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Countersinking Spar Nutplates Question

dwollen89

Well Known Member
So I'm just starting my wings and the first thing is to countersink these nut plates on the spars. My question is about the countersink depth. I can't seem to find a straight answer anywhere.

Vans countersink method is flush plus 7 clicks. If I do that, the #8 dimples are nowhere near flush. I saw some really old posts from the early 2000's that said the countersinks should be .365-.375. That seems to work, but the countersinks are huge and knife edged.

What have others done for depth?

Here are some pics I took of a practice piece.

https://photos.app.goo.gl/NffkJ2j5Pzhvnv4p7
https://photos.app.goo.gl/RF4yDs3SkxM7dzYp9
 
You countersink the center AFTER the nutplates are installed. This way, the pilot of the countersink is guided by the nutplate. It appears that the countersink to the left doesn’t have a nutplate and that explains the chatter you are receiving. Also, you should make some coupons out of scrap and dimple #8 (to simulate the tank skin) to test the nesting of the skin and screw, not just put the screw in the hole.
 
A good place to start might be looking through the stickied thread at the top of the Build tips and tricks forum

https://vansairforce.net/community/showthread.php?t=159343

I am not sure if it covers the tank screw dimples specifically, but it will help you get a good understanding of why you shouldn't strive for a countersink depth that appears too allow for the tank skin to lay entirely flush.
Additionally, I think there is a recommendation for the overall max. diameter of the countersinks but I do not remember at the moment where that is documented.
 
I pretty confident with countersinking for standard rivets and dimples. The problem here is that the dimples are for #8 screws and exceptionally large. Does the standard flush plus seven clicks (.007) still hole true here? Or am I better off dimpling a sample and trying to get it to within .002 of sitting flush? I dimpled a sample earlier today and the seven click method was nowhere near sitting flush. I have read everything I can find on this topic and watched any YouTube video I can find, but I can't seem to find a clear answer.

I did find this thread which references an article in a 2005 issue of RVator in which they discussed countersinking for screws. This says to countersink until the screw head is flush, then add two more clicks (.002) of depth. It says the countersink should be .365 to .375 across the widest part if I'm reading it correctly.

https://vansairforce.net/community/showthread.php?t=2740&highlight=countersink
 
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Use the measurements Greg shows. The lower edge of the countersunk hole will be knife edged (which is normal), even using the correct measurements, so you do not want to dimple the spar to the point where the dimple sits flush - This will result in a larger than spec lower hole diameter. Spars have been scrapped because of this. It all works out when you screw the skin to the spar later.
 
Thanks for the replies! So the second picture I linked above on the scrap piece is correctly countersunk? I'm assuming that's the measurement that is in the picture that Greg posted.
 
Thanks for the replies! So the second picture I linked above on the scrap piece is correctly countersunk? I'm assuming that's the measurement that is in the picture that Greg posted.

Well, your caliper shows your counter sink dimension diameter as .376 with the drawing showing the limit being .370, so technically you’re six thousandths over the limit.
I wouldn’t get too worried about it for just that one hole but I recommend you try and keep .370 as the limit for most of the holes.
 
Well, your caliper shows your counter sink dimension diameter as .376 with the drawing showing the limit being .370, so technically you’re six thousandths over the limit.
I wouldn’t get too worried about it for just that one hole but I recommend you try and keep .370 as the limit for most of the holes.

You're right about that countersink being six thousandths over. Fortunately, that's in a scrap piece, so I'll make the changes and practice a couple more holes before I pull the trigger on the spars.

Thanks everyone for the replies! I'm nervous about countersinking those holes. Not exactly cheap parts to replace :eek:
 
Also, a good lubricant is your friend here. Tool oil, 3-In-One, cutting fluid, and even engine oil will help immensely.
 
Lurking, good info here. About to do this on my -14 spars, so this is synchronicity at its finest.
 
Lurking, good info here. About to do this on my -14 spars, so this is synchronicity at its finest.

I'm jealous that you're building a 14. I'm building a 7A, which I'm sure I'll be happy with when I'm done. But at times, the directions leave a lot to be desired. I made numerous phone calls, texts and did a ton of online research trying to figure out how to countersink these nut plates without muffing up my spars. In the end, I'm using a diagram from the 14 instructions and an article from a 2005 edition of the RVator as my guide :cool:
 
I'm jealous that you're building a 14. I'm building a 7A, which I'm sure I'll be happy with when I'm done. But at times, the directions leave a lot to be desired. I made numerous phone calls, texts and did a ton of online research trying to figure out how to countersink these nut plates without muffing up my spars. In the end, I'm using a diagram from the 14 instructions and an article from a 2005 edition of the RVator as my guide :cool:

I built a -7 tailkit about 15 years ago, life happened and I sold it to a good home. The -14 is orders of magnitude easier to follow plans wise.

Just think about those scratch builders who put the RV-3 together from plans in the early days. That's impressive.
 
