Friday, May 10, 2013

Next Step on the Telescope

I assume that some of you are interested in this project.  At least I'm interested in telling you about it.  Not to worry, I'll get it done and start being a grumpy political curmudgeon again.

There were a couple of air bubbles that left serious depressions at the lower part of the Sonotube where I did the fiberglass cloth, and since I want this reasonably pretty -- and extra stiffness especially matters in this area where the mirror cell will be mounted -- I dripped some more resin in these holes, which are fortunately all within about 15 degrees of each other -- and left it to dry in the sunlight.

The next step is to reverse the mounting assemblies on the mirror cell so that I can insert the bolts that hold the cell in the tube from the rear of the mirror, not the front.  While disassembling these from the big plate that holds the springs and screws for the smaller plate that holds the mirror, I had one of those, "I did this?" moments.  I don't think I had the vertical mill yet, which explains why these parts are so crudely made.  But I must have obtained my first complete set of taps, because I clearly overapplied the, "I can put threads anywhere I want!" idea.

Threads are good in a deep object where it is impractical to run a through hole (one that bolt just slides through) all the through the object.  But threads in a thin plate, with a nut holding the bolt on the other side of that threaded thin plate, not so good, especially if you did not have a way to make absolutely perpendicular threads.  Then you just have bolts fighting threads in the object when they hit the threads in the nut on the far side.  So I turned all the threads in the mounting hardware into through holes.  Much better.

The other mildly surprising moment was that I remembered having bought aluminum 1/4"-20 screws for some project, long ago, but I could not remember for what I used them.  Then I discovered them.  This mirror cell is almost entirely aluminum, including the screws and nuts that hold the attachment hardware to the big bottom plate.  Why?  Because I was trying to make this really light.  Brilliant.  The downside is that aluminum isn't anywhere near as tough as steel, and the threading orgy above meant that some of these screws and nuts were slightly damaged; one had to be Vise-Gripped out of the aluminum to which it was holding on for dear life, and the head of this screw is behind redemption.  However, I was able to tap and die four of the six sets by turning a steel bolt through the nuts, and a steel wingnut on the bolts.  There are two steel 1/4"-20 hex head screws now part of the process, but I don't expect the weight difference to be serious!

I am painting all these fasteners black (again).  I really wish that I had a Boise source for black anodized bolts and screws, but I don't touch these very often, so I don't see it as a big problem to use flat black paint instead.  I am considering buying some of this black flocking material for the upper part of the tube interior, in the area near the eyepiece and diagonal.  (And read here to see why you don't really need to do the interior tube.)

UPDATE: As I do the arithmetic on placement of various parts, I am discovering that there really isn't room to put the mounting brackets for the mirror cell behind the main plate.  When I did this original design, I did not waste a single inch of length.  So how to solve this problem?  This has been a long gripe of mine about mirror cells -- many of them assume that you will find exactly the right spot on the outside of the tube to drill the hole for a bolt that screws into a blind hole on the mirror cell base.  This is not easy to do. 

But I have a clever solution.  What I am going to do is take the L-brackets that hold the mirror cell in the tube, tap the holes that I just made through holes into 5/16"-18 threads so that the bolts sticking down from these brackets will be captive (actually locked in place once screwed in).  Then I will install the L-brackets in the tube and they will be permanently located there; to remove the mirror cell for cleaning is just a matter of unscrewing the nuts and lock washers that will hold the base plate of the mirror cell to the bottom of the L-brackets.  That also means very deftly enlarging the holes in the base plate (since it is a big hassle to remove the mirror from the cell) to 5/16" size from their current 1/4"-20 thread.


  1. An alternative to painting the fasteners would be to blue them. You can use a commercial cold blue solution or heat blue. To heat blue, heat the fasteners to 700-1000F and quench in oil. Alternatively, you can dip the fasteners and burn off the oil. Lightweight oil like 3-in-1 or 20w motor oil mixed with linseed oil works fine. With small fasteners, it is a very quick process. Although it's called "bluing," the color is closer to black. The advantage over painting is that there is no thickness build-up and it won't chip or peel. Post-quench oiling helps prevent rusting. The oil quench method is practically free because it's likely that you have some motor oil around the house. If the fasteners are cad plated, you might have to burn that off first, or use metal etch.
    I got this info from "Gunsmithing," by Roy F. Dunlap. I've tried it and liked the results.

  2. You're making me nervous with the repeated use of 'put it in the sun to dry'. Polyester resin (like epoxy) does not need to dry, it needs to cure. The stuff I've worked with can be given a fairly wide range of the hardener (MEK in my specific case) based on the temparature and, to a lesser extent, the desired working time and curing rate.

    If you're putting it in the sun because you want to warm it up, so it will cure faster, ok; but otherwise all you're doing is causing the solvent in the resin to evaporate faster, without necessarily completing the polymerization reaction that will give the cured product its strength.

  3. It may be that the direct sunlight means more heat to operate the chemical processes. But I notice that there are polymerization reactions that take place in response to UV light as well. Perhaps UV is accelerating the process?

  4. Tailwind,

    How much flame to you need to get them to 700-1000? Can a plain propane torch do the trick? How red does the steel need to get?