears more because of the cutting The G76 thread cutting cycle uses an oblique feed method, as shown in Figure 4, with a single side cutting edge.
From the above comparison, it can be seen that it is not good enough to use only one instruction for turning threads. Using G92 and G76 for mixed programming, i.e. using G76 for roughing threads first, and then using G92 for finishing, will have two major advantages in thin-walled thread machining: on the one hand, it can avoid thin-walled deformation due to large cutting volume; on the other hand, it can ensure the accuracy of thread machining.
Optimize the fixture design
Due to the thin wall and poor rigidity of the workpiece, if the conventional clamping method is used, the workpiece will be bent and deformed by the axial cutting force and thermal deformation, and it is difficult to meet the technical requirements. In order to solve this problem, we designed a special fixture suitable for the processing of the above parts, as shown in Figure 5.
Figure 5
In which, piece 1 is the main body of the fixture, the material is 45# steel, the left end is clamped diameter of 80mm, available to clamp the workpiece bore diameter range of 20-30mm; piece 2 is the tie bar, the material is 45# steel, diameter of 21mm, it just correspond with the φ21 hole on the sheet workpiece, so that the workpiece in the fixture positioning and transfer of cutting force; piece 3 has been processed left end face and bore workpiece. When clamping, we should pay attention to the axial clamping fit of workpiece and clamping body 1. The small groove is designed to facilitate the control of the total length of the workpiece after the workpiece has been headset and clamped, and the size is 5mm×2mm.
The reasonable choice of tools
Machining steps selected
Clamping blank 15mm long with flat end face to machining requirements.
Drilling through holes with f18 drill, roughing and finishing f21 through holes.
Rough and finish machining of f48 outer circle, machining length greater than 3mm to dimensional requirements.
Turning head, using fixture as shown in Figure 2 for clamping, controlling the total length size 35mm flat end face.
Machining the thread outer dimension to f23.805; using G76 and G92 mixed programming for thread machining; disassembling the Workpiece and finishing the machining.
Cutting dosage selection
For rough turning of the inner hole, the spindle speed is 500-600r/min and the feed speed is F100-F150, leaving a finishing allowance of 0.2-0.3mm; for finishing turning of the inner hole, the spindle speed is 1100-1200r/min and a lower feed speed of F30-F45 is used to obtain a better surface roughness, and the machining is completed by one tool walk; for rough turning of the outer circle, the spindle speed is For external rough turning, the spindle speed is 1100-1200r/min, the feed speed is F100-F150, and the finish turning allowance is 0.3-0.5mm; for external finish turning, the spindle speed is 1100-1200r/min, the feed speed is F30-F45, and the machining is completed by one tool walk.
The internal boring tool adopts machine chucking tool, which shortens the tool changing time, no need to sharpen the tool, has better rigidity, and can reduce the vibration deformation and prevent the vibration pattern; the external rough and finish turning both use carbide 90° turning tool; the threaded tool uses machine chucking tool, with standard tip angle, so that it is easy to replace when worn.
The processing of several points of attention
The workpiece should be clamped to prevent it from slipping and flying out to injure and tie the tool when turning.
Use the appropriate coolant (such as kerosene) when turning, can reduce the heat deformation, so that the processing surface better to meet the requirements.
To pay attention to safe and civilized production.
Source: China Thin-walled Parts Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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