Surface finish issues are especially critical in aerospace and medical applications. Chips recontacting the work and high or unstable Built Up Edge (BUE) are the usual suspects of poor surface finish on machined parts, regardless of material. There can be other factors, such as a poorly maintained machine or exhausted metalworking fluids, but these are seldom the case when “the last job on this machine ran just fine.”
Here are our 5 tips that you can address on the machine to make poor surface finish go away:
1) Increase the speed SFM (especially on Carbide!). This will help reduce BUE.
2) Reduce the feed per revolution (IPR- inch per revolution). This will help reduce the flank wear.
3) Increase the top rake angle.
4) Add a chip breaker / chip curler.
5) Increase tool nose radius.
We have seen increasing speed to be especially helpful on aerospace and medical machining jobs on stainless steel. Increasing speed is also important when using carbide- carbide likes speed.
If you can see that the chip is recontacting the workpiece, then address your chip control issues first. Chip control is the first place to start. Adding chip control geometry on the tool is probably the easiest change on non CNC machines. Modifying the cam to break the chip should also be considered. On CNC’s, adding chip breaks into the program is also an easy adjustment. These are especially effective if the workpiece is a gummy material.
Built Up Edge (BUE) is impacted by three primary factors: material chemistry (which you can’t change- you already have the material); surface footage (slower speed means hot chip is in contact with tool longer, creating higher BUE); and tool geometry (the point is to slice or cut, not rub off the material).
Of course, you should make sure that your setup is rigid, your tooling properly seated, your coolant lines are delivering plenty of coolant to the tool/work interface, etc., etc.. But these 5 tips are ‘Tools You Can Use’ to improve the surface finish on your problem jobs, including stainless and other aerospace and medical materials.