Rods and Bearings: how smooth is smooth?

Hey all,
So I am slowly putting together a Prusa i3. I bought a set of rods off eBay for about $55 and a set of 10 bearings for something like $8 from China. I put the bearings in the plastic parts and tried these on the smooth rods. Some parts are -pretty- smooth and others have a little bit of a drag on certain areas of a given rod. I tried Teflon lube on the bearings and it seemed to help some, but I can still feel a slight dragginess in some spots with some parts. I have no idea if it is the bearings or the rods.

So my question is how smooth should the action be? Should the bearings roll down the rod without me touching? Is a -little- bit of drag ok? How much resistance, if any should there be? Am I worrying for nothing?

Thanks!

Id get out the acetone, Wash the bearings and rods, put the slightest amount of lubricant and see if it improves.

It might have nothing to do with smooth. Rods are centerless ground in long lengths and they could be varying in diameter or in cross section. It is common for rounds to be slightly triangular in cross section. You can't measure this with a 2 point tool like a caliper or micrometer. It can be measured with a dial indicator over the rod sitting in a V block.

Look at all the messages in the forum with issues all related to the bearings and rods. Cheap is only cheap if your time is worth nothing and you have an endless supply of patience. Look at the pile of bad parts you have printed and add that to the cost of the printer. Is it still cheap?

It may be warped prints. This can happen on the X-Carriage or after you attach the Y bearings to the frog. If it's your Y, make sure your linear rods are parallel and everything is square. Try each bearing separate/alone on the rods. If they are smooth, it's your part. Heating the part or a Dremel can fix this. When ordering bearings from China, the plastic ends can bind. Also, taper the ends of the linear rod so it does not push any bb's out the bearing.

Thank You All for the responses!

Learning by doing...

You all mostly were correct, it seems. It looks like I have a few things going on after further fiddling. I took the rods (lightly oiled with 3-in-1, recommended by the rod seller from ebay, bluscreen), and vertically dropped them through the bearings snapped in the parts one at a time. I found that a couple of the bearings are sticky. I also found a couple of the plastic parts are a partial hair off linearly when two bearings are lined up.

I found only one rod to be a partial hair over sized, which ebay seller bluscreen will be replacing for me. Overall I am quite pleased with the set provided by bluscreen.

I recommend his store here (I hope this is not a forum fauxpas):
[stores.ebay.com]

Oh, hey, @KingRahl, how do recommend heating the parts to get those suckers aligned? Dremel seems a bit hasty since the difference is just a few microns or so...

I've only done it once and don't recommend it necessarily. But with your bearings in place and part in place on linear rods, heat over stove for a bit until part moves smoothly. Keep moving part up and down linear rods while it cools. If all goes well, part will conform to new shape. I had slight warping to part when doing this so use caution and observe proper safety when working with open flames. About 8-10 inches above medium flame may be good.

So I figured out that two rods were slightly out of tolerance in a few places with the help of calipers, and the aforementioned seller is sending replacements, along with 4 LM8UUs that I need for the 'trouble.' Highly recommended seller. Thanks again for the ideas, everyone.

@KingRahl: I think I'll try that, but with a carefully placed heat gun for more careful control. Hope I don't end up needing a new part, but hey, if so, *I* did that.

Edited 1 time(s). Last edit at 02/07/2014 01:23AM by drastic00.