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20 vs. 12 pitch on a rack & pinion drive

i_r_machinist

Titanium
Joined
Apr 12, 2007
Location
Dublin Texas
Looking into building a plasma table for the home shop. Most of the designs I've seen use a 20 pitch rack and pinion drive system. The biggest rack I could find in this pitch is 1/2"x1/2". I want to bump up to a 1"x1" rack, but the finest pitch in that size is a 12. Would there be a big difference in backlash going with the 12 verses the 20?
thanks
i_r_

ps. If anyone has a better drive mechanism, I'd like to know. I want to limit my budget to <7k on this project.
 
What about using timing belts or roller chains?

AFAIK Timing belts are very accurate.

Depenpending on accuracy required, long ball screws can be surprisingly reasonable.
 
What about using timing belts or roller chains?

AFAIK Timing belts are very accurate.

Depending on accuracy required, long ball screws can be surprisingly reasonable.

You-tube is loaded with all kinds of drive systems. Can't seem to get good numbers on drive system vs. backlash, but the rack and pinion is what is on our table here at work. .03" out of round on a plasma cut hole seems pretty reasonable to me. I've been looking around for ballscrews, but they would eat up a big chunk of my budget.
Right now I'm drawing it up in solidworks. Took me a little bit to figure out how to configure a gear, but I got there.
have fun
i_r_
 
Why do you want 1"x1" rack? The load is not big.

Most of the backlash should be taken up by the controller. Timing belts are probably a bad idea where heat & sparks are involved. An advantage/disadvantage of the 12 pitch over the 20 pitch is the step count & distance of movement. The maximum size of the table is determined by the total number of steps your controller can count. Lets say your controller can count up to 10 and your step size (determined by motor and gearing) is 1". Your maximum table size is therefore 10", minimum res is 1". So for everything else equal, a 12 pitch will give you a bigger table with less resolution. A lot of people design these things first then try to find a suitable controller. My advice is find the controller you want first.
 
The usual CNC plasma table is simply made of rectangukar tubing fabrications tweakable to surprizing accuracy. If the rack is dialed into the axis motion, backlash once set will be fairly consistant. Backlash is usually compensated by a setting in the CNC control and if a little scientifically applied axis friction was part of the design backlash will play no significant role in the table's tracking and positioning accuracy..

Rack and pinion loads in a CNC plasma cutter are mostly incidental: inertia, out of level, accumulated dirt, and wire crag are probably the main culprits.

If backlash is a problem, simple spring loaded mounts holding the axis drive motor that act perpendicular to the pitch plane will tight mesh the rack and pinion ensuring zero backlash. The compliance of the spring loading ensures relief should it be necessary thereby avoiding mesh interferance and possible axis jamming..

If you're concerned about available rack lenghts being too short for the desired motion , don't be. Rack can be butted precisely by meshing and clamping a short length of rack remnant to the lengths being installed. If the butt ends are fitted with clearance at the tooth space root then the rack remnent spanning the butt will place both rack incorrect pitch relationship. Then it's just a matter of dial, clamp, drill and tap to attach and align the rack
 
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We have a Samson table at work. It works fine and does a good job, but has a few design flaws. I understand that the 1/2" wide rack and pinion is fine for the design envelope. My problem with that table is that it just looks cheesey. When I run the thing I feel like I need to be very careful not to bang around on it. You know, gently set the plate on the table and such.
I have not found a setting for backlash compensation in the Plasmacam software, which our Samson runs. The controllers that I'm looking at do have do have compensation. The homebrew plasma cutter crowd does not seem to be worried about backlash compensation.
I assembled our table, so I have experience matching the racks to get the length I will need.
I understand that excessive gantry weight is a killer. I know a man that built one here in Dublin. The gantry is heavy enough to mount a series 1 head. He has to slow the feedrate down so much that it is painful to watch.
customer
 








 
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