Frequency and Speed

Categories: Understanding Machining Dynamics

Imagine a rod protruding out of a holder. We put a dot on its end so we can see it rotate. We apply the right amount of force on the free end to make it deflect, rebound and return to center in exactly one second. We rotate the rod at exactly 60 RPM, or one revolution per second, so that one back and forth cycle equals one revolution. The dot ends up in the same spot in the back and forth cycle every second. Now let's extend that rod out further from the holder and apply same amount of force on the free end. Because it is now more flexible, it deflects further and takes longer to return. It now takes two seconds to complete one back and forth cycle. To make that one cycle equal to one revolution we now have to slow the rotational speed down to one revolution per TWO seconds or 30 RPM.

Replace that dot with a tooth or multiple teeth and you can see the challenge presented by milling tools. So why would be given the SAME speed recommendation for all the endmills shown at the bottom. How can that be? 

Fortunately, there are ways of measuring the tool point frequency and calculating stable speeds specific to the length of tool and number of teeth.