Duel Rack and Pinion Drives

[stevesplanes]

I've decided, due to cost and the fact I have two five foot racks, to dirve my gantry from both sides. This is necessary as I expect that my plunging z-axis will break the x-y plane. They are from Tourchmate and they are 24 pitch 14.5 degree power angle. I want to accomplich foam miling so speeds up to 50 inches per minute are not too fast. I am driving with a 254 Pacific Scientific M22 series stepper motor.

How does one tie the two sides together so that they don't work againts eachother.


Steve

[dneisler]

I am building the CadCut design router, they use racks on each side, and they have a drive shaft that extends all the way across with a belt setup in the middle.

[stevesplanes]

Belt Driven from the center. Can I get a picture or two.

[turmite]

Hi Steve,

I use an old Digital Tool machine that is huge. When configured properly it would cut anywhere on a 5'X9' sheet of mdf or other sheet material with two heads that were 8" apart. It is driven with one stepper using the two rack and pinion setup. My machine has the motor mounted on one side and the drive axle spans the width of the machine which is nearly 8'. The only problems I have encountered are:
1. At high rapids in excess of 300ipm the long shaft sets up a little vibration but not bad.
2. The way the company designed this interferred with the z clearence. I could have had about another 3-4" of z clearence if it had been raised up.

I currently carve foam with my machine at 200 ipm, at least on pieces large enough to get the speed.

Mike

[CAM_MAN]

I have built the Cad Cut Router myself. The design for the single shaft really doesn't work that well. There really is a lot of twisting going on with the shaft. Even though in the design the shaft is split into two pieces then tied together with a coupler it is still a nightmare to get the gears aligned right. The major flaw is that the shafts are fixed in the side panels and there is no way to adjust the pinion gear up and down. The only way to make adjustments is to move the gear rack up and down which is a very tedious project and will never be just right. I have had a lot of gear wear and probably only have about 100 hours on the machine. I constantly have to adjust the rack to keep any descent tolerance. I would advise you to go with a dual drive on the rack. When I have some time I will be redisigning mine to that. By the way I pity any poor soul who has bought the Cad Cut plans. I had to do a lot of redesigning and improvising to get mine together. And as far as any help from Cad Cut forget it, figure it out youreslf.

[stevesplanes]

Can I conclude that each side should have an independant gear height adjustment and that the lond connecting rods act as a torsional spring? Inquiring about driveing my x-axis with two steppers is my next path. Earlier advise was not to try to use two steppers to drive the x-axis, a thought I originally thought would not olny solve the drive height adjustement problem but also almost double the driving force.

Does anyone know anyone using two steppers to drive the x axis?

[ger21]

Originally Posted by stevesplanes Does anyone know anyone using two steppers to drive the x axis?

Lots of people drive gantry's with 2 motors. You'll find that of people that reply to this question, half will say it's a bad idea, and the other half that actually do it have no problems. Mach3 has a slaving option to drive the 2 motors together, always in synch.

Shopbot's have always used 2 motors to drive their gantry's, rack and pinion too. You can see how they do it in their assembly manual, page 24 I think. http://www.shopbottools.com/files/SB...y%20Manual.pdf

[Bloy2004]

I'm from the half that uses two motors and two rack/pinion sets on each side of the gantry. It works just great using Mach3 and "slaving" the motors together in sync as Gerry points out. Once set up, both fault together, both drive together, both align together when homed..... without any problems.......very reliable.

The advantage of this is no racking of the gantry due to load forces flexing through the cross drive shaft and extra drive force with the two motors. It may cost a little more, but you only have to duplicate your work on each end of the gantry rather than rigging the system with a drive shaft crossing to both sides.

John(bloy)

Here's my early construction of the dual drive axis. But I got lucky and found those 1:12(or is it 12:1?) motor/reducer units cheap on ebay.

[rugludallur]

I did a bit of research before building my plasma cutting table and I was unable to find any other solution with decent results.

Belts have to much stretch/play when dealing with these lengths
Chains have to be tensioned so tight that they wear out very quickly
Drive shafts are impractical because of weight, balancing and twist
Driving the gantry from one side only is impossible without adding massive weight.

In my case I have a dual pinion setup on each track. Both ends of the gantry have a motor with a pulley/belt system (3/1 redution) which powers two pinions on the track, this way I can tension the belt between the pinions to eliminate backlash/play and get a 3/1 reduction for cheap.

http://www.dallur.com/index.php?id=4...maxwidth]=1024
http://www.dallur.com/index.php?id=4...maxwidth]=1024
http://www.dallur.com/index.php?id=4...maxwidth]=1024
http://www.dallur.com/index.php?id=4...1;colrows]=1x1

Since these pictures were taken I have replaced the nylon pulleys with aluminum and the belts with low stretch kevlar reinforced urithane.

Jarl

[turmite]

I'm not sure I follow the logic that a motor on one side using drive pinions on both sides connected with a shaft allows racking of the gantry. It sure doesn't on mine! I have used this machine since 1991 and have never had a problem with racking, backlash or other problems I'm supposed to have according to everyone who has spoken on this issue. Are you guys speaking from experience, or rumor? A single motor setup like I have is the least expensive to build, but that does not make it any less efficient. Now, the fact the ways are Bishop/Wisecarver may make the design easier, but I doubt it.

Mike

[rugludallur]

I personally have no experience with driving a large table with a single motor on one end since I don't see any logical reason to do it.
I would love to see some pictures and get an idea of the accuracy. How big are you motors, what is the diameter of your drive shaft and is there gearing for higher rpm on the shaft ?
Asside from your Digital Tool machine I know Knuth also has CNC tables that have this setup but their Gantry is really big and really heavy.

It is possible to build a table in this manner but in the end I think that the price/effort of building it would be more than the extra motor/driver ($300). It might make more sense for a commerical factory which can probably source the driveshaft for much less than the electronics and motor.

When I considered using a shaft in my case it meant that I would need a 2.5m (8ft) shaft.
1 turn of the shaft translates to roughly 15mm travel on the table so twist is a problem.
The only way to reduce twist without adding weight is to gear up the shaft and spin it faster (torque*rpm=power -> more rpm = less torque = less twist) and anti-backlash 1/10 gearboxes which can handle up to 1000 lb/in cost around $800 each.

Since my lathe can only handle stock up to 1.8m and the drive shaft would be impossible to mount due to deflection I would have to buy a precision ground pre-hardened drive shaft 1" diameter and at 8' it's expensive and heavy (10kg. $250) (My gantry weighs 35kg)

So I'm either stuck with a 1" drive shaft or 2x $800 gearboxes both of which make me sad. For reference the total I have spent on my table so far is $3000 out of which the cable chains were $1000 but the drivers/motors were $500.

The way I look at it is that for $50 more than the shaft I can get twice the amount of power for my axis, eliminate all twist/backlash problems and save 8kg of weight in the gantry.

[CNC Pro]

Hey turmite,
I also have an old Digital Tool machine (84” * 168” table size). I recall seeing an optional drive axle available (I think it was 1” or 1-1/4” diameter) from DT to address the vibration issue.
I also have a Larken router (66” *126” table size) which has twin ball screw drive with a motor on each screw. I often run large orders on both machines with no discrepancies between parts coming off either machine.