Re: what gear ratio ??? - more

I understand the reason for the gearing down, just wasn't so sure the
encoder must be on the motor and not the screw.

Eric

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Doug Fortune <pentam@c...>
wrote:

> cnchomeman wrote:
> > Doug wrote:
> > > Let me further say, a 10:1 ratio might be best with those

smallish

> > > motors, to get decent machining power at typical machining

speeds....

> > > however the rapids might suffer. Also, to get more than 2.5:1

or 3:1

> > > you'll need more than 1 stage of belt drives, which complicates

things.

> > >
> > > So all in all, try the 2.5 to 3:1 first and see if that works

for

> > > you. Put the encoder on the motor, not the leadscrew.
> > >
> >
> > Why does the encoder need to be on the motor? For resolution?
>
> Because (if there is any gearing) there will be a discrepancy
> between the position of the leadscrew and the position of the
> motor, thus probably leading to nasty oscillations.
>
> True, mounting the encoder on the (geared down) motor shaft, will
> increase resolution - this is good (generally).
>
> > If he has 300 PPR encoders thats 1200 lines and 5 pitch screws
>
> I call that 300 CPR (cycles per revolution) and 1200 ciq (counts in
> quadrature per revolution)....
>
> > he is at .000 1666666" per step when its mounted on the screw.
>
> True.
>
> > it's .000 0555555" per step when mounted on the motor.
>
> Also true (with a 3:1 reduction).
>
> > He will need a lot of pulses to go 100" per minute.
>
> Also true. Say 120"/min == 2"/sec -> 36,000 pulse/sec
> which is easily achievable by Mach2 (http://www.artofcnc.ca )
> and achievable by TurboCNC (http://www.dakeng.com/turbo.html ) and
> probably also EMC/Linux.
>
> > Or is there some other reason?
> >
> > Eric
>
> The reason for the gearing down, is that a 40 oz*in continuous
> motor is woefully insufficient for heavy milling on a Bridgeport.
>
> Luckily, 400 oz*in peak (times 3) is OK and hopefully 40*3 = 120

oz*in

> will do for continuous light cuts. The real answer is either:
>
> - a bigger motor
> or
> - higher gearing (say *10 ) in conjunction with a lower resolution

encoder

> (like 100 CPR).
>
> But considering the difficulties (and extra backlash, setup &

expense) caused by

> compound levels of gearing, perhaps 3:1 will be sufficiently

powerful to do

> some useful cnc machining.
>
> ------------------
>
> ONE OTHER THING:
>
> Note that the above implies a 0.000 055555 inch/step accuracy.

That might

> be true if the motor driver kept the tracking (of the motor)

exactly in

> step with the desired position, but that doesn't happen. Some

people use

> a rule of thumb that says to expect around a +/- 10 count deviation
> (perhaps a LOT more during highspeed moves, but thats usually while
> rapiding and not cutting, so exact accuracy doesn't matter in that

case).

>
> Thus the expected accuracy is around 0.000 0555 * 10 or about 1/2

thou.

> Again, this is a great number, considering the conflicting

requirements.

>
> regards
> Doug Fortune
> http://www.cncKITS.com
>
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