Hello. I've been using ping pong balls to test my eggbot and have had some success. No matter the shape I use (egg, sphere, ornament) I'm seeing the same skewing. I've reset and tested everything that I can think of - even rebuilt the eggbot just in case I didn't seat something correctly. I get the same results.



I thought it might be the balls but I tried with eggs also (changing the motor position down to lowest position) with similar results.

Breaking down the pen arm, I think it could be one of two things: either the motor is canted slightly or the pen arm backer isn't flush. Since the shaft collar is glued into the backer piece, it might not be seated correctly but it's really hard to tell.

Any ideas on what might be causing this or how to confirm my hypotheses?

Thanks,

Don

Looks like the pen arm motor shaft is not perpendicular to the ball's axis of rotation. For instance, some grit is between the motor face and the chasis? Or an uneven build up of the "tinning" on that part of the chassis's heatsink surface which interfaces with the motor face?

If you have a small square (as in a machinist's square), you could start by seeing if the pen arm motor shaft is square to the chassis frame. If it is square, then with a spacer see if the proximal pen arm seems to be parallel to the chassis frame and, more importantly appears to be the same distance from the chassis at either (rotational) ends of its sweep. (Here, a set of feeler guages might come in handy.)

Anyhow, those are some things which might confirm your suspicion or otherwise help narrow things down.

Dan

Hi Don,
I'm not sure exactly what kind of "skewing" you're seeing, but I can make several suggestions based on what I see in the photo.

* As Dan was saying, it is important that the motor sits flush against the chassis, so that its shaft is absolutely perpendicular to the chassis. Make sure that it is.

* It is not important whether the shaft collar is aligned very well within the pen arm backer. Just make sure that the big thumbscrew holds the proximal pen arm to it securely.

* Hold the pen much lower, closer to its tip. Compensate by lowering the pen arm as well. This will improve the strength with which the pen tip moves, reducing skew due to the pen flexing.

* Once you've done that, adjust the position of pen arm backer, using the hex wrench, so that the pen points dead straight towards the egg-axis. (Right now, this alignment is pretty far off.) That will minimize the amount of frictional resistance to rotation of the "egg."

* It looks you may need to move the headstock closer to the center by about 1 mm on each side, to make the pen axis align up with the equator of the object.

* Make sure that both the headstock and tailstock are absolutely vertical, perpendicular to the chassis side walls.

* Hard to be sure with this angle, but I suspect that you need to move the tailstock to compress the spring more. You may need to compress it nearly all the way, so that the O-ring nearly touches one of the bushings. That will improve the coupling between the ball and the egg-motor.

* You may want to try one of the tricks from the appendix to the assembly guide that reduce wobble in the plunger.

* If the motors feel week, you may want to adjust their current setpoint, using the technique listed on our this page: http://wiki.evilmadscience.com/Making_your_first_plot

* And, the most important one: Make sure that you have solid coupling between the ball and the egg-motor. Before you try to plot, try to turn the object itself by hand. If it can rotate and "roll" out, it will do so while you plot. For better precision, I'd suggest holding the ball by the other end, too.

thanks for the suggestions. The skewing that I'm referring to is that the pen is not travelling perpendicular to motor 1 shaft (the pen stepper.) The picture is showing (as close as I can show) that when I push the pen back and forth over the sphere, it's at an angle. No power, just moving the pen back and forth and I'm getting a line at an angle. sphere is centered with respect to the pen motor stepper and the egg cups. I used a larger wooden sphere to show the effect in the picture. It does the same angle with a ping pong ball and egg also.

any plotted images are skewed at that 1-2 degree angle.

Thanks Windell. I experimented with your suggestions and found that moving the headstock closer to the center by very small increments changed the angle that the pen travelled over the ping pong ball. i found the sweet spot and now my text looks correct. I think I understand why but it completely blows my mind that such a tiny move can make such a difference. I guess I should have paid attention in geometry class?

Cheers,

Don