Roughing clearance perverted on inside profile of 3D surface

[lloydsp]

I just ran a roughing, then a finishing of the inside of an ovoid bowl using a 1/2" ball-nose cutter.  Upon finishing the surface, I notice that there were roughing artifacts (dimples) still left on the walls in a few places.  It appears that the roughing over-shot the actual surface boundary.

I did the roughing with 0.020" of roughing clearance, and the finishing with zero clearance.  Is this a bug?  Has anyone seen the problem before?

I can probably cure it by doubling my roughing clearance.  But why?

Lloyd

[dh42]

Hello

I did the roughing with 0.020" of roughing clearance, and the finishing with zero clearance.  Is this a bug?  Has anyone seen the problem before?

Yep I think ... I always use a larger roughing clearance to avoid this .. and I simulate with CutViewer with different colors for roughing and finishing tool to see if there is no problem.

The problem can also relative to the know problem that appears when resolution is different of the stepover value, so maybe it si not the roughing that go too deep but the finishing that is not deep enough ...

https://cambamcnc.com/forum/index.php?topic=10517.msg78203#msg78203

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David

[lloydsp]

Thanks, David.  I didn't know about 'pairing' the stepover and resolution.  I do very little 3D surface cutting.

Lloyd

[dh42]

The problem is only with "scanline" methods, if you are using the waterline mode, I'm not aware of such bug.

With WL mode, a problem can be the value for "resolution" that do not act on the toolpath resolution but on the tool shape resolution. If the value is large with a large tool diameter, the resolution can be insufficient to get the right shape, and uncut material can remain.

Some (long) time ago, Andy said me that the routine is done so in case of insufficient resolution, the tool cut less material that expected, so it is always possible to correct the problem with another cut with a better resolution.

http://www.atelier-des-fougeres.fr/Cambam/Aide_V1/tutorials/Usinage3D_overview_EN.html#waterline

Unfortunately, using a higher resolution increase terribly the computation time for the toolpaths !!

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David

[pixelmaker]

This is less of a problem with roughing and finishing. It is often a problem with the resolution. If you use a low resolution for roughing and a higher resolution for finishing, the finishing tool paths will follow the object better than the roughing tool paths.
Please try a test with two finishing operations, one with a scan angle of 0° and one with a scan angle of 90° on top of each other.
I am familiar with the artefacts in one direction. The double finishing pass takes time, but it eliminates the artefacts.
To prevent artefacts with a finishing operation, you can try reducing the resolution for both operations. Then only the calculation of the tool paths takes longer.

ralf

[Bob La Londe]

You can also have issues with large stepovers in three d operations and with crossovers that are less than the stepover.  I've documented it a couple of times here on the forums. 

[Bob La Londe]

Resolution can also be an issue. But you probably already know about that one.

[lloydsp]

Yes,
Because of the weird and excessive (and time-consuming) retracts and jogs made during waterline roughing, I DID do a horizontal scan line roughing pass.  So that concurs with your observations.  The waterline finish was a smooth operation, but of course it only followed the profile, rather than having to empty-out the entire cavity like the roughing does.

Lloyd

[Bob La Londe]

Yes,
Because of the weird and excessive (and time-consuming) retracts and jogs made during waterline roughing, I DID do a horizontal scan line roughing pass.  So that concurs with your observations.  The waterline finish was a smooth operation, but of course it only followed the profile, rather than having to empty-out the entire cavity like the roughing does.

Lloyd

I am sure you know this, but there are two issues.

1.  When using large stepovers you can get some really bad tool paths.  This one can't be fixed by a reasonable work around.  A time consuming work around is to create tool paths with a very small stepover, convert them to geometry, delete all the unwanted tool path geometry and then engrave the rest. 

2. There are issues when using crossovers smaller than the stepover or zero.   This also creates bad tool paths.  They can even be bad enough to break tools and create bad cuts.  If the boundary polyline is not along an axis you can get around this sometimes by using a max crossover only minsculey larger than the stepover.  It seems to then create decent tool paths.  Stepover 0.5 & crossover 0.5000001.  Unfortunately this is not always a solution.  In order to change the boundary relationship to the axis if you can get away with diagonal tool paths you can rotate the geometry, and then rotate the MOP back by the same amount.

All of these work arounds are poor choices, and sometimes they won't work for what you want to do, but they are what we have. 

This all drives up the cost of one off custom work, but for stock designs it can be worth it when it works. 

[lloydsp]

Yep, Bob.  And I'm always conservative with stepover values, because of that issue. 

Unfortunately, this IS a one-off job, and it's a gift for a 50-years friend, so I don't have any way to make up for the lost time, except to bitch about the bug! <grin>

Lloyd

[Bob La Londe]

Something I use from time to time for roughing are trochoidal pockets.  I'll take slices of the 3D mesh at my desired depth increments, and use a roughing clearance and a separate MOP for each depth. 

MOP 1
Depth Increment .125
Stock Surface 0
Depth of cut -.125

MOP 2
Depth Increment .125
Stock Surface -.125
Depth of cut -.250

MOP 3
Depth Increment .125
Stock Surface -.250
Depth of cut -.375


For a positive I'll add a polyline geometry one cutter diameter larger than the stock, and apply my roughing clearance to all the MOPs.

For a negative space I just apply my roughing clearance. 

It's not as good sometimes as horizontal or vertical for leaving a consistent amount for a finish pass, but quite often its good enough, and its pretty darn fast.