PRT stepper motor... What If Question

[pat_wardell]

I am working with a PRT 4 axis open loop shopbot with 1/8 stepping. The microstepping value is set to 509.2956. As I understand it this would work out to be 0.001963495 inches per step on the XY.

If the tool path program told the X axis to move .0005" would the stepper motor move? I am thinking no??? With that said, if the program had 2000 lines of code with each line moving X .0005", I thinking that there would be no movement in the X axis. This is my thinking and I may be wrong!

Now this is open loop running in absolute mode. If I am correct with the above scenario the X axis would now be offset by 1" and the rest of the X axis cutting would be in the wrong position.

With that said I think that a toolpath post created with resolutions less than the resolution of the machine could create additive axis errors that could be very large on a large part.

Finally I thinking that my tool path generation software resolution should be set to .002" to keep it from creating movements less than the resolution of the machine. This would also calculate the toolpaths much faster.

I hope this all makes sense. Can anyone verify, clarify or correct my thinking? Am I full of beans?

Thanks,
Pat

[richards]

Patrick,
You've raised an interesting point, but I don't think that you have to worry. In the third paragraph you said "absolute mode", which means that each move command includes the destination point for the axis. In other words, as the 'move' progressed, the software would compensate.

However, if you performed a series of moves in "relative mode", then you might have the problem that you described because those moves are not cumulative.

[pat_wardell]

Mike,
You are correct. I did some tests it looks like it wouldn't make any difference in "relative mode" either.

Here is what happens:
The Shopbot Position program will not do a move unless it is at least a stepper motor single step distance.

What this means is that my XY "Unit Value" is 509.2956. Divide 1 by "Unit Value" = single step move distance. 1/509.2956=.001963" so in my test I single stepped a file that started at MX,0.00000 and incremented every .00001". The X position display did not move until it reached .00196". I did another test that incremented at half the value of .00196 and the position display incremented every other step. The last test was to increment at .00196 and the position display incremented every step.

Ok with that said. My XY single step movement is .00196" and Z is 1/916.7324=.00109", this tells me that my CAM cut path tolerance setting should be set to .00109". This is the finest resolution of the machine. So by knowing this and setting the tolerance in my CAM program to the finest machine step resolution I am able to decrease the cut path calculation time and post file sizes by the following:

Bobcad
Tolerance_____Minutes_____Seconds_____Post Lines
.0001__________32:09_______1929_________43226
.00109__________4:16________256_________16666
---------_____-------_____------_______--------
Difference
.00099_________27:53_______1673_________26560

Performance
7.54 times faster calculation
2.59 times less post lines

MeshCAM
Tolerance_____Minutes_____Seconds_____Post Lines
.0001___________64_________3813________14260
.00109__________12__________715_________7049
---------_____-------______------_____--------
Difference
.00099__________52_________3098_________7211

Performance
5.33 times faster calculation
2.02 times less post lines

I do large 4 axis 3D parts with 12 to 15 cut operations. A 5-8X calculation time savings over accepting the CAM program defaults is huge with no loss in finish quality.

Im sure everyone does not use the default CAM tolerance, but this may be a good method for defining what someone’s CAM tolerance should be depending on the Shopbot “Unit Values”.

This is my engineering interpretation of how this all works and if I am not correct I am open to correction.

Thanks,
Pat

[jon]

I don't know if this is relevant, so forgive me if I am off base. I understand that the tolerance on most of our machines is; 0.02" I set my CAD/CAM package tolerances to the third decimal place to ensure that they do not limit the machine's resolution, or at least don't compound any errors due to low settings. I keep the limiting factor to the 'weakest' link. I have only noticed computer calculation times being affected by resolution settings, not actual machine times.