K8200 Z axis rework - cable driven

thingiverse

This is an evolution of my Z axis parallel drive, taking advantage of the cable to isolate oscillations caused by a bent or misaligned threaded rod, which normally appear as horizontal ridges in the print. The Z axis motor has been 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 was repositioned flush with the top of the columns, requiring two metal corner brackets to be removed. I added printed brackets that fit on both sides of the frames and also serve as supports for the small pulleys, which should restore lost rigidity. The cable used is a steel clothesline covered by a 4mm plastic sheath, and the parts were dimensioned to get a good grip on it, but will need to be reworked for a different diameter. The cable conflicts with stock bolts that fix LM10UU bearings to an aluminum plate on the left of the carriage, so I replaced mine with flatter ones that barely clear the cable, but using countersunk bolts would probably be better. I redesigned the pulleys to have a reasonable overhang, but am currently using the ones from my earlier mod, so cannot guarantee the new ones will work fine; they will likely require some adjustment. The SCAD file utilizes features 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 was 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 likely work better; I am still experimenting with this. Updates: 09-03-2020: Made major updates all around. Moved the Z axis motor and endstop to the frame's right side. Added carriage versions for T8X8 leadscrew and anti-backlash nut, as well as BSF12 ballscrews. Changed the cable grip system to use less hardware and be less bulky, and moved the tensioning to the left side of the Z axis horizontal extrusion. Added a leveling system using long M4 bolts on either side. Reworked the smooth rods to have one 10mm linear guide per side, so that the right-hand side 8mm rod can be moved to the top of the frame for anti-rotation. Reworked the leadscrew support brackets to use the ball-bearing portion of KP08 pillow bearings, discarding their zinc frame. The same bracket can also be used with KP000 for ballscrews, but I will probably add brackets for the purpose-made pillow blocks for it. I have not posted the new STLs, but it makes more sense to download the OpenSCAD file and play with it. 14-03-2020: Moved the smooth rod to the front of the frame, closer to the cable, to further reduce deflections caused by a bent threaded rod and to reduce friction. Reworked the carriage to use an LM8UU linear bearing; currently, I am using a printed PLA one. The plan is to eliminate the two aluminum plates on the left of the Z axis and move one of the M10 smooth rods to the right of the printer. This would free up the M8 smooth rod and linear bearing for the top of the printer, but will probably require a separate M8 smooth rod support to make use of the entire range of movement and recover the space used by the threaded rod coupler. 22-02-2020: Added a Z endstop switch bracket. Added small tools to insert and optionally extract 608 bearings in the pulleys, using a piece of threaded rod and a few washers. Modified the corner brackets to have slots in place of double mounting holes, allowing some adjustments in placing the upper pulleys and allowing the use of larger diameter ones if desired. Reduced the large pulley lightening hole diameter to make it simpler to slice. Added an unused stronger upper pulley, 3mm larger; to use these, the carriage tie point must be shifted upward by 3 mm. 18-02-2020: I reworked the carriage to run freely and made the SCAD file so that the modules follow more closely the names of the exported STL files.