Leadscrew stops and starts?

I'm trying to make a cnc plotter similar to this: http://hackaday.com/2013/10/17/unorthodox-home-made-cnc-machine/
I have changed some things though: I'm using aluminum rod with washers super glued to the end, another washer, and two nuts instead of flange bearings (brushings) and two nuts, I'm using 3/8 rod, pololu steppers (#1209), and slightly different conduit clamps.  I'm using a wooden dowel with a half-inch hole, and aluminum and nylon spacers tapped and with a setscrew to act as a motor coupler.  The problem I'm having is that as the motors rotate, even continuously in one direction, the gantry stops and starts moving.  It moves 1/16th of an inch per rotation, as it should, but within each rotation it moves quickly, probably 5/6 of the way within the first 180 degrees, then moves just the remainder in the last 180.  The motors do not stall, miss steps, or change speed, but the gantry does.  There is almost no backlash, and little play.  Any ideas whats wrong?

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rickharris3 years ago
Friction on slides and too much flex in the construction. If it were stiff it would just stop and stall the motors. Something is missaligned to give the friction..

jduffy54 (author)  rickharris3 years ago
How does that cause this issue? I know that the steppers are slightly misaligned, but how does that cause the gantry to stop moving?
friction will cause the motors to stall, if not too great then the frame can flex allowing the motor to pick up again but resulting in a slight jump.

We used to get this all the time on a cheap a) plotter the company I worked for illadvisedly decided to sell. If the frame was even slightly out of true the mechanism would produce jumps and sometimes a total stop. Usually because the surface the machines was on wasn't flat.
Eliminate slippage and motor issues, remove the motor and turn everything by hand. What do you feel and see ?
jduffy54 (author)  steveastrouk3 years ago
That the thing, without the motors it has much less, if any. I'm thinking it might be the motor couplers, which right now are fairly rigid, that messes it up. I've tried using a power drill to spin the rod, just to see if that works, problem it my drill is crap, and off-center itself, so the whole thing vibrates like crazy the whole time, and has the same issue.
How about some photos and video ?
jduffy54 (author)  steveastrouk3 years ago
First is the motor coupler and brushing assembly, second is the output (notice wiggles on curves), third is an "exploded" shaft coupler, fourth is the anti-backlash nut (white tube with grey epoxy on either end, held in the middle two conduit clamps).
The rest is almost identical to the hackaday one I mentioned above
jduffy54 (author)  jduffy543 years ago
forgot to add the pictures...
DSCN5562.JPGimage.jpgimage.jpgimage.jpg
This happened to me to, the connection between the motor and the gantry might be loose.
jduffy54 (author)  tanyutsharma3 years ago
I'm not sure, it's just a hunch, but what kind of motor couplers are you using?