Faceting on smooth curves?

[rewster]

My mill has started to do something quarky. When milling a 2.5d profile, the curves are coming out faceted instead of smooth. I've run these same parts many times before and never had this problem. I initially thought it must be due to the backlash settings, but they checked out ok. Any ideas?
x2, g540.

Thanks in advance for any advice.

-drew

edit: here's a pic

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[photomankc]

My mill has started to do something quarky. When milling a 2.5d profile, the curves are coming out faceted instead of smooth. I've run these same parts many times before and never had this problem. I initially thought it must be due to the backlash settings, but they checked out ok. Any ideas?
x2, g540.

Thanks in advance for any advice.

-drew

Check the G-Code. If it's not doing arcs and is instead generating piles of small straight segments around a curve then the it's the CAM or CAD that's causing it. I had several drawings that for some reason converted to DXF as just that (a polygon approximation) and the CAM package did exactly what it was told. I had to use the CAM drawing tools to replace the high-segment-count polygons to real circles and arcs. Also some CAM packages will have a post processor that doesn't use arcs so it forces the output to be the polygon approximation.

[rewster]

Check the G-Code. If it's not doing arcs and is instead generating piles of small straight segments around a curve then the it's the CAM or CAD that's causing it. I had several drawings that for some reason converted to DXF as just that (a polygon approximation) and the CAM package did exactly what it was told. I had to use the CAM drawing tools to replace the high-segment-count polygons to real circles and arcs. Also some CAM packages will have a post processor that doesn't use arcs so it forces the output to be the polygon approximation.

Thanks. That was also an initial concern, but I couldn't find any indication of it CAD or CAM (CamBam). The arcs appear to be smooth in the CAM viewer.

My dedicated pc is pretty old, haven't gotten around to swapping everything over to a newer machine. Could processor speed, or lack thereof, be to blame? I'm just confused as to why it randomly decided to start doing this.

[photomankc]

Are the facets uniform? Hard to imagine it being the machine if they all come out nice and uniform. Maybe a setting in the controller software? Does the faceted curve match the overall design? Other dimensional issues? If it's the machine taking a nap then I can't see how it would possibly not show up as all kinds of other problems.

[rewster]

Upon closer inspection, I think I found the problem. In AutoCAD, when the part is rendered the facets show. Come to think of it, the problem only arose when I switched from exporting dxf's to stl's. Dxf comes out nice and smooth, stl is faceted.

Anyone more knowledgeable than I in AutoCAD? I've adjusted facetres to 10 (max) and the facets remain, albeit finer. I also have pro-e and solidworks, but I prefer autocad for the more simple boolean stuff. Switching to one of the other programs may fix the problem, but I'd prefer to keep this part in AC.

[awerby]

Check the tolerance settings - it looks like your curves are being approximated in .01" segments instead of .001".

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[BobWarfield]

Yep, was gonna mention tolerance too.

Something to be aware of: STL has no way to represent a curve. It's just a pile of triangular facets. So any curves are going to be converted to flat-faced facets with STL.

Your parts are very 2 1/2D if the picture is anything to go by. I wouldn't use STL on them. Save STL for parts that are a more flowing 3D look. Going back to the DXF's should fix it.

If you want to insist on using STL's, you can certainly do it, but you're going to have to figure out how to lower the tolerances enough to reduce the faceting to where it is less visible. In the end of day your g-code file sizes will go up and your machine will be less happy making all those jerky little straight line moves instead of some arcs.

I don't know much about CamBam's simulator, but typically CAM simulators do not interpret the g-code--they interpret the internal geometry that was used to create the g-code. As such they don't really tell you what your machine sees, only what the CAM program thought it put out. A lot is lost from hand to mouth in that process.

Hence you may want to look into a good CNC Simulator that can give you "second opinions" for times like this. Try G-Wizard Editor--it's free during the Beta test:

G-Wizard CNC Simulator

It directly interprets the g-code. In addition, if you're not used to reading g-code, it has a "Hints" view that'll show you what the code is doing in plain English.

There are a lot of other CNC Simulators out there too. NCPlot is a good one, for example.

Best,

BW

[rpovey]

Upon closer inspection, I think I found the problem. In AutoCAD, when the part is rendered the facets show. Come to think of it, the problem only arose when I switched from exporting dxf's to stl's. Dxf comes out nice and smooth, stl is faceted.

Anyone more knowledgeable than I in AutoCAD? I've adjusted facetres to 10 (max) and the facets remain, albeit finer. I also have pro-e and solidworks, but I prefer autocad for the more simple boolean stuff. Switching to one of the other programs may fix the problem, but I'd prefer to keep this part in AC.

The DXF export retains curves from the CAD package. Cambam will actually generate G2/3 for circular segments.
The STL export has to approximate them with line segments,you should be able to change the tolerance and get it pretty smooth though.

[fragger6662000]

DXF and parasolids are the best export formats as they tend to loose the least data and represent true design intent.

[rewster]

Thank you all for the deluge of helpful replies, they cleared a lot up. When you say tolerance settings, do you mean in CAD or CAM? If I stick to dxf, I'll have to program two separate profile operations, no?

[rpovey]

When you export the STL file there are a host of options available, which largely control how it tessellates the curves in the model.

[CNCpatriot]

You can try the SPLINECAM & FontCAM app in Autodesk app store.

http://apps.exchange.autodesk.com/AC...ecamtrial%3aen
http://apps.exchange.autodesk.com/AC...tcamtrial%3aen

http://apps.exchange.autodesk.com/ACD/en/Home/Index

It converts splines & texts into smooth polylines & polyArcs which you can easily import into any CNC software.