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Countersink Diameter for Dimples..

TASEsq

Well Known Member
Patron
Hi,

Learning to use the countersink cutter over here, and i've made up a dimpled token in the skin thickness (0.025") that i am using (on the airfoil practice kit part 1).

I initially countersunk until a rivet sat flush (-3 rivet). Then i tested the dimpled token in the hole and it didn't sit well (Large visible gap). So i followed Section 5 and went 0.007" (7 clicks) deeper on the cage, but the skin still felt wrong and had a large gap. It didn't seem right.

The question is; there is a table which specifies the CSK diameter for making the rivets flush in the Van's rivet specs (table VI https://www.vansaircraft.com/public/Specs.htm).

Is there a similar table for the countersunk diameters for differing hole sizes and differing skin thicknesses?

In the end, i used the calipers to measure the diameter of the underside of the dimple, and countersunk until i had the same diameter. The skin felt like it nested ok, but the hole was almost knife edge! (which i don't think is right), especially since the material was angle that was 0.080"!

So i don't know if i've done the right thing in measuring the dimple, or do i just do 0.007" (even though there is a gap between the sheet and the CSK material), and assume it will close up when riveted?

Thanks all for the advice. Sorry if the question doesn't make sense (it makes sense in my head :eek:)
 
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I just saw a you tube video on the proper way to counter sink for flush rivets, however I cannot tell you the site address. My recommendation would be to search for "the proper way to counter sinking rivets." I think you will find it.
Dave
As i remember there was a different angle of counter sink for the bottom skin, than from the top skin. but I do not remember the angle sizes.
 
First, I think you're dropping a zero in your numbers. That would be a heck of a counter sink to be knife edged in something 0.80" thick - eight tenths of an inch.

The 0.007" measure - 7 clicks - works well. You can verify you are really that deep by putting a rivet in the counter sunk hole and using the depth gauge on most any calipers that measure thousands of an inch. I used seven clicks through the whole build when attaching a dimpled skin of either 0.025" or 0.032" material.

Sometimes I found that when using the cage the cutter didn't always go to full depth and a second shot would take care of it. I always checked a few holes with the method I described above. When you have a hole that measures the correct depth it is easy to see the difference in a hole that didn't quite go deep enough.
 
Here?s what worked for me depending on sheet thickness and type of dies used (standard, tank, or sub-structure). The next two log entries mention how I handled dimples for -4 rivets. YMMV...
 
First, I think you're dropping a zero in your numbers.

Oh yeah - oops!

A product of a life in a metric country thrust into the world of fractions! Should have listened to my maths teacher after all. Why everything isn?t just measured in 64th?s I don?t know...

I?ll have a practice and measure the 0.007? as you say and see how it fits a dimple.
 
Further to my last. Here’s the size of the dimple:
https://photos.app.goo.gl/89zOwbRFFrMN0zod2

Here’s the size of the CSK that put the dimple above so it nested such that the sheet around it was flat on the angle:
https://photos.app.goo.gl/BSh7F1ECzGdsMmgE2

Here’s a rivet in the above dimple:
https://photos.app.goo.gl/nEsLJUmZqjb6wC002

And here’s the depth:
https://photos.app.goo.gl/9QV6Dv7zCnnfprVC3

It’s a lot more that 0.007”!!

Thanks for everyone’s help so far. Tomorrow I think I will go to flush, then move 7 clicks, and measure the depth as above. Then try the dimple in the hole and report back.

It’s probably just that I don’t understand what a properly nested dimple looks like - and this valuable forum and all of your replies are very helpful in rectifying this knowledge gap! Thank you!
 
I couldn?t find a good picture of my countersink cage, but it is different from yours, having square notches instead of triangles, but the idea is the same. I only assume that one notch is 0.001 inch, as I?ve never measured it. I just used some pieces of scrap 0.125 (1/8-inch) sheet and various thicknesses of dimpled coupons until I found the right number of ?clicks? for each combination.
 
Thanks Miles,

I think my issue is I don?t really know what a dimple in a CSK is supposed to sit like. Did you see any gap between the skin and substructure with your settings?
 
Thanks Miles,

I think my issue is I don’t really know what a dimple in a CSK is supposed to sit like. Did you see any gap between the skin and substructure with your settings?

That is typical of all inexperienced RV builders and why Section 5 makes a very specific recommendation.

