Countersink Diameter for Dimples..

[jcarne]

Originally Posted by StressedOut

Trent,

I know what you're thinking and feeling regarding the dimples. I went through exactly the same thing a few months ago when I started my empennage kit. I was worried about every little thing and concerned that if I screwed up I'd eventually have a massive in-flight break up and end up a statistic. Eh, I got over it.

The bottom line is that there is plenty of safety margin built into these planes, and if you follow the guidelines in section 5 you will end up with a very good airplane.

Remember, "perfect" is the enemy of "acceptable". Unless you're building a show plane you should be shooting for "acceptable".

P.S. There are a couple of entries in my construction log on this very topic. Feel free to have a look.

This is a great post that was difficult for me to understand as I started my build. Building an airplane is so much more fun when you let the little things go. For example, the first time a saw a rivet slightly tilted I thought "oh ****, now what". The answer many times is, "move on". Kind of unrelated to the topic but it will undoubtedly make life more enjoyable later in the build! Not saying make all the mistakes under the sun but certainly there are those that are trivial. When in doubt just look at Vic's thread about the bad stuff he has found in flying airplanes!!! These airplanes have flown with some seriously major mistakes in them!

[squarebush]

Just following up from what Scott had mentioned a couple years ago on dimples fitting in a countersink. I?m well into the fuselage kit and noticed my dimples on the fuselage skin just doesn?t sit completely flush with the stiffeners countersinks. Typically if you go that extra few clicks it will be fairly good. Maybe i dimpled the skins a little more this time. I experimented on some scrap with a 120 degree countersink to see how it would fit with a dimple. It was actually shallower than using a 100 degree and it fit flush. That extra 20 degrees lets that dimple shoulder sit flatter. Any concerns with using a 120 degree countersink structurally? Sure fits nice...

[WayneJ]

Scott, I understand and agree with what you are saying with regard to the .007" deeper countersink depth. The dimple and countersink will lock together. There will be a small but invisible gap between the skin and the countersunk part and that gap is OK. But....is there an exception on the -14 rudder and elevator trailing edges where the gap will be visible?

[rvbuilder2002]

Originally Posted by WayneJ

But....is there an exception on the -14 rudder and elevator trailing edges where the gap will be visible?

No

In fact, in this type of situation I would say that it is more important to follow the recommendation of .007 deep for a strong rivet joint with it being a control surface trailing edge.

My recommendation would be to very slightly break the edge before dimpling (emphases on slight... you shouldn't be able to see with just your eyes that it was done). This should be enough to prevent any gap from being visable.

Since we now have an easy way on VAF to post photos, I will share some photos from the original testing that was done to determine the .007 deeper recommendation for countersinks that will receive dimples.

A basic rundown of the test details -

• Material was 6061-T6 .750X.750x.063 angle, and 2024-T3 alum. sheet .032 thick
• Holes were drilled #40 with both pieces of material in assembly, in a drill press (square to surface)
• Normal deburring techniques were used on both sides of all holes
• "Net" countersinks were a flush/net fit for the rivet head before inserting the dimple.
• "Over" countersinks were number of thousandths deep beyond the net depth
• Dimple countersinking was done with a new set of Avery Tools spring back dimple dies.
• "Poor" dimples were done using a hand squeezer. All others ("nominal) were done with C-frame tool and hammer.
• Machine countersinking was done with a hand drill and a #40 piloted countersink cutter in a .001" increment microstop countersink cage.
• All rivets were set using flush sets in a hand squeezer.
• Test sample was machined to expose rivet cross sections, then wet sanded 600 to polish the edge


I am not posting photos of all test samples.... just three that show the progression of the fit of a well formed dimple as the countersink depth increases. (some of the tests samples were with poorly formed dimples, which makes the poor fit even more pronounced)

The first photo is of a net depth countersink.

The second photo is of a countersink done to the recommended .007 " deeper.

The third photo is of a countersink done to .011 deeper than net (only .004 deeper than recommended)

If you click on and zoom in on the photos, you will see that by the time you get to .011 " deeper than net fit for a rivet, there is a noticeable gap opened up between the dimple and the inner bore of the countersink. It is not as apparent on the left side of the dimple because there is a small amount of material smear across the gap that occurred when the machining was done, but the gap is there.

I hope this makes it clear why the .007" deeper recommendation was made.

[rvbuilder2002]

I think it would be a good idea if a moderator would make this thread a Sticky in the RV Building Tips/Techniques forum, so that the countersinking depth for dimples info is easy to find in the future.

[ed. Done. V/r,dr]

[TASEsq]

Thanks Scott for your constant input on this forum and especially this thread. It helped me gain the confidence I needed in the early stages.

I’ve moved on from your advice (I did indeed begin a -14 kit) and in these cases I’ve made sure to only go 0.007” deeper. I was measuring this but now have a feel for what it looks like with a rivet in the hole. In some cases, such as the HS skin to HS rear spars, there was a fair bit of a gap when clecod together. Knowing I had gone to the correct depth, I sucked it up and riveted: the end result was a very small gap only. The rivets pulled it together much tighter than the clecos. There may be a gap under the skin but it’s no concern, as you say and you can’t see it unless you are really looking. I added a photo below of one such rivet - you can see the gap difference between a clecod joint and a riveted one.

For anyone reading this in the future I made a blog post summarising the experience of anyone wants to bore themselves to sleep:
https://tasrv14.blogspot.com/2018/04...rsink.html?m=1