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Flat undercut 82 degree countersink/ counterbore

Flat undercut 82 degree countersink/ counterbore

Flat undercut 82 degree countersink/ counterbore

(OP)
I have to splice a boom on a boat. This is going to be a butt splice with an internal sleeve.
The material thickness is .104" The total thickness is .208 to 210.
Ok so I go out and buy stainless flat undercut #8 screws,the depth to the base of the flat undercut is .062" so far so good.
Now I need a countersink for that profile. I have searched for two days through suppliers catalogs and on the internet.
No luck, I can find an 82 degree countersink which gives me the knife edge I am trying to avoid, or a counter bore.
My best option seems to be to use a micro stop countersink with a removable pilot , grind the bottom of the countersink flat, then put the pilot back in.
Does anybody know where to get a countersink/ counter bore with that profile #8 82 degree flat undercut head.
Or have a better solution.
B.E.

You are judged not by what you know, but by what you can do.

RE: Flat undercut 82 degree countersink/ counterbore

Since you are proposing a mechanical splice, I assume that the boom is made of aluminum.

A long time ago, I made an aluminum fan box prototype.
I would have secured it with spot welds in production, but I didn't have a spot welder.

So I clamped the parts in place, match-drilled and match-tapped a few holes (#6ish) through the places where spot welds would have been, ran in some aluminum Pem-studs with Loctite, then cut the studs off flush.

( What the hell we were doing with aluminum Pem-studs is a mystery to me, but they exist(ed)).

The resulting joint was interesting; it resembled a spot weld and was basically flush on both faces. I never bothered to analyze it, but it withstood normal handling well enough.

You'd need many such threaded pins to splice a typical boom, but with some Tap-Magic and a cordless drill, it wouldn't be that big a deal.



Mike Halloran
Pembroke Pines, FL, USA

RE: Flat undercut 82 degree countersink/ counterbore

(OP)
It gets worse Mike, I have three of them to do. I have spliced masts before in the same manner, but the extrusion has been a little thicker,
where I could get by with a conventional countersink. I like the idea of the Pem studs they are 2024 or 7075, however If I do that I can just rivet it, with a conventional flat head aircraft rivet 1/8" dia 2117 , If I have a long enough bucking bar. The material for the boom is 6063.

also the 100 degree countersink gets rid of the knife edge problem. Now I have to search for a 3'-0" bucking bar instead of a flat bottomed countersink.
B.E.

You are judged not by what you know, but by what you can do.

RE: Flat undercut 82 degree countersink/ counterbore

Quote:

however If I do that I can just rivet it, with a conventional flat head aircraft rivet 1/8" dia 2117 , If I have a long enough bucking bar. ... Now I have to search for a 3'-0" bucking bar instead of a flat bottomed countersink

Maybe try Cherry Max CR3212 Countersunk Head. 50 cents each - but if you only have a few dozen -- possibly worth it.

ice

Harmless flakes working together can unleash an avalanche of destruction

RE: Flat undercut 82 degree countersink/ counterbore

(OP)
iceworm,
I actually have a bag of a hundred of those.
However on considering the options I decided to make a flat undercut countersink, I got that done tonight, I removed the pilot from an 82 degree micrometer stop countersink, ground the bottom flat on the sink, re set the rake, re-installed the pilot, then tried it on a piece of spar, it works very well. I will install the screws with Loctite. I was hoping that somebody knew where to get such an animal , but no such luck I will have about 160 to 170 holes to drill.
B.E.

You are judged not by what you know, but by what you can do.

RE: Flat undercut 82 degree countersink/ counterbore

(OP)
All finished today.
Boom extensions in place, countersink worked well.peace
B.E.

You are judged not by what you know, but by what you can do.

RE: Flat undercut 82 degree countersink/ counterbore

I love a happy ending.

Mike Halloran
Pembroke Pines, FL, USA

RE: Flat undercut 82 degree countersink/ counterbore

So do I. Particularly in this forum. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Flat undercut 82 degree countersink/ counterbore

Gotta work on the water though. And during the storm.

RE: Flat undercut 82 degree countersink/ counterbore

(OP)
racookpe1978
I am fairly confident it will stay together. I used a rivet hole pattern from the tables in AC 43.13-1A then used stainless steel screws instead of aluminum rivets. the butt sleeves were 3 times the major axis of the oval on each side so a 14 " sleeve for a 3x2 oval.
The sleeve was " persuaded" into place with a very large hide faced mallet. The only think that will be a worry later down the road will be dis- similar metal corrosion between the stainless screws and the 6063 alloy. By then since these are 50 year old booms they will be replaced with new units. I did not have time to get new booms. These boats are sailing in a regatta tomorrow ( Saturday ).
B.E.

You are judged not by what you know, but by what you can do.

RE: Flat undercut 82 degree countersink/ counterbore

Berkshire, I'd like to contact you about something you posted in another (now closed) thread, concerning FRP houses. How do you contact other members? (Folks, kindly excuse my intrusion on his thread. I'm new, and trying to find my way around on this system.)

RE: Flat undercut 82 degree countersink/ counterbore

(OP)
Metaphorce,
That experience was over 38 years ago when building inspectors and codes had no experience with FRP at all.
There has been a lot of progress in that area over the years. Please start a new post on forum327: Composite engineering where you will meet experts in the composites field.
B.E.

You are judged not by what you know, but by what you can do.

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