This recommendation is based on testing by machining cross sections of rivet joints so they can be viewed and analyzed under high power magnification.

This is from a post about a year ago......

Edit: Here is another thread that discusses the subject (which it appears you (the OP) has commented in so you likely have read all of this already. http://www.vansairforce.com/community/showthread.php?t=157662

Use caution......

One of the more common critical mistakes that we have to help builders through is when they countersink too deep for dimpled skins.

If you go too deep, you will still have no gap between the skin and the sub structure but the strength is likely compromised, With countersinks that are slightly too shallow (as in the attached photo) the strength is no different, but there is assurance that it is not too deep. That is the reason for specific recommendations in Section 5 of the manual. The depth was derived from doing samples of different depths and then machining off 50% to allow magnified inspection of the rivet joints.
Countersinks only deep enough for a rivet head still produced totally acceptable riveted joints (though not quite as nice ascetically). Countersinks any deep than about .009 " beyond what would be needed for a flush rivet began to produce visible gaps in the riveted joint.


In a nut shell, the skin adjacent to a dimple can not lay entirely net to the surface below without making the countersink too deep. This is because the outer edge of the countersink has a sharp corner on it, but the angle at the outer edge of the dimple does not.

With proper technique used to form the dimple and install the rivets, from the outside, a rivet where the counter sink was done only 7 clicks deep vs one that was deep enough for the skin to lay 100% flush, the finished appearance will be indistinguishable but the over countersunk one will have a much higher possibility of problems in the future and is not as strong of a joint.

Edit: Here is another thread that discusses the subject which it appears you (the OP) has commented in so you likely have read all of this already. http://www.vansairforce.com/community/showthread.php?t=157662

Additional edit - I have added photos and a test description of the original testing that was done for establishing the .007 deeper recommendation that is in Section 5 of the manual, to Post # 24 in this discussion thread https://vansairforce.com/community/showpost.php?p=1456777&postcount=24
 
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Update after a second try..

Hi all,

Thanks again for all your advice. Here is a follow up post. Sorry for the links in lieu of embedding the photos - this is more likely to work.

I used the vans practice project and deriveted the angle from the 0.025" sheet. I drilled 6 x #30 and 6 x #40 holes, and dimpled the skin.

I then (starting with the #30) countersunk in a few stages so that a rivet sat flush:
https://photos.app.goo.gl/nhXBHK6x9DfiFLUW2

I then noted the position of the arrow on my countersink cage, and moved it 7 "clicks" deeper.
https://photos.app.goo.gl/HUxy1Ap6mTdEok282

I hit the hole again, and the rivet looks like this now:
https://photos.app.goo.gl/a0L1Y6N7okhSQ66g1
https://photos.app.goo.gl/731Q2kmkgR7qlHxl2
Is this roughly what others are seeing when they are countersinking for dimples?
I measured the depth of the rivet in the hole (it is a bit hard to measure accurately), but it was 0.007" or thereabouts.

I then clecod the token to the hole. As you can see there is a gap, and this is where i think i may have gone wrong (?) the first time. I assumed this was not right, so kept countersinking until this gap was gone. This resulted in almost knife edge in the angle (which is 0.060" not 0.080" as previously mentioned).
https://photos.app.goo.gl/w2pKkE1QETOxekLT2
https://photos.app.goo.gl/0feq8E7Uc1SQKYjE2
https://photos.app.goo.gl/YoAih4J36RDZ7ygB2
https://photos.app.goo.gl/0u6tjZrGtdLJK1kG2

I then countersunk all the other holes on the angle as above, obviously resetting the cutter again to flush, then 7 clicks more when i swapped to the #40 cutter. As you can see the gap is still there with all the celcos in:
https://photos.app.goo.gl/jaJ43uyJpyVymSqp1
https://photos.app.goo.gl/xTb6XheAXIXjjBC93
https://photos.app.goo.gl/EC5xUjRMM6srJ8sC2

Thinking that the gap may go away when i riveted them together, i got out the back rivet plate and finished the 12 rivets (gee i like backriveting! - helps my counting skills!)
https://photos.app.goo.gl/X28y0b7jc0vsuqKz1

Alas, the gap is still there after riveting:
https://photos.app.goo.gl/fm3HbRtIQ6k1HDHQ2
https://photos.app.goo.gl/HXOWAZT2J8npNv8y2
https://photos.app.goo.gl/uLcE5hf3RphQJVth1

So here is the question:
1. Is this gap acceptable?

