What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Countersink Enlarges Clearance Hole - Is that ok?

Saville

Saville

Well Known Member
Hi all,

I'm new to metalworking so today I'm practicing on scrap metal and running into a problem:

I'm trying to drill and countersink for an 8-32 flat head structural screw (1/2" long) in 0.063" thick material.

I'm using a nylon-faced micro-stop.

I start by drilling a clearance hole with a #19 drill. This makes a great hole - no slop.

Then I used a #20 Avery countersink in the micro stop and attempted to dial in the countersink depth.

I started shallow by (it turned out) about 5/1000's and so I increased the depth by 1000th increments to the point where I do not catch a thumbnail on the screw head BUT.....

....the clearance hole is now wider and there's a little slop in the hole.

Is this expected?

Or, should I start with a smaller drill and a smaller countersink cutter and then the increased hole diameter will work out to be perfect for the 8-32?

Please advise.
 
Last edited:
You are transforming your countersink into a router.

What you want is a pilot hole in a piece of metal to backup whatever you are countersinking. Then, the rule is "get in and get out". Creeping up on a countersink tends to have the problem you describe.

Get the depth set on a piece of scrap, then do the product.

Carl
 
Carl Froehlich said:
You are transforming your countersink into a router.

What you want is a pilot hole in a piece of metal to backup whatever you are countersinking. Then, the rule is "get in and get out". Creeping up on a countersink tends to have the problem you describe.

Get the depth set on a piece of scrap, then do the product.

Carl
Click to expand...

Hi Carl,

Thanks for the fast response!

To be sure I understand you, I should:

Drill a #19 hole in a scrap piece.

Drill a #19 hole in the piece to have the countersink where I want the screw to go

Stack the scrap piece UNDER the piece to be countersunk and somehow fasten them together (carpet tape?) such that the holes are lined up.

The scrap underneath will prevent the countersink from moving horizontally.

Is that it?

Thanks Again!
 
That's it. Or back your piece being countersunk with a wooden bench, drill the pilot hole through metal into wood, don't move anything then countersink. That will hold it in position as well.

Provided it can be placed on a flat surface of course
 
Saville said:
Stack the scrap piece UNDER the piece to be countersunk and somehow fasten them together (carpet tape?) such that the holes are lined up.
Click to expand...

This is where a few other holes and some clecos come in handy... or cleco clamps... or clamps... carpet tape is kinda bush league
 
It might help if you specified what holes you are working on.

There are some holes that are expected to get enlarged when you machine countersink (wing spar flange for tank screws is a primary one).
 
rvbuilder2002 said:
It might help if you specified what holes you are working on.

There are some holes that are expected to get enlarged when you machine countersink (wing spar flange for tank screws is a primary one).
Click to expand...

Sure:

I'm drilling and countersinking holes in a battery tray in the aft position of an RV-8.
 
If the head is flush with the surface then it is correct for a portion of the countersunk hole to show.

This is caused by the construction of the screw head where what would be the sharp edge on the flat top surface caused by the 100 degree countersink is removed in manufacture. It can be seen in these pictures -

counterSunk_3.png
 
az_gila said:
If the head is flush with the surface then it is correct for a portion of the countersunk hole to show.

This is caused by the construction of the screw head where what would be the sharp edge on the flat top surface caused by the 100 degree countersink is removed in manufacture. It can be seen in these pictures -

counterSunk_3.png
Click to expand...

Hi Gil,

It's not the upper portion of the countersink showing (i.e. a small "rim" around the screw head) that's the problem.


The problem I'm having is that when I drill the countersink, the original clearance hole diameter is increased. I would have thought that since the countersink doesn't go all the way through the thickness of the metal, that the clearance hole would stay the same diameter.
 
Saville said:
Hi Gil,

It's not the upper portion of the countersink showing (i.e. a small "rim" around the screw head) that's the problem.


The problem I'm having is that when I drill the countersink, the original clearance hole diameter is increased. I would have thought that since the countersink doesn't go all the way through the thickness of the metal, that the clearance hole would stay the same diameter.
Click to expand...

If it is a #8 screw and the material thickness is less than 0.072 then original hole should stay the same size.

