Need Help!- Slave axis problem

[Curtis Erpeldin]

I'm in the process of rebuilding my table top 24" x 36" router that I purchased a year and a half ago from a company which will remain unnamed. The design has two driven screws, one on each side of the gantry, driving the y axis (I refer to the x axis as the axis on the gantry and the y axis as the axis that moves the gantry). From the beginning I had problems with the gantry racking and I eventually decided it was a mechanical problem created by the manufacturer using linear bushings rather than ball bearings on only end supported 1" diameter shafting. Now I'm not so sure. I've replaced the linear motion with Thompson type bearings on fully supported shafting and before I installed the lead screws the gantry moved effortlessly. But when I wired everything back up I had the same problem with racking. What happens is the gantry binds and the motors stall. Pushing the gantry back and forth without the screws in place it's impossible to make it bind. The weird thing is if I remove one of the screws everything works fine. I can run the gantry back and forth at 150in. per minute on just one screw--either one. So the two screws together seem to be fighting each other. They are both Acme 3/4-6 running in delrin nuts. They are within a few thousands of parallel and one has a slight bend but runs fine by itself. My steppers are 156 in-oz Kelings which I realize may be a little underpowered but if one motor can drive the axis at 150in per minute why can two barely move it a 25in per minute without vibration and binding? Any ideas or advice would be dearly appreciated.

Curtis E.

[Big S]

Is one of the steppers running backwards by any chance?

Shannon.

[Curtis Erpeldin]

No, both motors have the same rotation because the gantry will move back and forth. It's just very erratic. It might move smoothly for 12 inches and then either stall suddenly or start vibrating until it stalls. It doesn't necessarily do this in the same locations of its travel so I don't think it's a parallel issue with the screws and as I mentioned in my first post, without the screws the gantry moves beautifully the whole length of its travel so the linear motion system is not at fault. The acme screws are not precision versions but they seem to match closely enough to one another (placed side by side the threads mesh evenly the full length). I made the delrin nuts using a 3/4-6 acme tandem tap. The fit is a little loose so I made the nuts extra long, slit the ends like a collet and put a tightened shaft clamp over the split end to take up the play. I can't tighten this down as far as I would like because of the limited torque of the motors, but I was wondering how important it is when slaving one axis to another to have equal torque being applied to both screws. Another way to put this question:"Just how mechanically fussy do you need to be to get a slaved axis arrangement to work?" If I've been fussy enough in that department, it seems to leave only the motors or controller that are at fault. Would it be unusual for two separate stepper motors getting the same signal stream to get out of synch?

Curtis E.

[Big S]

Very strange? On my Excitech router if the gantry gets out of sync it will bind like you describe, but thats only after hitting a clamp or something. When that happened that gantry was about 8mm out of square.

Are both of the stepper drivers set to the same settings? sometimes they have jumpers for step/half step. I would also try running it with just one motor, then measure a jog of say 24" then do the same with the other motor and see if they are the same. Other than that I am not sure.

Shannon.

[Curtis Erpeldin]

Well I isolated the problem. I ordered a long enough timing belt to run both axes from one motor. It arrived today, I hooked it up and no problem! I can run the gantry back and forth at 100in/min even with the little 156in-oz Keling and even with the timing belt bouncing up and down because I can't get enough tension on it. And either motor by itself will run both axes. They just won't work together. I'll try your suggestion of running each motor separately a specified distance and seeing if they're the same. It may be moot now since I've already decided to replace the motors with larger versions and will probably replace the controller as well. I just didn't want to do this before knowing whether or not my problem was in the mechanics of the linear motion and drive screws.

Curtis E.

[Big S]

Thats good, its always good to get things sorted out eh.

Shannon.