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1

General Mach Discussion / y drifting problem

« on: June 07, 2013, 04:21:21 PM »

I've been trying to solve this problem for a week to no avail, so I'm hoping somebody on this board can help. My system (XP on computer) is communicating through a gecko G540 to drive 4 stepper motors (a slaved to x). The problem is that the y position seems to change as I step down through a part. I will attempt to insert a picture that will hopefully show the problem. When the issue first occurred, as I roughed successive layers (3D roughing with a full retract) I noticed that the bit was off around 0.030 inches with each cut. The full roughing was about 1.5 inches deep and each cut was 0.25 inches in z. At the end of the roughing there was a series off steps vertically for a total error around 0.150 inches. Unfortunately I did not take a picture of that part.

During trouble shooting of this problem my coupler on the Z axis started to slip so I had to tear apart the Z drive to tighten up the coupler and then went through and realigned everything (to less than 0.1 deg error). The problem is still present but the steps (always to the -y direction) are much smaller. Somewhere in 0.007 inch give or take a mil.

I've tested slowing down the feed rates (typically I've run x, y around 90-120 ipm, and z around 50 ipm). The accelerations prior to the drift were around 0.0052 G's and have dropped down to around 0.0038 G's. Pulse width was 4 micro seconds before and was increased to 5 microseconds. None had any measurable affect. Presuming the picture insertion works, you can see the problem on this test run. The speeds are 120, 100, 75, and 50 ipm starting from the edge nearest my hand and moving upward. X is in the short direction while y is the long direction. This program sent the bit to an xy position dropped down in 0.25 inch steps and made a straight cut in x for 3.75 inches. The bit fully retracted and moved back to the x start position. It dropped another 0.25 and cut in x again. This process repeated multiple times. So in each slot there was never a command to move in y. The steps seem very consistent.

After the first slot was cut, the bit moved to a new y position and again cut a new slot at the next lower speed. The steps seem consistent for each slot and was not reduced by the slower speeds.

It seems to me that errant pulses are being sent to y with each z movement. I don't have an oscilloscope so I can't confirm this. My finger isn't apparently sensitive enough to detect it on the stepper motor.

I'd appreciate any suggestions as to how to track down the source of the problem and to resolve it with resorting to swapping everything out.

Regards,

Greg

2

General Mach Discussion / X and Y distances not tracking the DRO setting

« on: August 15, 2011, 05:22:11 PM »

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

3

General Mach Discussion / Z Offset issue

« on: August 14, 2011, 12:35:28 PM »

Even though I just recently started using Mach3, I thought I knew what I was doing on setting z work offsets, but that was clearly presumptuous. Previously, I'd use a 0.002 inch shim stock as the gage, but I recently misplaced it, so I grabbed a 0.5 inch gage block to set Z offset. The bit never hit the part and was at least a quarter inch above the part. I reset the offset but entered the gage as being 1 inch thick. I noticed that the z offset value in the DRO only changed by 0.1 inch which seemed odd. The bit still seemed to be about the same amount above the part surface.

So I'm guessing that the previous gage was small enough that I didn't notice the difference. Now I need to make some parts that mate together in places so the 2 mil difference might be a problem.

Is there some other setting that has to be made to invoke the Z part offset?

I can work around it, but would rather understand how to do this properly.

Greg