Solution for deformation during machining on multi-spindle machining centers

CNC machining centers are suitable for the production and processing of various parts, with commonly processed materials including die-castings, steel parts, and cast iron parts. Due to the large linear expansion coefficient of die-casting materials, thick-walled parts are prone to deformation during the production process. The following measures are adopted to reduce deformation of aluminum parts during production:

(1) Reducing the thermal stress of the blank

Natural or artificial aging, as well as vibration treatment, can partially eliminate the thermal stress of the blank. Preliminary machining is also a feasible processing method. For rough and bulky blanks, due to their large volume, the deformation after machining is also significant. If unnecessary parts of the blank are machined in advance to reduce the volume of each part, it can not only reduce the machining deformation in subsequent processes but also release some thermal stress after being placed for a period of time after preliminary machining.

(2) Improving the cutting performance of the cutting tool

The material and geometric parameters of the cutting tool have a crucial impact on cutting speed and cutting heat. Proper selection of the cutting tool is particularly important for reducing part processing deformation.

① Reasonable selection of cutting tool geometric parameters.

• Rake angle: While maintaining the strength of the cutting edge, a slightly larger rake angle can be selected. On one hand, it can sharpen the cutting edge; on the other hand, it can reduce cutting deformation, facilitate smooth milling, and thereby reduce cutting speed and cutting temperature. Negative rake angle cutting tools should not be used.

• Relief angle: The size of the relief angle has a direct impact on the wear of the rear cutting edge and the surface quality of the machined part. The thickness of the cut is a key criterion for selecting the relief angle. During rough milling, due to the high cutting speed, heavy cutting load, and large amount of heat generated, good heat dissipation is required for the cutting tool. Therefore, a smaller relief angle should be selected. During finish milling, a sharp cutting edge is required to reduce friction between the rear cutting edge and the machined surface, thereby reducing elastic deformation. Therefore, a larger relief angle should be selected.

• Helix angle: To ensure stable cutting and reduce cutting force, a larger helix angle should be selected whenever possible.

• Main cutting edge angle: Appropriately reducing the main cutting edge angle can improve heat dissipation conditions and reduce the average temperature in the working area.

② Improving the structure of the cutting tool.
Reduce the number of cutter teeth and increase the chip space. Due to the high plasticity of die-casting materials and significant cutting deformation during processing, a large chip space is required. Therefore, the radius of the chip groove bottom should be larger, and it is better to have fewer cutter teeth. For example, for cutters with a diameter below 20mm, two cutter teeth are used; for cutters with a diameter of 30-60mm, three cutter teeth are preferred to avoid deformation of thick-walled die-casting parts caused by chip clogging.

• Honing the cutter teeth: The surface roughness of the cutting edge of the cutter teeth should be less than Ra=0.4um. Before using a new cutter, the front and rear of the cutter teeth should be lightly honed with a fine whetstone to remove burrs and slight sawtooth patterns left during sharpening. This can not only reduce cutting heat but also result in less cutting deformation.

• Strictly controlling the wear standard of the cutting tool: After the cutting tool is worn, the surface roughness of the workpiece increases, the cutting temperature rises, and the deformation of the workpiece increases accordingly. Therefore, in addition to using alloy structural steel with good wear resistance, the wear standard of the cutting tool should not exceed 0.2mm, otherwise, it is easy to cause built-up edge. During cutting, the temperature of the workpiece should generally not exceed 100°C to prevent deformation.

③ Improving the clamping method of the workpiece. For thick-walled die-casting parts with weak rigidity, the following clamping methods can be used to reduce deformation:

• For thick-walled shaft sleeve parts, if they are clamped axially using a three-jaw self-centering chuck or a collet, the workpiece will inevitably deform once it is released after machining. In this case, a method of clamping the radial inner bore with good rigidity should be used. Position the part based on its internal thread, make a threaded shaft, insert it into the internal thread of the part, and use a cover plate to clamp the inner bore, which is then tightened with a nut. This can prevent clamping deformation when machining the outer circle, thereby achieving satisfactory machining accuracy.

• When machining thick-walled sheet metal parts, it is best to use a vacuum chuck to obtain evenly distributed clamping force and use a cutter with less durability for machining, which can effectively prevent workpiece deformation.

• In addition, the filling method can be used. To improve the processing rigidity of thick-walled workpieces, a material can be filled inside the workpiece to reduce deformation during clamping and cutting. For example, a urea melt containing 3%-6% potassium nitrate can be poured into the workpiece. After processing, the workpiece can be immersed in water or alcohol to dissolve and remove the filler.

④ Scientifically arranging the processing sequence. During high-speed cutting, due to the large amount of remaining material and intermittent cutting, vibration often occurs during the cutting process, affecting machining accuracy and surface roughness. Therefore, the high-speed cutting process in CNC machining can generally be divided into the following steps: rough machining, semi-finish machining, corner cleaning, and finish machining. For parts with high precision requirements, sometimes a second semi-finish machining step is necessary before finish machining.

After rough machining, the part can be naturally cooled to eliminate the thermal stress generated during rough machining and reduce deformation. The allowance left after rough machining should be greater than the deformation, typically 1-2mm. During finish machining, a uniform allowance should be maintained on the finish machined surface of the part, preferably 0.2-0.5mm, to keep the cutting tool in a stable condition during processing, thereby reducing cutting deformation, achieving good surface machining quality, and ensuring the precision of the product.

The key factors affecting the machining accuracy and surface quality of die-casting parts are the deformation phenomena that easily occur during the processing of such parts. There are many reasons for the deformation of die-casting parts during processing, which are related to the material, part shape, and manufacturing conditions. The main aspects include deformation caused by thermal stress in the blank, deformation caused by cutting speed and cutting heat, and deformation caused by clamping force. This requires operators to have certain practical experience and skills.