Prusa Z axis, so many problems yet one simple solution ! by MKSA

Last crawled date: 3 years ago

Note: This has worked perfectly for more than a year and is still valid. BUT, I wanted to go further and get rid of all the crappy linear bearings and poor leadscrew nuts. Done, I even managed to get rid of the vertical smooth rods, the lead screws can do the job (although TR10 would be more rigid, TR8 are acceptable) see here: https://www.thingiverse.com/thing:2594693
WARNING:
Don't be misled by the negative comments found here. This WORKS ! Unfortunately some people either misunderstand or simply lack the proper knowledge to understand, others are just expressing their anger for my critics of their poor, flawed "things". Just check what these people published and demonstrated. Some even created accounts, nothing published, just to spit their hatred. Amusing :)
PS: Beware, some are crooks, as they tried to call for funding or sell their wares I shot down ! See the first posts below for such piece of human garbage !
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The Prusa configuration is one of the most common, low cost, easy to build 3D printer, yet it has a few flaws and limitation but one that is not acceptable as it is easy to fix, two unsynchronized steppers driving a lead screw at each end of the X carriage. To make things worse, many kits are supplied with regular M5 or M8 threaded rod totally unsuitable for the task ( even often bent !).
You can find a myriad of poor attempts to get rid of the wobble and canted X carriage. Plastic braces to try to constrain the lead screw are just reducing it, not eliminating it.
Auto bed leveling gimmicks are just a waste, often introducing more problems ! And it adds weight (except the IR one from DC42 about the only decent one for the job) ! Note that auto bed leveling almost a must in a non Cartesian machine, is here a poor attempt at compensating a poorly designed, poorly assembled machine with dirt cheap poor quality components ! Better spend your time and money for real solutions.
Of course, what I propose is not the first set up making use of one motor, two screws with a closed belt and pulleys to drive them but is the simplest I have seen for a Prusa while allowing to tension the belt despite no idler pulley is used (can't make it simpler :) !)
It is designed for the Geetech Prusa Aluminum but can be adapted to many similar design. I arranged for the belt to be 760mm which is quite easy to find. You can make your own. I did it but it is a bit tricky to explain and do to get an almost professional splice.Here are links giving acceptable results to do it:http://www.thingiverse.com/thing:1814419http://www.thingiverse.com/thing:2162222
Yes, you have to buy two TR8 either 8mm or 2mm lead with nuts, three pulleys but you will save one motor, two couplings (most are so bad you can throw them away) and plenty of headaches and time ! Refuse them if they come bent !
Initially I used ball bearings but switched to Igus bushings I printed (see my other things).
You could even use bronze or delrin bushings reused from old motor, printers ... The ID is 8, OD 12, h 6.. A very common size but for the h, not a big deal if you can use a hacksaw and a file !
Or even print them with PLA, ABS, PETG but how long will they last ?
Although I do not recommend it, if you plan to use ball bearings (needs to make new plate to fit them), keep in mind that as the leadscrew is the moving part it has to be "loctited" to the bearing inner race. Make sure to provide clearance where required too.
The left and right plates holding the bushings are identical and snap in place on top of the Geeetech top plate. Make sure the smooth rod is about 4 - 5 mm higher to go through the plate with a snug fit.
The plates could be secured by screws or even glued to the aluminum plate underneath but not required in my case as the 8mm guide rods and frame hold them thigh. In addition the belt tension takes up minute play.
The bushing is 1mm above the plate and the pulley rest on it. Igus plastic is fine against Al.
To adjust the belt tension, you just slide the motor holder and secure it with two screws (here I used Nylon M4 but any M4 or similar self tapping screws will do). Just a moderate tension is enough to remove all slack, it doesn't need to be as tight as for the X or Y axis.
Adjust the motor pulley height for proper belt alignment.
I use a 8mm lead screw and three 36 teeth pulleys giving 400 microsteps/mm, 0.04mm per full step, a number I like. As is, the required torque to go up is about 10 N.cm. No big deal for the Nema17 40mm.
I have used it for months now. It is proven solution. It should be easy to adapt to other "metal" Prusa variations. I don't recommend it for "acrylic" or other flimsy frames, these may bend with the belt tension unless reinforced and anyway are junks not worth improving. Sell or give them to people you don't like :)
No wobble, vibration (all my bearings are of my own BTW) and no need to level the bed anymore unless I make change to the bed for ex.
Now I can print decent parts and why not for a more professional design than the Prusa.
I added a version of motor bracket to be able to mount a PM 48 series stepper one can find in many laser printers. You may need an 3mm ID 5mm OD adapter. Torque is plenty enough for a regular Prusa provided there is no binding or the X carriage doesn't weight like a dead donkey.
Note that I reinforced the frame as you can see in the picture. Just two Aluminum profiles, bent plate and bolts, attached to the vertical frame and the M10 threaded rods. Many ways to do it, here are some decent solutions. Must be rigid, avoid flimsy plastic parts, makes them beefy. Then you can bolt the whole printer to a heavy thick (40mm) flat piece of old kitchen table top or anything like that.
I even simplified the system by removing the right rod: https://www.thingiverse.com/thing:2504861.