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Countersinking aluminium skin


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Good evenening to everyone,

I have ordered a Savannah S kit and at the moment I m building (to make practice and exercise / I m a pharmacist ) a tail fin that I have bought separately. In the build instruction I read: Slightly countersink the holes for rivets and install  the rivets to have perfect flush head.

How should I do that without weaking the very thin alumiunm sheet?

With a drill bit? Wich Dimension ?  I have no idea.

Thank you very much in advance for any help.

Lucas from Switzerland 

 

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24392FD7-0EC0-4534-AB22-C50CD90541B5.jpeg

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Don’t countersink thin skins, use a dimpling tool. Several types available but the one I found most useful was from Aircraft Spruce, it utilises a male and a female die and has a 1/8” diameter nail through the middle and is operated with a set of pop rivet pliers. I also have a pair of dimpling anvils for a hand rivet squeezer but that is limiting for distance from edge. If in fact you need to countersink thicker material, buy a countersink tool specific to aircraft rivets because of the countersink angle. If dimpling you need to dimple both the skin and the rib flange behind it! I hope this helps.

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You can make your own dimple tool with a piece of aluminium plate, a 6 inch nail and a countersunk rivet. Drill a hole in the plate (should be 6-10mm thick) with the correct drill for the rivet. Countersink the hole so the rivet fits perfectly in it with no part proud. Cut the point off the 6 inch nail and drill a hole the size of the rivet stem in the centre to take the full length of the rivet stem. Then all you need to do is place the countersunk piece of plate under the sheet, put the rivet in the sheet and into the countersunk hole in the plate, put the nail with the hole in it on the rivet and tap it with a hammer. Voila a perfect countersunk rivet hole. One rivet will make about 20 or 30 countersinks in sheet before it becomes distorted & then just throw it away and use a new one.

 

Of course you can do this for all different rivet sizes and make special plates for hard to reach areas etc

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1. I'm sure Derekliston's reply above is the correct way to be doing it. However, as he points out, it is necessary to dimple both the skin and the part it is being riveted to, or the skin will not sit down flush

2. However,  at the very start of the Savannah manual (General Information, page 2/12) where it outlines various techniques,  it says for countersinking 'Just remove the material that is necessary to make the head of the rivet flush with a drill bit'.

While this may not be ideal, I expect it is what most builders do. And in the places where rivets are countersunk on the Savannah, it does seem to work okay.

3. If doing it this way:
Use a much larger drill (8mm or larger)

Ensure that the drill tip is the correct angle (not all drills have the same tip angle, see attached pic)
Turn the drill by hand and NOT with an electric drill. It's okay to have the drill bit in a drill, but turn it by hand, not with the power. The reason for this is that if you use the drill to turn it, it is very difficult to control the depth of the cut, and very easy for the drill to grab and pull entirely through, leaving you with a big hole.

4. While this does the job, if I was building again, I would buy a countersink drill bit: this has a tip that fits through the hole and keeps the bit centered. It was the one tool I did not have that I wish I had had.

 

DSCF1232.JPG

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PS while there are lots of countersink drill bits advertised, almost all of them (like this one) have the wrong countersink angle for the rivets we are using.

images?q=tbn:ANd9GcRGjF7XZYnmkoPVs0-3D95

Edited by IBob
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You just need an oversized twist drill to get a good countersink in the base material. Test it on a bit of spare material with different sizes till you get a good fit for the rivet. The rivet should go right in and be slightly below the surface of the material which will allow for the thickness of the dimpled skin. It is not hard. Practice a few times and you will be away.

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I really have trouble with  recommending using a 2 fluted twist drill to establish a countersink relief in thin al sheet.. It  will invariably chatter and any thinning of the metal ensures that weakness will initiate a failure event (Cracking) earlier than would otherwise be the case.   Rivets are localised stress points and frequently where cracks originate. Bonding as well as riveting helps there.  Nev

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You are absolutely right, Facthunter, and unless both job and drill are rigidly held, the result will not be a precise neat countersink.

So, it's certainly not best practise.

 

However, the tailfin Luca is working on does this in two places:

First to hold two nut plates on the back of the main post.

Second at the bottom of the skin on one side, where the rivets will be alternated with rivnuts, which then hold in place an inspection panel.

 

These are typical of the (limited) use of countersunk rivets on the Savannah: nowhere are they used solely to hold a skin on. Typically they are used where something else will be mounted over the rivets, which therefore need to be flush.

I am not aware of any failures due to this on Savannahs, or 701s.

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There is a tool available to dimple and also rivet about 450mm into a sheet. There is also a tool available to countersink, that lets you set exactly the correct dept to suit the rivet. I have both of these tools left over from building an RV.

Have a look at Aircrafft Spruce microset countersink I think it is called.

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Really nice looking tools, Yenn.

However, here again we run into the countersink angle problem: the tools offered by Aircraft Spruce cut a 100deg countersink, but the standard countersunk blind rivet has a 120deg head.

And this is what I kept running up against when trying to source individual countersink bits...

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Its called a Microstop

 

But be carefull the std set of countersinks that comes with it is 100 or 110 deg...NOT 120 deg...you have to buy teh 120deg set separetly

I had to get one for building my Rans S-21..Rodger who got his with mine when we bought them in found out the hard way about the wrong angle supplied bits. luckily for me as he had to put larger rivets in those spots

 

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3 hours ago, facthunter said:

I really have trouble with  recommending using a 2 fluted twist drill to establish a countersink relief in thin al sheet.. It  will invariably chatter and any thinning of the metal ensures that weakness will initiate a failure event (Cracking) earlier than would otherwise be the case.   Rivets are localised stress points and frequently where cracks originate. Bonding as well as riveting helps there.  Nev

You use a much larger HS twist drill like 10-12mm or so and only the very tip of the drill is used to create the countersink. You use hardly any pressure and it won't bite.  The D-box .025 sheet of both wings were CS riveted to the main spar (top & bottom) which is 4 x 40 angle this way & I had no problems with several hundred CS rivets. Just have a practice on some scrap material first

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28 minutes ago, kgwilson said:

You use a much larger HS twist drill like 10-12mm or so and only the very tip of the drill is used to create the countersink. You use hardly any pressure and it won't bite.  The D-box .025 sheet of both wings were CS riveted to the main spar (top & bottom) which is 4 x 40 angle this way & I had no problems with several hundred CS rivets. Just have a practice on some scrap material first

Spin it backwards any good. I don’t know for sure just asking.

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Don't use a regular 2 flute twist drill to cut the countersink - no matter how you operate it, or no matter how big the drill, it will grab the material virtually every time, and wreck your hole.

Use the proper countersink drill, that's why they make them.

 

Edited by onetrack
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You  can make your own Countersinking tool pretty easily and if the flutes are irrregular in spacing it will work all the better. Use silver steel and quench it in oil. Make a 1/4 shank with a bigger "head" depending on how close you have to get to a fold. Use a small (lightweight) drill Use trefolex or similar to stop it picking up. I work a lot in aluminium and it's a difficult material to predict and often a bit will pick upon the cutting tool. It's best with a fine point and a high cutting speed but that's not how you do countersinks in sheet. The sheet can't be held dead flat either most times.  I'd rather dimple it . I also realise some parts are of thicker material  but anytime you drill a hole or remove metal you create a weak(er) section .Nev

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9 hours ago, Downunder said:

Thanks for that, Downunder.

#40 and #30 only required for the Savannah (I think).
If I were starting again, I would definitely get them: as stated earlier, it was the only tool I did not have that I wished I had.

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