Rookie Countersink Question

Just trying to make sure I get off to the right start here. On the first countersink & dimpling instructions of the HS, where hs702(left & right) meets hs810 & hs814 (the middle 4 holes). Should the dimple completely fit inside the countersink prior to riveting? In section 5 it's stated to countersink only about .005" deeper (a few cage clicks) than flush. I did this but there is still daylight between the dimple on 702 and the CS on 810/814. Do I go deeper than .005 until the dimple is completely swallowed by the CS?

Thought I'd make my first post before taking off too much metal.

I countersink until the two pieces lay flat together (the dimple is completely contained within the countersink).

Have to ask about your handle. Falcon as in Falcon 50, 900, 2000?

Hey, Maybe he drives a FORD Falcon.
Mel...DAR
sorry, I couldn't resist.

your killin me Mel

I only have a few months of building experience under my belt, so take this as you will, but I use the 2-clicks method when countersinking for a dimple. That is to say I countersink (scrap) untill the rivet head fits flush, then I add 2-clicks on the countersink tool to get to the depth for the dimple. I too end up with a little "daylight" between the parts, BUT when you rivet the hole the parts squeeze together nicely with no space between.

On the parts your mention, countersinking a little deeper probably isn't a big deal since they are thick aluminum angle. That being said, there will be times that you will need to countersink thin material such as the trim tab spar, and if you countersink to accept the dimple completely, it will probably be too deep and thus the hole too wide.

From what I have read and from talking to other builders, this is a question that comes up often. I would suggest you practice different depths with some scrap. I think if you squeeze rivets in some of the countersinks you think are too shallow, you might be surprized to see how the parts come together.

Just my $.02, and good luck!

I have a little square of .032 about 1/2 inch square that I put a dimple in and keep in my toolbox. It's nice for checking countersink depth.

Steve Zicree

Daylight is no good

You don't want to see any daylight in there. Make it fit PROPERLY. There was recently an argument about this on the Matronics rv-list with little resolution.

My 2 cents is -- if the dimple does nest entirely in the countersink, when you rivet, something else will suffer. Metal *other* than the rivet will be deformed/stretched/stressed in some manner.

Again, just my 2 cents.

The suggestion of having "test strips" is a good one imho.

)_( Dan
RV-7 N714D
http://www.rvproject.com

I just use a rivet. After you get good at it (countersinking) you will begin to recognize the corect size "ring" around the rivet.

In the interest of science, or maybe it's that I am bored at work, I will attemp to play devil's advocate a little.

IIRC, one of the arguements recently proposed on the RV-list was that when countersinking, the edge of the CS is crisp and sharp, but when dimpling, the underside edge (for lack of a better term) tends to be somewhat less crisp with a slight radius. So what you end up with is the slight radius of the dimpled part won't allow the dimple to fit exactly flush with the CS hole even though the CS'ed hole may be of the proper size.

What do you do in this case? Do you make the CS deeper/larger so the the parts fit exactly flush at the risk of possibly making the CS too big? I don't know the answer, I wish I did, but that is why I am furthering this debate.

When I encountered this issue several months ago, I took my samples to the local builders (and EAA tech couselor), I was told that the issue of the CS/Dimpled parts not fitting exactly/prefectly/completely flush was fairly common and that I must be careful not to be fooled into thinking I need a deeper CS (my words not theirs). Now, maybe the use of "daylight" is not a good term as it may imply too much space between parts, but I will say that when mating dimpled to CS'ed parts I almost always have a *tiny* amount of space between the dimpled and CS part that disappears once riveted. My assumption, which may be incorrect, is that the CS makes the dimple more crisp once riveted together. I hadn't thought of stress related issues this slight adjustment might create, but aren't you deforming/streching/stressing the metal when dimpling in the first place?

Have I been led astray? Should I buy a new empennage? (just kidding)

Ok, sorry for the rant, but I am curious too!

My understanding is the same as your's, Brad. I always have seen a little "light" under my test patches. If, for example, I would have countersunk the tank attach screw holes on the spar until there was no light, I would have countersunk well beyond the .375" that Van's says is the max. That said, two more clicks on the countersink (as Van's recommended in RVator early this year) does not seem like enough, so I kind of split the difference. If I countersink until there is no light, that takes almost an extra 120 degrees worth (maybe 10-15) of extra clicks on the cage. I aim for about an extra 60-80 degrees on the cage (about 5-8 clicks) and that seems to work pretty well. Again, still a fair amount of light, but it squeezes together real nicely once riveted.

In all, I think the dimpled test piece method problematic because the test piece distorts when dimpled, making it hard to know how it would really fit once riveted together if it were completely flat and undistorted. Obviously, no one agrees on this issue entirely, but I would be careful about countersinking until there is no light, especially on the thin stuff (like that found on the emp. spars). That others have countersunk until no light shows without their planes falling apart, though, is also good to know.

I knew I could count on you guys. Thanks for all the input. I ended up practicing more than anything else this afternoon and before I knew it the sun was already down. I got the angles cs'd until they were ALMOST flush with the spar dimples. (There may be a reason this is the first countersink vans want us to do..ie. hard to screw up) I know 'daylight' may have been an ambiguous term, but I think you all seem to get what I was asking. As long as the 2 (or 3) pieces end up flush once riveted I think that will be more than acceptable.
This is a great site, I've been getting on here frequently for a few months now, and it's just one of the best resources around. And after walking by the Van's tent at Oshkosh everyday, I just had to have an 8 (quick build). The emp. got here a little over a week ago, and there's nothing too quick about it, but I'm having a blast. My new moto is: Full Time Project, Part Time Results. But what a cool kit.
Oh and it's the Dassault variety not the Ford kind. A straight 50 and a 900EX.
Thanks for the help everyone, I'm sure I'll be visiting with you all soon.

When I had just got started building I asked a mechanic in the prototype shop at Van's (it's the man pictured bending longerons in the fuselage plans) this same question. He told me that they had recently hired a man who had worked for Boeing, and he told the guys at Van's that Boeing only permits countersinking to the depth of a rivet head when countersinking for dimples. I asked him what he did at Van's. He told me he does the same as Boeing, plus a couple of clicks deeper. So that is what I have done- rivet depth plus 2-3 thousandths deeper.
I think it is a mistake to countersink any deeper. When a countersunk hole is deep enough, and therefore wide enough, to accept the shoulder of a dimple there will be a gap between the lower portions of the dimple and the sides of the countersunk hole. The danger is that the joint between the dimpled sheet and the countersunk structure will not be tight and the two could work against each other under load. I made some test joints to convince myself of this. You will notice that the bucktail of a rivet driven into a loose joint tends to disappear into the hole. It is trying to fill that gap. Oftimes the rivet will just not tighten up, and a loose rivet might not be noticed among many rivets. So, to restate, I think it is bad practice to countersink so deeply that a dimple falls completey into a countersunk hole. An undetected loose joint could result. Steve

The definite limit is the size of the countersink on the female dimple die. If you countersink larger than that, the joint will certainly be loose.