Sorting out how to route Kevlar cord to drive the Z motion of the print bed. Been through a few(!) iterations. Kevlar cord sliding on printed PLA turns out to be a non-starter - static friction is too high. Easy to source PTFE tubing (commonly used in Bowden setups), and PTFE makes a good bearing surface.
Integrated a tube guide into the printed brace for the (top-front) cube corner. Made a test-print to check fit, and the pragmatics of printing (or in other words, all I did wrong).
Will be glad(!) to have a dual extruder and dissolvable supports, someday. The more complex the part, the harder to do without supports and/or clean out the support structures. The pictured part … needs revision.
One topic of which I have not seem much mention: How to make or adapt designs so they are suitable for 3D printing.
Looking for the simplest possible fabrication (not the simplest possible design). Put differently, spending more time on the up-front design and first build, so later builds will be simpler.
Could use pulleys (and might), but end up with a higher part count, and more complex fabrication. Also the cord path is a little complex, so that would be a lot of pulleys.
PTFE tube (6mm O.D. in this case) is easy to source, inexpensive, and a single roll yields all the chunks needed.
The Kevlar cord is likely to cut though the PTFE in time … but that should(?) require quite a lot of time. The cord is meant to be taunt, but is not under large load. The duty cycles are slow and few for the Z-motion of the print bed. The movement of the cord against the PTFE is twice the height of all the objects you print, or more if the bed lowers between prints. Should require quite a number of prints to get much wear.
You might consider using Spectra instead of Kevlar. Spectra is very slippery, while Kevlar is very abrasive. UHMWPE on PTFE should be ridiculously long-wearing, particularly if you use a largish Spectra line and a gentle tube curve radius.
@Ryan_Carlyle Interesting choice. From a quick look, I have questions about creep under load, and relative stiffness.
Then again, the relative load is light. The ~1mm Kevlar cord I have is rated at 800 pounds, and the actual load on the taunt cord when in use is likely less than 10 pounds. The attractive property of Kevlar is the relative stiffness. The stiffness combined with the light load (print bed plus print versus 3200 pound rating for the combined cords) should mean very exact motion.
The friction and fatigue character of Kevlar is some worry. For the low and slow cycles in this use, raw Kevlar might be fine. Could use the old bicyclist’s trick of soaking the Kevlar cord in melted paraffin (used for bicycle chains) to reduce friction/fatigue … if needed.
@Preston_Bannister because this is a Z stage, I don’t think you really need to worry too much about the difference between “stiff” and “really stiff” line… unless you’re doing a lot of dynamic Z motion like Z-hop on retract. The Z load is more or less constant from layer to layer, so the line stretch doesn’t change from layer to layer, and thus no print artifacts will occur. (This is unlike XY stages that see high dynamic loads and ringing is a concern.) The weight-stress on the line will act more like a preload than a source of error. Basically it will shift the steps/mm by a very small amount, not distort/skew/ring anything.
In any case, a bigger spectra line will be stiffer than a smaller Kevlar line, so changing line spec is always an option if the price point makes sense.
Spectra DOES continuously creep under high load, but 1) this isn’t high load and 2) Zmin homing will calibrate out any creep stretch before every print. Kevlar also creeps, but it’s a thermoset so the creep is finite and somewhat reversible when load is taken off.
@Ryan_Carlyle I think we are saying roughly the same.
In this use, either choice is likely more than sufficient.
Note that ideally I want to build a printer that requires bed-leveling exactly once - during simple assembly - and not on every print. Whether that proves possible is a question.
