The causes and solutions of tool marks in metal processing
Precision metal parts are often manufactured using a variety of precision machining techniques, of which CNC machining is a common method. Typically, the dimensional and cosmetic requirements of precision parts are often very high.
Therefore, when machining metals such as aluminium and copper using CNC, the appearance of tool marks and tool lines on the surface of the finished product is a matter of concern. In this paper, we discuss the causes of tool marks and tool lines during the machining of metal products. We also suggest potential solutions.
Insufficient fixture clamping force
CAUSE: Some cavity metal products require vacuum fixturing, which may not produce sufficient suction due to surface irregularities, resulting in tool marks or lines.
Solution: To alleviate this situation, consider transitioning from simple vacuum suction to vacuum suction combined with pressure or lateral support. Or, explore alternative fixturing options based on specific part configurations to tailor a solution to a specific problem.
Process Related Factors
Cause: Certain product manufacturing processes may contribute to this problem. For example, products such as tablet backshells go through a series of machining steps that include punching side holes and then CNC milling the edges. This sequence can result in visible tool marks when the milling reaches the side holes.
Solution: This problem is common when aluminium alloys are used for electronics housings. The process can be modified to use CNC milling instead of punching and milling the side holes, and to ensure that the tools engage consistently during the milling process to reduce uneven cutting.
Inadequate programming of tool path engagement
Cause: This problem usually occurs during the 2D contouring phase of product production. poorly designed toolpath engagement in the CNC programme leaves marks at the entry and exit of the tool.
Solution: In order to solve the problem of avoiding tool marks at the entry and exit points, a typical approach is to introduce a slight overlap (approx. 0.2 mm) of the tool engagement distances. This technique helps to avoid possible errors in the accuracy of the machine lead screw.
Although this method is effective in preventing the formation of tool marks, it creates an element of repeat machining when the product material is soft metal. As a result, the texture and colour of this part may vary compared to other areas.
Fish scale pattern on flat machining surface
Cause: Fish scale pattern or round pattern appears on the flat surface of the product, the tools used in processing soft metals such as aluminium/copper are generally 3~4 edge alloy material milling cutters with hardness between HRC55~HRC65, these milling cutters are processed with the bottom edge of the cutter, and the surface of the product may have obvious fish scale pattern, which affects the overall appearance.
Solution: Commonly used for products that require high flatness and have a concave structure on the flat surface. The remedy is to switch to cutting tools made of synthetic diamond material, which helps achieve a smoother surface finish.
Ageing and wear of equipment components
Cause: Knife marks on the surface of the product are caused by aging and wear of equipment spindles, bearings, screws, and other components, coupled with poorly set CNC system clearance parameters, resulting in visible knife marks, especially when machining rounded corners.
Solution: These problems originate from equipment-related factors and can be solved through targeted maintenance and replacement.
Conclusion
When CNC machining metal, obtaining the desired surface requires a practical approach. There are a variety of ways to avoid tool marks and lines that involve equipment maintenance, fixture improvements, process adjustments and programming improvements. By understanding and correcting these factors, manufacturers can ensure that precision parts not only meet dimensional standards, but also have the desired aesthetic qualities.