Introduction: Simple Wooden Crayford-style Focuser
Crayford focusers have extremely smooth movements and do not require tight tolerances in production. I wanted a focuser for a 70mm refractor I was making with my six-year-old son. This focuser design replaces the ball-bearings with a PTFE tape surface, which makes it much easier to make. The PTFE on the aluminum draw tube is an idea from John Wall (the Crayford inventor).
The main part is a block of hardwood (cherry, from an old crib), 2.75"x2.75"x1" in dimensions. I used my drill press to drill out a 1.5" hole (A) in the middle for the 1.5" outer diameter aluminum focuser tube. I then drilled a second hole, 3/8" in diameter, all the way through from one side to the other (B), so that hole A and hole B would meet and overlap by 1/16". This second hole is for the focusing rod.
I drilled two more holes from one side to meet up with hole B (C and D) for adjustment screws that press on the focusing rod through PTFE pads, and tapped them to fit the adjustment screws simply by forcing a screw into the wood. These PTFE pads (E) are small strips of 1/16" PTFE, bent into a C-shape, with the back of the C facing the adjustment screws.
I purchased an 8" x 1" x 0.0045" strip of self-adhesive PTFE tape (F) on ebay from mousetape.com for about $3 shipped. I stuck a strip along one side of the focusing tube.
I then enlarged the 1.5" hole in the middle slightly. My method of doing this was to use a sanding drum mandrel in my drill press with a somewhat oversized sanding drum (which would stick out on both sides past the work piece), with the work piece resting on another piece of wood with a hole in it, which was on the drill press table. As a result, I could keep the work piece aligned at right angles to the sanding drum while moving it about so as to enlarge the hole. I enlarged the hole until it was about 1.56" in diameter. I then finished the exposed raw wood with Titebond II diluted with water and sanded. I stuck another strip of PTFE tape (G) inside the focuser hole, opposite where the focusing rod will go. Then I put it all together, and it worked just fine.
Eventually, I added knobs. I had some oak circles from using a hole saw on some piece of oak some time ago, and after sanding them (by putting on a bolt and spinning with a drill against a sanding block), I press-fitted them on the focusing rod. If it starts slipping, I'll drill a hole through the rod and knob and put in a screw.
And I added a screw on one end of the focuser tube as a stop, and I added a screw to hold the eyepiece. Because the focuser tube is only 2" long, so as not to compromise the length of movement, we drilled little depressions in the main block for the screws.
I had a bit of trouble with the wooden thread for one of the adjustment screws getting stripped. Treating the hole with CA glue helped, though.
The see-through diagram is not to scale.
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