This article is focused expressly on creating a better surface finish. Leading custom machine shop experts say it is important to keep the RPMs up where surface speed says they should be and back off the feed rate for a finer finish.
Use a Finish Pass with a Light Cut
According to custom machine shop machinists, it is good to keep a low cut width (or cut depth if you’re talking the floor of a pocket or surfacing with a Face Mill) and do a separate finish pass. The depth of cut has to be larger than the chip load, or you can get rubbing. Usually, something that will be right for the finish pass.
Clear the Chips
Chips down in the work will scratch it up as your cutter slides them around in the hole. With work hardening materials like stainless, you may as well scatter a handful of hardened steel chips to scratch up your work. Use your coolant or air blast to get the chips completely out of the way of the hole. Many say this is one of the first and most important things to attend to improve the surface finish.
Use Different Tools for Roughing and Finishing
Don’t finish with the same tool you used for roughing. Keep brand new sharp tools for finish passes and rotate them to roughing work after they’ve done a brief tour finishing. If you’re using an indexable tool, like a face mill, use two different sets of inserts.
Reduce Depth of Cut Slightly for Finish Pass on wall
For the finish pass on a feature with a floor, reduce your depth of cut versus the roughing pass by a few thousandths. This means you’ll only be using the side and not the bottom of the cutter, which results in a better surface finish.
Minimize Deflection and Chatter
You can’t get good surface finish if your tool is deflecting excessively or chattering. You’ll see every bit of that in the finish on the cut sidewalls. Use G-Wizard’s Cut Optimizer to minimize deflection, avoid chatter on finish passes, and do what you can to increase rigidity and minimize vibration of any kind. Chatter is the worst kind of vibration and will be very visible in the finish.
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