Recently I came across a design on Thingiverse to make a telescope focuser. Normal focusers use rack-and-pinion to move the drawtube (where the telescope eyepiece is inserted) in and out. Alternatively, there are helical focusers, that work like normal, camera lenses (essentially moving by turning a screwthread). In recent years, a type of focuser called Crayfords have become standard – these are similar to regular R&Ps, but the geared bits are replaced by a shaft that moves the drawtube using friction – the amount of friction is adjusted using a screw to push the tube against the turning shaft.
This new design on Thingiverse is known as a Helical Crayford, and works by pushing roller bearings (mounted on a collar) against the draw tube. The bearings are oriented at an angle, so that their rotation pushes the drawtube up and down. Loosening the collar locking nut allows one to quickly pull the tube in and out. It feels pretty counterintuitive, so I decided to build one. The 3D model on the Thingiverse link is good. I didn’t have to adjust anything, and got the thing working in one try. Here’s what it looks like moving:
The Amateur Engineer 