@carl I'm the chap who builds the MagBall Arms.

They are guaranteed to match within 50 microns. It looks like your set of six matched within 10 microns, which is pretty darned good!
You should split the difference tell your firmware that the length of your arms is the average: 288.065mm

Usually when people are printing the first layer, they can tolerate a variation of 200-300 microns over the surface of the bed, so ~5 microns is pretty small in comparison to that.

The most important features of the arms are that they are zero backlash and that the arms are the same length. Because they use ball-and-socket joints, it doesn't matter if the Delrin rod ends aren't perfectly co-linear with the carbon fiber.

You can demonstrate this for yourself by placing a sheet of paper on your print bed, commanding the tip of the nozzle to (x=0, y=0, z=0.1) and then trying to slide the paper. You should feel a small amount of friction. Now use your fingers to rotate any of the arms, and recheck the paper -- you'll notice that the friction is the same, because the length of the arms didn't change. This is what is important.

If you want to have fun, you can swap two arms, and you should still see that the friction on the paper is about the same.

I've experimented with a bunch of different jigs, and if I constrain things too much, then the accuracy of the lengths suffers. So that is why there is sometimes a slight lack of co-linearity between the Delrin and carbon fiber, however it doesn't affect the printing quality.

So, yes, it is within normal limits.

If you're upset by it due to aesthetic reasons, contact me directly, and I can build you a custom set of out 10mm CF tubing, and those always turn out perfectly straight.

I really try my best to make sure that every customer receives an excellent set of arms!

I'm working on designing an effector using a Titan Aero or Hemera with MagBalls. The six MagBalls each have 1.5kg of attraction, so with six of them there should be more than 9kg.

The trick is to have the extruder motor/body above the effector, and the nozzle stick down through it. This way if the nozzle bumps into something, it will have less leverage to knock the effector free. Minimize the vertical distance between the plane of the MagBalls and the tip of the nozzle.

Ultibots used to have an effector with the extruder hanging below it, and the hotend below that. It had problems because the nozzle was a long distance below the MagBalls.

@carl I'm the chap who builds the MagBall Arms.

They are guaranteed to match within 50 microns. It looks like your set of six matched within 10 microns, which is pretty darned good!
You should split the difference tell your firmware that the length of your arms is the average: 288.065mm

Usually when people are printing the first layer, they can tolerate a variation of 200-300 microns over the surface of the bed, so ~5 microns is pretty small in comparison to that.

The most important features of the arms are that they are zero backlash and that the arms are the same length. Because they use ball-and-socket joints, it doesn't matter if the Delrin rod ends aren't perfectly co-linear with the carbon fiber.

You can demonstrate this for yourself by placing a sheet of paper on your print bed, commanding the tip of the nozzle to (x=0, y=0, z=0.1) and then trying to slide the paper. You should feel a small amount of friction. Now use your fingers to rotate any of the arms, and recheck the paper -- you'll notice that the friction is the same, because the length of the arms didn't change. This is what is important.

If you want to have fun, you can swap two arms, and you should still see that the friction on the paper is about the same.

I've experimented with a bunch of different jigs, and if I constrain things too much, then the accuracy of the lengths suffers. So that is why there is sometimes a slight lack of co-linearity between the Delrin and carbon fiber, however it doesn't affect the printing quality.

So, yes, it is within normal limits.

If you're upset by it due to aesthetic reasons, contact me directly, and I can build you a custom set of out 10mm CF tubing, and those always turn out perfectly straight.

I really try my best to make sure that every customer receives an excellent set of arms!

A possible design for using direct belt-driven carriages for a Stewart platform would be to rotate the 20x40mm vertical extrusions by 90 degrees. Then for each tower, place one motor at the top and the other at the bottom. Place both limit switches at the top. One motor would be on one side, and could drive a carriage using three wheels, which would be carefully designed not to interfere with the other mirror image carriage on the other side of the vertical extrusion. If 20x40mm vertical extrusions make the spacing too tight, then you could use 40x80mm C-Beam extrusions from OpenBeams.

Simple, inexpensive, and quiet.

One relatively simple algorithm for slicing with a printer using a Stewart platform would be to use a "normal" slicer for everything, except to post-process the output. The algorithm would alter the slicing for the outside layers of the object. If the Stewart platform could tilt the effector by X degrees, then the algorithm would look for places where the outside layer was tilted by less than X degrees, and print that tilted, with the nozzle perpendicular to the surface at that location, thus helping minimize ridged/stepped layers.

I'm working on designing an effector using a Titan Aero or Hemera with MagBalls. The six MagBalls each have 1.5kg of attraction, so with six of them there should be more than 9kg.

The trick is to have the extruder motor/body above the effector, and the nozzle stick down through it. This way if the nozzle bumps into something, it will have less leverage to knock the effector free. Minimize the vertical distance between the plane of the MagBalls and the tip of the nozzle.

Ultibots used to have an effector with the extruder hanging below it, and the hotend below that. It had problems because the nozzle was a long distance below the MagBalls.

Is there anyone with a TEVO Little Monster on Maui?
I'd love to see one in operation (or if yours is broken, perhaps I can lend a hand)...

Mahalo!

--Haydn

I emailed back and forth with my machine shop in China, and they've created a new batch of ball studs.
These new ones are quite a bit more consistent than the previous ones (roughly 10x better).
Besides the ball being the same size and more spherical, I asked them to make sure they're a consistent height above the hexagonal base, and to make the threading match standard M3 more precisely (the threads used to be a little bit undersized).
Correcting the M3 threading should make these work even better with the M3 holes on the Duet Smart Effector.

Here are the results of measuring a random sample this morning:

–Haydn

I just picked 12 ball studs at random and measured them, and I'm afraid that my conclusion matches David Crocker's: I build the sets of arms so that they match within 0.05mm, and typically match within 0.03mm, however the ball studs have not been manufactured as accurately as the arms. The ball studs are manufactured in an machine shop in China, and I have been very thankful that this particular machine shop has given me very good service – I've dealt with others who were obviously not as good.

When I'm building the arms, I use G25 rated chrome plated ball bearings in my jigs, not the ball studs themselves.
This makes it easy to get the arms into and out of the jigs without varying the tension.

The 12 I just measured had the following diameters:
9.43
9.48
9.51
9.53
9.54 (three)
9.55 (two)
9.57
9.59 (two)
Nine were in the 0.07mm range from 9.53 to 9.59.

I also measured the distance from next to the M3 threaded stud to the tip of the ball on the same 12 ball studs. This isn't as precise as measuring the diameters, but I suspect this is more important. The results were more uniform:
12.46
12.59 (nine)
12.62 (two)
Nine were very close to 12.59, and two more were in the 0.04mm range from 12.59 to 12.62. There was only one outlier.

I expect that it is more important to have this distance consistent than the diameter.

So far, I've built almost a dozen printers using these and my experience has been that all of the printers have calibrated and printed well, so I'd guess that even though this isn't ideal, practically it doesn't cause problems. That said, for $12 extra, one can get two sets of ball studs and select the ones which match the best.

If I remember correctly from what other people have reported, the microswitches we use for the endstops have 0.05mm or so of variation from one triggering to the next, and many other parts probably have comparable variations.

Next time I'm placing an order for more ball studs, I'll ask them what would be the cost/tradeoffs to make these more uniform.
I'm sorry these aren't perfect and I feel a bit embarrassed!