JunkStrap, the bottom-up approach

After discarding RoStrap in 2015 it took me more than a year to get the parts sorted and finally use them again. Nevertheless I just had to utilize this remains to build a JunkStrap. After my experiences with XYZ-Head „Delta“ setup, where all axles combine their movement at the effector, and the XY-Head/Z-Bed setup of the Sumpod I choose to go for the latter. A Mendel90/Prusa i3-Style Y-Bed was still not trustworthy enough for me.

Again (like RoStrap before) this was meant to be a „how bad can you do it and still archive something“ experiment. If you want to see how to do things the proper way, read Mark Rehorst’s Instructable on the Son of MegaMax. 😉
(This build is already on it’s way to be discarded again as I am currently taking the time to write the build log while waiting for a kit from asia to arrive.)

Sorting the parts and rearranging some (after having visited the local waste-to-energy plant -featuring a really cool gantry crane- with the kids) I quickly realized that here I had to start with the Z-axis.

An M8 threaded rod was already there and now I wanted to use TinkerCAD for this project. I generated pulleys with droftarts OpenSCAD file and then imported them into my profile there. Here is the plain pulley for 608ZZ bearing (secured via M8 lock nut), this is a raw version of the base and this the final version of it. The motor mount also went through some iterations and in the end I had designed a spacer and a motor mount with idlers (bearings again) for burying the stepper in the base plate. The lift would drive some of the square tubing already o hand (via adapters). A bearing holder prevents the rods from falling over.

The wohle process took some time in multiple steps and rollbacks:

Synchronizing the rods and lifting weight was not a problem, the travel was enormous and the resolution good, but there are some flaws in this arrangement – more on that in a later blog post. First the Z-Axis lift in action:


A quick test to use a triagular frame with a single guide (from an old scanner) completely failed, so I went for a rectangular version and ordered two „MGN12H“ linear rail slides, which were comparably cheap (25,- € each) at that time. Probably not the „junkstrapping“ way to handle things, but now the next printer can run on rails salvaged from this one. ^^

Seemed to work good enough now…

But for those of you who didn’t already notice: this just screams „Z wobble!“
Not only the placement of drivetrain and guidance is extremely important, tight coupling in one direction and de-coupling in the other two also is. The fact that I used „proper rails“ and the reinforced, longer adaptor I installed later was not nearly enough…

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