Monday, November 26, 2018

Today's Machining Insight

I bought my first set of taps and dies about 20 years ago, and knew not enough at the time.  These are the tapered straight flute sets you find in DIY (Disaster is Yours) stores.  They are good for one thing: retapping through holes.  Otherwise, use spiral flute taps to remove the material.  Faster because you do not need to back the tap out to remove material every few turns.

This afternoon I returned to a hobby project that makes no economic sense: rebuilding a 3" f/4.5 reflector which was the first  (and only) telescope mirror that I have ever made.

For reasons that I blogged about in the past, a short focus parabolic mirror, especially of 3" aperture is always a bad idea.  Correcting it to a 1/4 wave parabola involves moving the knife edge on a Foucault tester such small amounts that you really should plan on machining the tester.

Anyway, today I was trying to make what must be the smallest telescope diagonal mirror holder made in Idaho ever.  (There are enough fools  elsewhere to limit this to Idaho.)  The hub which will hold the mirror assembly is .696" x .696".  There is a 17/64" hole in the center in which a 1/4" bolt will loosely ride.  There are 4 6-32 holes around it which will hold the collimation adjustment screws.  The mill made those holes easy to precisely locate.  On the drill press I completed drilling them to depth.  Then I started tapping the 6-32 holes with one of these tragic tapered straight flute taps.  This part was enough labor to make that I am terrified to break a tap, so I using just the minimum of force as I turn it, oil to lubricate it, and only about three turns at a time before backing out.  I am waiting for a spiral flute 6-32 tap to arrive before I resume this process.

I had originally planned to make this part of acetal, but attaching the legs of the spider involved  tiny  on the edges.  (You cannot epoxy acetal to anything.)  making this hub of aluminum means that I can epoxy .024" aluminum to the four sides on the hub.

When I say this makes no economic sense, I mean there are certainly optically superior small scopes that would serve the same function and at a pretty nice price.  But they would not be my work. Almost everything about this scope will be my manufacture: the mirror cell turned out of a  cylinder of aluminum. The spider. The primary mirror. Maybe even the focuser if I get ambitious.  The tube is PVC.  I am not sure I want to build a fiberglass tube this size.


  1. When possible, avoid the use of the 6-32 thread size. It is weak, and screws and taps break more readily than expected. 6-40 or 6-48 would be better. 8-32 is much stronger. In manufacturing, 6-32 is the most common size to fail. Unfortunately, designers don't consider this, as most don't actually work with their hands, and the hardware is commonly stocked.

    No more than 1 full turn before backing off to snap/clear chips. With a 6-32, I'd be inclined to hold it to 1/2 turn max. 3 turns is asking for trouble.

  2. Will: I would rather have used 8-32 but the diameter is a bit large for this hole. (Really tiny.) There is less than a pound of force involved so load isn't an issue. The spiral flute tap makes a world of difference. I used three turns before backing out and never felt at risk, unlike the straight flute tap. My big problem is that I would like to use 1" long 6-32 socket head black oxide screws (for reduced reflection), and that combination of length, head, and finish seem not to be stocked. I may get stainless steel and use flat black paint after everything is put together.