Re: [CAD_CAM_EDM_DRO] gearmotors,encoders

Hi Jack,

Funny you should ask about gear motors. We just had a mention of the
ShopBot, a BIG 4'x8' router table. I looked at the pix, and I think I
saw a Vexta motor with attached geared. There was a pinion gear
attached, and that drove a rack. I have a set of four PH266-01GK 83
oz/in motors attached to 2GK7.5KA gear heads. 'Been saving them for a
suitable project. I don't know what the gear ratio is, (easy enough to
figure out). Inline shaft, not 90 deg. I'm hoping to hear what is on
the ShopBot.

So I'd say SOME router designs use them! Encoders are BEST on the
actual table or item being moved. I can't speak from experience, but I
know the sherline DRO's are on the lead screw, it's closer to the table
then the motor! I would think that being at least on the lead screw
would remove the backlash from the motor/gearbox?

Alan

jmw@... wrote:

>
> Are gearmotors more or less verboten as axes motors in CNC
> applications? Seems like the answer has gotta be "yes" for the same
> backlash reasons that have landed acme screws and nuts in scrap bins.
>
> But a 90 deg gearmotor sure would make a neat X-axis installation on
> a Bport-class machine. It'd be fine on Y too, and no messing with
> wheels and belts.
>
> I've never heard of a no backlash gearset, except when someone messes
> up the shim pack rebuilding their diff ...
>
> ---------------------------------------------------------------------
>
> Apologies in advance for burning bandwidth; I've asked about this
> before--but I still don't get it. Why not put rotary encoder(s) on
> the non-motor end of the screw rather than on the motor shaft? It
> seems like one wants to know the position, direction and speed of the
> screw rather than of the motor per se. So if it's no worse, possibly
> even better, to have the encoder on the far end--then what would be
> wrong with a slightly flexible coupling (e.g. Lovejoy) between the
> motor and the screw? I can see that during accel and decel the motor
> and screw could get out of phase, and maybe in some bitter state of
> the world this could feed on itself, but couldn't this problem be
> tweaked away with a little gain correction? I like slightly flexible
> connections; they absorb vibration, correct for slight misalignment,
> would put some allowance for a few thou of axial heat-growth of the
> screw in the system, etc.
>
> Thanks.
>
> --Jack