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Author Topic: Strange cutting.... Circles for line segments...  (Read 8102 times)

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Strange cutting.... Circles for line segments...
« on: October 10, 2009, 11:40:26 AM »
I'm cutting a pumpkin out of wood on my CNC router. I didn't do the DXF file on my CAD, I got it from someone else. When it cuts the teeth of the pumpkin the machine moves in tiny circles as it cuts each line segment in the teeth. If I look at the DXF file the teeth look like straight lines but they are made up of tiny line segments. It appears that when it is being cut the router cuts a tiny line segment, circles around and cuts the next line segment, circles around again to cut the next segment and so on until it completes the line instead of continuously cutting the individual line segments. I've seen something similar to this before but only when I had line segments with directional arrows pointing opposite directions when I generated my G Code for the chain causing the machine to cut one line segment in the opposite direction while cutting a chain. I don't have this issue this time. I'm using BobCAD 21 to generate the G code. When I simulate it in BobCAD it cuts as I would expect it to. It doesn't cut that way when Mach controls the machine. Anyone know what might be wrong? I had been playing around over the last week with Backlash compensation and thougth that might be causing some issues but I have the same issue when it's turned off. Any help woud be appreciated.

Offline ger21

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Re: Strange cutting.... Circles for line segments...
« Reply #1 on: October 10, 2009, 12:38:58 PM »
BobCAD must be putting arcs in there, as Mach wouldn't "circle" if there were only straight segments. Can you post the .dxf and the g-code?
Gerry

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http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
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Re: Strange cutting.... Circles for line segments...
« Reply #2 on: October 10, 2009, 01:52:28 PM »
After looking at this further, BobCAD is definetly doing the Arc's. It does G02 the G01 then G02 then G01 throughout the line cut. I don't know why it is putting the Arcs in their since they don't appear on the CAD screen but I'll keep looking at it and see if I can figure it out. It's obviously not a Mach issue as I originally thought. It's almost as if the line segments are so short its having trouble generating the G code.

Offline ger21

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Re: Strange cutting.... Circles for line segments...
« Reply #3 on: October 10, 2009, 04:50:04 PM »
My guess is that the .dxf has the arcs, but you just don't see them.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Strange cutting.... Circles for line segments...
« Reply #4 on: October 10, 2009, 06:28:09 PM »
It's not in the original DXF. In BobCad I'm selecting the mouth of the pumpkin and creating a parallel offset for the toolpath (instead of using tool offset) and that is when the arcs are being added. Strictly a BobCAD issue....

Offline ger21

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Re: Strange cutting.... Circles for line segments...
« Reply #5 on: October 10, 2009, 07:27:59 PM »
Actually, adding the arcs when offsetting is the proper way to offset, as it gives a smoother toolpath. The issue is why Mach doesn't like the code and is doing circles. Are they IJ arcs, or R arcs?
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Strange cutting.... Circles for line segments...
« Reply #6 on: October 10, 2009, 11:35:52 PM »
I was only cutting air and using a 1/16" bit so it was really hard to tell the shape of the circle. The line segments/arcs were really small. I was able to get what I needed cut by just eliminating the offset. I really didn't need it with a 1/16" bit on artwork. But I will need to look into this further this week. I haven't had any issues with offsets in the past but this was by far the smallest thing I have cut on the machine and as I mentioned the DXF artwork had very small segments also.
Re: Strange cutting.... Circles for line segments...
« Reply #7 on: October 18, 2009, 02:41:38 PM »
BobCad can help you reduce the number of entities using the Cleanup/Optimize feature.  I've copied the help menu for you here.  This should reduce the number of entities in the file, thus providing a smoother toolpath.  Hope this helps you:

Cleanup / Optimize
Purpose: to reduce the number of entities within a drawing by joining contiguous selected segments that have the same X/Y/Z angle or share a common center point and radius into a single entity.

Use: The selectable entity types for this function are lines, arcs, and splines. The user is permitted to use the single selection, window selection, and chain selection methods with this function.

Gary


Re: Strange cutting.... Circles for line segments...
« Reply #8 on: October 20, 2009, 07:54:30 PM »
This is a BobCAD problem that I fought with for weeks while making a sign. This problem has been refered to as  'crop circles', I assume, because they appear from nowhere, and for no apparent reason. They destroyed a couple of signs I was making.  It has been a common problem in BobCAD for years. I tried everything to get rid of them. They do not show up when you view the cut path, but they are present in the GCode. I wrote a program to find and eliminate them, but finally got fed up and changed to VCarve Pro.
Re: Strange cutting.... Circles for line segments...
« Reply #9 on: October 21, 2009, 01:20:23 AM »
KT,

I'm wondering what you set your approach to in BobCad?  Did you use circular entry?  Thanks

I agree, for general sign making V-Carve is the absolute best, but there are some things I do that BobCad excels at.

I'm just guessing, but if the lines you are cutting are made of individual line segments, then the cleanup and optimize in BC will help that.  If you are set up in a profile feature and you select a circular entry - then (a question) would BC circle entry on every entity in the line?  I don't know.  Seem to me that the approach would have to be parallel.

Thanks

Gary