Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: MCONCEPTS on October 10, 2009, 11:40:26 AM
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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.
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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?
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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.
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My guess is that the .dxf has the arcs, but you just don't see them.
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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....
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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?
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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.
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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
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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.
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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
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My approach was a straight plunge.
The lines I was cutting were often multiple line segments, which I tried to clean up, convert to arcs, etc. but a crop circle would still sneak in on me occasionally. I used cleanup and optimize, which helped a little in some instances, but I did a sign using and Old English font, which drove it crazy. The crop circle (term used on their forum) was a random event. It you could have two identical letters and it would have a problem with one and not the other.
I found that in every case there would be an errant line of code that was easily identified. I wrote a program that would read the GCode file, identify the bad lines of code, and eliminate them. I just got tired of jumping through so many hoops for a simple 2D cut path.
The only thing I used BobCAD for was GCode generation. I do all of my CAD work in AutoCAD. BobCAD's CAD program was atrocious. It was very clumsy, non-intuitive, and generally poorly conceived. I was using version 20, and BobCAD's people told me that some of this was addressed in the newer versions, but they wanted more for the upgrade than I paid for VCarve Pro, and it has yet to have a single glitch. So far, I think I made a good decision on switching, and am looking forward to upgrading to Aspire when I can afford it.
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Thanks for the further explantion. I agree that the learning curve on BobCad is steep - very steep. Has BoobCad every addressed the issue? Or did they try to upgrade you? LOL.
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One other question. What post processor do you use in BoobCad? Forgive me - but I've had some bad experiences using the applicaton too. I'd tell you about them, but dragging up the old memories is just to brutal!
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The 'crop circle' issue seems to be one that has gone back for several releases. If you look on their support forum and use the search term 'crop circle' you will find MANY messages relating to this issue. The last contact I had with them stated that many of these issues have been addressed in later releases (again, I was using version 20 and they are up to 23). They wanted me to upgrade to (maybe) correct these problems (for the upgrade price of $800.00 or more).
I was using a Mach3 post processor.
As to the learning curve on BobCAD, that wasn't my issue, it was the clumsiness and unnecessary selections. For instance:
In BobCAD, to 'MOVE' an object you have to select the objects, then click on 'Change' then 'Translate' (what the hell does that mean, I want to MOVE an object), then select the method. You can select 'Increment' then enter an X coordinate ,then a Y coordinate, then OK to move it, or select 'Snap' and select the snap coordinates, etc.. In AutoCAD you select the objects, right-click, select 'Move', then you can (for increment) just type '@1,1' to move it over 1 and up 1 , or click on any 'snap' point and select another snap point and it is moved, or you can enter the new coordinates. All with a single mouse click and some typing.
In BobCAD, to trim/extend an object you go through the same thing. Several menu selections to get to the point that you can trim/extend anything. In AutoCAD you simple select trim or extend, select the trim/extend lines, the proceed to trim/extend as many objects as you want.
My biggest peeve is that in BobCAD the scroll button acts as a PAN function which is worthless (there may be a way to change this but I was never able to find it in their 'documentation'). In most other programs the scroll button acts as a zoom in/out. When trying to create a GCode file from BobCAD it would often not see a string as complete (usually a .005 or less gap in the string). I had to zoom in to EVERY intersection many times to find the break. Since the scroll button acted as a 'PAN' instead of 'ZOOM' I had to continually 'CROP' the intersection, zoom in, crop again, zoom in, etc. sometimes as many as 6 or 8 times to find the break. In AutoCAD I put my cursor on the intersection and used the scroll button to zoom in as much as necessary.
Don't get me wrong, BobCAD worked for me for several years. I used it from 2003 until just recently, but the more complex my drawings got the more glitches I found and the more work I had to do to get them to work without destroying material. I think the 'crop circles' were just the final straw to get me to find something better for my purposes. So far, VCarve has answered that. I have yet to have a single cut error using VCarve, and I'm cutting MUCH more complex drawings than I ever did with BobCAD.
Ken
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Can't speak of BobCad since I have not used it since Ver 17 which was a long time ago.
You may want to take a look at a tutorial in Memebers Docs called ARC Motion. ALthough
is was done specificaly for the lathe there is a lot of general information which pertains to a mill also.
Crop circles are covered rather in depth, and for the mill, you can change the programs shown for G17
along with the replacing the Z with Y. You need to have the correct Cam and Mach configuration
for things to turn out correctly. Take a good read of the info and you'll be able to identify and fix almost any crop
circle problem you come across. Unfortunately the Cad / Cam problem is a different story as all the programs have their quirks.
FWIW,
RICH
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I'm new at this game, but I feel I've come a long way in a short time. I got BobCad originally V23 and find the same issues with moving objects. I have read the other posts on crop circles (G0/G3). I've just never had it happen to me (yet). Of course I only use bobcad for "technical" drawings - lots of straight lines and an arc or two. For complex work I'm using vcarve pro 5, cut 3d and aspire.
I really appreciate you taking the time to write such a lengthy explanaton. It helps me (us) understand the code better. Thanks again!
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It's true that all software has it's glitches and quirks. As you said, once I knew what to call the problem (I found the crop circle reference after describing the problem on the board) I found PLENTY of information on them. This fact in itself would indicate that it is a problem that should have been addressed long ago. Oddly, the crop circles aren't even the real glitch, I'm sure there is a mathematical reason that the program sees these line segments as circles. The real glitch is the fact that they don't show up in the cut path, you only find them in the middle of a part when it destroys your work.
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kthomson,
'Oddly, the crop circles aren't even the real glitch, I'm sure there is a mathematical reason that the program sees these line segments as circles.
'
......There are explainations in the ARC Motion tutorial that address this.
"The real glitch is the fact that they don't show up in the cut path, you only find them in the middle of a part when it destroys your work."
......Wise to always dry run.
RICH