16x4 trapezoid screw hub using coins and bearings

thingiverse

This is my latest take on remixing "threadless ball screw" into threaded rod. Generally, this approach works incredibly well for frugal design - perhaps not ultra in durability but given how cheap and readily available parts are it's just great for many designs which require no backlash and high precision. This time I picked on a bigger screw - 16x4 trapezoid - which is very affordable to obtain where I live, is very straight (usually sourced in CNC stores for CNC purposes) with just one con of requiring a threaded 'nut' made from brass. For no-backlash you need spring-loaded two of them and that quickly becomes expensive and bulky. My first trials with using bearings riding on thread screws used tilted bearing approach to get it into the groove. This strips the screw slowly as it's made from softer metal than chrome-plated bearings. So this time I've set bearings at 0 degrees and used brass coins to ride the groove. I did use 1 "groszy" polish coin which until 2014 were made from brass. As far as I know they still can be found in circulation, though since brass value exceeded their nominal, they are made from steel nowadays, so check your coin stash with a magnet prior to use. Coins do not rotate and need to be filed slightly at the contact point to get a tight fit. Perhaps any piece of brass could be used because shape is not critical - purpose of the coin is just to ride the groove, bearings take all the load. Of course they need to be drilled with 3mm drill in the center, and you need a washer to separate them from bearings (so they can rotate). To drill I just did use the 2.5mm drill to pilot the hole and then re-drilled with 3mm outside. Original threadless ballscrew scad is modified so depth of each bearing is adjusted according to leadscrew pitch. Thanks to this for some designs you can skip bearings completely and use just coins (for low-durability and actually not so low precision designs where you just need to replace expensive brass nut, like telescope lens guides etc.). The holder itself is kind of a hack - once you decide on sizes, number of bearings, tolerances, tilt etc. - you need to create 'module' out of your scad file so it can be included as 'centerpiece' of the holder, and include it in the holder design. I did 16x4 3 bearings version as example. Perhaps should try making some more elegant solution but so far I focus on making it all run (there is still a lot to do to redesign drive of the screw as I can't just use nuts to fix it to bearings anymore). Note that to center properly you need to twist to a screw first and then screw the screws fixing it to the holder - mind that all bearings should have same gap so all runs true on the screw. It is not really difficult, just mind you that fixing it to holder first means doing it wrong. Also I think using two sets of shorter screws is better than long screws all thru the holder, especially that way You can adjust 'preload' a little bit. In final design dimensions of the holder centerpiece should be ideal to not cause too big preload and allow bearing holder to rest firmly. It will take much testing to find those dimensions though :).