I am just now finishing printing my parts and tested the fit of a few of them on the rods of my other printer and I found the rod holders to be rather loose. I haven’t measured anything yet, as I had to leave for work. Is this common? Is there something I’m not realizing about the assembly?
The X rods, Y rods, or Z rods? Do the bearings have play or just the 3D parts where they mount?
The Z rods will be held in place against the motors by gravity. Mine don’t have any lateral play, but I can slide them up and down easily.
The X rods on mine are sort of sticky, but I can easily move them. I was going to put a small piece of tape to increase their size, but I haven’t had any problems, really. On the side with the bearing idler, the screw that holds the bearing will tighten that a little (but don’t overtighten 
).
The Y rods are loose on mine. I put a bunch of hot glue on each end to keep it from moving.
They should all have very little force on them, so it’s just to absorb any vibrations. Even the X axis rods aren’t fighting the belt tension, it’s the lateral movement of the Z rods that fight the tension.
If the bearings are moving laterally, then I don’t think there’s any way to fix that, short of replacing the rods and/or bearings.
It’s the Z and Y rod holders. I have ways of securing them, I was mostly curious if this was common. I’m printing my parts on two different printers, one of which is not amazing. I’m glad to hear that this may be by design, rather than my own user error.
I’m thinking of turning a groove in the rods under each holder and printing a sort of snap ring to fit in it. After positioning the rods and holding them in place, I will use hot air to soften the PLA and then press them together. Might be over-complicating things, but I’ve been wanting to try the process anyway.
Well, I can’t eliminate user error :). But I can tell you having them loose isn’t a big deal, except that the Y axis did come loose on me once, then I globbed on some glue, and it’s been fine.
I would recommend trying it without those steps first, so you can judge how much effort to put into it. Especially the Z axis doesn’t need it. I do think it’s possible the X axis could become slightly shorter, if the Z rods bent in the middle, but having them rigid causes other problems too, if they are not square. The other nice thing about waiting is you can tell if it made any difference (and share).
I’ll let you know. I was really considering just using zip ties as shaft collars just inside the rod clamps. I don’t anticipate much of an issue, it’s not like this machine needs to pass a certification at ±0.001".
Don’t you want your fidget spinners to be balanced?
If you have any issue use a drop of superglue. It is temporary on the rods. For me the rods are tight enough to stay and if a bearing starts to go bad it will move a rod, that is when I know it is time to replace 1. I actually just replaced 2 bearings on the first MP3DP, so they last a good while since it is almost always printing.
Oh balanced spinners are my main priority. But I usually test my spinners on a prop balancer, then file, hand scrape, and lap down to a mirror finish before I retest and send them off in an argon filled positive pressure vessel to be certified by NIST. 
And Ryan, that’s good to know. My rod holders aren’t tight enough to stay put without help, so maybe I’ll just do some super glue. I like being able to take something apart without worrying about adhesive bonds, so we’ll see. I am not so concerned about $.75 bearings because replacement cost isn’t super prohibitive. With more expensive bearings though, I do have a habit of letting them go until they sound like a loose bag of marbles and have visible slop.