SORRY - BIG TYPO ERROR - should say "more than 0.072"

The screw head spec. is here -

http://glftechnical.com/PDF/MS24693.pdf

If the thickness is only a bit above the minimum you probably need a scrap piece of material with a #19 hole clamped behind the work to act as a guide.
 
Last edited:
Why countersink at all?

BTW - no real reason to ever countersink for #8 screws if you can dimple. Such things as the spar fuel tank screws holes of course get countersunk.

As example, the RV-10 wing root calls for countersunk screw, but I dimpled instead. You may need to do a slight countersink touch on the dimple to get a perfect fit for the #8 screw - but that is about it.

I believe this provides a much more repeatable process, looks just as good if not better, and if anything is stronger than doing the countersink.

Same goes for the RV-10 plan call out to countersink the top skin for rivets for the wing doubler plate. Just dimple instead.

Carl
 
Carl Froehlich said:
BTW - no real reason to ever countersink for #8 screws if you can dimple. Such things as the spar fuel tank screws holes of course get countersunk.

As example, the RV-10 wing root calls for countersunk screw, but I dimpled instead. You may need to do a slight countersink touch on the dimple to get a perfect fit for the #8 screw - but that is about it.

I believe this provides a much more repeatable process, looks just as good if not better, and if anything is stronger than doing the countersink.

Same goes for the RV-10 plan call out to countersink the top skin for rivets for the wing doubler plate. Just dimple instead.

Carl
Click to expand...

Can't dimple - 0.063 aluminum
 
Carl Froehlich said:
Correct - but not what I was recommending.
Click to expand...

Well I misunderstood because when you said:

"BTW - no real reason to ever countersink for #8 screws if you can dimple.
..................
I believe this provides a much more repeatable process, looks just as good if not better, and if anything is stronger than doing the countersink.
......................
Just dimple instead."

But I understand you now. Thanks!
 
az_gila said:
And that is below the 0.072 head thickness of a #8 screw and will open up the original hole.
Click to expand...

ok what you wrote above makes sense to me. Earlier you wrote:

"If it is a #8 screw and the material thickness is less than 0.072 then original hole should stay the same size."

And I didn't understand that at all.

So the upshot to all this is:

There is no way to avoid increasing the diameter of a #19 hole in 0.063" thick material if you are countersinking for an 8-32 screw because the head height is greater than the material thickness. If you want it flush - the hole gets widened.


Which means I was doing it right all along.

So now my questions become:

How does one calculate the shear strength of the screw?

What is it that holds the parts together under stress assuming the screw is holding two plates together with a nutplate underneath the bottom one? Is it the sliding of the upper plate against the inclined plan of the screw head thus forcing the two plates together?

It can't be the upper plate hole edge bearing against the grip because it's not touching the grip.
 
Saville said:
ok what you wrote above makes sense to me. Earlier you wrote:

"If it is a #8 screw and the material thickness is less than 0.072 then original hole should stay the same size."



SORRY - BIG TYPO ERROR - should say "more than 0.072"



And I didn't understand that at all.

So the upshot to all this is:

There is no way to avoid increasing the diameter of a #19 hole in 0.063" thick material if you are countersinking for an 8-32 screw because the head height is greater than the material thickness. If you want it flush - the hole gets widened.


Which means I was doing it right all along.

So now my questions become:

How does one calculate the shear strength of the screw?

What is it that holds the parts together under stress assuming the screw is holding two plates together with a nutplate underneath the bottom one? Is it the sliding of the upper plate against the inclined plan of the screw head thus forcing the two plates together?

It can't be the upper plate hole edge bearing against the grip because it's not touching the grip.
Click to expand...

See correction above
 
Yes, and it is no different than what the resultant holes were for all of the fuel tank attach screws on the wing spars.
Even with the hole enlarge slightly the retention strength is quite high.

BTW, dimple countersinking .063 thick material doesn't work very well at all, and is why if you read about dimpling in Section 5 of the manual it recommends that when you can, to dimple countersink everything up to and including .040, but machine countersink anything thicker than that.
 
You must log in or register to reply here.
Back
Top