How to improve the machining accuracy of thin-walled parts

Because of its light weight, material saving, compact structure, etc., thin-walled parts have been increasingly widely used in various industrial sectors. However, the processing of thin-walled parts is more difficult, because thin-walled parts have poor rigidity and weak strength, and are extremely easy to deform during processing, and it is difficult to ensure the processing quality of the parts. How to improve the machining accuracy of thin-walled parts will be a topic of increasing concern in the industry.

The processing problem of thin-walled parts has always been difficult to solve. Thin-walled parts are generally processed by CNC turning. For this reason, it is necessary to test the clamping of the workpiece, the geometrical parameters of the tool, the programming of the program, etc., so as to effectively overcome the deformation in the processing of thin-walled parts and ensure Precision. There are many factors that affect the machining accuracy of thin-walled parts, but they can be summarized in the following three aspects:

Force

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Due to the thin wall of the workpiece, it is prone to deformation under the action of clamping force, which affects the dimensional accuracy and shape accuracy of the workpiece, as shown in Figure 1. Heat change

Because the workpiece is thin, the heat of cutting will cause thermal deformation of the workpiece, making it difficult to control the size of the workpiece. Vibration deformation

Under the action of cutting force (especially radial cutting force), it is easy to produce vibration and deformation, which affect the dimensional accuracy, shape, position accuracy and surface roughness of the workpiece.

Now that the factors affecting the machining precision of thin-walled parts have been found, how will we improve the machining accuracy of thin-walled parts? Next, the author will introduce the measures to improve the machining accuracy and efficiency of thin-walled parts through specific examples.

The thin-walled parts shown in Figure 2 are the most difficult parts in the external processing products of our school's CNC lathe. The equipment used is a CNC lathe equipped with Guangzhou CNC system GSK980T. In order to improve the qualification rate of products, we comprehensively consider the workpiece clamping, tool geometric parameters, programming and other aspects. Practice has proved that the accuracy of the parts is effectively improved and the quality of the product is guaranteed.

1. Analysis of workpiece characteristics

From the perspective of part drawing requirements and materials, there are two main difficulties in processing this part:

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 how to ensure the positioning accuracy of the workpiece during processing, as well as the convenience and reliability of clamping. Usually turning is processed by the clamping method of clamping the outer circle or supporting the inner hole with a three-jaw chuck, but this part is thin, the turning force point is relatively far from the tightening force action point, and it is necessary to turn M24 thread , The force is large, the rigidity is insufficient, and it is easy to cause shaking. Therefore, the problem of how to clamp and locate must be fully considered. The thickness of the threaded part is only 4mm, and the precision is high.

At present, the GSK980T thread programming instructions of the Guangzhou CNC system include G32, G92, and G76. G32 is a simple thread cutting, obviously not suitable. The G92 thread cutting cycle adopts the straight-line feed method. As shown in Figure 3, the cutting force on both sides of the tool is cutting at the same time, and the cutting is difficult. The two cutting edges are easy to wear during cutting, and the cutting pitch is relatively large. In the case of threaded threads, due to the larger cutting depth, the blade wears faster, resulting in errors in the pitch diameter of the thread, but the tooth profile processed by it has a higher precision. The G76 thread cutting cycle adopts the oblique feed method. As shown in Figure 4, the one-sided cutting edge cuts the workpiece, and the cutting edge is easily damaged and worn, but the processed thread surface is not straight, and the tool tip angle changes, resulting in relatively high profile accuracy. difference.

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From the above comparison, it can be seen that it is not perfect to simply use one command to turn the thread. Using G92 and G76 for programming, that is, first use G76 for thread roughing, and then use G92 for finishing in thin-walled thread processing. There are two major advantages in this: on the one hand, it can avoid the thin-wall deformation caused by the large amount of cutting; on the other hand, it can ensure the accuracy of thread processing.

2. Optimize fixture design

Due to the thin wall and poor rigidity of the workpiece, if the conventional method is used for clamping, the workpiece will be bent and deformed under the influence of axial cutting force and thermal deformation, which is difficult to meet the technical requirements. To solve this problem, we designed a set of special fixtures suitable for the processing of the above parts, as shown in Figure 5.

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Among them, piece 1 is the main body of the fixture, the material is 45 gauge steel, the left end is clamped with a diameter of 80mm, and the diameter of the inner hole that can be used to clamp the workpiece is 20-30mm; piece 2 is a tie rod, the material is 45 gauge steel, and the diameter is 21mm, it just matches with the φ21 hole on the sheet workpiece to position the workpiece in the fixture and transfer cutting force; part 3 is the workpiece with the left end face and inner hole processed, pay attention to the workpiece and the clamp concrete 1 when clamping Axial clamping fit. The small groove is designed to facilitate the control of the total length after the workpiece is turned around and clamped. The size is 5mm×2mm.

Third, the reasonable choice of tools

Processing step selection

The clamping blank is 15mm long, and the end face is flat to the processing requirements;

Use f18 drill bit to drill through hole, rough and finish f21 through hole;

Rough and finishing F48 outer circle, the processing length is greater than 3mm to the size requirement;

U-turn, use the clamp as shown in Figure 2 to clamp, control the total length of 35mm flat end face;

Process the thread outer circle size to f23.805; Use G76 and G92 mixed programming for thread processing; Disassemble the workpiece and complete the processing.

Five, cutting amount selection

When the inner hole is rough turning, the spindle speed is 500～600r/min, the feed speed is F100～F150, and the fine turning margin is 0.2～0.3mm; when the inner hole is finished turning, the spindle speed is 1100～1200 r/min, which is to obtain For better surface roughness, choose a lower feed rate F30～F45, and complete the machining with one pass; when the outer circle is rough turning, the spindle speed is 1100～1200 r/min, and the feed speed is F100～F150, leaving for fine turning The margin is 0.3～0.5mm; When the external circle is finished turning, the spindle speed is 1100～1200 r/min, the feed speed is F30～F45, and the machining is completed in one pass.

The internal boring tool adopts machine clamping tool, which shortens the tool change time, does not need to sharpen the tool, and has good rigidity, which can reduce vibration deformation and prevent the generation of vibration marks; both the outer diameter and the fine turning are made of cemented carbide 90° turning tools ; The thread cutter is a machine-clamped tool with a standard tip angle, so that it can be easily replaced when worn.

Six, several points of attention during processing

The workpiece should be clamped to prevent it from slipping and flying out, hurting people and sticking a knife during turning;

Use proper coolant (such as kerosene) during turning, which can reduce thermal deformation and make the machined surface better meet the requirements;

Pay attention to safe and civilized production.