(X=0, Y=0) mismatch

[apimentel]

Dear All,

I am fighting with this problem for a long time.
I hope someone can help me.

First of all, I am sorry for the long e-mail.

I am using a Minitech Mini-Mill Pro3 controled by Mach 3. The axis are actuated by stepper motors.

I am milling 6 sets of pockets using a TR-2-0.0100-S (0.0100 inch Tool) . The sets are disposed as a polar array with a 60º spacing and are all the same.

Each set is composed by two pockets further designated by structure 1 (50 um deep) and structure 2 (200 um deep) that partially overlap.
The Gcode was created using Master Cam. It was created a Gcode for structure 1 and another Gcode for structure 2.

The part was milled (running first the Gcode corresponding to structure 1 and then the Gcode corresponding to structure 2) and the two structures in each set are misaligned.

In order to perform a quick test, I have generated 12 independent Gcodes for the same geometry (6 Gcode for structure 1 and 6 Gcode for structure 2) and I have used a thicker tool, 0.0200 inch, and the same depth in both structures (depth of structure 1=depth of structure 2 = 50 um).

Before milling, the point X=0.0000, Y=0.0000, Z=0.0000 (localized in the center of the part) was set by placing the tool over the reference point on the milling platform.

After the milling of each Gcode, it was verified the position of point X0, Y0 by moving the tool to point X=0.0000, Y=0.0000 (appearing on the screen) and confirming by visual inspection the position of the tool with the reference point X0, Y0. There was a good match between the point X=0.0000, Y=0.0000 (appearing on the screen) and the reference point.

The Gcodes corresponding to structure 1 were first milled (clockwise sequence) followed by the Gcodes corresponding to structure 2 (also clockwise sequence).

After the milling of the 12 Gcodes it was observed that structures 1 and 2 are aligned.

The 12 Gcodes were again generated on MasterCam for the same geometry, this time for the tool TR-2-0.0200-S (0.0200 inch Tool).
The Gcodes were milled for each set (from 1 to 6), first structure 1 followed by structure 2. It was verified between each Gcode the position of X=0.0000, Y=0.0000 relativelly to the reference point on the milling platform and the tool was misaligned relativelly to the reference point. This mismatch was not corrected during the milling of the structures.

Since the milling sequence was:
set 1: structure 1, structure 2;
set 2: structure 1, structure 2;

...

set 6: structure 1, structure 2.

structures 1 and 2 are aligned relatively to each other but the sets (from 1 to 6) are not in the correct position.

Can anybody explain me what is going on? It seems that the longer the Gcode, the more pronounced the mismatch between the point X=0.0000, Y=0.0000 (appearing on the screen) and the reference point X0, Y0 on the milling plattform.

The G's I have been using are: G0 G1 G2 G3 G17 G40 G49 G80 G90.

Regards

apimentel

[mcphill]

Sounds like you are losing steps. What feedrate and stepover are you running? What is the material? Slow the feedrate by 1/2 to 1/4 and see if it does better. May want to slow your rapids as well. Finally, are you plunging or ramping down in to the material? Ramping requires less torque and will lessen the risk of losing steps.

[apimentel]

Hello mcphill,

Thank's fot the reply!

When I was using the tool TR-2-0100-S the feed rate (xy plane) was 244 mm/min and the plunge speed (z axis) was 60 mm/min.

For the tool TR-2-0200-S the feed rate was 488 mm/min and the plunge speed was 120 mm/min.

I guess I am not using ramp, since the gcode sets first the depth and then moves along the xy plane.

The milling was performed on poly(methyl methacrylate) (PMMA).


The milling of both pockets (structures 1 and 2) are misaligned when using the tool TR-2-0100-S (and also with more depth steps)
but when using the tool TR-2-0200-S (and also less depth steps) both pockets are aligned. Do you have any idea why this happens?


Regards

Alexandra