Crop Circles are...

[stevespo]

a) wasting hours of my time
b) causing me to go prematurely gray and bald
c) a feature of BobCAD!

If you're not sure what these things are, they appear to be BobCAM V20 generated arc instructions with bad/bogus values that cause seemingly random circles to get created when trying to simulate or machine a part. Sometimes the simulation catches them, sometimes Mach catches them, sometimes you just ruin a part.

They may occur because of a chain gap, because an arc has a Z != 0 value, or for other reasons not known to me.

I seem to run into these almost daily. I plot/generate a series of 2d points, connect them with a spline, verify. Everything looks good. I generate my gcode, and then all heck breaks loose. I can reorganize, regenerate, etc - still random circles. I have played with my accuracy settings with little luck.

The only true "solution" I've found is to generate the splines as a series of straight lines (avoiding arcs entirely!). However, if I create an offset from this "straight line spline", sometimes arcs make their way into the new offset I've created. What a great feature that is! I want straight lines, I get arcs. If I then try and perform a pocketing operation, guess what? Crop circles!

Can anyone provide a consistent solution for this? Sometimes, I can generate my arc-based splines and everything works flawlessly. Other times, it's hours of frustration. I'd like to use arcs, because they generally create nicer looking curves and less gcode. I can't put my finger on what is different, or why it happens. Sometimes, I'm working in 2d, and I get very small Z values on my generated splines. I want Z=0, and get Z != 0. Bugs, bugs, bugs.

Sorry for the sarcasm,

Steve

[The One]

Steve,

Here are some steps that may provide helpful for you as they have worked in the past.

Routine #1:
Most "crop circles" are caused by non-tangent arc segments, because BobCAD uses the corner rounding method rather than the straight corner method. That is it creates an arc around a corner to make a corner. The following will work in most cases involving splines.

1. Select All of your geometry.
2. Click on Change => Reorganize => Clean-up and Optimize (to get everything in sync)
3. Click on Change => Reorganize => Make Arcs Tangential
4. Input .001 for the Minimum acceptable angle and 15 for the Max Angle for correction.
5. Click OK.
6. Now generate your Toolpaths.

Routine #2 is to eliminate the use of arcs completely by interpolating the Toolpath, which removes all arcs from the program and replaces them with faceted line moves (which can be small enough to provide clean arcs on the finished part). I will not post this one today, but if you would like to know how, just say the word.

Regards

[ger21]

Make sure your IJ mode matches from BobCAD to Mach.

[stevespo]

Gerry, I was aware of the IJ mismatch and do check for that. Right now, I'm just working in the BobCAD simulation mode, and still seeing circles. Most of the time I don't catch them until I view the toolpath visually in Mach.

TheOne, thanks for that advice. I gave it a quick try and it definitely helped, but not completely. I still get some small circles in the toolpath generation, but not in the original outline. I will continue to work on it.

The only way I know to convert from arcs to line segments is to select, choose Other -> Spline and deselect "with arcs". If you know of a better or more reliable technique, I would definitely appreciate hearing about it.

The one upside of having these problems is you really learn the software, because you're forced to try all these different techniques to find a workaround. You can then apply the techniques to other drawings and gain some knowledge.

Steve

[The One]

OK. To get Line segments out of a Toolpath do the following:

Generate your Toolpath (method does not matter)
Click on 3D => More => Interpolate (this is an action/object command, so any selections do not make a difference.)
Input .0001" or .001mm for the Accuracy (leave the other values as is)
Select Chain or Region (with Region you can just drag a box around the entities you want to interpolate, with Chain you will need to do a Chain selection. I normally use Region because I am, admittedly, a little lazy.)
Click OK.
Then select the Toolpath. The system will handle the rest.

Regards

[tobyaxis]

Originally Posted by stevespo a) wasting hours of my time b) causing me to go prematurely gray and bald c) a feature of BobCAD! If you're not sure what these things are, they appear to be BobCAM V20 generated arc instructions with bad/bogus values that cause seemingly random circles to get created when trying to simulate or machine a part. Sometimes the simulation catches them, sometimes Mach catches them, sometimes you just ruin a part. They may occur because of a chain gap, because an arc has a Z != 0 value, or for other reasons not known to me. I seem to run into these almost daily. I plot/generate a series of 2d points, connect them with a spline, verify. Everything looks good. I generate my gcode, and then all heck breaks loose. I can reorganize, regenerate, etc - still random circles. I have played with my accuracy settings with little luck. The only true "solution" I've found is to generate the splines as a series of straight lines (avoiding arcs entirely!). However, if I create an offset from this "straight line spline", sometimes arcs make their way into the new offset I've created. What a great feature that is! I want straight lines, I get arcs. If I then try and perform a pocketing operation, guess what? Crop circles! Can anyone provide a consistent solution for this? Sometimes, I can generate my arc-based splines and everything works flawlessly. Other times, it's hours of frustration. I'd like to use arcs, because they generally create nicer looking curves and less gcode. I can't put my finger on what is different, or why it happens. Sometimes, I'm working in 2d, and I get very small Z values on my generated splines. I want Z=0, and get Z != 0. Bugs, bugs, bugs. Sorry for the sarcasm, Steve

I hear you and suggest not using I's and J's but R's at 90 Degree Quadrants.
Personally I hate I's and J's unless doing a 360 Circular Interpolation.

