Re: Newbie question

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:

> 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

Well, thanks, Jon. That was certainly informative. I didn't realize a
lot of that. What I've now done is dig out my Home Shop Machinist last
4 issues covering the Grizzly mill/drill CNC conversion. This looks
do-able and someone else has already done the research so I don't need
to re-invent the wheel. Think I'll take this route and just take my
chances with an open-loop system. They must work OK as everyone seems
to be going in that direction. Anyone else follow that series of
articles? If so, waddaya think?
Bob