I’m kinda surprised one of the plastic bearings has slop in it already after a month and a half of owning the Mini. One corner of my bed wiggles now.
Guess I should order several to keep on hand.
I have a TAZ, but I imagine the bed is mounted similarly on the Mini. The plastic bearings Lulzbot uses are already slightly oversized, so they slop a little by nature. I have found that I can greatly improve the stability of my Y axis by loosening the bearing mount bolts and rotating the bearings against the rails until they are snug, but not binding. Proceed as follows…
Looking from the front of the machine complete in order:
Loosen all of the bearing mounting bolts.
Push the bed as far right as it will go on the loosened bolts.
Snug (not tight) the Back Left and Front Left bearing bolts
Twist the Back Right bearing Counter Clockwise until it is snug against the rail. Snug the bolts.
Twist the Front Right bearing Clockwise until it is snug against the rail. Snug the bolts.
Loosen the Back Left and Front Left bearing bolts.
Twist the Back Left bearing Clockwise until it is snug against the rail. Snug the bolts.
Twist the Front Left bearing Counter Clockwise until it is snug against the rail. Snug the bolts.
Slide the bed along the full distance of the rails. Make sure it moves smoothly with no binding.
Tighten the bearing bolts on one rail at a time. Be sure not to over tighten and pull out the threaded inserts.
Again check that the bed moves smoothly along the rails.
Tweak as necessary to get the bed moving smoothly.
PRINT!!!
Mine has lots of slop in X and Y axises. This is pretty standard for those bearings, it was like that out of the box, but it still prints damn well.
I have LME8UU bearings on the way to replace all 8 of the plastic ones. They are cheap at least.
(EDIT…Note, the LM8UU standard bearings are 15mm OD, you need the 16mm OD LME8UU bearings for a direct drop in)
Be sure you also replace your rails with ANSI 1050, HRC 62+/-2 Induction Hardened Steel Shaft, as those linears will shred the softer steel that Lulzbot uses with those plastic bearings. Also make sure you are buying high quality linears, as some of the cheaper Chinese versions are not well made and stick quite a bit.
Here’s what I bought from Amazon to upgrade my TAZ machine.
WV 10X0.5M Ametric® Metric 10 mm Diameter, 500 mm Long, Material ANSI 1050, HRC 62+/-2 Induction Hardened, Solid Steel Linear Shafting, ISO Tollerance h6,(Ametric® 1-003) - from American Metric Corp.
4pcs LM10UU Linear Bearing Ball Bushing for 3D Printer, MendelMax, Prusa, Reprap, CNC - from Easy RepRap
I am sticking with the 8mm shafts that the mini comes with…If I shred them, so be it. I will replace them when they look or feel worn. I am really not too worried about shredding them as the forces on the rods are fairly low…but we will see.
BTW, the standard LM8UU (with 15mm OD) are actually too small and are not a drop in replacement. it should be the LME8UU with the 16mmOD that needs to be ordered. They are a little more expensive, but still not that bad.
The TAZ uses 10mm rods. I’m guessing that the Mini must use 8mm then. 
Confirmed that the Mini uses 8mm rods with my calipers.
Finally got around to swapping out the plastic bushings for the LME8UU.
Did the X and Y axis (8 total) , and man what a difference in stiffness. before I could move both axis’s by hand a decent amount. I actually measured .014" of side to side travel of the Y axis plate. Now with the actual bearings I cannot feel any movement at all.
I will re-measure it after I am done printing my calibration cubes. did 1/2" cubes in ABS in standard, High speed, and High Detail before the swap, and will do all 3 after the swap to compare.
1013, how your bushings are doing? I am thinking whether I should go for plastic or metal.
Thank you!