I spent some time trying to work my way through the feeds and speeds section of the Machinery's Handbook. This is a thick read, but it is informative. For one it becomes clear that all the feed/speed info out there is really focused on getting the fastest cut out of the machine at the expense of tool wear. They want you to use the highest spindle speed and fastest feed rates possible.

One output of all this is that the limiting factors include overheating the cutter, stalling out the spindle motor, and chatter/flex in the frame of the machine. I did a quick power calculation and for the bits we can use (1/8" single flute) our 500 watt spindle should be more than strong enough for anything but steel. That leaves chatter/flex of the frame and burning the cutter. Burning can be minimized by taking a correct amount of material on every pass of the cutter (having a proper chip load), and flex is down to experimenting with feed rates to minimize chatter.

After running many different calculations using values from many sources I have come up with the theory that all that really matters is the chip load. Once we have an idea of how much of a cut we want to make on each revolution (how large the chips will be) we can then take a stab at the best feed rate and work out the matching RPM (or the other way around) to produce a clean cut within our machines capacity.

I made a new calculator (on sheet two) that can calculate feeds and speeds based on only the number of teeth on your bit, a percent of chip load (0 percent is a very thin chip, 100 percent is a very thick chip), and either the desired feed rate or RPM. The output will be the missing value, all limited to the ranges that your machine can support.

I need to sit down and run these numbers through some real world testing to see if my theory holds any water, but if it works this would be a much simpler way to think about all of this for our particular machine.

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