Finishing Pass Problem:

1. ## Finishing Pass Problem:

I am roughing, then planar finishing a 2" diameter cylinder to a 1 in 10 tapered surface with a 1/2" bullnose mill with a 0.060" corner radius. The bull nose finished so-so with significant imperfections in the finish.

The finish pass is only removing the last 0.010" left from roughing using a 0.005 stepover, climb, outside-in path.

In reviewing the G-code, both the roughing and finishing passes work by cutting sequential, concentric circles. I noted a that there are very few z level changes as the roughing pass runs, but during the finishing pass there are a very large number of Z moves ... almost one for every line segment in each concentric circle. The moves are as small as a few tenths up to three thousandths as it moves around a single concentric circle ... which seemingly should be a constant Z level.

Here is the section of code where the roughing pass finishes and planar finishing begins:

N3466 X0.02246
N3467 X0.06739 Y1.25783
N3468 X0.11231 Y1.25459
N3469 X0.15724 Y1.24973
N3470 X0.20216 Y1.24333
N3471 X0.24709 Y1.23520
N3472 X0.29201 Y1.22529
N3473 X0.33693 Y1.21373
N3474 X0.38186 Y1.20033
N3475 X0.42678 Y1.18514
N3476 X0.47171 Y1.16797
N3477 X0.52369 Y1.14558
N3478 X0.56156 Y1.12750
N3479 X0.61247 Y1.10065
N3480 X0.65141 Y1.07810
N3481 X0.72056 Y1.03428
N3482 X0.74126 Y1.01847
N3483 G00 Z-0.17400
N3484 G00 Z0.21000
N3485 G00 X-1.05222 Y0.53810
N3486 Z0.10000
N3487 Z-0.20242
N3488 G01 Z-0.30242 F40
N3489 M08
N3490 X-1.03890 Y0.56363 Z-0.30258 F80
N3491 X-1.02990 Y0.58022 Z-0.30303
N3492 X-1.02090 Y0.59681 Z-0.30434
N3493 X-1.00779 Y0.61837 Z-0.30387
N3494 X-0.99402 Y0.64102 Z-0.30503
N3495 X-0.97901 Y0.66313 Z-0.30407
N3496 X-0.96400 Y0.68523 Z-0.30486
N3497 X-0.95048 Y0.70351 Z-0.30425
N3498 X-0.93078 Y0.72944 Z-0.30436
N3499 X-0.91853 Y0.74443 Z-0.30366
N3500 X-0.89435 Y0.77365 Z-0.30431
N3501 X-0.87821 Y0.79146 Z-0.30342
N3502 X-0.86206 Y0.80928 Z-0.30394
N3503 X-0.85430 Y0.81786 Z-0.30472
N3504 X-0.83608 Y0.83608 Z-0.30389
N3505 X-0.81786 Y0.85430 Z-0.30472
N3506 X-0.80928 Y0.86206 Z-0.30394
N3507 X-0.79146 Y0.87821 Z-0.30342
N3508 X-0.77365 Y0.89435 Z-0.30431
N3509 X-0.74443 Y0.91853 Z-0.30367
N3510 X-0.72944 Y0.93078 Z-0.30436
N3511 X-0.70351 Y0.95048 Z-0.30425
N3512 X-0.68523 Y0.96400 Z-0.30485
N3513 X-0.66313 Y0.97901 Z-0.30407
N3514 X-0.64102 Y0.99402 Z-0.30503
N3515 X-0.61837 Y1.00779 Z-0.30387
N3516 X-0.59681 Y1.02090 Z-0.30433
N3517 X-0.58022 Y1.02990 Z-0.30302
N3518 X-0.55261 Y1.04477 Z-0.30249
N3519 X-0.53050 Y1.05618
N3520 X-0.50840 Y1.06759 Z-0.30409
N3521 X-0.49093 Y1.07535 Z-0.30307
N3522 X-0.46419 Y1.08722 Z-0.30325
N3523 X-0.44208 Y1.09636 Z-0.30315
N3524 X-0.41998 Y1.10550 Z-0.30445
N3525 X-0.39788 Y1.11320 Z-0.30322
N3526 X-0.37577 Y1.12090 Z-0.30336
N3527 X-0.35708 Y1.12731 Z-0.30424
N3528 X-0.33156 Y1.13516 Z-0.30448
N3529 X-0.30946 Y1.14112 Z-0.30374
N3530 X-0.28735 Y1.14708 Z-0.30428
N3531 X-0.26525 Y1.15209 Z-0.30342
N3532 X-0.24315 Y1.15710 Z-0.30383
N3533 X-0.22104 Y1.16138 Z-0.30342
N3534 X-0.19894 Y1.16567 Z-0.30425
N3535 X-0.17958 Y1.16859 Z-0.30366
N3536 X-0.16021 Y1.17152 Z-0.30400
N3537 X-0.13263 Y1.17478 Z-0.30346
N3538 X-0.11052 Y1.17728 Z-0.30406
N3539 X-0.08842 Y1.17898 Z-0.30361
N3540 X-0.06631 Y1.18069 Z-0.30436
N3541 X-0.04421 Y1.18171 Z-0.30431
N3542 X-0.02210 Y1.18272 Z-0.30545
N3543 X0.00000 Z-0.30484
N3544 X0.02210 Z-0.30544
N3545 X0.04421 Y1.18171 Z-0.30431
N3546 X0.06631 Y1.18069 Z-0.30436
N3547 X0.08842 Y1.17898 Z-0.30361
N3548 X0.11052 Y1.17727 Z-0.30405
N3549 X0.13263 Y1.17478 Z-0.30344
N3550 X0.16016 Y1.17152 Z-0.30398
N3551 X0.17955 Y1.16860 Z-0.30364
N3552 X0.19894 Y1.16567 Z-0.30427
N3553 X0.22104 Y1.16139 Z-0.30344
N3554 X0.24315 Y1.15711 Z-0.30385
N3555 X0.26525 Y1.15209 Z-0.30344
N3556 X0.28735 Y1.14708 Z-0.30428
N3557 X0.30946 Y1.14112 Z-0.30373
N3558 X0.33156 Y1.13516 Z-0.30448
N3559 X0.35708 Y1.12731 Z-0.30424
N3560 X0.37577 Y1.12090 Z-0.30335
N3561 X0.39788 Y1.11320 Z-0.30323
N3562 X0.41998 Y1.10550 Z-0.30445

