Friday, December 14, 2018

Adhesives and Aluminum

I am approaching the final point of assembling the diagonal mirror spider.  This is a .5" x .800" x .800" piece of 6061 aluminum to which I am going to (hopefully) glue .5" wide 6061 aluminum legs to hold it in the telescope tube.  Because the square is tiny and has four 8-32 adjustment screws distributed around a .265" center hole, trying to use screws to attach the legs is impractical.  My plan was to use an adhesive, like epoxy.  I would also like a non-reflective black surface.

Two choices: flat black paint or black anodizing.  Obviously, the bare aluminum needs to be properly degreased and perhaps roughened with some #80 sandpaper to get a good grip with epoxy.  It appears that black anodized aluminum does not do well with adhesives because the sealant coat on top prevents a good grip: too smooth.  I have never tried to anodize, and I do not think I want to try that now. I suspect that anodizing after gluing the legs on will fail, because anodizing is done in an acid.

I really do not like how flat black paint, applied with a spray can, comes out.  It thickens the threaded holes so screws do not go in smoothly.  (I suppose that I could retap the holes after the paint dries.)  I am using black oxide screws, so they are not in need of paint.

Powder coating has the disadvantage that it typically involves heating the material to 400 degrees F.  Will the epoxy lose adherence, or just bake a little harder?  Epoxy bonds benefit from heating when curing, but what if it has already cured?  I suspect that getting everything glued, clamped, and then baked in the oven for a few minutes, then taking it to the powder coater that I use (Signs by Smith) in Boise would work.

2 comments:

James Gibson said...

Try pre-installing the screws to use them to keep the paint out of the threaded holes. That or you can insert cotton into the holes to absorb the majority of the paint: remove cotton after painting.

I don't know what size screws you are using, but if they are big enough the brush attachment for a .223 cleaning rod is a nice way to remove paint and debris from a threaded hole. Used similar brushes to clean fastener holes of old-locking compound.

Heating the epoxy may or may not cause a problem. I had locking epoxy's for screws that were rated to rather high temps. It all comes down to the epoxy and most people tend to use the low temp stuff to make construction with it easier. Not everyone has their own autoclave. You need to check the specs on your epoxy.

John Dough said...

You might try J-B Weld steel epoxy. Goes to 550ºF.
https://www.jbweld.com/collections/epoxy-adhesives/products/j-b-weld-twin-tube