Countersink/Dimple Math?

[Golf Echo]

Dear =VAF= Brain Trust,

I had some minor frustrations in the countersinking of my first part (HS-710) to accept the 1/8" dimpled spar and an AN426AD4-6 rivet. It appears this was actually due to not squeezing the dimple dies quite enough. Regardless, I'm left with a countersink which is too deep. Deep enough to require a new HS-710? I dunno... that's a topic for another thread (and probably a question for Van's).

In any event, the "guess and test" method failed me. The Van's manual says "a couple of clicks", or "about 0.005" below the surface". I figure there has to be a better way to at least ballpark the depth the rivet should sit below the surface if dropped into the countersink. Here's my go at deriving a ballpark depth (d) based on skin thickness (t) and a countersink angle of 100?.



So, given a skin thickness of 0.032" (HS-702 spar thickness), the calculation says the rivet should sit approx. 0.010" below the surface if dropped directly into the countersink in order to get a perfect[ish] fit.

Make sense? Has anybody done/used something similar as a starting point?

[Golf Echo]

Deep enough to require a new HS-710? I dunno... that's a topic for another thread (and probably a question for Van's).

The thread dealing with my specific goof-up: http://www.vansairforce.com/communit...ad.php?t=94662

[az_gila]

As the skin is dimpled it stretches and thins out a bit.

If it just gets 0.005 thinner, then Vans estimate is correct....

[aturner]

This thread may be useful:

http://www.vansairforce.com/communit...288#post472288

[AF_Alan]

An alternative to the math/measurement method to determine CS depth, I made and use templates (for various skin thicknesses) as suggested in threads similar to this one:

http://www.vansairforce.com/communit...ad.php?t=32283

Also learned that after changing the CS bit in the cage, it may tighten with use and need very slight adjustment deeper after a few holes.

Good luck!

[terrye]

Yes I did much the same as the OP. I eventually made tables for all standard skin thicknesses and the -3 and -4 rivets. Part of the difficulty is deciding just where to take the intersection point on the inside of the dimpled skin, since it is really a radius at this intersection. So my table has a "fudge factor" or tolerance on the countersink diameter. The second part of the difficulty is deciding if the dimpled skin fits the countersunk hole. It's easy to convince youself that the countersunk hole needs to be "just a little deeper" when it really doesn't. Again I rely on the large scale paper layout, the math and my tables that are derived from them. The final difficulty is precisely measuring the diameter of the countersunk hole. There are cone gauges that do this but they're $$$. I use a magnifying glass and a digital caliper with a strong light. I also do test holes in scrap material for the initial setting.

[Golf Echo]

Originally Posted by az_gila

As the skin is dimpled it stretches and thins out a bit.

If it just gets 0.005 thinner, then Vans estimate is correct....

True, but that would be a 16% change in thickness due to deformation. I might be off the mark, but I wouldn't estimate that much of a change occurs during dimpling/riveting.

[Golf Echo]

Originally Posted by aturner

This thread may be useful:

http://www.vansairforce.com/communit...288#post472288

Originally Posted by terrye

Yes I did much the same as the OP. I eventually made tables for all standard skin thicknesses and the -3 and -4 rivets. Part of the difficulty is deciding just where to take the intersection point on the inside of the dimpled skin, since it is really a radius at this intersection. So my table has a "fudge factor" or tolerance on the countersink diameter. The second part of the difficulty is deciding if the dimpled skin fits the countersunk hole. It's easy to convince youself that the countersunk hole needs to be "just a little deeper" when it really doesn't. Again I rely on the large scale paper layout, the math and my tables that are derived from them. The final difficulty is precisely measuring the diameter of the countersunk hole. There are cone gauges that do this but they're $$$. I use a magnifying glass and a digital caliper with a strong light. I also do test holes in scrap material for the initial setting.

Sounds quite similar. I tried to measure the countersink diameters with my digital calipers, but I found it quite difficult. Conversely, it's very easy to get an accurate measurement of how far below the surface the rivet head sits when dropped in.

I suppose a few more calculations would correlate the depth below the surface with the countersink diameter at the surface. All you'd need to know is the diameter of the factory rivet head.

Sounds like something to do over a cup of coffee in the morning...

[Golf Echo]

Ok, so my inner engineer came out again...

I took Terry's diameter numbers and figured out how those numbers would relate to the depth below the surface the rivet would sit if it were dropped directly into the countersunk hole. Note: calculations are based on a 100? countersink angle.

The orange and green columns are the depths based on Terry's range of diameters and the min/max head diameter specs for an AN426 rivet. The far right column are the numbers based on the formula in the first post in this thread.

Interesting that Terry's numbers are also only dependent on skin thickness (t).

[Smilin' Jack]

If you do this math on every dimple you'll be flying in about 100 years

Vans show you pictures, every time you have to machine dimple it is usually in a different circumstance and several times I have had to take the depth gauge of the machine countersink just to be able to get into where I have had to go.. Use the pictures as a guide and make them all the same, one last item, don't tilt the drill when countersinking,you'll really be upset then.