Cetus Z axis Clamp
Description
Improved high performance clamp with smooth friction adjustment. Aug 9 2019: Added a reverse flexture socket feature. This geometry tends to seat the sprag bearing better. Added clearance on the back side of the ball part to better seat the sprag bearing (minor interference eliminated). (Added a flexture feature version to the socket part to allow some in-out movement) The problem source is the axis of the Z stepper motor shaft and the axis of the outside diameter of my sprag bearing are not parallel. With the original cetus Z clamp the clamping force reduced the Z layer accuracy. To have a consistent braking force the bearing clamp needs to follow the sprag bearing surface and constrain a single degree of freedom, rotation. With this design I can now adjust the clamp to slowly over 3 or 4 seconds land or park the extruder head on the build surface. Three implementations: Use with the original cetus screw. Get a new 4-40 nut and bolt and a spring which could be a deforming washer type spring. Add shim brass or some other thin wearing liner. Fully assemble and work the Z axis up and down by hand 30 times or so while adjusting the screw. This polishes the PLA clamping surface. Repeat the above adjustment after a day and then if you want you can lubricate the surface with a minuscule drop of silicone grease. Don't use grease with PLA, the increased tension causes the PLA clamping band to stretch. I tested a few greases that I had on hand and the more Non-Newtonian the grease is, the better it works. Grease with long strings or whiskers if you touch it. In my very limited test a 20 year old tube of Apiezon grease worked the best, but clean brass on steel with a compression spring is what I use. Ball to socket radial clearance is 0.05 mm in the "two parts in one print" STL file. I prefer to print the parts separately. Then for assembly twist one part 90 degrees and slip them together. Equally important is this approach makes two threaded stepper holes free for a Z axis brace. The Z axis shakes whenever the extruder stepper motor is accelerated and the vibrations are frozen in the print surface. Print on fine with a 0.1mm layer thickness. Use a 0.4mm nozzle but choose a 0.3mm extrusion line width. The PLA is thin 0.6mm around the sprag clutch bearing to distribute the clamping force more uniformly over the bearing surface. If used a stiff spring with about 2 lbs force when compressed, a 4-40 or M2.4 nut and bolt, and a small strip of metal. I used 0.005 thick shim brass cut with scissors. I folded the end to double the thickness then poked a hole with a nail for the bolt to pass through to preventing the brass from shifting or rotating in the final assembly. In the photo the steel bracket firmly connects the Z stepper motor face to the printer base. Measuring deflection with a Dial indicator the top of the Z axis deflection is now 0.003" over the travel limits of the shifting extruder motor weight. The bracket also makes tramming the printer possible (make the three-axis perpendicular to each other and level the build surface) should you be so inclined. Designed in Fusion 360. -Peter
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