Obviously if this were a real piece, i would have broken the edge and the gap would have likely not been there visually. But is this correct strength wise?

Can i go a little deeper, if the material was a little thicker to make for a nicer fit?

Thanks all - sorry for the long post! Appreciate all replies and assistance in the learning curve i am on before the emp arrives.
 
Not mentioned

What nobody has mentioned is one needs to make sure the dimple is very crisp, to minimize the radius trying to contact the countersink top edge. For critical items, like the tank dimples to the spar, i broke the edge of the countersink with some 220 grit sanpaper to help this meshing of the dimple and countersink; but this is not required.
 
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What nobody has mentioned is one needs to make sure the dimple is very crisp, to minimize the radius trying to contact the countersink top edge. For critical items, like the tank dimples to the spar, i broke the edge of the countersink with some 220 grit sanpaper to help this meshing of the dimple and countersink; but this is not required.

I would be leery of thinning the material at its highest stress point. A few dollars invested in a high quality set of springback dies is money well spent. The thickness is not reduced and the fit is better.

3/32 in. skin dies (rivet sits flush with skin)
3/32 in. substructure dies (0.011 in. deeper than skin die - use on interior parts that mate to exterior parts e.g. skin)
3/32 in. fuel tank skin dies (0.007 in. deeper than skin die - allows for sealant under rivet so you don't have to shave the head flush)

There is a video on these pages that go into detail about each of these dies. I haven't used the tank dies yet, but the substructure dies do result in a better fit.
 
if you made a countersink that is just too deep i know you can dimple a thin material [.016-.020 ] and then cut, beltsand around that dimple until you have a ''cup'' that will nestle between the countersunk hole and the bottom of the dimple. a touch of superglue holds it in place until the rivet is set. works well and is easy. i would never be so inept as to over countersink a hole, i just heard about this somewhere.
 
A few dollars invested in a high quality set of springback dies is money well spent. The thickness is not reduced and the fit is better.

Thanks Art - those are the ones I?ve been using, and I agree, you can tell the difference when you use the SS dies on the ribs.

One other poster on another forum made a comment that no dimple should be wider than the female dimple die! Makes sense to me, but I would still like some
guidance on how much gap is acceptable around the skin either side of the dimple. It?s surprising to a newbie - so much of this is scientific, then something like this is like using an uncalibrated hammer. I get where vans is going with the 0.007? but section 5 doesn?t really give any indications on what gap is appropriate either.
 
Makes sense to me, but I would still like some
guidance on how much gap is acceptable around the skin either side of the dimple. It’s surprising to a newbie - so much of this is scientific, then something like this is like using an uncalibrated hammer. I get where vans is going with the 0.007” but section 5 doesn’t really give any indications on what gap is appropriate either.


That is typical of all inexperienced RV builders and why Section 5 makes a very specific recommendation.

This recommendation is based on testing by machining cross sections of rivet joints so they can be viewed and analyzed under high power magnification.

This is from a post about a year ago......

Edit: Here is another thread that discusses the subject (which it appears you (the OP) has commented in so you likely have read all of this already. http://www.vansairforce.com/community/showthread.php?t=157662

Use caution......

One of the more common critical mistakes that we have to help builders through is when they countersink too deep for dimpled skins.

If you go too deep, you will have no gap between the skin and the sub structure but the strength is likely compromised, With countersinks that are slightly too shallow for the skin to lay entirely flush (as in the attached photo) the strength is no different, but there is assurance that it is not too deep. That is the reason for specific recommendations in Section 5 of the manual. The depth was derived from doing samples of different depths and then machining off 50% to allow magnified inspection of the rivet joints.
Countersinks only deep enough for a rivet head still produced totally acceptable riveted joints (though not quite as nice ascetically). Countersinks any deep than about .009 " beyond what would be needed for a flush rivet began to produce visible gaps in the riveted joint.


In a nut shell, the skin adjacent to a dimple can not lay entirely net to the surface below without making the countersink too deep. This is because the outer edge of the countersink has a sharp corner on it, but the angle at the outer edge of the dimple does not.

With proper technique used to form the dimple and install the rivets, from the outside, a rivet where the counter sink was done only 7 clicks deep vs one that was deep enough for the skin to lay 100% flush, the finished appearance will be indistinguishable but the over countersunk one will have a much higher possibility of problems in the future and is not as strong of a joint.

