K8200 Z axis rework - cable driven by Brandano

Last crawled date: 3 years ago

This is an evolution of my Z axis parallel drive, taking advantage of the cable to isolate the oscillations caused by a bent or misaligned threaded rod that normally show up as horizontal ridges in the print.
The Z axis motor is moved to the top of the frame, with the threaded rod placed horizontally and pulling on the cable sideways. To make space for it the top element of the frame is repositioned flush with the top of the columns, which means that two of the metal corner brackets have to be removed. I have added printed brackets that fit on both sides of the frames and also double as supports for the small pulleys, that should restore the lost rigidity.
The cable I used is a steel clothesline covered by a 4mm plastic sheath, and the parts are dimensioned to get a good grip on it, but will need to be reworked for a different diameter. The cable also conflicts with the stock bolts that fix the LM10UU bearings to aluminium plate on the left of the carriage, I have replaced mine with flatter ones that barely clear the cable, but it would probably be better to use countersunk bolts.
I have re-designed the pulleys as well to have a reasonable overhang, but am currently using the ones from my earlier mod, so can't guarantee the new ones will work fine. They will probably require some adjustment.
The SCAD file makes use of some features that are only present in the latest development snapshots, specifically the rotation angle in the rotate_extrude() function, so thingiverse will not render it properly in its previews.
The part that holds the nut is built to house two of them, to handle the twisting load caused by the weight of the z axis. Currently it causes a fair deal of friction, and I had to increase the driver current to the z axis motor. I will probably rework it for a deeper and looser fit over the nut. Nylon M8 nuts would probably work better, I am still experimenting with this.