For parts with precision as the core requirement, how to control dimensional stability is always an issue of interest to metal machining manufacturers. Aluminum alloys have much higher thermal expansion coefficient values compared to other commonly used metals. Therefore, significant deformation problems are faced in various machining operations such as thin-wall machining and forging. In addition to the deformation caused by the internal stress and cutting force of the material blank, the heat dissipation problem during the cutting operation and the clamping force during the processing can also cause deformation. Therefore, it is crucial to control these deformations and optimize the dimensional stability of aluminum parts. Here are several strategies to achieve better dimensional stability:
Eliminate The Internal Stress Of The Material
Aluminum stock material may contain internal stresses that can cause deformation during machining. Pre-treating the material through stress-relieving processes like annealing or thermal stabilization can help minimize residual stresses and improve dimensional stability.
A common method for eliminating internal stress in aluminum parts is to use natural or artificial aging and vibration treatment. Natural aging refers to placing the workpiece under natural conditions such as outdoors, so that the internal stress of the workpiece is naturally released, thereby eliminating or reducing the residual stress. Artificial aging is a man-made method, usually heating or freezing treatment to eliminate or reduce the micro-stress and machining residual stress in the workpiece after quenching to prevent deformation and cracking.
Improve The Clamping Method Of Workpieces
Securely fixturing the workpiece is essential to prevent vibration and movement during machining. Well-designed fixtures with adequate clamping force and support can enhance stability and accuracy, particularly for complex or thin-walled parts.
If a three-jaw self-centering chuck or spring chuck is used to radially clamp a thin-walled CNC machined bushing component, the workpiece will undoubtedly deform once released after machining. Therefore, an axial end face compression method with good stiffness should be used. According to the location of the inner hole of the part, make a threaded mandrel to find the inner hole of the part. It should be inserted into the inner hole of the part. The end face and cover plate are pressed tightly, and the nut is tightened backward to prevent loosening and deformation when processing the outer circle and achieve processing accuracy.
For the milling of thin-walled aluminum parts with poor rigidity, the following clamping methods can be used to improve the processing accuracy: use vacuum suction cups to clamp the workpiece to obtain evenly distributed clamping force; another method is to use filling inside the workpiece Liquid medium method to improve the rigidity of the workpiece to reduce workpiece deformation during clamping and cutting. For example, molten urea containing 3% to 6% potassium nitrate can be poured onto the workpiece. Alcohol or water can be used to rinse the final parts to ensure that the liquid media is completely washed away.
Improve The Cutting Performance Of The Tool
Select appropriate cutting tools and machining parameters to minimize heat generation and mechanical stress on the workpiece. Using sharp, high-quality cutting tools with proper coatings and geometries can reduce tool wear and prevent dimensional inaccuracies.
The correct selection of tool parameters directly affects the quality of cutting force and heat dissipation. The material, geometric parameters, and tool structure of the tool have an important impact on the cutting performance. The correct selection of the tool is crucial to improving the dimensional stability of processing.
Tools used for processing aluminum alloys should be properly ground. This results in greater caster and roll angles. Tools used for finishing should use larger rake angles. In addition, a larger rake angle is also beneficial for machining softer aluminum alloys because a larger rake angle means the cutting edge of the tool is sharper. A smaller rake angle is beneficial for roughing, deep machining and high feed rates. Rake angles vary from 0 to 40 degrees and should never be negative.
The size of the relief angle should always be larger because it directly affects the wear of the relief tool surface and the quality of surface finishing. The clearance angle depends on the feed rate and depth of cut. If rough machining and high feed rates are required, the relief angle should be smaller. However, in precision CNC aluminum machining operations, it is necessary to reduce elastic deformation and ensure reduced friction between the tool and the workpiece surface to obtain a high surface finish. For this purpose, the size of the rear corners should be as large as possible.
In addition, the clearance angle is also important for the proper operation of the tool. If the clearance angle is too small, the back (side) of the tool will rub against the workpiece, causing heat loss. An excessive clearance angle will cause the tool to penetrate the workpiece too deeply, causing chatter. Therefore, the optimal gap angle needs to be selected. In most applications, the optimal angle is 6 to 10 degrees.
Finally to ensure smooth milling and reduce the milling forces required for the application, the helix angle should be as large as possible.
Improve Tool Structure
Reducing the number of milling cutter teeth is very important for aluminum machining. This is because aluminum has high plasticity and therefore undergoes large deformations during processing. Aluminum chips are sticky and can interfere with the production of delicate parts with required tolerances. Increasing the spacing between cutting edges allows larger chips to escape.
The cutting edge roughness must be kept below 0.4 µm and machined by appropriate grinding operations. This will eliminate all unnecessary burrs, ultimately reducing heat dissipation and cutting distortion.
Replacement or refurbishment of cutting tools should be carried out in accordance with standards. Therefore, when the surface roughness value exceeds 0.2 mm and the cutting temperature value exceeds 100 degrees Celsius, the tool must be replaced or replenished.
Use Appropriate Operating Methods
Utilize CAM software to optimize toolpaths and minimize tool engagement, particularly in areas with high machining loads or complex geometries. Adaptive machining strategies can help distribute cutting forces evenly and reduce the risk of distortion.
Use symmetrical processing methods on the front and back of the workpiece to avoid concentrated processing that is not conducive to heat dissipation.
Use layered multiple processing methods for all cavities of the workpiece to evenly stress the parts and reduce deformation.
Avoid excessive cutting speeds and feeds that can generate heat and cause thermal expansion of the workpiece. Optimize machining parameters to achieve efficient material removal while minimizing heat generation, especially in critical areas where dimensional accuracy is paramount.
When processing deep cavity parts, the drilling first and then milling method is used to avoid poor chip removal, which may cause parts to overheat, as well as tool collapse and breakage.
Proper cooling and lubrication help dissipate heat and reduce friction during machining, which can minimize thermal expansion and distortion of the workpiece. Consider using a coolant system with adequate flow and concentration to maintain stable machining conditions.
Reasonably arrange the processing technology, the CNC high-speed cutting process is generally: roughing – semi-finishing – corner cleaning – finishing and other processes. For parts with high precision requirements, it is necessary to increase the number of semi-finishing operations and retain a uniform machining allowance.
By implementing these strategies, manufacturers can improve the dimensional stability of CNC-machined aluminum parts, ensuring tighter tolerances, better surface finish, and overall higher quality of the finished products.