Amazing how attempting to make larger parts with higher precision unleashes the gremlins.
First a bit of system description. This is a home built gantry system with the A axis slaved to x axis. All axis are stepper motor driven with rack and pinions on x and y and a ball screw on the z axis. The racks/ pinions on x,a, and y are the same pitch and ratio's on the gearing.
I'm starting with a stock that is 24 x 26 inches (x,y). I've zeroed the x and y axis to the edges of the stock -that is the control point is (-0.25, -0.25). In a move where the bit (0.5 inch diam. end mill) should end at (23.3277,25.0698) [DRO -control point readings] it actually is located at (23.6875, 25.625). The actual measurements were made with a 2 foot vernier caliper, so the y dimension could be off say 0.020 since I had to align it a second time to a mark on the stock. The vernier graduations are at 1/ 128 but I could easily estimate to 1/ 256 (but that is splitting hair relative to the differences I'm seeing).
Originally during a dry run above the part, I noticed that the bit actually moved beyond the far edge of the stock when it should have been inside the 24 inches by more than 1/2 inch. This lead to recalibrating the x and y axis. The x was off a bit but both now seem to be reproducible to within 0.001 (The accuracy of my cheapo dial indicator) over a 1 inch span. I also ran some cal runs at 3 inches, but didn't have better than 0.020 " repeatability (due to technique rather than the system I suspect). These recal's essentially made a trivial difference (bit in x direction was beyond the stock still).
Next, I reset the accelerations and velocities for these axes. Originally,x,y,a were nominally 100 mm/min, 3 mm/s^2, 4 micro sec pulse widths, and 2000 steps/ unit. The recals seemed to have changed the steps/ unit to 1925.42 (x) and 1953.049 (y). Is this odd that x, & Y are not the same since the drive systems are? I did not recal A since I assumed as a slave it is sent the identical pulses as x. Is this correct? Also when I modified the accel and velocities, I did not adjust the A axis.
The one thing that did help was bumping up the acceleration to 20 (it at least got the bit over the stock and produced the data given above). I tried up to the max (196) but that made no difference ( and I am sure it is not good for the sytem long term). I moved the velocity up and down with no measurable improvements.
What else should I be investigating?
Greg