Cracked dimples

Hello all thogether!

I found a real showstoper after I started dimpling baggage ribs and seat ribs of my center section.

I could not beleve what I have seen, but look yourself. 2/3 of all the dimpels are cracked




More pictures later in the thread!

Yes I had the correct drill, I deburred with a good CS tool, no damage before dimpling in the hole, ... all was fine as I have done it 1000th of times during the build up of the empenage, wings and aft fuselage ...

I broke the pin of a dimple die a few month ago, and this could be the first time I'm using the new one! I bought it from a very well known tool supplyer, so it should be ok, but lets check this first. I dont think it can be the material,
because it happend on different parts, different thicknes of material, prepunched and drilled by my own holes.

Does someone can give me mesurements how the dimple dies should be?

- mesurement of the conical part at the bigest diameter?

- height of the conical part where the pin start?


I think I have to buy new baggage and seat ribs ... I can not think about to deburr, straigthen and flute these parts AGAIN

Thanks for your inputs,

regards, Dominik

RV-7A slow build
Switzerland

Check your dimple die.

Are you sure your dimple die is 100 degree? Those dimples look more like 82*. 2024-T3 will not dimple to 82* without cracking.

I'll bet you drilled those holes out using the recommended #19 drill bit. (It sucks when you get bit by following the directions). Holes for flush #8 screws need to be drilled using a #17 drill bit. The extreme stress caused by the dimple dies will concentrate that stress into the screw hole. The larger #17 hole will prevent that cracking.
I AM assuming that you are properly de-burring those holes first. I recommend de-burring with a ScotchBrite disc, not those Mickey Mouse de-burring tools.
Charlie Kuss

whats the shop/ material temperature? the few metal troubles i had were when the shop temps were 40 deg f or less. however, i did alot at those temps with no probems though.

You may want to check this out. I had similar results after dimpling my fuel tank skins.

http://www.vansairforce.com/community/showthread.php?t=13696&highlight=cracked+dimples

Thanks for your inputs.

Sorry I forgott to mention, this are holes for a AN426AD3-3.5 so the should really not crack and be easy to do. I have done a few thousends of them, empenage, wings, aft fuselage, without any problem. Drilled with #40 and deburred with a CS tool until I see a very small surface, then run a scotchbrite a few times over it, clean it an the dimple it.

Shop temp was 70?F, so it was not cold.

The angle should be 100?, the dimple dies are engraved with 426-3, but I will check them.

Will come back with mesurements.

Regards Dominik

Now my eyes are not what they use to be, but that doesn't look like a crack. It looks like there was a chip of material on the die that made an impression when the dies came togeather.

Are you sure that is a crack?

Yes it is a crack, the dies are clean.

It was not so easy to make a photo, let me try another one.

Will be at the building place tomorow mesure and checking things.

Dominik

no need for another photo those are coming in loud and clear. something is amiss but i dont know.

Sorry for the delay with more information, was a busy week.

Here are pictures from the male dimple dies:











To the X in the sketch I have mesurements, I checked different dimples, Nr. 1-3 are the NEW/BAD one, Nr. 4 is the one that I used for years, but is now broken, Nr.5 is a dimple from a buddy and Nr. 6 is a dimple from an other forum member, Nr. 7 is a Tank dimple die:

1: 5.06mm / cone height 1.28mm
2: 4.98mm / cone height 1.12mm
3: 5.15mm / cone height 1.17mm / rivet sits 0.27mm below the surface
4: 4.76mm (old, broken one)
5: 4.62mm / cone height 0.95mm / rivet sits flush with surface / dimples from a buddy
6: 4.85mm / cone height 1.10mm / dimples from another forum member
7: 5.01mm / cone height 1.18mm (Tank dimple)

All these dimple dies are from the same manufacturer!

Look at Nr. 1-3,if put a rivet AN426AD3-3.5 in the dimples hole, it sits 0.27mm deeper than the skin around! This is to deep.

1 Inch = 25.4mm

So the base of the 100° cone is slightly biger, close to an tank dimple die!

Also the NEW/BAD one have on the transition from the centring pin to the cone very bad tooling marks (the tool was not sharp anymore, the material is riped out/pressed foreward) and a step is formed. Like in the 4th picture and the sketch, you can see the sideline of this transition. This step will produce much more pressure to the aluminum, the tooling marks and the bigger diameter give them the rest, so that cracks are started.

I will become other dimple dies later today from a friend who build an RV7 with them, so I can test.

Also I will contact the manufacturer of the dimple dies and listen what he has to tell about. Hopefully there are not big series out of them

regards, Dominik

Hi Dominik, my dimple dies are the highly polished ones from Cleaveland, but I have one from ATS that looks like plain, unpolished steel. I bought it for its height to use in an alligator squeezer for dimpling steel platenuts. After dimpling lots of platenuts, this die looks like yours under magnification except for that ridge! I just chuck it in a drill and buff it gently against the fine Scotchbrite wheel, and it looks better than new. I've learned to keep the dies polished and not use the platenut dimple die for aluminum. The die with the ridge is bad, just replace it.

Hi Ron

Thanks for your input and the picture. I normaly do not dimple the nutplates .. for this you should really use hardened dimples ...

I will redo mine as you have done and check them before on spare material.

I'm still waiting for dimple dies from my RV-Friends, but those are lost!!! I only get an damaged envelop which was empty, the sorting machine eat up my dimple dies ... so I have to order 2 more.

So I have still no example that I can mesure.

I also found that all the cracked dimples are quite deep, if I put an 3 CS rivet in, he sunks below the surface! this is not good!

will contact the supplyer of the dies, they are from CAT. I really liked them, they had a much better quality then dies from others.

well see,

regards, Dominik

swisseagle said:

Hi Ron

Thanks for your input and the picture. I normaly do not dimple the nutplates .. for this you should really use hardened dimples ...

I will redo mine as you have done and check them before on spare material.

I'm still waiting for dimple dies from my RV-Friends, but those are lost!!! I only get an damaged envelop which was empty, the sorting machine eat up my dimple dies ... so I have to order 2 more.

So I have still no example that I can mesure.

I also found that all the cracked dimples are quite deep, if I put an 3 CS rivet in, he sunks below the surface! this is not good!

will contact the supplyer of the dies, they are from CAT. I really liked them, they had a much better quality then dies from others.

well see,

regards, Dominik

Click to expand...

Hi Dominik, I think you just got a bad die. That ridge is really overworking the metal, I believe. You say the die is new, so it really shouldn't have those tool marks. I think a bad one just got past inspection. I'd replace the set, in case the female die is bad also. Can you buy from Cleaveland or Avery where you live?
The measurements you asked for are approx.: 4.85mm at the widest point of the cone, and 1.10 mm for the height of the cone on a Cleaveland 426-3 die.
Regards,
Ron

I got a few of these ones yesterday:




Any idea how it is caused?

I drilled, debured then dimpled.

I can feel the corrugations with my finger nail.

Any idea how to fix it?

What is your method of de-burring? It looks to be caused by chatter of the de-burring tool.

I used a flute http://www.averytools.com/pc-430-43-hss-three-flute-burring-cutter---100.aspx

I was using the http://store-planetools.com/speeddeburringtool.aspx but the pilot gets a bit caught in the #40 holes

I put the flute into the speed deburring tool handle.

How should i fix it?

Those ridges are almost certainly caused by the deburring tool. In my experience, a single flute deburring tool is far less likely to exhibit chatter like that.

So is this bad? I have ran another turn on the first deburrer and ran the scotchbrite pad over it. It hasn't made a whole lot of difference to the chatter marks. Is it structurally unsound?

Do the parts need to be replaced?