Helical Crayford Focuser - v2

thingiverse

This Focuser is adjustable, in that you can use various diameter focuser tubes (within a certain range). It can be tough to source the proper sized tube in short lengths in a cost effective way, so this design allows you to use what you can find locally. Probably the best source is PVC tubing. A zip tie is used to close the 'petals' onto the tube. Perhaps you can use a large rubber band as well, which would be a bit better for instances where the tube isn't consistent in OD. Maybe one of those silicon bracelets? A large hose clamp could also work. Half of the bearings are also inverted. This moves the bearings into two planes, so that it only moves along one axis and doesn't tilt to the sides. In v1 this was done with steps, but I think this design is cleaner. One revolution yields about 35 mm of travel with the standard petals, which have a 15 degree angle. The fine petals have a 5 degree angle, and I assume should be ~11.5 mm/revolution. All the sketchup files are in the zip below. Add a comment if there's any trouble opening it. Video: http://youtu.be/bW7UEPq3LA8 Instructions Print 6 petals, 1 Focuser base, and an eyepiece lock (which you might want to redesign to fit your tube, so source your tube first). Assemble as in the pictures, except use smaller screws :-). Everything is M4 except the M3 on the eyepiece screws. I think m4x30 is a bit conservative, and you might be able to use smaller, like 20-25mm. In the pictures of mine, i used 40mm 'cause that's what i have on hand, but there's at least 15-20mm extra. You might also like a focus tube lock, and if so print out one lock petal and only 5 petals, replacing the 6th petal with the lock petal (only in the fine pitch for now) There is a base plate for a 6 inch tube like a PVC drainage tube, and one for a truss like design, which has a flat bottom face. The truss base plate is more multipurpose; it can be used on all sized telescope tubes and trusses. You just need to print an adapter to attach it to, so i would recommend the truss design.