Dimpling, need advice...

Hello guys,

I started riveting the left HS skin and noticed something on my dimpling.
I've always done countersinking and not used to dimpling.

So, when I'm using the squeeze for dimpling, the installed rivets looks way better and sits perfectly on the skin. (2 first rivets on the left)
When I use the C-Frame, with the same die, it looks under done and the rivets are not perfectly flush.

Not very clear on the pictures but trust me, it doesn't look that good.

Any advice on how to do the dimpling with the C-Frame?
What's the tolerance according to flushness?

Thanks a lot!
Here's the pic :

Counter sinking should only used when the skin thick enough to avoid leaving a knife edge in the whole. This is why most of the airplane requires dimpling.

Are you sure you're whacking the c-frame hard enough with the mallet? Perhaps try on some scrap to get a feel for it. When I was still using the c-frame my whacking was done with a 5lb plastic mallet. I know there's a vocal micro-minority here that does not like the drdt-2, but that is what I use for dimpling now and rarely use the c-frame for dimpling anymore.

I use the C-frame dimpling tool and a 2 lb. hammer, not a soft one. One good whack gives one good consistent dimple.

A friend has the DRDT-2 and it's okay, but I this way. This is certainly easier on my wrist.

Dave

sahrens said:

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As for dimpling versus countersinking, 0.040 is the minimum thickness for countersinking. Some would rather dimple 0.040 than run the risk of a knife edge. As most of the skins are thinner you will want to ensure your dimpling technique is well developed so you get the finish you want.

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Actually the FAA uses a 0.032 minimum thickness countersink for a 3/32 rivet in AC 43.13, as does Vans instructions Section 5.

However the mil spec uses the 0.040 you mention.

I have seen an Art Chard RV-6 fuselage that had countersunk rivets in the 0.032 skin section and it was great. For the rest of us dimpling is easier...

I use a 5lb steel sledge hammer. I was told that you want to shock the metal into form when using the c-frame. I doubt that's actually true but I have no doubt that you do need a BFH to impart the same force that a squeezer can. In anycase, my dimples are the same with either tool. But one needs ear and eye protection when using the c-frame.

You are right, those are under done. +1 on using a steel hammer, not plastic or rubber. Mine is not that big, 13 oz I think, but it does the trick. If you are already doing this then you may need to consider better dies.

Are you going to polish or paint? I can see where a C frame would give a moore even dimple for the award winning polished plane, but I went the other way and painted.

Hands down I would..

Recommend getting a DRDT 2. I started out using the C-Frame and was very unhappy with the results. I was able to get my hands on a DRDT as well as a set of Cleveland dies. The dimples are very consistent as well as it is must faster to set them. I'm sure it's the combination of the two. Cleveland makes a great set of dies.

Solid base

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.

bhester said:

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.

Click to expand...

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

Dimples

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.

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!

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

Thanks guys!

Hooo and one other thing...

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

Test

lShadowl said:

Hooo and one other thing...

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

Click to expand...

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.

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.

FWIW

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.

sahrens said:

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..."

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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-HDBK-0001-0099/MIL_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.