Wooden Crayford Focuser
(Click for larger image)
I wanted to try to make a Crayford without using bearings, so I tweaked the design from the aluminum Crayford from my TriDob to build this creation. So far so good.
Quick and Dirty Construction Notes:
1. Laminate several pieces of 3/4" Mahogany together with epoxy.
2. Trim to the correct size on the tablesaw.
3. Drill hole for drawtube.
4. Drill clearances for knobs.
5. Cut and assemble the shaft, knobs and pressure contacts.
6. Glue teflon bearing inside draw tube.
7. Expand the existing hole in the tube to accommodate the 2" drawtube.
8. Figure out how to match the base to the scope's tube.
(I've made it this far!)
9. Varnish and tweak the unit.
10. Mount that sucker!
Here are some images of the current progress:
How do you like that fit to the tube? It was pretty easy using a technique called "COVING" on a tablesaw.
You set up an angled fence (angled with respect to the saw blade) like this:
I used a 7.25" continually adjustable ("wobble") style dado blade. The angle is approximately 40 degrees and I determined it "experimentally" (thanks to the folks on the ATM list!). I just traced the tube onto the focuser base and then held the 7.25" blade up to it and tilted it until the blade profile matched the trace. Then a quick trial run on the saw and I was good to go. I removed about 1.25 inches of sagitta from the base of the focuser with about 10 passes on the saw.
IMPORTANT! No doubt about it, this is a DANGEROUS technique, especially with small chunks of wood. I was careful and everything went smoothly. A little sanding against the tube and it's nice and flush...and should be very square to the tube also.
I'm a bit worried about backlash. I'm using a small piece of latex tubing on the focuser shaft to generate enough friction to overcome the friction between drawtube and the teflon 'bushings'. Won't know for sure til I get the scope back together. By eye it looks fine right now. If it fails, I'll have to find an alternative to the latex tubing, or even revert to bearings. Time will tell.
UPDATE:
October 2007: The Teflon bearing surfaces collect dirt too easily and it causes major issues with the mechanical smoothness. I'm replacing this focuser with an aluminum version with bearings. There's just no substitute for ball bearings! Here's Woody's Replacement:
