The finishing cut is the last step before a part reaches inspection, and after all the time and effort that goes into making a precision part, it is critical that this final step produces a quality surface finish. Incorporating a few key practices can help prevent the need to scrap parts in this last stage and ensure your surface finishes are always up to snuff.
Feeds and Speeds
It may seem like an obvious point, but dialing in the correct feed and speed is essential for achieving a high-quality surface finish. Increasing the tool speed reduces the built-up edge (BUE), while reducing the feed decreases flank wear. Both can reduce the possibility of tool failure and prolong the life of your tools. A word of caution, however: don’t dial the depth of the cut back so far that the tool rubs against the surface instead of cutting it This will result in a smeared finish.
Although seeing a long-coiled chip come off a tool tip can look satisfying, those long chips may mean the cutting pressure is too high. This can result in accelerated tool wear and create a mess of metal strips that damage the part surface. Adding a chip breaker improves chip removal and control. Factors such as part ductility, tool shape, milling setup and coolant usage all influence chip breaker selection, but once the right one is implemented, it’s easy to clean away chips before they contact the finished surface.
Positive Rake Angle
In addition to reducing the BUE and breaking up chips with a chip breaker, increasing the rake angle helps produce more manageable chips as well, thereby improving cutting efficiency and decreasing tool wear for a more precise surface finish.
It is important that the toolholder remains rigid throughout the cutting process. Any pockets of movement in the setup can cause chatter and result in a scalloped surface. In addition, using best practices like a short tool reach and proper feeds and speeds to minimize tool deflection will also minimize chatter.
Use a Finishing Tool
Roughing tools and finishing tools are distinctly different beasts. Although it may seem like a cost saving measure to use the same tool for both, it can result in greater volumes of scrap after the finishing stage. Additionally, the desired surface finish may require different cutter and insert geometries than those of roughing cuts. A different nose radius, a finer pitch or a wiper insert might be considered for the finishing cut.
If your finding your surface finishes lacking, try implementing some of these practices to improve your final products.