Re: backlash

Tony Jeffree wrote:

>Tim -
>
>That is only because the crude approximations to backlash compensation
>start with the assumptions that:
>
>(a) a given program will not do both climb milling and "normal" milling, and
>

No, the problem with climb milling is that the rotating cutting tool can
pull the work into the
cutter, causing a broken cutter, wrecked workpiece, damaged machine, etc.

>(b) the backlash is constant over the travel of a given leadscrew, and
>(c) the backlash value is accurately known.
>
>

No, it still doesn't work. The problem is that when an axis reverses
while the cutter is making a
cut, the machine cannot get from one side of the backlash to the other
side instantaneously, so it
allows the table to be unconstrained by the motors for a moment. This
is a situation whenever
you are milling a circular hole or pocket, a wonderful thing to be able
to do with a CNC machine.

>It would be possible to provide sufficient information to the control
>software to allow it to do a significantly better job of backlash
>compensation than most controls manage currently; however, the real
>question is whether it is a worthwhile thing to do, given the alternative
>of building a better machine to start with.
>
>

The problem stated above just can't be fixed with software, no matter
how hard you try.

Jon