Problem Cutting curves from AutoCAD generated DXF file

[AndyBuck]

We have drawn arc shaped counters in autocad. When we save the drawing as a dxf file and send to the cnc machine, they're cut in straight segments. It used to work, i.e. cut perfect curves, the expert left the company and now it doesn't.

What's wrong?

 

[berkshire]

AndyBuck (Industrial)
It sounds like you are using a polyline instead of a spline curve. use Pline edit.
B.E.

 

[AndyBuck]

We've done it two ways, neither with polyline or spline.

We've started with a circle and with a 3 point curve. Both ways were saved to a dxf file.

Could it be the way we're pulling it into the cnc machine (scm)?

Andy

 

[berkshire]

AndyBuck (Industrial),
It may be. Does your CNC machine use a post processor? If it does, it may be how the information is handled from the DXF file. I do not know what kind of machine you have, however some machines will give you a printout in"Notepad" of the G codes generated by the post processor.
You can then look at this and see if the machine is cutting arcs or going point to point.

 

[AndyBuck]

I'm just starting in this position (sort of a software debugger for the shop) so I'm picking up pieces as I go. Turns out ASPAN is converting the dxf to G codes, so I'm chasing that down next.

Thanks.

 

[MikeHalloran]

Most CNC controllers are primitive, which is a two- edged sword.

Example: Some 'see' only layer 0, which leaves you free to put identifying information, or even the base cad model, in other layers.

Example: Some, maybe many, don't 'see' AutoCAD 'spline' entities at all, but will do you the courtesy of interpolating between their ends with a straight line. Ellipses can also be problematic.

The simplest trick is to just export splines as polylines with a lot of segments, which I think is a menu option somewhere in the dxf export process. Actually, that's two options; curves as polylines, and many or few segments, etc.



Mike Halloran
Pembroke Pines, FL, USA

 

[AndyBuck]

Mike -

We did finally discover that. We're trying to figure out how to set the number of segments or polylines during or before saving as dxf. You mentioned Export - when we Save As there's no intermediate dialog box - it just saves it. We'll see if there's something different exporting...

Thanks,
Andy

 

[AndyBuck]

None of this seems to make sense (to me). I have a simple drawing of an under counter support with 3 1/4 circle curves. All doc I find talks about converting polylines to arcs, not vice versa, which seems to be what I need to do to get a smooth curve cut after exporting to DXF. Also, I can't find any way to set the segement length once I get it there.

Thanks,
Andy

 

[berkshire]

AndyBuck (Industrial)
Do a machine test.
Draw a 4 or 5 inch circle on a 10 inch square plate.
Export it to the cnc router. If you can look at the G-Code it should then say something like G2 X5.0 y5.0 R2.5 ----go in an arc from X5.0, Y5.0 to X0.0 Y0.0, with a radius of R=2.5 The actual X and y positions will depend on where you start the cutter the g2 or g3 code is the circular cut code clockwise or anti clockwise. Other machines will take your curved lines and break it into poly lines at 0.01" increments i.e. G1 X0.0 y0.0 X0.01 y0.0 and so on. I do not use the ASPAN post processor however I understand it has a built in editor for DXF files. So it should show you what the machine is getting.
B.E.

 

[MikeHalloran]

Arcs should be okay; CNC controllers mostly understand them. The various menu options for adjusting how things really work seem to change appearance and location with every release of AutoCAD, so I can't tell you exactly what you'll have to do, and even if I could, I'm working from memory right now.

The DXF file format is documented, and with practice you can learn to read them pretty well. For simple planar objects that are typical of what gets sent to a CNC machine, they aren't very big, so even NotePad can handle them.

As berkshire says, find or synthesize a simple part that exhibits the problems you have, and beat on your process using scrap workpieces, until you can reduce it to a rote procedure. Be sure to write down the procedure _exactly_, and keep copies in more than one physical location, and test the procedure on someone who knows _nothing_ about what you do or how you do it. When _any_ literate person can follow your procedure and get a good G-code file and a good part, the procedure is okay.




Mike Halloran
Pembroke Pines, FL, USA

 

[AndyBuck]

Thanks to both Mike and berkshire. I will try all suggestions. And Mike, your suggestion about documentation is a very familiar one for me. I come from a business/IT job to this one. There, and in my photoshop experience as well, what you're talking about is a well documented workflow, an essential tool to getting complicated processes done repeatedly correctly. I'd already started one! Thanks.