Yeah, that is really really strange. The consistency is good, it’s just consistently not right which isn’t good.
I will run the same exact cut right now and see what I get
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Thanks, Bar. I am really hoping its the gcode LOL
I’ve got bad news…mine are coming out round 
At least as best I can measure with a tape this looks to be the same in each direction.
Here’s the horizontal measurement:
Here’s the vertical measurement:
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Thanks. I am completely stumped then.
Every dimension I adjusted, distance between motors, rotation radius, motor height only served to make things worse.
PS, those turned out exactly as I had imagined they should 
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Ok so we know it’s not the file…what now?
My instinct is to say that something is up with the linkages…but that’s probably because that’s the only part I didn’t make 
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Haha. Could be!
I am going to have to take a break for the afternoon, the kiddos have awaken from their naps.
I am going to check the chain as well.
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It’s probably the perspective, but the mounting points for the linkage looks “closer” together than the mounting points at the end of the arms. What distance from the router bit are you using for the mounting points?
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So those mounting points are exactly 4.5" from the center of the bit.
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I assumed you had it right on… must be the perspective.
Okay…I believe I have made a bit of a breakthrough. I was thinking about the things that would have kept the error consistent through all the changes, the only things I didn’t change were the linkage and the chains. I had my wife help me accurately space the motors at 3010 mm by loosening the screws and tapping the mounts in and out with a hammer and tightening them down.
I then did the automatic measurement 4 times with these results: 3006.26mm, 3005.47mm, 3005.57mm, 3006.01mm = Avg. 3005.83mm
I back calculated using the “stock” chain pitch of 6.35mm and got ~473.36 links. If I then map that to my “known” length I am getting a chain pitch of 6.3588mm.
With that being said (and I am looking for a logic peer check here), I THINK that would explain the consistency of the error. I never changed the chain or the chain pitch therefore, when it calibrated it would always be off by essentially the same amount. However, its a double whammy because its measuring the distance between the motors to be short which puts all of the calculated measurements out of whack.
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Could you chain be bad? It doesn’t look like the stock chain (longer)
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So, I did buy a little extra chain but I verified that the only thing the extra chain is being used for is for the length that provides tension, none of the different chain is being paid out over the motor sprockets.
just went through all of that with the new chain pitch value. I get an average across 4 automatic measurements of ~3010mm which is the correct motor distance. The calibration calculated exactly 270.0mm as my rotation radius which to me indicated to me I was on the right track.
I cut my circle, still EXACTLY 10" wide and now 9.75" tall. The width never seems to change.
*Revisiting the simulator I guess this is actually good news, as it would indicate that I have perturbed the chain pitch in such a way that I am getting an increase in chain length error ~equal to 2/3 of a chain link on each side. However, it does not explain why I am not getting the right measurement between motors. I know this was an issue that already got fixed based on the incorrect motor gear ratio.
I am pretty stumped too.
Good thought to check out the chain pitch to make sure it is correct.
When you move up 12 inches manually, does it move up exactly 12 inches?
Good question, I had done this before but good to check twice esp since the sled had changed. So, I got a result, I moved in 6" increments up and down from the center of the sheet.
At the center I am off about 1/16" towards the top and towards the bottom. Near the edges, my error multiplies to almost 1/8" between the 6" marks.
This is good. That is pretty much what I am seeing when I cut these circles.
I am going to go back to standard chain pitch, set my motor distance at 3010mm and then try again.
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I can’t imagine its possible to be wrong, but could you measure the linkage kit arm lengths and make sure they are correct? Also, I know the distance from center of bit to the mounting point is 4.5 inches (as you checked earlier) but what’s the distance between all four mounting points (i.e, check that they are all square)? I keep coming back to the linkage kit because you’ve seen to have eliminated everything else. I’d be interested in seeing how a ring kit behaves with your setup.
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I measured and drilled those holes extremely accurately at 4.5 inches to the center of the bit. The holes had to be dead center in those upright rails I made.
The one question I have is, the rotation radius that is listed in the calibration is ~260mm. When it calculates, it’s calculating 270.0mm. If everything were perfect (and I could go draw this up) what should the rotation radius actually be?
The linkage kit is also what @bar suspected, I was going to reach out to him to see if he had enough spares to sell me a standalone ring setup.
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Someone can correct me if I’m wrong, but I think the rotational radius is just the amount that’s added to the chain length to account for the linkage kit. It should be, I believe, precisely equal to the distance from the end of the chain to the center of the router bit.
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Yup, that is what I have with the top-mount kit.
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