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Messages - GAK801

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1
General Mach Discussion / Re: y drifting problem
« on: June 08, 2013, 08:18:34 AM »
John,

you make a good point. While we didn't remove the rails on the z, we also did not check them after reassembly. I will do so later today. Not sure how that would have happened to create the original event but maybe I'm getting stronger in my old age and torqued the chuck too much when putting in the bit.  So long as I can banish the demons for a while and make some dust, that is forward progress.

Jim,

in theory there is always deflection when a force is applied, but I think (hope) that it is minimal as I typically only cut low density modelling board. I will also run the test you suggest as  a thousand theories are not as good as one set of measurements in my experience.

I appreciate both of your suggestions.

Regards,

Greg

2
General Mach Discussion / Re: y drifting problem
« on: June 07, 2013, 06:21:31 PM »
That is what I thought at one point, but I was at the machine when it originally happened and didn't see or hear anything out of the ordinary that indicated the z axis mechanism shifted. The Z axis was perpendicular to within 0.1 deg (the limit of accuracy on measurement tool). I always check when I put in a new tool but I did not check after this happened. That was silly.

For the part in the picture the bit is also perpendicular to within 0.1 deg. If I did my calculation correctly a 0.1 deg error over 0.25 inches would be on the order of 0.0004 inches. These steps are more than 10 x larger and would suggest an angle error more like 0.4-0.5 degrees.

The original problem would have suggested a misalignment closer to 7-8 degrees or have I made a mental error? I think I would have seen that happen. I will try to check my angle measurement tool by buying another one tomorrow just to be certain.

I didn't show the other side of the groove but it is straight as expected. The groove is formed by a single pass with a 0.500 end mill. As it steps in y minus direction it just keeps cutting the opposite side.

I might have better said that something seems to happen to y when there is an x and/or z command given.

Any other ideas come to mind?

Thanks,

Greg

3
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


4
Rich, BR,

thanks, actually moving with just a G code call gave did fix the problem. Just to clarify, I only told the calibration operation to make movements of 12 and 6 inches after calibrating at 22 inches.  It seemed logical that that particular function would just ratio the number of steps determined in the 22 inch calibration to make each distance movement, but that was not the case. I wonder why. It is moot since I have a proper procedure that works.

One thing I also noticed is that originally I did a G code move of 22 inches and took the actual distance moved and used that to ratio the pulse number in the motor tuning configuration page. I then repeated the 22 inch movement and the travel distance was unchanged (it essentially ignored the pulse number I typed into that field). Is this typical? That particular config window does respond to speed and acceleration numbers entered into their respective fields. When I did the calibration at 22 inches in the settings tab, it ratioed correctly to the previous pulse number ignoring the value I had typed into that field that was around 2% different.

So much to learn and so little time... thanks for your help.

Greg

5
Okay, I performed the calibrations at 22 inches in x and y and followed up with 3 repeats. At 22 inches each repeat went 22.000 inches (within my measurement accuracy of say 8 mils). I then ran a calibration at 12 and 6 inches for each axis. I did not accept the new count number during either of the calibrations. What I got was:
x set dist = 12, actual movement=11.813;  set dist=6, actual movement= 5.688
y set dist= 12, actual movement= 11.813;  set dist=6, actual movement= 5.712

Does this nonlinearity suggest some other source of error?

Greg

6
Rich,

thanks, I'll try that but my ability to measure accurately over 24 inches is probably in the 4-8 mils range. In the interim I did run a small design of experiment on velocity (100, 75, 50) and acceleration (3, 26.5, 50, 100) I also tested V=150 @ accel= 125, 30, 3 mm/s^2. The bit stopped in the same place for each run so there is no correlation between these values in the ranges tested.

One thing I did notice during another go at recalibrating the axes is that the variability in the steps/ are correlatable to acceleration (30, 10) at a speed of 150 mm/min. Of course the steps variation is due to the variation in distance traveled. The 10 mm/S^2 had a much smaller standard deviation. This was run over only a 1 inch distance (but at the other end of the system than the previous runs). Given this I am very surprised I didn't seem some change in the experiment above. It may be that starting and stopping within such a small distance amplifies the effect. I'll have to ponder this one a bit and will likely repeat it tomorrow with more data points just to be certain.

Greg


7
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




8
General Mach Discussion / Re: Z Offset issue
« on: August 14, 2011, 03:15:19 PM »
I think I understand. So in the upper left hand region of the offset page (z offset block), I ignore the gage thickness field and just enter it into the DRO field directly, correct?

Greg

9
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

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