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Slightly Over Countersinking

Jarrett

Member
Today I was countersinking the front spar doubler for the horizontal stabilizer and one of the holes was slightly over countersunk (on the right):
https://photos.app.goo.gl/C9Cdg5EfmnjB8qYv5
It's a pretty thick part so I'm not too worried, but I haven't countersunk before, so I want to get some other opinions. Should I worry about it or build-on? Thanks!
 
It's hard to tell from a picture but I think that it's Ok. Get a local to look at it. The trick for seriously deep counter-sinks is to refill the hole with a structural epoxy (not glassing resin or epoxies from a hardware store, which are either too brittle or too soft). Once it cures, smooth to the surface and then redrill and re-countersink. This is an acceptable fix only for an isolated oops. If the underlying hole was also widened, then you will need to supply a backing tab to keep the rivet from pulling through the back. If the underlying hole has been widened to 150% original size, then you should also add additional rivets on either side and possibly just fill the bad countersink without putting a rivet there. Good structural epoxy can take the compression of a rivet head, up to a point. Another good possibility is using an 'oops' rivet or simply a larger size rivet. It's an extraordinary person who can spot that one bad rivet out of thousands unless they go over your project rivet-by-rivet - which will give you plenty of time to plant a boot in their overly critical behind.
 
I've replaced a few parts countersinking. The toughest spots for me are the ones on the edge of a part. There is a tendency to roll off the part and gouge out the side of the countersink nearest the edge. Using the press helps prevent it. Knowing it is likely to happen helps a lot too. I also have started using the narrow micro-stops more than the wider stops because being narrower they sit on most faces completely.

Rolling off also happens on a narrow flange; these also can be ever so slightly rounded in fabrication and that also makes a good c/s a technical challenge sometimes.

Your slight overage here may be from slippage. As you countersink you can go to deep; you can also find the hole slips or drifts from elongation and the pilot moves.

I'd wager your small imperfection here isn't material. But some items can be critical. A reassuring email to Van's is always good. I've a stack of them.
 
The main issue when machine countersinks are too deep (assuming the material is more than .032 thick so that the hole is not enlarged and/or knife edged) is that when setting the rivet it will not be tight to the bottom of the countersink.

The outer face of the rivet will be flush with the outer surface of the material which will leave a gap under the rivet head.

What you can do to mitigate this is use something as a spacer over the rivet head to assure that it is pushed to the bottom of the countersink while the rivet is being set.

I have made spacers by punching a small aluminum disk of appropriate diameter using a Whitney punch and place it over the rivet head before squeezing the rivet. I have never tried it driving with a gun but if the spacer was taped in place, that might work as well.
 
A tip on how to set up for countersinking

When I started this venture, I was always puzzled when trying to figure out how to make a countersink the correct depth.

First of all, test on a scrap piece! That is the most important step! Hardware store aluminum angle works fine for this.

Also, always back the piece up with something if the piece is too thin to stop the countersink from wandering. If you don't, it will meander all over the place, making the hole to big and oddly shaped. The little nub on the bottom of the countersink tool should be fully in some material. You can use aluminum/wood/thick plastic or whatever drilled to the hole size. This will be important when you do the counter-sinking on the wing spar to attach the fuel tanks to the main spar flange.

A friend showed me that the easiest way to set the depth of the countersink is to measure the width of the hole. I'm talking about the top of the surface where it is widest from counter sinking it.

I do this using a dial caliper (micrometer) to check it. It should be the same width as the widest part of the flush rivet that you want to put in there. You can get it very close to the correct depth this way. Then test it by sticking in your rivet to fine tune the depth. After it's good to go on your scrap piece, it will be good to go on your plane.

It works for me. Your mileage may vary as they say. I hope this helps you for future holes... Happy building!
 
One way to set a rivet in a too-deep countersink is to punch a 3/16 hole in .032 aluminum with a Whitney-Roper punch. Place the punch-out disk, which won't be flat, over the head of the rivet and set the rivet.

It's left me with good sets and, it doesn't matter, it flattens the disk. Only reason to mention the disk's flatness or lack of it is so that you know what to expect.

Don't do this often!

Dave
 
Same problem

I've had the same issues, and the backing piece made a big difference to me. I no longer trust the countersink cages. I've already replaced two pieces due to this, which I now use for scrap to drill test countersinks.
 
That looks OK to me. If you look at commercially set rivets on say an Airbus, there is a tiny ring around the head of the rivet. I always set the countersink so I get this tiny silver ring - then the rivet sets perfectly every time.

Countersink cages are the very spawn of Beelzebub - they seem to re adjust themselves on the shelf ! I have a 6" square piece of .063 with several #30 and #40 holes in them. I also have a small coupon of .025 with a corresponding dimple. Before using the cage, I always pop a countersink in and gauge it either against a rivet or the dimple. Takes seconds and always worth it.

For the odd countersink - say for nutplates, I just use the 3 flute deburring tool and a calibrated wrist :D
 
I've had the same issues, and the backing piece made a big difference to me. I no longer trust the countersink cages. I've already replaced two pieces due to this, which I now use for scrap to drill test countersinks.

Where did yours come from? Mine had a spring that was too long and caused the cages to jump if pressed hard enough worst case or were inconsistent to within a couple thou best case. The solution was to clip off a couple of turns. After that they worked perfectly for hundreds of holes. Push HARD and if it jumps you have the same issue I had.
 
Countersink cages are the very spawn of Beelzebub - they seem to re adjust themselves on the shelf!

This is so true! To minimize the problem, I have one cage set for 1/8-inch rivets and another set for 3/32-inch rivets, but they still need to be slightly adjusted almost every time I use them.
 
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