Open a drawing then open the CAM Side by selecting the Green Machine Icon.

Setup>Driver Setup>ISO Tab and set it like the one below. No More Crap Circles.

When you get better with the software experiment with the I's and J's to get the results your looking for. This is just a quick easy Fix so you can Program your Mach Control for now.

Cheers!!!!

[stevespo]

I'm still using BobCAD V20.

I've gotten a lot better at finding/fixing the 'crop circles', but I'm still very frustrated that I have to deal with them at all. I think the issue is that G02/G03 instructions are occasionally generated with no X/Y values. Although these values should be optional, this is the culprit at least part of the time.

It doesn't matter if I'm in R or IJ mode, the results are still the same - one of the arc instructions generates a complete circle, instead of a partial arc. It could be that the size of the arc is so small that there is some type of rounding error and it results in a full circle. That is one of my theories.

These particular errors show up during software simulation, but others don't surface until you actually run the part (with Mach 3).

If anyone has any other ideas I would love to hear them.

Thanks,

Steve

IJ Mode - line 154 is the culprit (one circle)

N150 G01 X+1.1503 Y+1.5028
N151 X+1.1749 Y+1.4595
N152 G02 X+1.1631 Y+1.3829 I-0.0544 J-0.0309
N153 G01 X+1.1389 Y+1.3604
N154 G02 I-0.0426 J+0.0457
N155 G01 X+1.1042 Y+1.3284
N156 X+0.9724 Y+1.1988

R Mode - line 155/156 is the culprit (actually generates 2 circles)

N151 G01 X+1.1503 Y+1.5028
N152 X+1.1749 Y+1.4595
N153 G02 X+1.1631 Y+1.3829 R+0.0625
N154 G01 X+1.1389 Y+1.3604
N155 G02 R+0.0625
N156 R+0.0625
N157 G01 X+1.1042 Y+1.3284
N158 X+0.9724 Y+1.1988

[Allen123]

N151 G01 X+1.1503 Y+1.5028
N152 X+1.1749 Y+1.4595
N153 G02 X+1.1631 Y+1.3829 R+0.0625
N154 G01 X+1.1389 Y+1.3604
N155 G02 R+0.0625
N156 R+0.0625 <--------------------- That's your crop circle
N157 G01 X+1.1042 Y+1.3284
N158 X+0.9724 Y+1.1988


Are you working on text or something like that?

What you can do is change you code from non modal to modal ( in the post setup under drive change the " show only change in" )

This way you can see the values being posted for every movement. If you see repeated movements then you know that's where your problem is. Most of the time this comes from the lines or arcs you are cutting.

A good test to see if it's the tool path or part path, it to cut the path with our an offset. If you don't get a problem that this is a good sign. If you do you know the cleanup options can work.

Next if you look at the offset that BobCAD creates when you do a pocket or profile. If you find an issue there you know it's from BobCAD. It may be the environmental setting. I don't like the change those from default but sometimes that helps.

Most of the time you don't have an issue with things like this unless you are cutting smaller arc segments

[vlmarshall]

Crop Circles! Great name for them.

I've had issues with MillWrite generating wild arcs in the wrong direction, and now I have something to call them.

[stevespo]

I am often working on text or hand drawn artwork. I scan this in and trace with Illustrator to generate the vectors. Then I import into BobCAD, so some cleanup and generate my pockets and toolpaths.

I believe there are several problems here.

1) The smooth bezier splines in Illustrator get turned into many, many small arcs and line segments when imported into BobCAD. (Maybe it's the export to AutoCAD format).
2) Many BobCAD V20 functions will barf when you chain select more than a thousand elements, so I have to simplify the artwork with spline and then offsets.
3) Even after using the spline and then offset functions, BobCAD will often generate very, very tiny (> .001") arc segments.
4) These segments are so small that the difference between the begin and end points approaches 0, so the end values are omitted when the gcode is generated. This is an accuracy setting or rounding error.
5) When the end point is missing, Mach3 (and other machines) will rightly assume that a full circle is being machined. It's just doing what it has been told.

The arc commands are simple. They will move from the current location, to a specified point (XYZ), using a specified radius (R) or an (absolute/relative) centerpoint (IJK). If the endpoint is omitted (XYZ), you get a crop circle.

I wrote a small script to look for G02/G03 commands with no XYZ endpoint and comment them out. The output from this script runs perfectly, because the arcs are so small that they're basically irrelevant to the quality of the final part. It's a solution, but not a great one because it introduced an extra step and is not foolproof. Even so, it has helped speed things up greatly and I can cut these complex pockets with a much higher degree of confidence.

Many people encounter this problem with other pieces of software, so it is a common thing - not specific to BobCAD. I still view it as a bug and hope it has been fixed in later versions.

Interestingly enough, I installed the trial version of VCarvePro, where I could directly import the Illustrator file. It generated the pocket and simulated beautifully (no circles), with no tweaking and messing around. I could not test the output (trial version), but this was a 2 step process, instead of the 10 step process I've gotten used to.

Hmnn...

Steve

[alexlis3]

Hi, all!
sh

[Jim Estes]

I have fought those dreaded crop circles since version 17. They are sneaky little devils.