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Author Topic: Stepover problems with 3D MOP  (Read 479 times)
GeoffreyGRoy
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« on: May 19, 2019, 07:24:00 am »

I have a very simple 3D object (a prism, x130,y10,z10)), and have created a 3D MOP for the top surface.  As I want to limit the carving to the extent of the top surface I have tried both a Bounding Box and Selected shapes.  I am using a 6.35 mm diameter end mill.

If I choose a stepover of 0.53 the cut paths are correct.  If I choose a stepover of 0.54 the cut paths are incorrect (see attached images and cb file).  I think that there are also some other values that give incorrect results (e.g. 0.45, 0.40)

I would welcome any ideas, suggestions.

thanks
Geoff

* StepOverTest.cb (5.12 KB - downloaded 19 times.)

* StepOverTest-53.png (28.38 KB, 1477x212 - viewed 49 times.)

* StepOverTest-54.png (40.26 KB, 1482x275 - viewed 44 times.)
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dh42
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« Reply #1 on: May 19, 2019, 21:08:10 pm »

Hello Geoff

The toolpaths are done so the tool stay IN the boundaries, and no extra toolpath will be added if the given stepover is too large to achieve the boundary width.

A good trick is to use a boundary = the workpiece contour (as you did on the example) but with a  Boundary Margin = tool radius, so the center of the tool go to the edge on the workpiece and so the whole surface is cut.

++
David
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GeoffreyGRoy
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« Reply #2 on: May 30, 2019, 04:23:52 am »

Dear David

Thanks for the feedback.

I have done some further tests see attachments.  I think there is a real bug here.

In my test I have polyrectangle for a MOP pocket (at the top) and a 3D surface (at the bottom)  with a rectangle boundary (same size as the top one).  Using the same tool diam, and step overs for both cases you can clearly see the differences in cut paths.  The Pocket MOP is always OK, but the 3D Mop cut paths do no always align to the required boundary.  The results depend on the StepOver value, I suspect it is some sort of numerical/round-off error.  For the cases in the attachment the cut path does not reach the specified border.

One would expect that the cut paths for the 3D MOP should be similar (in terms of spacing and distance from boundary) to the Pocket MOP for the same step over.

Geoff

* StepOver.pdf (506.19 KB - downloaded 14 times.)
* StepOverTest.cb (8.03 KB - downloaded 15 times.)
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dh42
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« Reply #3 on: May 30, 2019, 06:33:59 am »

Hello

With 3D mop, the boundaries are not an absolute limit to reach as with a pocket, you can't use this method if you want very accurate "pocket" (*). With boundaries and 3D mop, the "rules" is "stay IN the boundaries and use the given stepover, not follow exactly the boundaries.

If you look at our PDF, you can see that the toolpaths of the 3D mop are spaced from the given step over, but seems to be centered in the boundaries

Note also that the toolpath position can be affected by the 3D object shape because CB calculate first the toolpath for the whole 3D object (it start at a side of the object and offset the toolpaths from the given stepover), then it remove the toolpaths that not fit in the boundaries, so the toolpath not necessary follow an "edge" of the boundary rectangle depending on where is the rectangle in the 3D shape.

(*) it can be accurate "along" the toolpath by using a small resolution value, so the only way to get an accurate result with 3D MOP (even when not using the boundaries) is to cross the pass with a small value for resolution ; in your case if you use another MOP with vertical scanline, you'll get an accurate size in Y (but not in X)

++
David
« Last Edit: May 30, 2019, 06:43:31 am by dh42 » Logged
GeoffreyGRoy
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« Reply #4 on: May 30, 2019, 07:47:07 am »

Dear David

Thanks for the explanation,  I can now see what causes the problem.  Solving it is another issue. With some trial and error it seems possible to keep the cutting paths aligned to, and inside, the boundary.  I will just have to take more care to check the computed paths before machining

 Thanks again

Geoff
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dh42
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« Reply #5 on: May 30, 2019, 20:37:44 pm »

Hello

Yes on this case, because the bottom of the part (in Y) is also the boundary, we can easily see how that works by adding the toolpath that exists before they are removed.



it's the 0.35 stepover case.

the first TP at the bottom is offset of 0.35 * tool Ø from the bottom (Y) of the 3D shape, then all other are offset from the same value.

you can see that the first toolpath on the bottom can't be used because the tool not fit in the boundaries, same things with the top most.

In this case, and only because the bottom of the 3D shape = the bottom of the boundaries and because the Y size is divisible by the step over, a step over of 0.5 , 0.25 , 0.125 ... or any halve of those values will give bottom and top toolpaths that follow exactly the boundaries.

So I think it's not a bug but a limitation of the 3D scanline machining, we have to do with.

++
David
« Last Edit: May 30, 2019, 20:47:18 pm by dh42 » Logged
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