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Countersinking depth 1/8th to accept a dimple

jamcgee1978

Active Member
Afternoon all,

So after making good progress on the vertical stab on the RV10 I have come upon my first of many issues, in relation to countersinking on the Vertical Stab 6-4 Step 2 . The plans call for you to machine countersink 4 holes on the front spar doubler and dimple the corresponding holes on the front spar in prep to accept a AN426 AD4.

In the manual it states:
"a skin riveted onto a machine countersunk surface the countersink must be slightly deeper as mentioned earlier.Proper depth is .007 deeper than when the rivet head is flush. "

So despite doing this on the 3/32 holes with no issue i am unable to get the CS holes to accept the dimples without in my opinion over countersinking the holes. i guess iam asking how much material should you take away. Im using a std 1/8 CS 100 degree and similar dimple die set but for some reason i cannot get the two skins to match together and can see slight daylight between them. I asked my inspector to have a look this evening and he agrees the holes are over countersunk but agrees they still are not accepting the dimples? Issue with my die set or CS?

I carried this out on scrap beforehand and was unable to get the 2 pieces to be flush, am asking for advice as although this is early doors in the process i ultimately want to iron out common builder issues early as CS successfully is going to be crucial to the success of the project.

little steps and all!





 
I have struggled with this also, I came back and touched the machine countersink with a 120 pulled rivet countersink which just broke the very top edge of the countersink, I think what is holding the pieces apart is the slightly rolled edges of the dimple. I have also used a 135 degree bit to do the same thing. I don't know if this is a good way to go but it seemed to work for me. I am interested to hear what others have to say about this
 
Did you try to rivet your test articles together? I had essentially the same question on my 7 and they tightened up under pressure.
 
Sum of symptoms sounds like the thin piece is under dimpled. If it is, you'll have a rounded corner & the 120 degree c/s rounds the bottom piece to match.
 
Sum of symptoms sounds like the thin piece is under dimpled. If it is, you'll have a rounded corner & the 120 degree c/s rounds the bottom piece to match.

Agree. Your dimples don't look crisp. If not, they won't nest properly. My pneumatic forms a much better dimple than my hand squeezer ever could.
 
The "bottom" of the dimpled skin is at a wider angle than the 100 degree dimple die because the skin thins as it is stretched forming the dimple.

I second the observation about inadequate dimpling. You need to apply sufficient force to "coin" (my term) and develop proper edges.

The material should "nest" such that a .002" feeler gauge cannot be inserted between the sheets.

I ordered "custom" 110 deg dimple dies and countersinks as I believe that "stacking" 100 deg dimpled skins replicates stacked "tea cups" Regardless of the quality of the riveting, the dimples do not nest well - promoting the "smoking rivets" that bedevil and befuddle many builders.

Onward and upward
 
Thanks for your responses, v much appreciated

i understand now what is happening im off to check if the corners are rounded on the dimple, i will try another test piece today and increase the force on the pneumatic squeezer for the 1/8 dimple and attempt a test rivet to see the results. Glad to see others have had similar problems.

like the idea of increasing the angle of the CS with 120 pulled rivet countersink or 110 degreee set also to accept the dimple

I see Cleaveland stock Single Flute Countersink Cutters at 100 degrees and 3 fluted 120 Degree #30 Pilot

"Your dimples don't look crisp"/ "coin" and develop proper edges.

- im using a pneumatic squeeze, as i understand to set the correct squeeze and ensure proper edges you want the yoke to slightly push away during the force and to leave a circle ensuring your dies have come together - is this correct?
 
If you don't set up an air squeezer correctly, it will do a (much) poorer job than hand squeezing.

Put a piece in your C frame, & whack it with a big rubber mallet like you're driving a big nail. Twice. Compare to the air squeezed sample.

There's a utube video made by one of the tool vendors showing what a poor dimple vs a proper dimple looks like, and how to achieve it. He uses light to show the difference.

Using a wider angle c/s for the substructure is hard for me to understand. Sure, the flat surfaces may touch now, but you'll have a single ring of contact around the dimple, instead of a real 'nest' of dimple & c/s. The extra strength of a dimpled joint comes from the increased contact area between the sheets' cone'd areas, and that goes away if the angles are different. (Never think that sliding friction is what keeps flat stuff from moving.)

Charlie
 
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Gages.

Make yourself a set of gages for each size of rivet and screw heads dimples out of some strips of Alclad. One for each thickness of sheet you will be riveting. Dimple them with the dies that you will be using. When you countersink take the gage for that hole and check the fit down into the cone tell the gage just fits with both face surfaces touching flush and the dimple fitting snug in the cone at the same time. That's where you want to set your countersink depth at. You don't have to make all these gages at once you can make them as you go. They can be made of strips 3/4-1 inch wide by 4-6 inches long with a dimple on each end. Take a scribe mark them and put them with your tools. Just an idea for your consideration, Yours, R.E.A. III #80888
 
Thanks Robert and Charlie for your help, will certainly make a set of gauges and i checked out the cleaveland you tube vid also regarding dimpling,
James
 
Charlie,

I'm understanding that you don't agree with my 110 deg statement?

If so, please re-read the first sentence to my post.

The skin, as it's dimpled "down" is stretched. Poisson's ratio instructs us that the material will thin as a result of the stretch.

The dimple die compels the top side to assume a 100 deg angle, the thinning material results in the bottom side of the dimple assuming a net 110 deg secondary to the stretching/thinning as a result.

FWIW
 
Thanks for the help guys, all points noted. I used a set of cleavelands spring backs today and the dimple sat snug in the countersink first off! Im now replacing both dies with cleaveland products to avoid this in the future
 
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