Alignment issues

Hi,
I have got my Reprap Mendel MAX up and runing. It has bottom mounted motors and uses Ramps1.4 with marlin firmware.

Now that I have some experience I am trying to tune my machine to improve my print quality. I recently changed out my printed z couplers for flexible aluminum couplers hoping that would improve quality.

What I am noticing is that my layers are shifting on my prints at a frequency that basically matches the pitch of the lead screws. In other words I have essentially a silhouette that for all intents and purposes mirrors the 8mm pitch on the lead screw. It appears it is following the line of the screw thread if that makes any sense?

Does anyone know how I can correct this, as I am not really sure what to do here.

Thanks for any help

Hi Simono, people call this problem Z wobble. And there will be 100's of cases on the internet (thingiverse, g-groups etc.)
I suggest googling 'Z wobble, Mendelmax' through the mentioned sites and see what solutions others have come up with. I have dealt with this in the past by moving to smaller threaded rods, but you probably have Acme rods and may want to continue using them.

Good luck!

it sounds like your z axis motion is wobbly. normally this can be dampened out by the bushings along the rods that guide z axis. you may need to replace those, or increase stiffness to those bushings, or use the old printed couplers you had before. also be sure the motor alignment to the z threaded rods are aligned and the motor is not forced to compensate when the threaded rods turn. if the threaded rods on your z axis are acme and are precision, then the issue may be from your motor alignment. best of luck!

If you are using 5/16 rods, and 5mm-8mm flex couplings, you now know where the problem is. Those 8mm couplings have to be tightened pretty hard to clamp onto 5/16, and it makes for an off-center fit. As a temporary fix, you can try adding a turn or two of aluminum, or copper tape, or in a pinch, kapton to the threaded rod, and then clamp it only lightly.

Also, if your rod is captive at the other end, for example in a 608 roller bearing, try removing the bearing, so that the rod may orbit freely.

All of this demonstrates how little a stiff frame contributes to good prints.

If you want to try one other thing before taking everything apart, find the outermost travel of each coupling, and place those 180° from each other so that they cancel each other at the extremes. This may minimize the effect of wobble, but not totally eliminate it.

I replaced one of the aluminum shaft couplers with the original printed plastic one and it is much better now.

Kinda annoying as I bought the aluminum couplers thinking it would improve things...