Countersink depth question

prestwich · Post by **prestwich** » Tue Nov 08, 2005 11:59 pm

Well, I'm working on the front spar of the tailplane and just tried out my new microstop countersink tool on the reinforcing pieces (HS-710 and 714.) Van's instructions say to go a 0.005 under flush to accomodate the dimpled skin (in this case, not a skin but the HS-702.) However, this leaves a gap between the parts of somewhere around 0.020, judging by eye. Do I have to go quite a bit deeper with the countersink, until the dimple will drop down in all the way, or am I doing something else wrong?

captain_john · Post by **captain_john** » Wed Nov 09, 2005 5:41 am

p-wich,

I think the next time I measure a countersink it will be the first time. So long as the only thing keeping the 2 materials from nesting completely (barring any misalignment, debris, etc...) go deeper.

Some of the "stand off" is due to the dimple not being exactly the same shape as the cavity of the countersink. When the three materials are compressed (sheet, angle and rivet) they will all form up together to a point. That point is where the cavity is filled and the sheet is too large to fill the recess and the sheet will not lay flat.

Of course, you know this. My goal is to dwell in minutia so that you know what to look for when inspecting your methods. If after riveting, the sheet stands off from angle... drill out the rivets and go deeper.

Hope this helps.

CJ

Post by **Spike** » Wed Nov 09, 2005 7:37 am

Ah, the good old countersink problem set. Yuck, I hate counter sinking. Dont continue to take out material until the dimple sits perfectly flush, you will take out more material than you should. DAMHIK.

jim_geo · Post by **jim_geo** » Wed Nov 09, 2005 2:39 pm

On flanges and like surfaces I use the debur tool. The crank looking one from Avery, to counter sink with. I get much more control than with the Mombo Horse Power micro tool. I use the micro adjustment tool mostly on my drill press on large flat surfaces, and find it works very well that way. I test the depth of the counter sink as I go and then try to make it fit with no gap between surfaces and no woble between parts, as in, they don't slide back and forth on each other. At all. In fact I did the AEX wedge for the rudder this exact way. It took about half an hour.

cjensen · Post by **cjensen** » Wed Nov 09, 2005 3:54 pm

i've never measured the depth either. but, i also use the same method as jim, and countersink most parts with the duburring tool. my countersink cage is more or less used in the drill press. lack of control is the reason. it's easy to get sideways with a hand drill.

go deep enough for it to sit right. i wouldn't worry too much about actual measured depth.

jim_geo · Post by **jim_geo** » Wed Nov 09, 2005 6:11 pm

There is also a rule of thumb that you shouldn't leave a knife edge at the bottom of the hole. From what I've seen the folks at van's applies this rule only when it suits them. The flange material on the left rear elevator spar for example is only JUST thick enough to machine countersink regardless of the bottom of the hole.

captain_john · Post by **captain_john** » Wed Nov 09, 2005 7:58 pm

It's a Zen thing.

Be ONE with the tool!

CJ

prestwich · Post by **prestwich** » Wed Nov 09, 2005 9:38 pm

Thanks for all the input. Just to be clear, in this case, I'm talking about countersinking a hunk of aluminum that's 1/8" thick, so I wouldn't use the deburr tool for that, right? Detecting a little disagreement in the responses from this group, I emailed Van's customer support and got this response:

"There will be a visible gap until the rivet is set. This is what leads
people to do the wrong thing, which is to keep countersinking until
the gap goes away. Stick with the instructions in Sect 5.

Vans"

This sounds like what Spike warned me about. Since I'd rather not experiment with real parts, I think I'll take some scrap and try out the situation the way I have it now. Of course, I ain't ready to rivet a damn thing yet, because although I have a squeezer and six yokes, I still ain't got no flat sets. I guess I'll order that adjustable set holder and, uh, try not to break it. By the way, how many extra rivets do they give you in these kits, anyway?

(When I first started thinking about a five-year build plan, I wasn't thinking just the empennage....)

captain_john · Post by **captain_john** » Wed Nov 09, 2005 10:10 pm

There are LOTSA extra rivets in the kit. I was worried about that too when I was doing my emp. Not that I am afraid to order more, just didn't know what to order!

When I was doing the emp, I am not ashamed to say that I didn't know a 426 from a 470 by sight. After all, this is the first plane I have ever tried to build!

Trust me, things have a way of working out.

Even an idiot like me can make a passable part!

Feel free to ask questions. I will help out when I can. Just keep in mind, my opinions are worth every penny you pay!

CJ

cjensen · Post by **cjensen** » Wed Nov 09, 2005 10:38 pm

i've paid him a bunch of pennies, so i can vouge for him!

just for the record, i always have gap between the flange and bar before it's riveted. it disappears after it's set.

btw, rivets are cheap. they do send more than enough, but sometimes (in my case) it's not enough and more are ordered. i just ordered some today as a matter of fact!

i needed some more blind mk-319-bs for my litte rib that i fabed up tonight!

order more if ya need to!

