Friday, September 22, 2017

My Current Shop Project

I mentioned a few days ago that as part of rebuilding my 3" f/4.5 telescope, I was building a new mirror cell to replace this tragedy made when I did not even have a drill press:
The black part is a too large (3 3/8" ID) part that holds the mirror.  The base (below the springs) is made of oak and slides into the telescope tube.  The mounting screws go through the tube and into the threaded holes in the oak.

So far, I have made the aluminum base:

This started as a 4" OD piece that was too large to fit in the Sherline 3-jaw chucks.  In any case, I needed to turn it down to 3.93" OD to fit in the tube.  So I drilled a 1/2" hole in the center.  To find the center quickly, I used one of those "scratch lines across diameters" tools and drilled a 1/2" ID hole, which really means 1/4", then 3/8", then 1/2" drill bits in the drill press.

Then I put it in the 6" Sears chuck to bore that hole large enough to mount the interior hole on the outside of the jaws of the 2 1/2" chuck.  The 6" chuck was really intended for woodworking and likely not very accurate.  This requires putting two headstock and tailstock spacers on the Sherline lathe.  (Do not tell Sherline; I am sure they would disapprove.)
Boring from 1/2" to 1 1/2" is very boring, so I dug around my drill bits collection.  You will see why I ignore the spam offering tool enlargement:
That is 1 1/4" diameter.  At 500 rpm, it rapidly enlarged the hole.  I don't have any larger drill bits but boring 1.25" to 1.5" was not too bad.

Squaring the faces was hard because acetal spoils me into forgetting the need for lubrication of the workpiece.  One that was solved, the surface came out adequately smooth, or at least with the spirals of proper machined aluminum.

Now I had something to  mount on the outer jaws of the 3" 3-jaw chuck so that I could turn it down to 3.926" OD.  It just barely slides into the tube, requiring some slight pressure to do so.

Next step will be setting up the rotating table on the mill table so that I can get three holes 120apart for the collimation screws.  Last time I used a protractor with only so-so accuracy.


  1. Have you made a mirror for a telescope yourself? Way back when Halley's Comet came by and I subscribed to Sky and Telescope magazine I had thought about making a telescope and the mirror, but other things came up and it got pushed off the back of the desk. It still seems interesting to grind it yourself and do the test to see how the mirror shapes up.

    1. The 3"mirror is the only one that I have completed.