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Best method to countersink/install Tinnerman washers?

MikeinPhoenix

Well Known Member
This is probably a dumb question, but I wanted to find out what is the best way to countersink holes in fiberglass so I can install Tinnerman washers? I'm installing #8 screws. I believe the washers are 100 degrees, but it appears most drill bits are either 82 or 90 degrees. Would welcome any recommendations. Many thanks!

Mike
 
General hardware store countersinks are a different angle than aviation countersinks. Get the right ones.
 
Dremel

Buy a few Dremel grinding bits. Use the stone from the bench grinder (not spinning). Run the Dremel and grind the point down to the desired countersink angle. Grind the countersinks close and finish off the last few thousandths with the countersink tool so they are perfect.
They work great and don't mess up your expensive countersink cutters.
 
Buy a few Dremel grinding bits. Use the stone from the bench grinder (not spinning). Run the Dremel and grind the point down to the desired countersink angle. Grind the countersinks close and finish off the last few thousandths with the countersink tool so they are perfect.
They work great and don't mess up your expensive countersink cutters.

I want to say that I bought one of these ready-made 100* countersink grinding stone things from somewhere, back during the build...but I can't figure out where I got it from, *if* I'm remembering correctly.
 
Or just skip the tinnermans.

Instead use AN526 or 526C screws with a nylon washer, in a plain, straight hole.

Fiberglass does not play well with knife-edged countersunk holes. No bearing strength; the holes eventually grow bigger and bigger.

The edges of most tinnerman washers stick up into the boundary layer anyway, so it's not like there is a huge comparative drag penalty to using 526's.

Your fiberglass will thank you.
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Another way

I used a Dremel with stone that I ground to the proper countersink. Not that critical with the method I'm going to describe.

1) Get the countersink done so the washer fits flush.
2) Scuff the bottom of the Tinnerman. Scuff the glass
3) Use epoxy/micro/flox to bond the tinnerman to the glass
4) Sand the edges, then fill with edges so the tinnerman is nicely blended to the glass
5) Put a layer of light glass cloth over the filled area. Use Peel Ply over the cloth.
6) Sand after cured. Cut the hole out where the screw goes.
7 Use a high build primer to fill the glass weave. Sand, repeat as needed.

Now you have a nicely fitted Tinnerman that won't cut into the glass. I used this process on my tail fairing on my 7. Only 8 total. You'd never know there is Tinnerman there but yet I have a solid base for the screw.

If you'd like pictures, send me an email to [email protected]
 
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