Dimble Issues

Having just ordered the empennage kit for a -14A while at AirVenture, I thought I would do a bit of practice on the Van?s tool box kit. The instructions direct me to match drill (#40) the end pieces and sides then to dimple BOTH with 3/32? dimple die. Rivet together using AN426AD3-3.5 flush rivets. I followed the instructions and worked carefully but the end results were not as nice as I expected.
My rivets are flush, no smiley-faces or dents in the sheet metal, the shop heads are all straight and they?re sized properly. But . . . the rivet heads don?t fit the dimple. There is a ?ring? around the head. I dimpled more holes using my c-frame but every one I tried resulted in the same issue. After some thought, it stands to reason there is a ring because the dimpled hole has a radius. I did more testing and realized a couple more things. When I match drilled the holes with the #40 then dimpled, the hole got bigger. While the rivet wouldn?t fit in the factory punched hole prior to drilling, it did if I skipped the drilling and simply dimpled the piece. When I compared the non-drilled with the drilled holes after dimpling, the ones I drilled are significantly larger. The rivets are pretty loose when I check them for fit prior to riveting.
Third, when I check the fit of the dimpled skin and rib (end and sides on the toolbox), they don?t nest perfectly. It seems to me if I dimple each with the same size dimple I will have gaps in the joints. The only way I can see to fix this problem is to use ever greater size dies on each piece below the top one. So my questions are, should I use one dimple die size for each layer or different size for each piece? Should I skip drilling them if the rivet will fit without drilling after it?s been dimpled? Is the ?ring? around the rivet head just something I?m having to live with? And finally, am I looking for perfection where it?s not possible?

Hard to say without photos, but, typically, people new to the C frame don't hit it hard enough. Dimples should be crisp.

Answer kinda depends on what you mean by a 'ring'. Properly set dimples will display a concentric circle around the dimple, roughly the diameter of the die. If by 'ring', you mean a slight 'volcano' shape, with a ring shaped bump around the crater, then you need a bigger hammer. Seriously, most of us didn't hit the c frame hard enough when we were learning. Can you run your fingernail across the set rivet & catch your nail on the edge of the rivet? That's another indication that you didn't hit the c frame hard enough. Not nesting well is another indication of ... Well, you get the idea.

Try laying a straightedge across the heads. If it doesn't lie flat with no air between the rivets ...

Charlie

Congratulations on being a keen eyed observer. Three things I found: 1. The head never set in the dimple no matter how hard it was it or squeezed. I put the deburring tool in and with a practiced force spin it 2-3-4 times. It does not take much but all the rivets will nestle perfectly. 2. Drill the holes first to ensure alignment, then dimple. Only dimpling will stretch the inside of the hole, possibly initiate cracks, and provide aluminum transfer to the die pin. Fact. 3. No the parts might not nestle perfectly with two dimpled pieces. While there are dies for the lower piece with appropriate angles (Cleaveland), it is of no matter to the integrity of the joint. Others have sectioned the variables and shown that it is not needed, although geometrically it is imperfect initially. Covered in detail in other VAF posts. Lots of debate, as expected.

One note on #3. When doing multiple pieces like the firewall to skin, I used a dimple die on the stack to ensure the settled together, a hardened tapered pin for alignment and the rivets set this way pulled all the pieces tightly together.

With your keen eye, keep paying attention as you build and will find ways to address the tiny differences you are finding.

Happy building.

Here is a good video from Mike at Cleaveland tools
https://www.youtube.com/watch?v=Qo9QCMaNSoA