I have tried changing the tool tolerance to as tight as 0.0001" as well as experimenting with different mesh parameter settings and I am unable to smooth this Z-process out at the G-code level.

I am puzzled why the Z level would be moving around by a thou+ during a finishing, concentric cut. It seems possible to me that this could be contributing to the roughness of the finish pass I am seeing.

Can anyone suggest what I am doing wrong in the setup that is causing the Z value to not be constant during the planar finishing pass?

Best regards!

Bruce

2. ## Re: Finishing Pass Problem:

Hi Bruce, I think it may be that the algo in your CAD/CAM that generates the coordinates, which are I think incremental (?), for some reason is generating a Z coordinate that has a small and varying error. I'm surprised that it is not just generating G02/3 commands that produce circular interpolation, but seems to be approximating circles with short chords generated by G01 moves.

I would be inclined myself either to write an Excel sheet that will generate the g-code for using circular interpolation; or using Excel to clean up the coordinates such as you list above, so that for each segment that ought to be at a fixed Z level it replaces each Z value with the mean of all the Zs in the segment. I would have more confidence in the first as one has more control, but you need to understand a bit about how to construct g-codes in Excel.

One thing that might work is to actually loosen the tool tolerance so that it is larger than the Z variations - that might persuade the CAM to generate a constant Z level.

4. ## Re: Finishing Pass Problem:

Hi John,

I actually loosened the tool tolerance and it smoothed out 99% of the z variation. Turning on control points in Rhino helped visualize this greatly. Sky Greenwald had a helpful article of tweaking madCAM toolpaths:

I believe the G02/G03 exists in the 2.5D axis part of the program and is planned for 3 axis sometime down the road.

I was experimenting with using a 1/2" ball to do a fine finish on a 9" round using a .005" stepover to try for a super smooth finish. The finishing pass post alone went over 100,000 blocks of line segments (but only about 2.6 Mbytes). That was enough to exceed the Okuma P200 controller buffer. (School MV2040 VMC). It is probably a good 8-10 yr old. I looked up specs on the latest Okuma controller ... P300 ... and surprisingly the buffer there is still the same! As memory is so cheap relative to big buck, heavy hardware, why they don't have at least 100Mb for a buffer is a good question I would like to hear an answer to. I would like to know how big of a post other mill controllers can handle. Maybe Okuma is just exceptionally small. Dunno. If this is typical for the industry, CAM packages without G02/G03 capability will be precluded from running larger area, low stepover jobs because of the mass of line segments created ... unless I am missing some other workaround here being relatively new.

I ended up raising my stepover to 0.025" for my 9" round to get the post size to fit the VMC buffer. I skipped going for a smooth surface and chose a scalloped finish using a 1/8" ball to get it moving. It produced a finish without the problems I had before and was still comfortable to the touch. Maybe even a non-slip texture which is really pretty good to have in my idea tool box.

I would like to see madCAM implement a spiral machining strategy for this type of facing work. Joakim helped me get the tool lifts correct for each concentric milling circle by adjusting the 'Cut-Link' parameter, but if you look real close at the finished round you can see the start/end point for each circle. Very faint, but it's there on close inspection. Probably more due to the variability in the machine than the math in the software ... but a spiral toolpath would cover a multitude of hardware sins. :^) Maybe there are other ways to handle that as I am just on the learning curve.

Adding Circular Interpolation would probably be the most bang for the buck to avoid having to break a larger workpiece/small stepover operations down into regions and separate posts to fit a small machine buffer ... at least from my limited viewpoint right now. I will come back to a smooth finish surface project later as I want to find what the real limit there is with the tooling ... not computer memory. It's good to learn what the limitations are before I plunk hard earned \$\$\$ down for a machine.

Much to learn yet.

Cheers!

B

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