more questions on rack n pinion

[stirling]

I've done the searches and can't find the specific answers to my question so...

With steppers (not servos) has anyone used direct drive and if so was it successful for your app. If so can you tell me a little about it e.g.
The application - router/mill/plasma etc.
your travel per rev
your motor torque
your achievable accel

Next: with a rack n pinion drive, is it best to accurately machine your mounting system such that the pinion is "exactly" the correct distance from the rack (pitch circle touching pitch line)? OR - is it best to make it adjustable? - your opinions much appreciated.

Thanks

Ian

[Greolt]

For direct drive pinions on steppers look at Mechmate and older Shopbots.

My R&P is 4:1 reduction via timing belt and that is not what your asking for.

The only way to mesh them IMO is spring loaded.

Unless you want to go a lot more money on anti backlash pinions. Would be overkill IMO on a router or plasma.

I'm very happy with the accuracy and performance I'm getting.

Greg

[stirling]

Hi Greg - thanks.

out of interest though - whats the linear travel per motor rev?

Cheers

Ian

[Greolt]

Approx 19mm

Greg

[High Seas]

You may have looked already but you can find more MechMate details at:
http://www.mechmate.com/forums/index.php
Gerald has done a lot of work sorting out R&P setup. And a lot of free plans too.
You can find a lot of the info for a R&P setup there - calculating motor/speed/etc/etc.
Jim

[stirling]

Thanks greg. Just a quicky about your spring loading - doesn't that mean that as load increases, the pinion will tend to try to climb out of the rack?

Thanks Jim - I'm still working my way through the Mechmate stuff, but I think I'm getting some good new info. I'm thinking direct drive is looking good - particularly for my latest venture - plasma cutting where resolution doesn't perhaps need to be as fine as routing for example.
Cheers
Ian

[ger21]

Originally Posted by stirling Thanks greg. Just a quicky about your spring loading - doesn't that mean that as load increases, the pinion will tend to try to climb out of the rack?

You need a strong enough spring to prevent it from climbing.

[stirling]

you guys are worrying me now just done a bit of looking at this and...
If you use a spring then arn't you going to mesh too close i.e. INSIDE the pitch circle/line?

[stirling]

OK - that killed it moving on...

it seems not many are using direct drive - for those using reduction of one sort or another, what travel per rev are you using and what sort of motor power? any info gratefully received.

Thanks

Ian

[Greolt]

Ian

I am using 4:1 toothed belt reduction

495oz steppers, two driving the gantry

Approx 19mm/motor rev

Greg

[Gerald_D]

Originally Posted by stirling it seems not many are using direct drive -

I have two productive MechMates with direct drives, (about 70mm/rev) cutting wood productively. Maybe about 2000 older shopbots with direct drive. (their "Alpha" model was direct drive for maybe 5 years?)

For a plasma cutter I can't see you needing anything finer, or being concerned about exactly where the pinion rides in the rack. (we only get concerned when the pinion wears and rides against the root of the rack).

[stirling]

Thanks again Greg - useful info.

Originally Posted by Gerald_D I have two productive MechMates with direct drives, (about 70mm/rev) cutting wood productively.

Thanks Gerald - this is the sort of stuff I need - I'm going to be using Mod1 rack and am trying to figure out what pinion. The smallest possible is 12 teeth with (being Mod1) a 12mm diam so therefore about (pi x D) 38mm travel/rev.
But I'm thinking it would be better to use a slightly larger pinion if poss which will give me more pinion/rack contact and therefore presumably less ware ergo longer life. 20 teeth for example (20mm D) gives me (pi x 20) around 63mm travel/rev but I wondered if this was too much so seeing your 70mm/rev makes me happier.

Originally Posted by Gerald_D For a plasma cutter I can't see you needing anything finer

Agreed

Originally Posted by Gerald_D or being concerned about exactly where the pinion rides in the rack. (we only get concerned when the pinion wears and rides against the root of the rack).

All I was thinking here was that as I understand it pinions are designed to mesh with the rack where their pitch circles/lines meet. The way they are cut means that they CAN actually mesh closer BUT you then have reduced the diameter of the circular contact and therefore affected the travel / rev. However on second thoughts - I'm wondering if I'm talking horse ****

Thanks

Ian