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Wing spar countersink

LAMPSguy

Well Known Member
In the plans, Vans says to rivet the nut plates to the spar flange, then use a #40/30 CS and that it will center well enough to drill the CS for the #8/6 screws. Is this just to save the builder from buying multiple CS bits?

I have a #27 and #19 CS bits, could I just use those, get all the countersinking done, prime once, then be done.

Per plans it requires drilling, CS, prime, rivet on nut plates, CS again, prime again.
 
I think you're going to run out of material for the pilot to run in, regardless of CS size. That's a guarantee for chatter and a very ugly countersink. Instead of running a pilot in the nut, I made a 1/8" backing plate to clamp behind the spar to provide a pilot hole for the countersink. I needed several to take care of the various orientations and sizes required for the RV-7 wing. I used some very short rivets taped into the plate to accurately locate the pilot hole under the hole in the flange.

FP28042012A0007G.jpg
 
Similar approach

I re-used the aluminum angle that I used to set the rudder trailing edge in a similar way. I could do three feet or so worth of holes without having to move it. I didn't like the idea of running a counter sink in a already set plate nut.
 
Miles is right on about countersink quality. If you loose control of the pilot by going too far through the material, the countersink will chatter and MOVE off center. A BAD thing that can make a mess.

Vans method of installing the nutplates first is a simple way to provide pilot control for the countersink as you intentionally "go too far" to make the required countersink size. The pilot engages with the threaded part of the nutplate and is held on the hole centerline.

To some people, running a pilot in the threads is against their nature so one can make a jig that you clamp onto the spar (before installing nutplates), registered with the nutplate rivet holes, so you can drill and control the pilot. That is what the previous posts are showing. I made mine out of a 1/8" steel scrap. Even with that, it will wear on the multitude of holes.
 
I just finished doing this to the second spar on my 14. The tank attach screw holes most definitely end up larger than the pilot, that is the reason for the countersunk nut plates, and you must have some sort of guide. The access plates use regular flat nut plates and if I got into them with the counter sink it was just barely, so if your brave and careful you might get away with doing them with the #27. I followed the plans method and saw no ill effects on either the countersink pilot or the nut plates.
 
I also followed Van's standard instruction for the big countersinks and had no issues.

Just make sure that the screw holes are drilled to the right size BEFORE installing the platenuts...
Ask me why I know...
PICT4255.jpg


Good luck !!!
 
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