Wicked Stick · Post by **Wicked Stick** » Thu Nov 10, 2005 1:01 am

prestwich wrote:Well, I'm working on the front spar of the tailplane and just tried out my new microstop countersink tool on the reinforcing pieces (HS-710 and 714.) Van's instructions say to go a 0.005 under flush to accomodate the dimpled skin (in this case, not a skin but the HS-702.) However, this leaves a gap between the parts of somewhere around 0.020, judging by eye. Do I have to go quite a bit deeper with the countersink, until the dimple will drop down in all the way, or am I doing something else wrong?

Here's my 2 cents worth.

Using a microstop countersink cage will provide a more precise result than using a deburing tool. I've found that approx 5 to 6 clicks/notches deeper than flush is about right for me. I've also gotten good results without having to place it in a drill press. You just have to be sure there's enough material left in the part to hold the stem of the countersink bit in place. If the material is too thin, then it could wander on you creating an oval or larger hole than intended. So, when I countersink on smaller material, simply place it on top of some compressed wood-board, drill through the holes into the wood, and cleco the part to the wood, then use the countersink and the stub or protruding part of the coutnersink bit will remain stationary in the wood hole and you will get a nice round/perfect countersink every time.

I have pictures of when I was countersinking my elevator trim tab spar on the RV-8 yahoo group. The link is here.

[SPIKE EDIT: Shortened the URL up to get rid of the page scrolling]

jim_geo · Post by **jim_geo** » Thu Nov 10, 2005 2:58 am

Chad I also had to buy the same rivets you did. There just aren't enough sent with the kit.

captain_john · Post by **captain_john** » Thu Nov 10, 2005 6:53 am

I had way more MK-319's than I needed.

They must have been having a bad fiscal year and had to cut back somewhere in order to buy some RV-11 supplies!

CJ

Post by **Spike** » Thu Nov 10, 2005 9:55 am

After reading WS's response it reminds me of a picture that I took in my builder's log of a solution that I used when countersinking the HS spar flanges for my 9. I had some real difficulties with it.

We have a decent sized thread on countersinking here in the screwup chronicles thread.

Heres the picture of my solution to keep the darned things from walking when you get close to the bottom of the material.

-- John

captain_john · Post by **captain_john** » Thu Nov 10, 2005 3:39 pm

Oh now THOSE were the days!

Back when Spike was an airplane builder and we would toss a few squirrels around just for fun!

CJ

Post by **Spike** » Thu Nov 10, 2005 4:07 pm

OUCH. That hurt.

We have plans for recovery. We are hosting Thanksgiving, and as soon as that is over we are putting together a workshop in the new house. I should be back to playing with aluminum at the end of November, beginning of December

captain_john · Post by **captain_john** » Thu Nov 10, 2005 4:12 pm

Niiiiice!

Way to go!

Gonna need pics of the new aerie!

CJ

jim_geo · Post by **jim_geo** » Sat Nov 12, 2005 7:53 pm

There will be a visible gap until the rivet is set. This is what leads
people to do the wrong thing, which is to keep countersinking until
the gap goes away. Stick with the instructions in Sect 5.

Thinking to myself that I had better find out what I missed I went back and reread section 5. On page 4 paragraph 2 of section 5 there are instructions of less than ideal detail about counter sinking in conjunction with dimpling. Although, it says to set the micro stop tool three clicks deeper than a rivet requires to be flush nowhere does it say anything about leaving a gap between parts that will later be closed after setting the rivets. I believe that after having read the manual, once again, I'm going to continue counter sinking the materials the same way I have been.

prestwich · Post by **prestwich** » Mon Mar 06, 2006 2:36 am

Finally got around to doing a test on this. I took a couple pieces of 1/8" aluminum, about 1" x 2", and some small pieces of skin to match. Clamped, match drilled, then dimpled the skin and countersunk the thicker piece. Two holes on each sample, about 1/4" from each end.

On one piece, I countersunk it until a rivet would sit about .005" under flush, as Van's suggests. On the other piece, I kept going deeper in small increments until the dimple sat fully into the countersunk. Then I riveted the things together.

I took the sample pieces down to the shop and cut through one rivet on each piece with a band saw. The saw left such a ragged finish that I couldn't see a damn thing.

So I put the things on the mill and took several passes with a nice 1/2" cutter until I got to the center of the remaining rivets. It turned out that the shallower countersink was sufficient. Everything pulled together nice and snug. I could just see the outlines of each component.

On the deeper countersinks, there was a bit of an air pocket under the dimple. My concern about that is twofold. First, it could be a place for corrosion to take a nice foothold. Secondly, I think it's possible that the rivet, in an attempt to pull the dimple into a hole that's too deep, could tear the skin. I couldn't tell whether that was happening, so it's just speculation.

Anyway, I'm glad I did the test, and I'll feel better now about following Van's guidelines, at least on that issue!

Wicked Stick · Post by **Wicked Stick** » Mon Mar 06, 2006 9:13 am

For #40 size holes, I usually go about 3 clicks deeper than flush for countersinking areas where thin skin dimples will go into it. (.024) But I prefer about 5 clicks deeper than flush, for skin dimples where the dimpled material is .032 to .040.

For #30 sized holes, I almost always have found that 6 clicks deeper is much better. I have found that if I can just see a sliver of light between the two parts, I'm about right when it gets riveted together.

I suppose if you cannot slip a 2 or 3 thousands feeler gauge between the two parts when riveted, it should be fine.

RivetBangers.com

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