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View Full Version : Countersinks.... there sure are a lot of sizes, some I don't understand.



J Tiers
11-29-2013, 12:03 AM
I was cleaning up, my once per decade cleanup, and decided to go through the misc countersink box.

I was very surprised. I figured to find both 82 deg and 100 deg, maybe some 90 deg.

Well.... I found 60 deg, 82 deg, 90 deg, 100 deg and 120 deg.

OK I figured that teh 60 deg have the obvious purpose. And the 82, 90, and 100 deg I know about. But I had never heard of a 120 deg countersink, even though I have some.

What uses a 120 deg countersink?

Mike Burdick
11-29-2013, 12:17 AM
... What uses a 120 deg countersink?

Most likely for flush rivets used in the aerospace industry.

ahidley
11-29-2013, 12:37 AM
They are used on THIN sheetmetal. This allows the countersink to spread out without countersinking into the screw diameter hole.

J Tiers
11-29-2013, 12:43 AM
I'm familiar with 100 deg for aircraft. And metric are 90 deg.

With 120 deg it isn't just the sheet metal that is thin, the countersink head is mighty thin, too. I'd suppose it would be for rivets, or allen/torx/Robertson screws, because there isn't much depth for a drive means that spreads out like pozi/phillips or slotted.

Tilaran
11-29-2013, 01:23 AM
Airboat hulls.

Mike Burdick
11-29-2013, 01:38 AM
... I'm familiar with 100 deg for aircraft. ...

I'll have to check this to be sure, but for flush mount rivets, I believe the aviation industry now favors the 120 degree as the standard due to its increased bearing surface. If the materials are very thin, the surfaces are then dimpled" instead of using a countersink. Either way, machining a countersink or dimpling, the 120 degree argument for increased bearing surface still applies.

EVguru
11-29-2013, 07:13 AM
If the materials are very thin, the surfaces are then dimpled" instead of using a countersink. Either way, machining a countersink or dimpling, the 120 degree argument for increased bearing surface still applies.

You often countersink the underlying material, then dimple the thin skin for riveting.

J Tiers
11-29-2013, 09:41 AM
I was under the impression that c-sinking into the support and dimpling the skin was pretty standard. But the 120 deg c-sink head, whether rivet or screw, is pretty darn thin even then.

Obviously they are used. It doesn't seem that the area is really THAT much larger between a 100 deg and a 120 deg head, but it may be that even a fairly small percentage is enough to be desirable. the added material is around the OD, where the maximum area per unit radius is the most. But the head is so thin there, almost a knife edge, if you want to use that added area, that the strength increase doesn't seem like much, particularly when the rivets are the same basic material as the skin (even if a higher strength alloy).

Stu Miller
11-29-2013, 12:51 PM
You are missing a 78 degree countersink in your set. I have one that I think was a wood screw countersink at one time.

J Tiers
11-29-2013, 06:30 PM
You are missing a 78 degree countersink in your set. I have one that I think was a wood screw countersink at one time.

And likely a dozen other sizes that somebody somewhere has used at some time for some special purpose.......... In the words of the F.I.L, "I ain't too worried about that".

;)

TRX
11-29-2013, 07:13 PM
Unfortunately, I've run into both 80 and 82 degree countersinks. Arrgh...

Paul Alciatore
11-30-2013, 03:28 AM
Unfortunately, I've run into both 80 and 82 degree countersinks. Arrgh...

If they are that close, does it really matter? Except perhaps for NASA.

J Tiers
11-30-2013, 08:58 AM
It's enough to alter the "hold". Although the steeper one will at least still be held at the OD by a typical 82deg or shallower head. A steeper head and shallower c-sink will be held at the ID, which is undesirable.