Z Motors and Threaded Rods

I am in the process of assembling the Z axis on a linear prusa with Z stabilizers. The instructions call for two 11 5/8" threaded rods that attach to the two Z motors. I was wondering if I can I use 12" threaded rods instead of 11 5/8“ Rods? I know the Z axis goes up and down, but I am not sure if it's supposed to be free once it raises or if it was of if it stayed in the 608 bearing of the 2 Z Stabilizers.

Around here, you will learn quick. Don't put the 608s in the Z stabilizer. Leave the bearing out so the bottom of the threaded rod is unrestrained and can wobble freely.

As to the length of the Z rod, a little longer won't hurt anything except your threaded rod may be so long that it touches the table your reprap sits on, which you don't want. If anything you can make it shorter if you want, since we don't want the bottom ends locked into bearings. Not really sure why you'd want it longer.

Edited 1 time(s). Last edit at 01/13/2013 03:06PM by xclusive585.

Roger that. Take the 608 bearings out and as long as the threaded rods aren't long enough to touch the table, they should be fine. Oh and by the way, I ordered all my rods off eBay and they all came a littler longer than called for. I've had to cut a few already, just wondered if i needed to these is all. Thanks for the advice xclusive585.

The threaded rod on a Prusa should be around 9" long.

I don't think I have seen a design yet that properly makes use of flex couplings, and roller bearings, so leaving the rods free will minimize the Z-wobble, assuming the rods are perfectly straight.

If you do ever want a tighter z-axis, you would need a stack something like this:

NEMA17 motor M5 Shaft - M5/M8 Aluminum Flex Coupling - M8 Nut - 608 Roller Bearing (or a thrust bearing) - M8 Nut - M8 threaded rod - M8 Nut - 608 Roller Bearing (or a thrust bearing) - M8 Nut

In this configuration, the motor is fixed in place, the rod is fixed in place, and the flex coupling allows the two to interface without introducing wobble.

Many of us using SAE 5/16" threaded rods, incorrectly assume that these are a good fit for 608 bearings, but they are not. The fit of the bearing on the rod should be very snug, in order for all of this to work well.

Even with all of this, if the frame lack a certain stiffness, YMMV.

M8 threaded rods aren't a good fit in a 608 bearings either. Proper lead screws have an un-threaded section that fits the bearing.

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Wait, should I or shouldn't I be using the 608 bearings?

I also have another question. What's the proper way to install the 8mm flexible couplings? Am I supposed to stop the threaded rod short of the spiral cut or is it suppose to touch the Z motor shaft?

What we are advising is that you NOT use 608 bearings, because they cause MORE, not LESS Z wobble.

As Nophead has pointed out, even the M8 threaded rod is not a perfect fit, so even if you had the rod captive at both ends, you probably would have a small amount of acentricity (is that even a word). It probably wouldn't bother me, but Nophead strives for better results, so you are getting slightly different but compatible advice from each of us.