Originally Posted by big_mak  ...Do you really think taking 0.01" off the diameter of a 0.75 endmill is gonna make a big difference stiffness wise?... |
I wondered so I did a calculation. Treat the cutter as a simple cantilever beam (although I admit I have never seen a beam with flutes and cutting edges
). The deflection depends on the
moment of inertia which varies with the fourth power of the radius for a round solid and the third power of the depth for a rectangular beam. Fourth power gives a reduction in stiffness of about 5.3% and third power about 4% and it is probably safe to say that the cutter will fall somewhere between these two. So if the 0.75 cutter was deflecting 0.005 you may see the difference which is about two tenths.
But I don't think it matters because the deflection is going to be constant and when you have the upper end of the cutter relieved you can offset the cutter path to compensate for the deflection because there is clearance. With a parallel cutter a compensating offset is not possible.