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.
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
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
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
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
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
Boom extensions in place, countersink worked well.
B.E.
You are judged not by what you know, but by what you can do.
RE: Flat undercut 82 degree countersink/ counterbore
Mike Halloran
Pembroke Pines, FL, USA
RE: Flat undercut 82 degree countersink/ counterbore
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Flat undercut 82 degree countersink/ counterbore
RE: Flat undercut 82 degree countersink/ counterbore
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
RE: Flat undercut 82 degree countersink/ counterbore
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.