User Guide

surface, occasionally digging in. The surface of the tool that is doing the most cutting will
find a frequency of vibration that is a product of all the variables involved. This can cause
anything from a high pitched whine on light, high speed cuts to a resonating racket that
can rip the work out of the chuck on heavy cuts. If you maintain the same feed rate and
reduce the RPM, the feed will increase because chip will be thicker. (If that sounds
wrong at first, think of it this way: At the same feed rate, if you cut the RPM in half,
twice as much metal must be removed with each rotation to get to the end of the cut in
the same amount of time. The chip is twice as thick, so the feed is GREATER at lower
RPM if the feed RATE stays constant.)
When a tool chatters it gets dull faster because it must keep cutting through the
previously machined surface that has been "work hardened" by machining. As you can
imagine, there are limits to how much you can increase feed rate, so the answer lies in
adjusting both speed and feed to achieve the proper cut.
Proper cutting speed is the rate a particular material can be machined without damaging
the cutting edge of the tool that is machining it. It is based on the surface speed of the
material in relation to the cutter. This speed is a function of both the RPM of the spindle
as well as the diameter of the part or size of the cutter, because, as the part diameter or
cutter size increases, the surface moves a greater distance in a single rotation. If you
exceed this ideal speed, you can damage the cutting tool. In the lathe and mill
instructions, we give some examples of suggested cutting speeds, but what I wanted to
get across here is that it isn't a slow process. A tool can be destroyed in just a few
seconds. It isn't a case of getting only one hour of use instead of two. The cutting edge
actually melts. If you machine tough materials like stainless steel, you will ruin more
tools than you care to buy if you don't pay a lot of attention to cutting speeds. Charts
showing suggested cutting speeds for various materials are included in both the lathe and
mill sections that follow.