Scalloping path not smooth

[Anonymous]

I have this custom circle-diamond-square model that I want to mill (to test my machine). In the center I have subtracted a slightly flattened hemisphere (could not fit the entire hemisphere so I scalled it down in the z direction). I first do a pocket clearing pass to clear out most of the material and then I do a scalloping pass (.25" ball end mill) to smooth out the walls. The problem I am having is that the scalloping paths do not conform to the hemisphere walls. The paths are jagged- shouldn't they be smooth? I have tried using the "smoothing" option in the "passes" pane, but that did not help much (plus my TinyG doesn't seem to like it when I smooth the paths, so I don't use this option). Whenever I pocket or profile round things, the paths are smooth, hence the machine runs smoothly, but with these jagged scalloping passes, my machine is very clunky and sound like it is stuttering (I have not run this file on real material yet, just in the air). Is there a more appropriate operation for what I am trying to do?

[Steinwerks]

Try reducing the tolerance in your toolpath to .001mm, that will help with the toolpath conforming to the model. Smoothing is essentially a G-code affecting filter, and in other CAM software is known as Arc Fitting in which the software works within a specifield tolerance band to create an arc instead of single point moves. If your machine is having problems, it's likely a block processing limitation in which it doesn't handle long blocks well. The flipside is that without Smoothing enabled, many machines have problems processing short blocks quickly enough and translating them to machine movement and stutter, known as data starvation. 3D toolpaths can be a guessing game until a good medium is worked out. Here's a quick rundown of how it can affect a machine's performance: http://www.hsmworks.com/docs/cncbook/en/#Ch09_Tolerances

[Anonymous]

Thanks for the suggestion, but that did not solve my problem- it did smooth out the tool path a little but did not eliminate the stutter.

After taking a closer look, the tool paths seem to jitter up and down. My Z axis is slower than the X and Y hence the stuttering. I'm scalloping a hemisphere, shouldn't the path be planar concentric circles? Is there a more appropriate operation for a situation like this, where I want my paths to be concentric planar shapes?

[Steinwerks · Accepted solution]

The stutter is something you'll have to figure out with your particular machine and its ability to process code.

That said, try a 3D Contour path instead, with smoothing set to .01mm I get a 27kb path instead of >100kb with the Scallop path. Getting it to run well on your machine may take some trial and error unless someone can come in here and recommend proven parameters. This is often the way of 3D machining on less-than-stellar machines, even dedicated cnc mills from the mid-90's and earlier.

I've done a bit of a rework, no speeds and feeds attention, and I don't work in metric so some things may be imperfect, but it's more of the workflow I'd take. Hopefully it helps a bit, and good luck.

[HughesTooling]

Something to remember, with scallop it's a 3d offset so one path is an offset of the previous one, a small deviation is going to get bigger and bigger. That said the surface you're machining will still be accurate as the toolpath is projected to the surface. In your example where the toolpath rises up it will go out further to compensate.

You will get a more circular path if you use Contour instead. Another toolpath you might want to try is spiral.

One last note on scallop, smoothing doesn't work to well because of the slight wander in the Z axis.

[Anonymous]

Another vote for 3D Contour. This is a constant Z approach which will keep the Z axis still except for when transitioning. This will make smoothing more likely to create arcs, and will also take significant burden off your machine/control. Use Shallow cut controls to reduce stepover in shallow areas.

[kb9ydn]

For whatever reason scallop seems to produce jittery paths sometimes and it's more fussy to get right.  For this reason I mostly use 3D contour and parallel for finishing.

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