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What to do about a few over-countersunk holes...

missile29

Member
I’m looking for some advice on what to do with a few (3 actually) holes that I over-countersunk on the horizontal stab long stringers. The holes in question are the ones at the tips of the long HS stringers. To be more specific, the holes are not next to each other. Rather, each hole is at the very tip of each end of the two long stringers. So both ends of one stringer, and one end of the other, have screwed up holes. Grrr...

Since these are holes for skin dimples, I had my C-sink cage set to flush + .007”, and I was merrily countersinking away when I got to the last hole near the end of one of the long stringers. At that point, the cage isn’t supported well by the part itself (shame on me for not shimming under the unsupported areas). I was using the countersink cutter with a single cutting edge. This cutter makes a nice clean cut and doesn’t chatter, but it sure takes a big bite out of the aluminum, no matter how slow I try to go. Long story short, the bit bit into the aluminum and the unsupported part of the cage let the whole thing torque in my hand, and the end result is a countersink that’s way bigger than it should be for an AD3 rivet.

Embarrassingly, this happened to some degree on 3 out of 4 of the holes at the tips of both of the long stringers. I thought I learned my lesson after the first one, but try as I might to go slowly and shim and support the cage on subsequent tip holes, the only way I got the last one done properly was to back the cage out some, make a first pass, then come back with the proper depth in a second pass.

So my question is, what should I do about the over-countersunk holes? Again, these are at the very tips of the long stringers on the Horizontal Stab. The rest of the holes are fine.

Should I:

A) trash everything and start over on the whole plane (kidding, but I feel that way after a screwup like this),

B) start over on the horizontal stabilizer (also kidding, I hope),

C) order two new stringers and do those over (this raises the question of how I would match-drill the stringers to skins that are already dimpled,

D) close my eyes and pretend it didn’t happen, it’s just a few holes, only at the tips, they are not next to each other, and everything will be fine, or

E) drill the offending holes out to an AD4 rivet, drill out the corresponding holes in the skin and in-spar ribs, and use a bigger rivet, or

F) … something else???

I’m obviously hoping for D or E. The holes in question really are out at the tips of the stringers, so I don’t think there is any structural compromise. My concern with just plowing ahead like nothing happened is that when I rivet the skin into these over-countersunk holes, the skin will be stretched and stressed and it could lead to a crack. At least I could inspect for such.

Answer E (next size rivet) seems like it would solve the problem of the skin stretching, and that’s the solution I’m leaning towards. The C-sunk holes are almost the diameter of the top of the head of an AN426AD4 anyway, so that seems like a workable solution. But I’m anxious to know if there’s a reason this is a bad idea.

I could stomach C (reordering and remaking the stringers) and I know people reorder things all the time, but I’m curious what folks do about match-drilling when they have to reorder a parts this far into it. It isn't really feasible to reassemble the whole structure and match drill and new set of stringers since (per the order of the plans) everything, including the skin, is already dimpled.

As for F, something else, I’d love to hear what other folks do in these situations. Surely people don’t toss a whole part and start over just for one or two (or 3) oversized holes or countersinks. I toyed with the idea of trying to think of some kind of shim, goop, epoxy, pro-seal, or any other material that would safely serve the purpose of filling the gap, but other than (maybe) Pro-Seal, that seems like a real hack.

Sorry for the short novel, and thanks for any advice!

—mgm
 
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E, go with an AD4 rivet. Drill the skin with a #30 bit and dimple with the 1/8” die set.

You will never notice the different size rivet head.

Carl
 
If you are going to make a mistake on a rivet, and there are 18,000 rivets, the place to make it is at the very end of a long string of stressed skin load bearing rivets. That rivet is the least important one in that string, and these rivets are normally loaded in shear. When you think about it, if that load line flexes, that maximum deflection is in the middle of that string of rivets - like between ribs, stringers, longerons, etc. The one at the end has the least amount of shear load. Your over countersunk flush rivet should still have the same shear strength as a normally countersunk rivet. If it’s loaded in a way that would require tension strength, then you’ve lost some of that, but at the tip end of a row of rivets, I’d say build on and keep your eye on that anomaly going forward. I’m not an aeronautical engineer, just using common sense, so listen to others more qualified, but it sounds like you are analyzing things to a degree that guarantees success. Good question though.
 
Thanks folks for the advice! I took a step back from it last night and am trying to come up with a way to NOT have that happen on the spars themselves. I sure wish I had a way to put the whole thing in a drill press, but I can't think of a good way to make a contraption to hold it. I'd settle for a jig that just holds and keeps the countersink cage square when using a hand drill.
 
MGM,

E is best option.

To remedy.
Use 1x4 wood cut to size under project piece.
Use scrap piece of metal same size of piece, fasten all to table, drill in place.

Will keep all square and level end out so cage will not tilt.

Boomer
 
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