Converting Splines to Arcs in CAD

      Skilled users describe ways to turn an AutoCAD spline into a set of arcs or line segments that CAM software can process. February 13, 2006

Question
I drew a profile for a countertop using SPLINE in AutoCAD 2004, but my CAM program will not read splines, so I need to convert the spline into arcs. How do I do this?

Forum Responses
(CAD Forum)
From contributor A:
Try drawing an arc over the spline, then use the grips to match up the arc with the spline. There might be a slight difference between the two entities, but a spline really isn't an arc. Next time use an arc.



From the original questioner:
I was trying to save some time - drawing arcs over a long weird shape profile will take a long time, whereas if you use the spline, you can do it quicker, but it only helps if there is a quick way to turn them into arcs.


From contributor B:
I can't get your spline to convert to arcs, but I do know how to get it to convert to small line segments.
Save the drawing as Autocad R12/LT2 DXF.
Close the drawing.
Reopen the DXF file.
This changes the spline to a polyline.
Explode the polyline if needed, and you have lots of little line segments that your CAM program can read.


From the original questioner:
I guess I just need to trace my profile with a series of arcs. The problem is that I have to keep making sure that it is all smooth. I have one more question - for example, if I draw a series of arcs to create an inside cut out shape (sink cut out), is there a way to check to make sure that all those polylines or arcs are all indeed connecting? Is there some sort of command that would automatically advise you that any lines, arcs or polylines are not connecting?


From contributor B:
There are a couple of ways to do this. Use the region command and pick everything that you want to be joined. If they are drawn correctly, meaning all ends connected and no overlaps or extra pieces, they will be connected and you will see a message that says one loop created, or something like that.

Another way is to turn one of the lines into a polyline by using the Pedit command, then join this polyline to all the other entities. Check this by picking the polyline and moving it. If everything is connected properly, the entire thing will move together. Otherwise you can see where the pieces failed to connect properly.

I'm not sure why you don't just use the method I suggested earlier. It will be very smooth. After all, your CNC machine only moves in straight lines. A circle made on a CNC is just lots of little line segments.



From the original questioner:
Yes that's true. The only problem with the Spline to Polylines is that it creates millions of little segments in order accomplish the smoothness of the curves, and it takes my CAM program forever to read line by line when I am programming the profiles.


From contributor C:
Create your spline to the shape that you want on a separate layer. Type *divide* and select the spline, pick a number depending on how accurate you need to be. I usually try 12-16 as a start, and hit enter. Set OSNAPS to endpoints and nodes only, select polyline using the required layer for your machine, then start at one end of the spline, and around you go. After you’re done, highlight the polylines and edit them, snapping each midpoint on your polys to the perpendicular of the spline. If you see you're not accurate enough, start over and increase your divisor. This works like a charm.


From contributor D:
What CAM package are you using? In MasterCAM you can use the MODIFY-BREAK-SPLS_TO_ARCS function in order to produce an extremely close approximation of the original spline using tangential arcs and lines. You specify the tolerance used. This represents the chordal deviation. I like 0.0002". Once you map your toolpath to this new line/arc representation, the resulting NC code with the tangential G01s and G02s and/or G03s should run very smoothly.


From contributor E:
If you have the AutoCAD express tools loaded, you can use the FLATTEN command and it will convert that spline into a polyline with arc segments.


From contributor F:
To contributor E: Great call! We make custom moldings, and show all these pieces in 3d. I usually use polyline arcs to make these, since I didn't know of a way to *explode* and *pedit* a spline. The flatten command worked great.

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