Problems cutting circles

[44henry]

Hello, I have had our Shopbot for a little over a year now and, due to time constraints, I haven't had much chance to use it until now. I am trying to cut a series of circles that I designed in PW. When I go to cut this file on the shopbot it is producing more of an oval than a circle with definite flats on the sides. Can anybody give me some reasons that it might be doing this? I have had no problems Vcarving fonts with the machine and this is mainly what we have been using it for until now, though another professor in our department had experienced problems cutting circles in the past when using MasterCAM files on this machine, but had never had that issue resolved. Any information would be greatly appreciated.

Regards,

Alex Johnson

[myxpykalix]

Try saving your file with the "shopbot arcs inch" postprocessor. It worked ok when i tried it....

3815

[44henry]

The file that I cut had been saved in this format, I tried it again and it is still giving me the same oval type form instead of a true circle. Are there any settings that might not be set correctly that could cause this?

[jsfrost]

Slipping X or Y pinions can cause distorted circles. Check that yours are tight.

[dana_swift]

Does anything else cut correctly? If you zero the table, then move X 12.0 inches, does it go 12.0 inches? Same with Y..

If those are not correct the values in "VU" need to be updated to get the correct motion for your router. Check with tech support to get the correct values for your stepper/pinion combination. These values have everything to do with the motion scaling of each axis.

Hope that helps..

D

[bill.young]

If it's just a little bit out of round I'd agree with loose pinion gears, or maybe the motors not engaging the pinions fully in the rack?

One way to narrow it down would be to cut a sample circle using one of the built-in circle commands... I find the [CP] version is the easiest to use. If that doesn't have the flats then it's most likely a file issue...if the same flats are there then it's probably mechanical.

Bill

[44henry]

Dana, I everything else that we have cut on the machine seems to cut correctly. We have mainly done signs with the Vcarve fonts provided in PW, but I would like to go into other areas with the machine. The only issue I have had is with the machine losing its position occasionally on long signs, it will cut all the deep parts of the letters and than somehow it resets itself to zero somewhere in the middle. On shorter signs this has never been a problem, but when they go much over 3' the problem does occur.

I did check the table movement with a dial indicator and all three axis are within .001 which seems pretty good.

Regards,

Alex Johnson

[dana_swift]

Alex, that eliminates scaling issues.. have you looked at the SBP file to see what was generated? Feel free to post your SBP file for the circles you show above and I will be glad to look at it and see what it is doing. That should separate out whether your problem is on the PW side or the SB side.

Also, if you include a square box in the same cut file does it come out as a rectangle?

D

[dana_swift]

Afterthought to my previous post.. the SB preview shows circles to the naked eye.. how far out of round are your cut circles if you measure x and y with a caliper?

Also.. how is your material held down? Could it be shifting?

D

[harryball]

Gantry deflection can cause oval too. As the tool cuts force on the bit changes directions pushing dynamically on the gantry. Anything loose or flexing will cause an issue.

One test... cutting foam circles might be round while cutting plywood with the same file produces ovals.

Just another thought.

/RB

[44henry]

Dana,
I checked the feed rate per ShopBot's technical support and changed the rate from 25 ipm (with a 1/8" end mill) to 1 ipm and have the router set to maximum RPM's. The material is 3/4" mdf with a 1/8" depth of cut. The cutting file is:

'----------------------------------------------------------------
'SHOPBOT ROUTER FILE IN INCHES
IF %(25)=1 THEN GOTO UNIT_ERROR

[dana_swift]

Alex- the GC commands you provided give following values with a 0.125 end mill (see graphic).


3816

The only unusual thing that sticks out is that MasterCam seems to be optimizing the hole entry points for toolpath minimization. That could create a very tiny error in "ovalness" due to the circle interpolation routine in the SB3 program. That shows up in the spreadsheet as "start angle". Making start angles of 0, 90, 180, may give better results, I dont use MasterCam so I cant make any suggestions there.