Edit: Here is another thread that discusses the subject which it appears you (the OP) has commented in so you likely have read all of this already. http://www.vansairforce.com/community/showthread.php?t=157662

Repeating my previous post...... it is still a valid answer to your question. I have highlighted a few key points. Bottom line is you can not have the skin lay flush with no gap, and have the sides of the dimple fit tightly within the countersink. The tight fit is much more important. The slight gap you see will not have any effect on structural strength or appearance quality. Van's doesn't specify an allowable gap because if you follow the recommendation of Section 5, whatever gap there is, is considered acceptable.
 
Thanks Art - those are the ones I?ve been using, and I agree, you can tell the difference when you use the SS dies on the ribs.

One other poster on another forum made a comment that no dimple should be wider than the female dimple die! Makes sense to me, but I would still like some
guidance on how much gap is acceptable around the skin either side of the dimple. It?s surprising to a newbie - so much of this is scientific, then something like this is like using an uncalibrated hammer. I get where vans is going with the 0.007? but section 5 doesn?t really give any indications on what gap is appropriate either.

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.
 
Thanks Gents for both your feedback.

Seems sticking with the Vans recommendations is the way most people go of course, it just doesn?t stick well with me that 0.007? is correct for all skin thicknesses. Anyway..

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.

Fair enough! Thanks. I?ve looked at your log a few times - especially your paint booth. Re the CSK, have you tried an electric drill? I?ve tried both, and it is easy to get too fast with the air drill and it starts to screech. I?ve found the electric drill is better, and is mostly what I?ve read people use. Heaps easier to go lightly and slowly than the air drill, but harder to keep normal to the surface than an air drill as it?s heavier.

Repeating my previous post...... it is still a valid answer to your question. I have highlighted a few key points. Bottom line is you can not have the skin lay flush with no gap, and have the sides of the dimple fit tightly within the countersink. The tight fit is much more important. The slight gap you see will not have any effect on structural strength or appearance quality. Van's doesn't specify an allowable gap because if you follow the recommendation of Section 5, whatever gap there is, is considered acceptable.

Thank you Scott. I appreciate the time you?ve taken and I do take your points and understand the reason why vans uses the 0.007?. Thanks again. I?m just a ?why does it work that way?, or ?why do they ask us to do it that way? type of guy. Better to have these questions answered now on the practice kit than go too deep on a real part - the shipping down here is a bloody killer.

I think overall this is a good experience and something I could do. Almost got the minister for war and finances convinced we should have an emp send to Aus!
 
Thanks Gents for both your feedback.

Seems sticking with the Vans recommendations is the way most people go of course, it just doesn’t stick well with me that 0.007” is correct for all skin thicknesses. Anyway..

...

Your instincts are correct. Using a 0.007 in. additional depth is not perfect for all skin thicknesses. If you were to model the dimples using Catia or SolidWorks you could create an assembly showing this.

However, the differences are very small for the range of skin thickness used in RV's. It's a little like picking the fly poop out of the pepper. Recommending various depths for different combinations of thicknesses will get very complicated very quickly. In the end it's better to use the recommended 0.007 in. additional depth because it's simpler and gives acceptable structural performance.

Just for fun, here's a link to a WW2 training film about riveting. Go to the 4:13 mark to see how they do dimpling in two thin sheets. Their approach is a little different than what we use today.
 
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From the mouths of gods.

Thanks very much guys.. I had a response from Van's (which is impressive given Sun and Fun is on!)..

You guys were spot on:
Use the directions in Section 5 to avoid countersinking too deep.
There may well be a gap ? the idea is to make sure the dimpled skin and countersunk part are locked together. A countersink that is to deep will prevent that, and though there is no gap between the parts, the joint is not so strong.
 
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!
 
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...:)
 
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?
 
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.
 

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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]
 
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Follow up

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/learning-to-coutnersink.html?m=1
 

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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.

Very useful info! Do you have the in between shots between 0.007” and 0.011”? 0.011” produces a gap that is clearly undesirable but when does that gap form? I think it would be extremely useful to figure out what the tolerance is beyond 0.007”.

A builder would want to target 0.007” but if they slip and countersink to 0.008” or 0.009” or 0.01”, when should they consider it a mistake that eats in to their structural margins?
 
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