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| PeterE builds a 3d Printer |
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| PeterE:
Will do. |
| nrml:
The problem I found with my kit was that the combination of the poor tolerances of cheap LM8UU bearings and the use of just 3 bearings for the bed plate mean that the bed is not well supported on the smooth rods. I bought replacement bearings and these improved things slightly, but even a fraction of millimetre increase in tolerance translates to a significant wobble once the bed is fully assembled. There was very little slop when running the replacement bearing by itself on the rod, but it can rock side to side a bit after bed assembly. The X and Z axis haven't been a problem because the use of two closely spaced bearings on each rod provides a much greater area of contact. If you haven't already considered it, using two sets of bearings together or the long version of the linear bearings on the Y axis might save you a lot of frustration . Again, if you haven't already considered it, using 10mm smooth rods and trapezoidal leadscrews (dirt cheap on ebay) would be a big improvement over the average prusa i3 kit. |
| PeterE:
nrml, Hmmm, see what you mean. The bed on the Y motion only bears on three points and the lonely point is of course on one rail. But I think I have seen someone make another type of bearing holder for the lonley side which is longer and have two bearings. That will make it work more like on the X and Z motions and then probably be stiffer and less prone to wobble. I must surf around and see where I saw that. I'll be back. /Peter |
| nrml:
Within the bearing, the area of contact between the balls and the rod is miniscule. Poor quality bearings and unevenly ground areas on the rods make the contact area even less. I think the 4 bearing kit supplier (if I remember correctly) is a spanish company selling steel framed Prusa i3 kits. |
| PeterE:
Yep, it's a Spanish Co. I got all the bearings and the toothed belts and gears in one go at a fairly good price. Cannot complain about that. When assembling I have heard that the linear bearings are sensitive to forces trying to flatten them and that may be a reason for them to get out of order. I have been thinking about using a piece of round bar stuck into the bearing while assembling. That should limit the ovaling forces. But yet again, I might very well make a couple of aluminium bearing holders instead. That will totally avoid risk for out of round. If I do that I will also look into the possibility to have two LM8 bearings close together instead of one lonely at one side. That should also stiffen things up. But we will see, I'll start with the basic originals to begin with. /Peter |
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