Dimpling, need advice...

[az_gila]

Originally Posted by bhester

My dimples where not forming correctly when I first started. I had a pocket in my table that the c-frame was in. I also had a 2x4 leg directly under it to the floor, but that was not enough. I took it out put it on the concrete floor, and made perfect dimples. I got a 1/4" steel plate, put it in the table pocket under the c-frame and made perfect dimples using my table. Problem solved. Hope this helps.

I found the same - I put the C-frame on the concrete floor instead of the workbench and then two whacks gave good dimples.

[wirejock]

Looks like under dimpled holes. No burnish ring. It also looks like you may want to use some heavy packing tape over the rivet to protect the skin from the mushroom set burnish marks.
I'm with the few who use a c-frame on a table directly over a leg on concrete. I use a 2lb dead blow. I'm on my third but Harbor Fright has a lifetime warranty. I blow a hole in the hammer after a few skins. Gives you an idea how hard the hammer is hitting.

[Tom Martin]

I am working on airplane #11. All dimples done with an inexpensive C frame and good dies. By good, I mean Avery or Cleveland dies. The key is at least one sharp blow. Typically I do one or two light taps followed by a firm hit. The firmness of the hit depends on the thickness of the sheet. It "sounds like" tap,tap, kerthunk, with the kerthunk almost a sharp ringing sound.
Too hard will deform the metal sheet, expanding the metal around the hole.
Too little will look exactly like your pictures.
A good row of dimples is easy to see by looking down a row of dimples. Do ten dimples in a row. Now hold the sheet up and rotate the sheet so your eye is looking along the row of dimples. Keep rotating the sheet until it is flat and you can no longer see the dimples. If they are formed correctly the sheet will look perfectly flat as if there are no dimples at all. Any ripples in the skin are an indication of under or over dimpling a rivet. Typically the error is not hitting it hard enough. Wack a few, in a scrap, really hard to see what it looks like when you over dimple. It takes a pretty hard hit to damage metal .032 or thicker.
When you get the "feel/sound" of a correct dimple the process becomes really fast and consistent. The sound, when you get it, is almost as pure as a musical note. It is also kind of satisfying to hit an expensive airplane part with a hammer!

[lShadowl]

That gives me a few things to try.
I will practice on scrap and see how it goes.

Thanks guys!

[lShadowl]

Hooo and one other thing...

What should I do with those?
Leave them as is?
Remove all the skin and re-dimple?

[wirejock]

Originally Posted by lShadowl

Hooo and one other thing...

What should I do with those?
Leave them as is?
Remove all the skin and re-dimple?

It's your plane. If you're ok with them, leave them.
If you want to try and fix them, make a test piece and duplicate the dimples then see if a good whack or two fixes them. Test first.
If you decide to drill, practice that too. Search VAF for techniques.

[Lemmingman]

Hi Martin,
You may want to reveiw section 5E of the manual (page 5-4 in mine) that describes countersinking and dimpling where, why and how. It discusses thickness and technique.

[Bill Boyd]

I encountered sort of the opposite. Worked on my first skin this weekend, and noticed as I was shooting the first few rivets that the dimples I made with the C-frame on a non-reinforced bench with a rawhide mallet were much crisper than the ones along the edge I made with the pneumatic squeezer. After a few flush rivets where I could feel the edges with a fingertip, I stopped what I was doing, got out the squeezer and set it up more properly, and re-dimpled the edge holes I could still reach.

It is possible to make good crisp dimples with a C-frame, and pretty poor ones with a pneumatic squeezer if you aren't careful. Shallow dimples will doom your sheet metal job to looking pretty crappy no matter how good you are with a rivet gun/squeezer. They won't sit right and there will be a wavy surface around each factory head.

Bench reinforcement may be necessary for best C-frame dimples in some cases, but my EAA workbench gives good results in .032 without.

[az_gila]

Originally Posted by sahrens

Gil,

Thank you for pointing that out. I searched through chapter four of 43.13 but could not find a specific minimum thickness for countersinking. Can you tell me where you found that information. Here is the Vans data:

"For AD3 rivets, a total material thickness between .016 [.4 mm] and .032 [.8 mm] must be dimpled. Material thickness between .032 [.8 mm] and .040 [1.0 mm], should be dimpled, but a countersink may be used if necessary. Finally, for a thickness of .050 [1.3 mm] and above the material must be countersunk.

For AD4 rivets, .050 is the minimum thickness that may be countersunk. Metal thicker than .040 is difficult to dimple, so it is common practice to machine countersink any material thickness that is more than .040. There are a couple of reasons for this. First, although RVs are designed so that a countersunk joint is acceptable, the interlocking nature of a dimpled rivet line is stronger. Second, dimpling leaves a thicker edge for the shop head of the rivet to form against, meaning that the metal deforms less and there is less chance of "working" rivets later.

This recommendation is meant as a guideline, not an absolute rule. There may be instances where countersinking 0.032 stock makes the job easier..."

I believe it was in the old version of AC43.13 but will still check.

I thought it strange that the FAA and the mil spec differed, but in researching the Mil Hndbk 5 reference I see this footnote -

b Values above line are for knife-edge condition and the use of fasteners in this condition is undesirable. The use of knife edge condition in design of military aircraft requires specific approval of the procuring agency.

NOTE 0.032 with a 3/32 rivet is "above the line"

Page 1254 of this big file - http://everyspec.com/MIL-HDBK/MIL-HD...L_HDBK_5J_139/

Older FAA documents did not differentiate and said a countersink of 0.032 was OK, which I'm sure is where Vans paragraph came from.

My old FAA "A & P Mechanics Airframe Handbook" (hard copy) graphically shows the 0.032 countersink and says "..permissible and should be avoided".


So the references I find all allow a 0.032 countersink, but suggest not to use it.