Bearing blocks for Z screws

Hi,

After making very little progress over the summer I'm finally getting back to my rep-strap build. It's broadly based on a Prusa I3 with a few changes to suit the materials I have available. One of the changes I decided on was to take Mark's (digital dentist) advice about using a single motor to drive both Z lead screws. I'd never particularly liked the original design of taking the weight of the X axis directly on the stepper motors, so I'd been thinking about adding some thrust bearings to take the load. With the change to a single motor and belt drive I realised I'd not only need thrust bearings for the weight but also normal bearings to take the lateral load from the belt tension. To do that I decided to make up a bearing block that would hold a pair of radial and thrust bearings at each end, with a drive shaft through the middle to transmit the torque from the belt to the lead screws. I have one inch square aluminium bar which seems ideal to hold the bearings, but is a bit expensive. It seemed like a good idea to make two end caps out of the square bar that would hold the bearings, and then space them with 1" aluminium tube which is much cheaper and locally available. I should have thought about it a bit longer, because once I'd finished it I realised I'd saved myself all of about 1/2" of solid bar for a lot of extra work. I made the second one out of a single piece of solid and it was much easier and intrinsically more accurate as well.

Here's a photo of the block in the lathe after boring out the holes to take the bearings



I thought I might have to scrap the first one and make a second solid one, but after assembling them they both seem to run equally smoothly so I think I'm good to go. There's very little play in either direction, so I'm happy with how these turned out.



The bearings are quite small, 6mm inner diameter. The shaft was 1/4" so I turned down a 6mm section for the radial bearing, a slightly less than 6mm section to clear the thrust bearing (which also is a loose fit in the bearing block), and then a 5mm section for mounting the pulley/flexible coupler. I've laid out the bearings for one end of the block in the following photo, there's an identical set for the other end.



Now that these are done I can start thinking about how I'm going to make the vertical part of the frame and mount the lead screws and the smooth rods.

James.

Very nice! The Z axis assemblies in my machine have normal axial bearings at each end of the screws. The weight of the X axis really isn't that great and shouldn't wear the axial bearings out, especially considering how little the Z axis moves over the lifetime of the printer, but hey, there's no kill like overkill!

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I got the frame finished and the smooth rods mounted for the Z axis, and the gantry runs nice and smoothly on the rods. But when I mounted the bearing blocks and fitted the lead screws I couldn't get things lined up without binding. I made the mounts for the bearings adjustable in X & Y, so it should have just been a case of bringing the Z axis close to the bearings with the screws loose and carefully tightening everything up to lock the position in, but no such luck. My current theory is that the flexible couplers are non-concentric, forcing the lead screw off centre and causing it to bind. I've never been convinced by those couplers, it looks like they would introduce variability into the Z axis, so I'm going to try making up some solid replacements.

I may have been a bit quick to blame the flexible couplers - it was a lot more difficult to get the lead screws running smoothly than I expected. My first attempt at solid couples didn't work either, I couldn't get the 8mm and 5mm holes sufficiently concentric. In the end I turned down 1/2" of the lead screws to 5mm so that I could use a single bore straight through the coupler. Once I got the mounts for the bearing blocks lined up with those it ran smoothly enough that with the belt off it will fall under gravity. Today I got the mount for the Z motor done, along with the mount for the endstop. Much happy running of all three axes followed

Next up will be mounting the heat bed, and then I need to work out how I'm going to manage this mess of wires!

Your shaft coupler also compresses from the weight and inertia of the x-carrier.
I've put a 4.5mm steel ball ( airgun ammo ) between stepper shaft and lead screw to eliminate this unwanted degree of freedom.
-Olaf

I think the problem is that you're trying to use the motor's bearing to support the lead screw. If the lead screw has it's own bearing in its own mount, then the motor can be coupled with a flexible coupler and a little offset at the motor won't matter. Look at industrial positioners that use lead screws. The motor mount always has a bearing for that end of the screw.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Yes, when I was talking my design over with a machinist friend of mine, he was very sceptical about the whole layout. His recommendation was to build one-piece mounts that firmly hold both the smooth rods and lead screws at both ends, and I think that for sure I would go down that route if I was doing this again. Constraining both ends of the lead screws does raise the bar on the requirements for straightness and the accuracy of the machined ends, so would probably push the costs up a bit, but I guess that is the trade-off for higher quality.

First prints last night - feeling very pleased indeed. Needs some fine tuning on the extrusion, but it was surprisingly close right off the bat.

Now to join the merry throng of posters in the printing section, and start honing my hotend unblocking skills!