Beginner: Countersinks and Enlarged Clearance Holes

Hello all,

I've run into a situation which can easily place the beginner (me) into a situation where the builder does not know what to do and does not know what matters and what does not.:

With regard to machine screws, when is it ok for the clearance hole to be enlarged by countersinking, and when is it not?

The plans call for AN509-10R9's (MS24694-S51), 10-32 machine screws to be used to fasten a 0.072" thick plate to a flange using plate nuts. According to the Mil Spec for 10-32 the head height is 0.080". The screws essentially fasten a flat plate to a flat plate.

The screws have to be flush with the plate surface and therefore countersunk.

So if you countersink the match drilled holes the resultant clearance hole will be enlarged such that the screw grip is no longer in contact (or even close to) with the edge of the material.

From Van's Section 5.5:

"Machine countersinking removes metal and is done with a stop countersink
tool and a drill motor . Machine countersinking can only be used in
areas where the skin thickness is sufficient.

Where the skin thickness is insufficient, the countersunk hole for the
rivet head enlarges the original rivet hole and no longer
supports the shaft of the rivet. Only a portion of the rivet head is now
contacting the skin, so it cannot achieve its design strength
which is based on full head contact. See Figure 2, Example 1."

Admittedly this section is talking about rivets and not screws. But I did not see a section on screws, clearance holes and countersinking.

The section also says:

"There are a few instances on RV's where it is considered acceptable to enlarge holes when machine countersinking."

Ok so the beginner is presented with a dilemma:

Is this one of those instances? Can the builder assume that Van's knows the holes will be enlarged. As this application is for a battery, there is considerable weight, and therefore force, being applied to the joint.

More generally:

What is it about countersunk screws that give the joint it's strength in shear?

Is it the grip resisting the sliding of the top plate as it tries to move under a side force?

Is it the friction generated because you are squeezing the two pieces of metal together tightly?

Is it the resistance generated by the inclined plane of the underside of the head?

Is it all three? (Probably)

Is there another factor I've failed to list?

Which of those three is the primary means of making the joint strong?

Which can be discarded under what circumstances?

Yes I know these are detailed questions and that the novice builder can simply follow the plans and be ok. But I have a curiosity about these things and so I'd like to know. If anyone knows the answers and would care to share I'd appreciate it.

I put "Beginner" in the heading to alert the reader as to the level of the questions.

Thanks

Well, I'm not an engineer, but I'll take a layman's swing at a few of the questions.

Enlarging the hole with a countersink: This is obviously one of the exceptions, since the plans show it. My interpretation of 'exceptions' would be where continued structural integrity isn't compromised. If you do it in a rivet hole, the aluminum knife edge that's created will be bearing against an aluminum rivet. Continued pressure of the knife edge will eventually cut through the rivet. If you have a whole line of holes like that (wing skin, etc), eventually the flexing of the wing could cause that whole line to fail, failing the wing.

In your example, the aluminum knife edge will be bearing against a steel bolt. The joint will never be significantly stressed in shear unless you're in a crash that's bad enough to destroy the airframe. And even then, it will likey just deform the knife-edge hole; it's not going to shear that bolt. Total failure of the attachment because of those knife edged holes is extremely unlikely in any scenario that you survive.

Friction: I *think* that an engineer will tell you that joint friction in flat mating surfaces is rarely, if ever, included in designing a structural joint.

Loads on the battery mount are:
Positive vertical (pulling G's), but in that case it's crush loads on the underlying structure that's signifcant.
Negative vertical (pushing G's), rare and airframe limited to 1/2 the positive limit.

Lateral, in any of the 4 directions, which will only happen in a crash, and only needs to contain the battery; deformation/damage is allowed (consider the 'crush zones' in all modern cars).

Negative vertical (pulling the head through the panel) and lateral are the only ones at risk, and Van's engineers are obviously confident that there are safe margins in the design.

Maybe a real engineer will jump in and correct me if I'm wrong.

Charlie

I don't think a new thread on the same subject as your other
http://www.vansairforce.com/community/showthread.php?t=145712
that so far is at 69 posts, is likely to resolve this for you any more than the other thread did.

But to directly answer your question...
There are a number of places that the plans specify a process that will end in the result you describe.
Another is on the wing spar screw holes where the fuel tank attaches.

In these areas, the enlarged hole is expected.

So the only time you actually need to make a decision on this is when you are going off plans and making your own engineering decisions.
It is entirely possible that in at least some of those cases, answering your questions is not possible because it will depend on a lot of factors (loads, etc.) that can't easily be resolved with a generic yes or no.

Bottom line is the holes in the battery tray were expected to be enlarged slightly.
If you are uncomfortable with that then you need to do some engineering analysis to decide what you are comfortable with. A few questions in a thread here on VAF is probably not going to provide a training course in engineering that can provide you with enough knowledge to properly make that decision.

So what it then comes down to is do you have enough desire to learn that you are willing to read a bunch of books and maybe take some classes, except advice from others here on the forum that may already have the training and experience to make a correct recommendation, or just do the work to make it match the plans......

Your choice.

rvbuilder2002 said:

I don't think a new thread on the same subject ........is likely to resolve this for you any more than the other thread did.

I've resolved the issue of 10-32's vs 8-32's in my mind if that's what you are referring to. Sorry but this thread has nothing whatsoever to do with that one. Zero. Zip. Nada. it's not the same subject. The subject here is countersinking enlarging clearance holes. That never came up in the other topic.

But to directly answer your question...
There are a number of places that the plans specify a process that will end in the result you describe.
Another is on the wing spar screw holes where the fuel tank attaches.

In these areas, the enlarged hole is expected.

So the only time you actually need to make a decision on this is when you are going off plans and making your own engineering decisions.

Sounds like a general rule of thumb. Thanks

Bottom line is the holes in the battery tray were expected to be enlarged slightly.

........
A few questions in a thread here on VAF is probably not going to provide a training course in engineering that can provide you with enough knowledge to properly make that decision.

Not looking for an engineering training course from here.

So what it then comes down to is do you have enough desire to learn that you are willing to read a bunch of books and maybe take some classes, except advice from others here on the forum that may already have the training and experience to make a correct recommendation, or just do the work to make it match the plans......

Your choice.

Click to expand...

HI Scott,

Thanks for giving me an answer regarding how one should interpret the plans as against Section 5.5 - Van's expects the holes to be larger.

Now if a newbie builder has studied Section 5 and starts on the wing spar and notices that the clearance holes are enlarged. And maybe starts to fret because we are, after all, talking about the wing spars, They can come to the forum, and search on "countersinking clearance holes" and they will see a topic heading t]hat clearly shows that this is being discussed....

...and they will see Scott Daniels saying that if the plans say to do it, Van's has thoroughly vetted the step.

They can build on in confidence.

And I'm not looking for an engineering analysis - just a simple explanation as to how a machine screw does it's job. I have a lot of curiosity and I like to learn through discussion when I can. I'm looking for "Yeah it's mostly the grip that does the work" or "Grip and the action of the head". Or something like that . Something simple.

Thanks for your answers!