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Match drilling replacement part to dimpled skin?

missile29

Member
After doing a not-so-great job countersinking the holes in my HS stringers, I ordered replacement stringer parts and am going to remake them right. But now I’m faced with the problem of how to match drill the new stringers to the already drilled and dimpled skins.

My plan is to skip trying to match drill the stringers to the skins and go straight to countersinking the holes for the dimples.

To do that, I’ll be hoping that the non-match-drilled holes are still large enough to accept the C-sink cutter’s pilot. I think they are, but will be close. If they’re not, I may drill the holes out with something smaller than #40, either a #41, or something smaller, that's just large enough for the pilot to follow but not quite the final size.

Once the holes are countersunk, I plan to go ahead with final assembly of the HS as normal, but check carefully that the skins and stringers line up. Basically, I’m hoping the holes are so close as they come from the factory that they didn’t really need to be match drilled anyway.

But if I find that there are holes that aren’t lining up, I can match drill those at that point during final assembly, since the dimpled skin should fit into the countersunk hole in the stringer flange.

If I do need to match drill a hole or ten, I should be able to reach in and clean up or debur the hole after the fact.

Any thoughts or advice here? Am I going about this all wrong? In general, is this kind of "do-over" fairly common and consequence free? Or is my HS going to look like cr*p and have a bunch of oil canning because the new stringer holes didn't quite line up? (Sure hope not...)

Thanks!
 
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How about the a/c model, and some part numbers? Photo? I'm confused. If you are talking about what I think you are, those are normally dimpled not countersunk. I must be thinking about the wrong part.
 
What are the “stringers” in the HS you are talking about? The spar? The ribs? I have had luck using the spring loaded drill stop things, and adjusting it so the tip of the drill just barley sticks out. The ball of the drill stop will center in the dimple, and the drill with go through when you press a little and the spring compresses.
 
The holes needs to be large enough for the countersink pilot so that it does not seize and snap off.
As a first step, I would place the new spar beneath the already dimpled part and insert a few rivets or clecos to keep it in place. Then sight down through all of the dimpled holes to see if they are concentric with the underlying holes in the new spar. If they are aligned, then just enlarge to #40 the holes in the spar - you may be best off using a drill press to ensure they remain perpendicular.
 
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Match drilling

I've done similar. If they are prepunched, it's probably not a problem. Just use a lot of clekoes. If no match hole, you need accurate marks and clamps to get started so the part doesn't wander. I use a short piece of brass tubing. 1/16" I think will accept a #41 bit. Round the tip so it nests centered in the dimpled hole. Use it to center the bit.
 
Thanks to those who've replied, and sorry for not providing more specifics.

A/C is an RV-10. The parts in question are the HS-1014 and HS-1015 stringers that, along with the HS-1016 stringer web, form what you might call a "mini" spar in the center area (inboard and mid-chord) of the horizontal stabilizer. The stringers are formed from L-channel-like strips, and come pre-punched with holes. Material is same thickness as a spar, but it's not a full spar.

Since the holes are pre-punched, I'm hoping (as wirejock says) that it'll just work out. But I'll definitely lay the parts on the dimpled skins first and align with a few clecos and drop some rivets into the other holes and see how they're going to line up. If some holes look way off, I'll use wirejock or Taltruda's methods to help center the bit in the dimple.

Thanks!
 
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