Sunday, June 13, 2010

Absolutely riveting

AC 43.13 Is a book written to cover a great many subjects - and most every subject in it has plenty of complications and nuances, from riveting to welding to wood repairs. Unfortunately, when writing the book, all the subjects therein got simplified by committee, which makes it full of not-quite-wrong but certainly not really right information.

This is why it's stated as "one way, but not the only way to do repairs." For more in-depth information, like riveting distances on capstrips (built-up ribs), there are more obscure sources like Piper Service Memo #10. (Why it was a Service Memo instead of a Piper Service Bulletin, I'm not sure.)

A few notes before I dive in: always wear gloves (thin leather ones recommended by my IA for safety & minimal tactile loss), safety glasses (a broken drill bit in the eye is NOT something to risk), and make sure your pieces are securely clamped when drilling. Cap strip material is light enough to hold by hand, but if it starts to helicopter, let it go and stop the drill. Don't try to catch it! Like any other tool including guns, finger off the trigger until you're ready to use it. If you are drilling multiple pieces at the same time and they aren't cleco'ed or clamped tight, a chip chaser is your friend. And don't forget to dress the faces of the holes with a larger bit so your rivet isn't cocked on a burr!

Rivets must be at least 1/8in from the edge of the rib and (if used) from the edge of the reinforcement / overlay. If you are riveting in the same holes that the factory did, just use those holes.

When splicing, you need two rivets, with at least 3/8in distance from the end of the overlap, and 1in distance from each other. Cap strip may be spliced anywhere except at a cluster or forward of the front spar.

When reinforcing with an overlapping piece of material, you need at least four rivets, cut the edges of the patch on an angle to be wider toward the base and narrower toward the edges of the stamped capstrip. All distances are measured in a stright line from the narrow edge in, ignoring the angled bits.

When reinforcing a fracture or crumple on a section without a connecting web, the patch needs to be at least 1.75in, as you need 3/8in distance from the outside of the patch to the rivets, 1/2in distance between rivets (two rivets per side of the patch), and 3/8 distance from rivets to the point of fracture or crumple.

When reinforcing a fracture or crumple at a section with connecting web, it's basically the same with 6 rivets - 3/8in distance from outside edge, 1/2in between two rivets on each side, 3/8in distance between rivet and web, and two rivets in the connecting web itself.

If you get your hands on the actual service memo, this is all in illustrations and numbers to make life simple.

For the record, 43.13's advice for single-row rivets was that rivet edge distance must be at least twice the diameter of the rivet shank, and spacing must be at least three times the diameter of rivet shank. But it has a nifty chart showing all the ways that rivets can be bucked wrong...

fortunately, you buy rivets by the ounce, and which means I have a lot of solid brass 3/32 SO rivets. I also saved the too-corroded, crumpled, and fractured sections that were cut out and repaired with splices, so I have plenty of beyond-repair rib material to practice on.

Oh, and when it comes time to be hand-squeezing your rivets in tight spaces - find a friend with a lathe and have them turn down the surfaces that that actually squeeze your rivet head and shank until they are small enough to easily fit against the capstrip, instead of crushing down against it or squeezing off-center. When tackling the inside of the reinforcing web, I cut the head off a bolt, wrapped a lot of duck tape around one end so it won't shift and threaded the other with a castle nut so it sticks out at the right length to be a very surface to buck the rivet in the hand-squeezer.

All that said, My left tip rib is now repaired. Next rib, I slipped when drilling out the bad rivets, made the holes too big, and am going to splice a new section and start over tomorrow. When working with soft rivets - use a punch and make a dimple in the top before you start, so your drill doesn't wander off the curved surface!

1 comment:

Old NFO said...

Oh that brings back memories... A-school 1971, NAS Memphis... I wondered if I would EVER pass the riveting test! :-(