CAM model to level sacrificial bed

Hi All

I'd very much like some advice on what is happening with my model and subsequent Gcode.

I want to face a new sacrificial board on my Chinese 3040 router. The model I've created allows for the four corners on the sacrificial board not to be machined as I have four bolts located at each corner. I'll machine those areas as a secondary function.

The model when running in Fusion looks good and simulates as designed. However when I generate the Gcode and run it on mach3, it adds a significant Z axis command that sends the spindle into the stratosphere, across then back down to begin the machining.

Any ideas why this would occur? I haven't added a command for the z axis to rise.

I do wonder if it is related to G28 G91 Z0 in line 5 and setting a home position.

I'd really like at attach the .tap file but cannot. Hope it's ok to post the Gcode below.

Thanks in advance. Apologies for any misuse of terms...I'm very much learning my way around Fusion 360 and Mach 3.
Shane

(CNC SURFACE CUT)
(T1 D=10. CR=0. - ZMIN=0. - FLAT END MILL)
G90 G94 G91.1 G40 G49 G17
G21
G28 G91 Z0.
G90


(2D POCKET2)
M5
M9
T1 M6
S5000 M3
G54
M8
G0 X78.694 Y-0.5
G43 Z20. H1
Z15.
G1 Z12.5 F1000.
X84.25 Z9.909 F333.
G3 Y0.5 I0. J0.5
G1 X75.75 Z5.945
G3 Y-0.5 I0. J-0.5
G1 X84.25 Z1.982
G3 Y0.5 I0. J0.5
G1 X80. Z0.
G3 Y-3.015 I0. J-1.758
G1 X82.696 F1000.
G3 X83.015 Y-2.696 I0. J0.319
G1 Y2.696
G3 X82.696 Y3.015 I-0.319 J0.
G1 X-82.696
G3 X-83.015 Y2.696 I0. J-0.319
G1 Y-2.696
G3 X-82.696 Y-3.015 I0.319 J0.
G1 X80.
G2 X85.448 Y-5.754 I0. J-6.789
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G1 Y10.918
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G1 X90.918
G3 X91.237 Y-10.918 I0. J0.319
G1 Y-3.837
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G1 Y-18.274
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G1 X98.281
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G1 X63.258
G2 X56.688 Y69.177 I0. J7.118
G3 X50.117 Y73.557 I-6.571 J-2.738
G1 X-129.641
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G3 X-153.209 Y-43.987 I0.655 J0.
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G1 X129.641
G3 X130.075 Y-73.222 I0. J0.449
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G1 Y43.408
G3 X153.209 Y43.987 I-0.655 J0.
G2 X130.075 Y73.222 I21.791 J41.013
G3 X129.641 Y73.557 I-0.434 J-0.114
G1 X50.117
G2 X43.705 Y77.832 I0. J6.947
G3 X37.292 Y82.107 I-6.412 J-2.672
G1 X-136.891
G3 X-137.244 Y81.783 I0. J-0.354
G2 X-161.766 Y49.493 I-37.756 J3.217
G3 X-162.107 Y49.002 I0.183 J-0.49
G1 Y-49.002
G3 X-161.766 Y-49.493 I0.523 J0.
G2 X-137.244 Y-81.783 I-13.234 J-35.507
G3 X-136.891 Y-82.107 I0.353 J0.03
G1 X136.891
G3 X137.244 Y-81.783 I0. J0.354
G2 X161.766 Y-49.493 I37.756 J-3.217
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G1 X37.292
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G2 X-145.284 Y85. I-139.39 J-2.505
X-169.999 Y55.753 I-29.662 J0.
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X-145.306 Y-87.505 I-139.412 J0.
G3 X-145.329 Y-90.009 I139.39 J-2.505
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G1 X145.009
G3 X145.329 Y-90.009 I0. J0.319
X145.306 Y-87.505 I-139.412 J0.
G2 X145.284 Y-85. I139.39 J2.505
X169.999 Y-55.753 I29.662 J0.
G3 X170.329 Y-55.363 I-0.067 J0.39
G1 Y55.363
G3 X169.999 Y55.753 I-0.396 J0.
G2 X145.284 Y85. I5.01 J29.3
X145.306 Y87.505 I139.412 J0.
G3 X145.329 Y90.009 I-139.39 J2.505
X145.009 Y90.329 I-0.319 J0.
G1 X24.96
G2 X18.793 Y94.44 I0. J6.68
G3 X12.627 Y98.551 I-6.167 J-2.569
G1 X-153.231
G3 X-153.551 Y98.231 I0. J-0.319
G1 X-153.506 Y85.
G2 X-175. Y63.506 I-21.494 J0.
X-176.616 Y63.528 I0. J58.021
G3 X-178.231 Y63.551 I-1.616 J-57.998
X-178.551 Y63.231 I0. J-0.319
G1 Y-63.231
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G2 X-175. Y-63.506 I1.616 J-57.998
X-153.506 Y-85. I0. J-21.494
G1 X-153.551 Y-98.231
G3 X-153.231 Y-98.551 I0.319 J0.
G1 X153.231
G3 X153.551 Y-98.231 I0. J0.319
G1 X153.506 Y-85.
G2 X175. Y-63.506 I21.494 J0.
X176.616 Y-63.528 I0. J-58.021
G3 X178.231 Y-63.551 I1.616 J57.998
X178.551 Y-63.231 I0. J0.319
G1 Y63.231
G3 X178.231 Y63.551 I-0.319 J0.
X176.616 Y63.528 I0. J-58.021
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G3 X-12.047 Y115. I-6.17 J-2.571
G1 X-170.
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G2 X-175. Y80. I-5. J0.
G1 X-195.
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G1 Y-115.
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G2 X175. Y-80. I5. J0.
G1 X195.
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G2 X170. Y85. I0. J5.
G1 Y115.
X-12.047
G3 X-12.303 Y114.967 Z0.034 I0. J-1.
G1 X-12.418 Y114.929 Z0.084
X-12.527 Y114.878 Z0.134
X-12.615 Y114.823 Z0.213
X-12.697 Y114.76 Z0.293
X-12.755 Y114.706 Z0.396
X-12.809 Y114.648 Z0.5
X-12.87 Y114.569 Z0.741
X-12.889 Y114.54 Z1.
G0 Z20.