Minimizing the circle interpolation error can be done from the menu Values:Units (VU) and changing the Circle Resolution value. According to the manual that value can be reduced to 0.005. That should reduce the interpolation error to a few thousandths. I found the documentation for the VU command in the COMREF.pdf file in:

C:\Program Files\ShopBot\ShopBot 3\Help

How far out of round are the holes?

D

[44henry]

Dana, though we use MasterCAM in the department this file was generated by PW directly. The circles themselves are over .050 out of round. I'll try changing the values to .005 and see if that does the trick. Thanks again.

Alex

[dana_swift]

Alex- Once PW generated the CG command, its up to the shopbot to cut the circle. Conceptually it makes a perfect circle, in reality it makes a polygon approximation. So the problem is either the 0.005 or mechanical.

Interesting correlation to the 0.050 and the default setting of 0.050 in the VU command. Sounds like that may have been a good place to look.

D

[44henry]

I tried adjusting the value to .005 and did another test cut which did not seem to have an effect on the circle. The test circle,which was 4" diameter was .060 out of round. I'll try to take pictures of it tomorrow and show them on the board.

Alex

[dana_swift]

Alex I wrote a short C# routine to generate a circle with a known interpolation error. This cuts a circle with a 4 inch diameter with a known interpolation error of 0.0015. I included the program I wrote, as well as the SBP file. Can you run these and measure your results? The only errors remaining is bit flexing, router mounting issues, runout, etc. The traditional error suspects.

(( Note: the forum does not support Unicode letters, so look at the attached files to see what I did.. I use Greek characters where that makes better sense in my programs.))

If the circle is out of round in the X and Y axis this same program can be re-written using the polar coordinate formula for an elipse and that error removed. This same routine can be updated to correct for runout errors, non-orthogonality of the gantry, etc. I included the source so you can use it for inspiration to write some equivalent routine.

I like partworks for quick building generic shapes, but when I want to get the accuracy to 0.0005 generating my own toolpaths is the only method I have come up with. As a result I have ended up writing a set of ShopBot classes that the following code is based on:


testprogram
CircleTestExtract.cs (http://www.talkshopbot.com/forum/messages/26/CircleTestExtract-30468.txt) (3.2 k)

[44henry]

Dana, thankyou for the file information, I am busy grading student work right now, but I hope to try it out this weekend and I'll let you know how it turns out. Thanks for all the help.

Regards,

Alex Johnson

[44henry]

Dana,
I finally had a chance to run the program you gave me. I'm still getting the same result, a circle with two flat spots on it. The one it cut was approximately .050 out of round. I'll get some digital photos of it and post them here, maybe this would help to describe it. I appreciate the help.

Regards,

Alex Johnson

[dana_swift]

Clearly there is a mechanical problem as a circle cut from that file is going to be within 0.0015.

I suggest you check for backlash in the rack/pinions gears, especially in the axis with the flat spot. Check both axis while you are at it. Do you have a dial indicator? That can help find those problems..

Too bad its such a long walk to North Dakota from here and gas prices are so high! This would be fun to look at in person!

D

[jsfrost]

Alex,

I suspect you have checked based on previous posts. But my out of round issues, caused by slipping pinion, sound much like yours. I had been V carving lettering and doing general cutting without problems. I was asked to cut 2 inch diameter wheels for kids toys and could not get them round enough to roll. My problem was a Y pinion with set screw not fully tighton the flat. Y would slip a tooth or two only when the Y direction reversed passing the circle center in X. Briefly, the Y motor moved, the pinion shifted, and the router/spindle followed only in X causing a small flat. Try pushing both directions in both X and Y while the SB is powered but not moving, and look for a brief,subtle lack of resistance. If you have the same issue, the problem pinion has two stable states, and shifts between them as the stresses reverse.

Bit flexing or router/spindle bearings could do something similar, but this would tend to cause flats in both X and Y.