Re: [CAD_CAM_EDM_DRO] Newbie question

bdarin0108 wrote:

> New to this board, but have been lurking for a while and now I have a
> question. Been seeing a lot of stepper motor conversions etc., and I
> was wondering, from all you gurus out there, can CNC be done using
> regular DC motors, such as used in a table power drive, and the output
> from a DRO as feedback info. Seems like a natural. I have a Jet JMD15
> mill/drill with a Shooting Start DRO on X, Y, and Z. Can the DRO
> output be used to control the motion motors, and would it be better
> than or worse than stepper motors without feedback? It seems that
> ballscrews wouldn't be necessary since backlash wouldn't be relevant
> as the actual table movement would be used to set position. Anyone
> else going in this direction? TIA.

The regular DC motors may well be suitable. Where the problems come in
is that for closed loop operation, the backlash between the motor and
the position sensor must be very small, otherwise the servo loop 'hunts'
at a high rate, wearing out not only the drive but your nerves, because it
is noisy. Most power feed drives have gears with a fair bit of backlash,
and would therefore be unsuitable. DC servo motors with toothed belt
drive work well, however.

It is standard wisdom in the industry that you want to have 10 x the resolution
available as you have displayed. That means .0001" resolution for a .001"
display, which is not too hard to do with shaft encoders. But, it is a bit
tough with linear encoders.

The DRO readout box is too slow to be of any use in a CNC servo system, but the
sensing scale might do fine in that use.

Backlash is VERY relevant, as you need to maintain tight control of the
table position, otherwise the motor keeps banging from one side of the
backlash to the other, trying to hold position of the table.

Jon