I want to add one thing that I learned when I did the spar countersinking. My first C/S chattered like a set of wind-up false teeth at the beginning. I stopped well before I reached full depth and spent several hours testing scrap trying to get a good countersink without chatter by varying pressure and speed.

Long story short, use an electric drill and keep at it until it reaches the limit. I got significant chatter at the beginning of the process and as it got deeper the pilot started to engage the nutplate hole better. At the end the chatter settled down and the resulting countersink, while not quite perfect, was well within the acceptable limits.

Edit: Also I used a single-flute (some places call it a zero-flute) countersink. Cuts like butter if you use the right pressure and don't forget to use Boelube on every hole. It'll keep the C/S quality high and the cutter will last much longer. You'll need a #40 and #30.
 
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Countersinks

<snip>
Just think about those scratch builders who put the RV-3 together from plans in the early days. That's impressive.

This issue comes up often. Part of me wonders why Vans has never done anything about it like a huge warning in the manual or even cutting the countersinks before shipping. Yes, I know, expensive.
Another part of me wonders about all those birds flying around before this became so Vans Builder public. How many birds have countersinks too deep? I know walking through the Vans area at Airventure, I rarely saw a fuel tank skin that wasn't perfectly flush. Hmmm.
I'll get back to work on my 7A now. :D
 
This issue comes up often. Part of me wonders why Vans has never done anything about it like a huge warning in the manual or even cutting the countersinks before shipping. Yes, I know, expensive.
Another part of me wonders about all those birds flying around before this became so Vans Builder public. How many birds have countersinks too deep? I know walking through the Vans area at Airventure, I rarely saw a fuel tank skin that wasn't perfectly flush. Hmmm.
I'll get back to work on my 7A now. :D

It's funny you say that. I was talking with one of my builder friends yesterday and we said the same thing. I've also watched some YouTube videos of guys that were countersinking their spars and commenting that they were going until the skins were flush. If that's the case, they are way too deep. But, you don't hear of wings coming apart so obviously it must not be a problem. Although, if you look at the spars, those huge reinforcement bars are where the tanks and countersinks are, so that's probably where the strength comes from. It's thoughts like that that reassure me the occassional bad rivet or oversized holes aren't going to cause my plane to turn into confetti mid-air
 
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Can someone help?

There are many ways to countersink the spar flange - all with the same aim - to keep the pilot centered - so whether it is using the nutplate screw how or a backing piece of aluminium - they all do the same.

Where I am confused - is what drill and countersink do you use for an #8 screw - for the tank nutplates.

Where you have a piece of stock under the flange, it suggested you drill and open up the hole to a #19 and then use a #19 countersink. When using the nutplate as a pilot guide, you're using a #30 countersink (after drilling a #19 hole). Is that correct?

Given the flange is less than 0.07" thick, it is accepted the hole will become larger - is that why a #19 drill is preferred - because of how much material the countersink removes - creating a knife edge?

Does it really matter what size you open the hole to - provided the c/sink pilot is the same - and that it has a backing plate?
 
Can someone help?

There are many ways to countersink the spar flange - all with the same aim - to keep the pilot centered - so whether it is using the nutplate screw how or a backing piece of aluminium - they all do the same.

Where I am confused - is what drill and countersink do you use for an #8 screw - for the tank nutplates.

Where you have a piece of stock under the flange, it suggested you drill and open up the hole to a #19 and then use a #19 countersink. When using the nutplate as a pilot guide, you're using a #30 countersink (after drilling a #19 hole). Is that correct?

Given the flange is less than 0.07" thick, it is accepted the hole will become larger - is that why a #19 drill is preferred - because of how much material the countersink removes - creating a knife edge?

Does it really matter what size you open the hole to - provided the c/sink pilot is the same - and that it has a backing plate?

Caveat: the following is based on the RV-14 plans, so you may need to adjust / ignore per your situation.

I would not use a #19 for this. But maybe that's just me.

On the 14 wing spar, the manual instructs the builder to use a #30 countersink for the K1100-08 nutplate attach holes (i.e., the tank rivet nutplates). You end up countersinking deeper than what you normally would with that countersink and the holes will be larger than a #30. What I did was put the cutter in a microstop counter cage, and slowly approach the dimensions listed in Figure 3 in the attached graphic.

Take your time with these, there are a lot, but the time spent will be worth not worrying about the spar later.
 

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Thanks TYConnell.

The same is recommended for the 7, in that you install the nutplate and use it to centre the pilot on a #30 c/sink. There is however no way of removing shavings completely after the cut. The #8 screw has a diameter of approx 0.1610 (#20) so with clearance the bottom of the hole MUST be greater (ie: #20).

All piloted 3 flute 100 degree countersinks cut the same - so whether it's a #30 or #20 or #19 it makes no difference other than the pilot. The pilot will be determined by what is being used for the backing plate.

