This is my Z axis which maybe takes a bit of explaining. I didn't fancy a cantilever design - they just don't "feel" right, especially with my bed weighing around 7kg. So, I elected to have 3 screws to take the weight and provide lift. I also don't believe that lead screws should be use for guidance. Their job in this application is to provide lift only and I have separate linear guides to keep the bed in place and prevent any wobble in the XY plane. I also intend to use the lead screws for bed levelling. Initially, this will be manual but a future upgrade will be via software when DC42 gets around to writing the code. At that point, I'll change the design to have separate motors for each screw but for now, I'm using a single motor with one continuous looped belt. With that in mind, I elected to use 1mm pitch screws. I really don't get this 4mm pitch lead screw thing that people are advocating for the Z axis. In my view, a course pitch is solely to give a greater linear motion for a given angular motion, so effectively speed but at the expense of lower resolution. I can't think why I'd want a fast, low resolution Z axis.
Anyway, the philosophy behind this design is that the screws will provide lift, and the linear guides will do the guiding, so it's important that the screws don't impart any sideways constraints on the guides. That is to say that they are fully "floating".
Here is a bearing block to take the bottom of the screw.
You'll see that it has a blind hole. This is for a thrust bearing to take the weight. Here is the same block with the lower part of the bearing and the ball race in place. The top part of the bearing goes in next and the end of the rod will sit on that.
Next comes an 8mm flange bearing. So I have a thrust bearing taking the vertical load and a separate ball bearing taking the sideways load of the belt tension.
That takes care of the bottom of the screws. This is how I set about fixing the screw nuts to the bed. Firstly, having printed all the nut housings, I fitted and "O" ring to each nut. These are all the parts.
The reason for the "O" ring becomes clear in the next picture. I fixed the brass nut to the housing with 3mm bolts and Nyloc nuts such that the bolts are just starting to compress the "O" ring. This will allow a small amount of flexibility for levelling without impacting on the linear guides.
This next picture shows the screws in place and you can see the linear guides at either side towards the front. These are basically small Open Builds "Gantry plates" fitted with Delrin wheels which ride on 2020 V slot. A "rod end" ball joint is fixed to each gantry plate and this is how they connect to the bed rails.
This next picture shows the connection from the underside. A bolt goes through the rod end ball joint, then a couple of washers, then into a "Tee" nut in the extrusion. Again, this allows some flexibility for bed levelling but constrains the bed from any XY movement or wobble (well almost)
I reality, I found that it was possible to induce some sideways wobble by gripping the rear of the bed and moving it side to side. It was caused by the 2020 vertical guides twisting about their single bolt fixings. If I was starting again, I would use 2040 or even 2080 for these guides and fix them with 2 bolts at each cross member. In this instance, a quicker and cheaper fix was to add a third guide at the rear of the bed and it is now rock solid from a "wobble" point of view.
This next picture shows the single ZX motor mount.
It looks like a forest of bolts. Basically the Nema 17 motor is slung underneath (4 bolts) a separate carriage with slotted holes (2 more bolts) so that it can be moved back and forth to set the belt tension. The belt goes around two OpenBuild idlers which are raised on bosses (2 more bolts) which form parts of the mount which is bolted to the frame (6 bolts).
Finally, here is a picture of the entire Z axis. Note that there is no connection at the top of the screws. The screw nuts are fixed to the bed frame. The bottom of the screws sit on thrust bearings and are constrained from any sideways movement caused by the belt tension. Other than that, the screws are fully floating and the linear guides take care of any "wobble".
Edited 1 time(s). Last edit at 07/21/2016 02:53PM by deckingman.