M9
G28 G91 Z0.
G90
G28 G91 X0. Y0.
G90
M30

[QUOTE=ShaneMinnear;2313240]Hi All

Try removing "G28 G91 Z0" and run again.

Yes, remove all the G28 lines, and add manually any moves of your own for the Z. I ran into the same issue and this fixed it for me. Note, I’m a beginner at Fusion and Mach 3 so don’t take what I wrote as gospel:)

Edit: for me fusion also ads the two g28 lines you have at the end of your program, every time. I edit those out also.

The whole reason for having the G28 G91 Z0 line is for SAFETY, but it only works if the user has correctly setup their machine and it has been correctly "Homed".

Regardless of where the machine is at when it starts up the first move must be to a safe place particularly on the Z axis which should go to it`s highest level so that regardless of what moves the machine might make in the X and Y directions the head has been moved up so it will then miss any fixtures, clamps etc, etc.
That is why the Post processors are configured to output the G28 lines.

However if the actual machine has not been setup correctly then yes, as others have said the G28 G91 Z0 lines and any other G28 lines will need to be removed that`s for sure.
Not the best way to use a machine but people do "get by" doing it but it is what is commonly called the "half-assed" :D method !!

Rob

[QUOTE=Jmhx2;2313248]

Quote:

---

Originally Posted by ShaneMinnear

Hi All

Try removing "G28 G91 Z0" and run again.

---

Thanks, that was what I also thought to try. Reading through I will try and rehome my machine and get it working that way.

Thanks for your reply daytrader. I'll try homing my machine then if that doesn't help I'll remove the g28 lines.

Thanks for your reply Rob. I'll try and get the machine homed correctly as you advise.

When generating your post in Fusion you can turn G28 off in the parameter box in the lower right of post dialog window. Just scroll down and about half way down is the G28 parameter.
Regards,
Arie.