The nutplate itself is 0.286 thickness if being compared to a piece of sheet metal. The nutplate thread commences approx 0.134 below the plate. It's this which allows the cutter to go deeper thereby opening the hole (and why the #30 is recomended by VANS).

If you use the top screw head c/sunk diameter (ie: 0.37 or the range given by VANS - and it makes no difference whether you use a #40, 30, 20, 19 c/sink), then the corresponding bottom hole size will be approx #19 - hence the recommendation by some to open all holes to #19 / #20/#21 and use the correseponding c/sink pilot (if you are using a backing plate).

At least that's my take on things. There is a post by Rudi Greyling back on 8 Aug 2005 with the heading 'Drilling Spar for Tank Platenuts" which is responsible for sending me down this path, but it's also a learning curve and the desire to understand things in more detail.

I would appreciate confirmation by others that my undertsanding is correct - if you're reading this post.

In summary - it makes no difference what 100 dgree c/sink you use - but for a #8 screw the hole WILL be oppened up to a minimun of #21. The question is how you do it.
 
A few things:

Yes you are right that it dosent matter what pilot countersink you use. No matter what happens you’re getting the same knife edge in the spar flange.

I don’t see what the problem is with shavings. Even with the nutplates in they’ll just fall out the bottom of the nutplate. The only thing you should worry about the shavings is taping up the spar channel to spar bar gap as shavings get in there otherwise and they’re hard to remove.

If I was in your position (having used another method for countersinking)
I’d rivet in the nutplates and hopefully you still have a few thou up your sleeve to get up to the max 0.37. That way you can guarantee that your countersinks are in exactly the right spot. Concentric with the nutplate.

The reason I tend to respond to this particular issue regularly is that I’ve seen quite a few people over the years go off piste with this using a home brew piloting method and end up with pillowing along the screw line because they’re countersinks aren’t perfectly concentric with the nutplate. It’s one spot I strongly recommend doing it as instructed.

Give me a call or PM. I’m happy to come have a look if you like.
Cheers
 
A few things:

If I was in your position (having used another method for countersinking)
I’d rivet in the nutplates and hopefully you still have a few thou up your sleeve to get up to the max 0.37. That way you can guarantee that your countersinks are in exactly the right spot. Concentric with the nutplate.

The reason I tend to respond to this particular issue regularly is that I’ve seen quite a few people over the years go off piste with this using a home brew piloting method and end up with pillowing along the screw line because they’re countersinks aren’t perfectly concentric with the nutplate. It’s one spot I strongly recommend doing it as instructed.

Give me a call or PM. I’m happy to come have a look if you like.
Cheers

Hi Richard, that makes perfect sense. Drilling into a backing plate assumes you are / and stay properly centered. But if you're not and you proceed as others do, then there's no way of correcting the lack of concentricity - other than now c/sinking with the nutplate and hoping you don't go over the 0.37 mark (in which case you wouild of been better going with that approach in the first place). Using the nutplate as a guide is almost a fool proof way of ensuring you don't have the issue in the first place. Thank you so much for your input. Greatly appreciated. PS: not in Sydney but thank you for the offer.
 
Hi Richard, that makes perfect sense. Drilling into a backing plate assumes you are / and stay properly centered. But if you're not and you proceed as others do, then there's no way of correcting the lack of concentricity - other than now c/sinking with the nutplate and hoping you don't go over the 0.37 mark (in which case you wouild of been better going with that approach in the first place). Using the nutplate as a guide is almost a fool proof way of ensuring you don't have the issue in the first place. Thank you so much for your input. Greatly appreciated. PS: not in Sydney but thank you for the offer.

No worries. See how they look when the nutplates are all riveted on.
If any of them look a little askew (test by running a screw in) then maybe you can ask vans whether going up a few thou is doable or not.
Or even a slightly out of round countersink if that makes sense.
 
One last question - for the K1100-08 tank attach nutplates on the wing spar, each of the two attach rivet holes are drilled out to #40 and then machine countersunk to accept a AN426 3-4 rivet.

Is the depth of these countersinks only to flush?

With the recommended +7 clicks only for machine countersink / dimpled parts?

Thanks!
 
... then there's no way of correcting the lack of concentricity - other than now c/sinking with the nutplate and hoping you don't go over the 0.37 mark (in which case you wouild of been better going with that approach in the first place). Using the nutplate as a guide is almost a fool proof way of ensuring you don't have the issue in the first place.

This seems to have been sufficiently answered, but while the dead horse is still in the open...

I didn't mention this, but one thing the #19 countersink might not do well is "seat" in the nutplate hole - i.e., the pilot is too big and won't help you stay concentric. I'm not sure I have a #19 csink to test this. While I was countersinking with the #30, in "the middle" of countersinking each hole, there were times I would get noticeable wobble while drilling (variations in pressure and RPM with a hand drill). However, as the pilot began to line up / go into the nutplate, it would straighten out, the wobble would disappear, and the holes turned out looking good.
 
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