Countersink Diameter for Dimples..

[TASEsq]

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 )

[DaveO]

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.

[n982sx]

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.

[longranger]

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...

[TASEsq]

Originally Posted by n982sx

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.

[TASEsq]

Just so we are talking the same language, by 7 clicks you all mean 7 of the little triangles?

You can see how I tried to count these using an arrow and a pen..

https://photos.app.goo.gl/oMd5LOFaQg5PQArg2

[TASEsq]

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!

[longranger]

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.

[TASEsq]

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?

[rvbuilder2002]

Originally Posted by TASEsq

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/communit...d.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/communit...d.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/s...7&postcount=24