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I needed a locking focuser knob for my Celestron C9.25 Schmidt-Cassegrain telescope, so I decided to machine two from aluminum.
I started with 1" diameter stock about 5" long, chucked it in the lathe, and took a finishing cut along it. Then I knurled about 1¾" on one end, and turned the remainder to a slightly smaller diameter.
I drilled a large hole into the stock, then used a boring tool to enlarge it to slightly larger than ½", so the knob would fit over a brass tube in the telescope. I cut off the stock near the end of the knurling, then took a finishing cut on that end. The knob was finished after I drilled and tapped a hole for a set screw to secure the knob on the brass focuser mechanism.
With the knob finished, I cut a 6" length of ¼" threaded brass rod, drilled and tapped a ¼"-20 hole in the closed end of the knob, then machined a knurled knob from Delrin. I drilled and tapped a hole for a set screw in the knob, and threaded the rod into the knob. When tightened against the telescope's focusing mechanism, this rod locks it in place.
I repeated the procedure to make a second knob, to give to a friend with the same model telescope.
Here is the locking knob installed on the telescope.
Updated September 7, 2016