Aluminum alloy is an important industrial raw material. It has relatively low hardness and a high coefficient of thermal expansion. The thin-wall or sheet parts, it is easy to be deformed in the CNC machining process.
In addition to improving the performance of the cutting tool or eliminating the internal stress of the material by aging treatment in advance, we can adopt some machining technology to reduce the deformation of the part as much as possible.
For example, when machining aluminum parts with a large processing allowance, to create better heat dissipation conditions and reduce thermal deformation, we should adopt the symmetrical machining method to avoid heat concentration as far as possible.
Here to introduce some tips to reduce the deformation of aluminum parts in CNC machining:
1. Method to Machine Multiple Cavities Aluminum Part.
For aluminum parts with multiple cavities, if one cavity is machined in one cavity in turn, it is easy to make the cavity wall deformed due to uneven force.
The best solution is to cut a small layer of each cavity one by one, then repeat the processing layer by layer till getting the required size. In this way, the force of the parts will be more uniform, and the probability of deformation is smaller.
2. To Choose the Right Cutting Amount on Machining
Selecting the proper cutting amount can effectively reduce cutting force and the heat generated in the cutting process.
In the CNC machining process, a big cutting amount will lead to a large cutting force, which is easy to cause the deformation of parts, and also, affect the rigidity of the machine spindle and the durability of the tool.
3. Improve Cutting Ability of the Tool
Tool material and its geometric parameters have an important impact on cutting force and cutting heat, choosing the correct tool is very important to reduce parts deformation during the machining processes.
Some advice is listed below for reference.
1). Reasonable selection of tool geometric parameters
a). Radial Rake Angle
To maintain the strength of the cutting edge, it is better to choose a bigger Radial Rake Angle, if so, it can not only grind out a sharp cutting edge but also reduce the cutting deformation, so that the chip removal is smooth, hence reducing the cutting force and cutting temperature. Never use a tool with a negative Radial Rake Angle.
b). Clearance Angle
The clearance angle has a direct impact on the wear of the back tool surface and the quality of the workpiece surface. The cutting thickness is an important factor in selecting the clearance angle.
As for rough milling, due to the large feed rate, heavy cutting load, and more heat generated, it requires good heat dissipation of the milling tool, therefore, the back Angle should be selected smaller.
Fine milling requires a sharp cutting edge, needs to reduce the friction between the tool surface and the work-piece surface, as well as to reduce the elastic deformation. Therefore, a larger clearance angle should be selected.
c). Helix Angle
To ensure the milling process is smooth and reduces milling force, a relatively large Helix Angle should be chosen.
d). Entering Angle
Appropriately reducing the Entering Angle can improve The heat dissipation condition, and lower the average temperature in the machining area.
2). To improve tool structure
Reduce the number of cutter teeth and increase the chip space.
Due to the large plasticity of aluminum alloy material, it has large cutting deformation in the machining process, hence larger flute space and fewer milling cutter teeth are better. For example: if the milling cutter diameter is less than φ20mm, it is better to adopt two cutter teeth, if the milling cutter diameter is between φ30 and φ60mm, it is better to adopt three cutter teeth, it will avoid the deformation of thin-walled aluminum parts caused by chip blockage.
Sharpen the cutter teeth: the roughness of the cutting edge of the cutter teeth should be less than Ra=0.4um.
Before using a new tool, we should gently grind the front and rear surface of the cutter tooth several times by using a fine whetstone, it will help to eliminate residual burrs and slightly serrated lines when sharpening the cutter teeth. In this way, it can not only reduce the cutting heat but also lower the cutting deformation.
Setting a strict tool wear standard.
if the tool is worn, it will increase the roughness of the work-piece surface, rise the cutting temperature, then increase the work-piece deformation.
Therefore, in addition, to choosing good wear resistance tool materials, tool wear standards should be set to not greater than 0.2mm, otherwise, it is easy to produce debris.
4. To Select Suitable Cutting Sequences.
Rough machining and fine machining should adopt different cutting sequences
Rough machining requires the fastest cutting speed and the shortest time to cut off the excess material on the raw materials to form the geometric profile for the fine machining process. Therefore, machining efficiency is most important in rough machining, it pursues high materials cutting rate per unit of time, so it should adopt up milling.
However, fine machining requires more precise and better surface quality, it should adopt down milling.
Because the cutting thickness of the cutter teeth gradually decreases from the maximum to zero during the down-milling process, the phenomenon of work hardening is greatly reduced, and the deformation of the parts is inhibited.
5. Clamping Method for the Thin-wall Aluminum Part
When machining the aluminum thin-walled parts, the clamping force is also an important reason to cause the deformation, which can’t be avoided even if the machining accuracy is improved.
To reduce the work-piece deformation due to clamping, the pressed part can be loosened before reaching the final size, release the clamping force, so that the part can return to the original state, and then slightly press again.
The force point of secondary clamping is best to be on the supporting surface, and the clamping force should act in the direction with good work-piece rigidity. The clamping force magnitude should be determined by the strength of the clamping workpiece without loosening, it has higher requirements for the experience and feeling of the operator. The compression deformation of the parts is small if it is machined in this way.
6. Adopt Drilling then Milling Machining Method
For the parts with a cavity, machining it by using the milling cutter directly will result in poor chip removal due to lack of chip space, thus the parts accumulate a lot of cutting heat, then the expansion deformation, In the worst case, it may cause accidents of cutter collapse or cutter broken.
The best method is to drill before milling, that is, to drill the hole with a drill bit greater than the cutter size, and then use the cutter to insert the hole to start milling, which can effectively solve the above-mentioned problems.
