Watch Solutions For Hard Milling This MMS inMotion presentation from Makino demonstrates the latest techniques and technologies for high-speed machining of hardened materials for die or mold applications. Check out tips, tricks, and new technologies you can use to mill the toughest steels, shortening your lead times, lowering your costs, and eliminating bench work.
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The best way to figure out feedrate is to use the guide that the manufacturers give to you as a starting point, but then acquire your own knowledge. Take good notes and pay attention to processes that have worked in the past. Every hard milling application is different, and feedrate is really where machining experience comes into play.

Some of the other considerations to throw a knuckle into our feedrate include how long and how far-out your tool will be. The depth of cut is going to play a part. If I go deeper then I'm not going to be able to go as high of a chip load. The run-out of the tool is also going to play a part because the further out that my tool is and the worse the whipping effect is the less likely I am going to be able to run at a higher feedrate. The tool will play a part because if the tool holder has got good TIR or it is a highly balanced then I can get the rpm out of it that I want, so I can increase my feedrate. A low quality or improper tool holder will also slow the feedrate.

So here is an example of a feedrate calculation. Feed equals the rpm multiplied by the chip load multiplied by the number of flutes. So as an example here we are going to multiply our quarter inch diameter ballnose endmill. We are going to use a chip load of about two and a half thou, and rpm of 19,589. This is a two fluted endmill. So our feedrate equals rpm multiplied by the chip load, multiplied by the number of flutes. We throw in their rpm and the chip load and the number of flutes and it tells us that we can run 97 inches a minute with that particular diameter of cutter. When I am looking at these speed rates and I'm looking at the rpm I have to evaluate my part and see if it makes sense that I can run 97 inches a minute. Don't assume the calculation will give you the feedrate infallibly. Use the machining experience at your disposal and notes you've taken during the process to scale the number down or up as is appropriate.