Crayford Focuser

Bearingless 1.25in Crayford focuser

The idea behind this focuser was do do away with any bearings and make it as simple to make as possible. This focuser is all aluminum. Its’ weight is 165g. The pinion is 6mm diameter. These are all the parts in an exploded view.

Instead of bearings the drawtube slides on etched virgin Teflon. It is also important to acid etch the aluminium so the etched side of the teflon bonds well. I used slow curing thick superglue. The drawtube has a flat 10mm wide machined off to give the pinion more grip.

The pinion has 9mm heatshrink tightly heated to the pinion. This gives excellent grip and no ‘springiness’ when racking in and out. Using heatshrink doubles the lift capability. However if the focuser is on the side of a telescope (as opposed to say 45deg), the heatshrink is not needed as all forces are acting perpendicular to line of focus. I really only included it as an experiment.

The right side pinion support is fixed so only 1 spring is needed. The left side pinon pivots around a fixed point near the base using 6mm aluminium dowel. This gives movement on only 1 axis of rotation. A spring pulls the pinion onto the drawtube.

 

A small yet critical detail in this design are the pinion and pivot holes that accept the 6mm dowel. For a tight (interferance) fit on the pivot dowel and focuser knobs, holes are drilled to exactly 6mm so the pinion grabs the knobs and pivot supports. For a free rotating fit on the pinion supports the holes are drilled to 6.1mm. Engineering drill bits are supplied in these small incremental measurements. (even 6.05mm is available). Using these fine tolerances allow the pinion to rotate freely but without any radial play. A small amount of grease is applied to the inside of the 6.1mm holes to aid smoothnesss of rotation.

The tension can be varied by changing springs. The spring shown lifts up to 750g vertically and horizontally, probably even a bit more than that in the vertical axis.
Big enough for a small camera, barlows etc…

Movement is precicse and there is no backlash.

This was a test run for my 12in F3.3 scope. I ground most edges but will be using a milling machine next time as aluminium will ruin your grinding stone! Any drilling was done in a bath of oil. Also the parts were put in oil aswell:-) The alloys in this focuser are mixed. It is better to use all 6062 or 6082 aluminium as this can be easily machined, annodized and welded and doesn’t corrode too quickly . I will be making a 2in version for my 20in scope using the same format – teflon and single spring.