Saturday, May 8, 2010
Inkscape. (The annotations are for the benefit of the blog--they weren't there in my original file.) I am very bad at measuring (but getting some cheap digital calipers for $8 from a Hong Kong seller on ebay helped a lot--and then I got $4 back as one of the jaws was chipped and a set screw was crooked, so it was a great deal) so my best way for precision is to print out a cutting template, attach it to the piece and drill and cut right through it, or at least make marks with a pencil or drill bit through it.
There would be a rectangular baseplate made from a piece of scrap red oak that I had. The baseplate would be mounted in the tube, near its bottom (not quite at bottom, as the plywood reinforcement ring on the tube would I rounded the corners of this scrap piece with the router using approximately the right diameter to fit in the tube, bearing in mind I could always sand it smaller (as I indeed had to) but couldn't make it larger. (By the way, here is a hint for how to measure things off from the router. I cut things with a 1/4" up-cut spiral bit. For measuring, I replace the spiral bit with a small length of 1/4" stainless steel rod, and measure from that with the calipers.) Notice from the diagram that the rectangle would be somewhat off-center in the tube--this was on purpose, to fit better with the collimation screws. I drilled holes for the collimation screws (actually, I did that while drilling them in the triangular cell, because they needed to line up exactly).
I also cut three ventilation holes in the baseplate with a hole saw. The result coincidentally (honest!) looks like a certain rodent from a company that protects its intellectual property with a zeal that one may think violates the Greek maxim meden agan. In defense of this design, I will say that the placement was entirely functional. I made the inner hole as large as I could while keeping strength around the collimation screw holes, and the small holes are also located in such a way as to be fairly symmetric, and not too close to any of the alignment holes or the inner hole. Four wood screws hold the base plate to the tube.
The mirror is glued to the cell with three 3/4" blobs of silicone sealant, with the positions optimized with Plop. I used some removable 1/4" particle board spacers to make sure the blobs wouldn't flatten out. One wants the mirror to float on the blobs, and not have stresses put into it by the differences between thermal expansion of wood and glass. By the way, when I removed the mirror from the original cell, I was horrified to see it was attached with two blobs of 1/16" thick rubbery stuff.