Saturday, May 25, 2024

What I Have Been Machining When Not Working on Declarations for Court Cases

What us that white thing (the one attached to the black tube, not my Jaguar in the background).  The piece of white acetal is an adapter from the black tube that the maker of the telescope used to mount the finderscope.  (Attaching finderscopes when there is no solid telescope tube is often an interesting mechanical design.  The black aluminum tube is parallel to the optical axis and the picture below shows the maker's original design (sort of).
As you can see it uses a rectangle of Baltic birch plywood with a hole and a thin slice so that turning the wing nut tightens the wood onto the aluminum tube.  The original finderscope had a base that attached to the wooden part nicely but when it came time to upgrade the finderscope,  that base was too narrow.  The replacement piece made of black acetal turned out to be too narrow to get a proper coaxial relationship to the optical axis and it was pretty much hopeless to get the crosshairs to line up, so first I tried to replace the black acetal with something wider but the hex head bolt just kept splitting the plywood.

Natural materials have many virtues but they are not very good at being threaded within the grain, so after several bad design starts, I took a piece of acetal,  cut into an L, drilled and tapped holes for the scope base, then cut a 1.256" hole through one of the legs.  Then I drilled and tapped a 1/4"-20 hole for a thumbs crew to secure the adapter to the 1.2515" OD aluminum tube.

gCode has G02 and G03 commands for cutting arcs and circles.  I spent a lot of time experimenting (learning from my failures to understand the command) before I got a command that should have cut a 1 25" hole.  It actually came out more like 1.247" ID, too small to fit.  I wonder if my 1/4" carbide end mill might be slightly smaller than. 25".  Anyway, I enlarged the radiua of the hole .022" and ended up with not quite a press fit.


  1. Clearly the Telescope people need to standardize their attachment systems, with something like Picatinny Rails....

    1. There are several standards for attaching scopes to mounts (Losmandy and Vixen) and finderscopes to bases (Vixen). Dobsonians are all over the place because some are solid tubes and some are half-Suerrier trusses.