My PrintrBot Simple Metal has long suffered the dreaded Z WOBBLE ! I have seen others postings of their solutions to the problem, but they all seem to miss the same contributing factor that PrintrBot themselves missed from the beginning, the free floating end of the Z axis lead screw.
I decided if I was going to fix the problem of wobble, I also wanted to upgrade my printer from the 1/4" imperial lead screw with a single thread, to an 8mm trapezoidal 4 thread (4 start) lead screw and reduce the rotation my Z axis stepper made per unit of vertical movement.
I am a fan of embedded nuts for mounting hardware, and my lead screw guide nut mount includes them. For those who do not like embedded nuts, I would consider them in this application, because if threaded plastic fails here your entire print head will come crashing into the print bed. The guide nut mounting holes can accept up to 3.5mm hardware, but I was not able to get the nuts to fit due to the collar size of the guide nut, so I used 2.5mm hardware to secure the mount and guide nut to one another.
Each time I have replaced my guide nut in the past I have had difficulty getting it aligned due to PrintrBot’s placement of the mounting holes forcing a small amount of the Delrin nut into the face of the stepper motors center alignment guide extrusion. This left the Delrin nut with a crush mark (noted with a red arrow in the pictures) , put unnecessary strain on the aluminum threads of the stepper motor and applied tilting pressure (torsion) on the Delrin nut likely being part of the cause for the Z wobble. For this reason I did not want to use PrintrBot's own new 8mm 4 start Delrin guide nuts. Printrbot also once again only faced half of the Z wobble problem with their 8mm lead screw upgrade, leaving the end of the lead screw loose once again.
Many 3D printers include a lead screw bearing or guide, and PrintrBot's reason for not having one has always left me wondering. So while I was considering an upgrade I decided to solve this problem as well, and have included a Z axis lead screw bearing.
I could not be happier with the results
I have included, as always, source STEP files for those who may want to customize the parts. These can be opened with a variety of CAD packages, such as AutoDesk's FREE to use Fusion 360.
Enjoy, I hope this design can make your PrintrBot Simple Metal's Z Wobble OBEY like it has for me.
I used Silver (grey) ABS for the Z nut mount, as this part sits directly against the Y axis stepper motors face, which does get hot. ABS has a high TG glass transition temperature (105ºC) so is a good material for parts that otherwise could be deformed by heat. This part will not require any supports, but you will need to pause your printer to install your nuts at the end of the last layer before they get printed over.
I used Taulman's Nylon 645 in Black (actually a very dark purple) for my Z axis bearing mount, as it is incredibly strong, not to be confused with rigid, and offers just enough flex to dampen vibrations this axis may produce. This nylon has a low TG glass transition so is not suitable for hot places. The part itself could also be printed in ABS, PLA or any other material you use for printing. I have been very happy with my nylon 645 part. The nylon prints fairly hot, 255ºC, and it produces much more stringing than ABS or PLA, which is hard to remove due to nylon's flexibility and strength. I used supports on this part for the bearing opening as the bearing will cover the rougher surface left by the supports.
After your parts are printed and assembled, disassemble your Simple Metal's Y axis arm and Z axis lead screw. Install the new Z nut mount leaving the stepper motor bolts somewhat loose to finger tight. Run your 8mm lead screw through the nut and couple mount it to your Z axis stepper motor with a 5mm to 8mm coupling, again leaving any bolts just finger tight to snug. Using your fingers to turn the lead screw, drop your Y axis arm as close to the Z axis stepper motor and coupling as you can get it without running your hot-end into your print bed. Now install your bearing mount on your Z axis guide rods while you align your lead screw through the guide bearing. Leaving your bolts loose helps everything get aligned, and should now be ready to be tightened up with all your parts in place.
This is the 8mm 4 start lead screw I used here. It was shipped in a small PVC tube and arrived very straight. I also ordered a few extra lead nuts, and found some fit better than others, but I think all of them will work fine.
If you are upgrading to an 8mm lead screw from an imperial 1/4" lead screw, you will need to change your steps per millimeter (steps/mm) settings for your printer. Enter the following command lines in your control software.
Your Z stepper motor will turn much slower now as it only turns 400 steps instead of 2020 steps for the same amount of travel.
I would put a little bit of lube on your lead screw and try it out.
These two fairly small parts greatly improved the surface quality of my prints.