Countersinking and dimpling are not the same thing. In countersinking, you are removing metal to form a shape for a flush head rivet.

In dimpling (as we refer to it), you are changing the shape of the metal to accept a flush head rivet.

I don't know what a spot rivet is either, but I can offer this advise: Don't overthink this. Remember, a lot of those things aren't about matched-hole tooling in which the hole has already been drilled and matched.

Line up 'em up, cleco, drill to full size with the bits in your aircraft tool kit, debur, dimple, rivet.

Repeat about 10,000 times.

Fly.
 
Dimpling always expands the hole. By not dimpling without deburring, you get a slightly tighter hole. The dimple dies sorta swage the hole, leaving less to deburr afterwards. There've been posts about cracks forming out of dimples that weren't deburred. I've never seen that. I have seen cracks from holes that were dimpled without final drilling first. Pretty sure that's from the pilot stretching the hole before the dimple forms. There just isn't much to deburr after dimpling. A quick swipe to chase loose chips is usually enough.
 
One thing to keep in mind is the difference between predrilled and prepunched. As Bob said, drill them to full size. This does a couple of things, obviously makes them very slightly larger, aligns them perfectly (although they usually are anyway,) AND removes the tool marks that are perpendicular to the surface. These are the marks that can cause cracking when the material is dimpled. As the material is dimpled, it is stretched. #40 dimples aren't likely to crack, but #30 and especially larger dimples for screws are more likely to.

There has been some debate as to whether it is necessary to drill out 3/32" holes to #40 before dimpling. It is your airplane, but I think Bob said it right--cleco, drill, deburr, dimple, and rivet, in that order. Cutting corners can save time, but can lead to problems later. The old "Pay me now, or pay me later." can apply here, in matters of time as well as expense.

Bob
 
I took a piece of scrap stiffener angle (tank stiffeners, IIRC) that already had prepunched holes, and performed all the different permutations of ways to get to a dimpled hole. From dimpling the prepunched hole to drill, debur, dimple. I also tried the debur process with a one-hole debur tool in addition to the 'industry standard' 3-flute debur tool.

I'd suggest trying it yourself, then inspect each hole under low power magnification, & report back. Also, search the archives for previously given advice on deburring, & compare the advice to what you see.

Charlie
:-}
 
You must log in or register to reply here.