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-   -   Machine Countersinking .032? (https://vansairforce.net/community/showthread.php?t=120578)

HiPlanesDrifter 12-12-2014 08:21 PM

Machine Countersinking .032?
 
So this has me a bit baffled. Horizontal elevator plans, Page 09-10, step 4 has you to countersink the rear spar on the holes which are common to the trim tab hinge.

http://www.vansaircraft.com/pdf/revisions/RV-14/09.pdf

The rear spar is only .032 thick. I have not proceeded yet, but under very careful examination, it appears the countersink would be very slightly too deep, causing a knife edge around the inside of the hole.

I do realize that section 5.5 does say you can countersink .032 thickness material as shown here:

"For AD3 rivets, a total material thickness between .016 [.4 mm] and .032 [.8 mm] must be dimpled. Material thickness between .032 [.8 mm] and .040 [1.0 mm],should be dimpled, but a countersink may be used if necessary. Finally, for a thickness of .050 [1.3 mm] and above the material must be countersunk."

So .032 is on the very low end.

I am very new at all this, and the only other time I have encountered needing to recess holes associated with hinges was on the tool box practice kit. Interesting, the lid is also .032 thick, and it had you to dimple it and the hinge for the flush rivets.

I hate the thought of going against the plans, but have any other builders entertained dimpling the .032 spar at the hinge rivet holes rather than countersinking, thus avoiding any potential knife edge? Perhaps they don't want us dimpling the hinge causing weakness or misalignment of some kind???

Thanks!
Chuck

az_gila 12-12-2014 08:51 PM

Special case
 
Special case here. Like the other RV kits the softer aluminum of the hinge does not dimple well - it buckles a bit and deforms.

Countersinking the spar lets you use simple holes in the hinge for a much better job.

The plans are correct even if you do make a bit of a knife edge in this case.

rvbuilder2002 12-12-2014 08:58 PM

+1 Gil's comment, and the knife edged spar flange is sandwiched between the hinge and the skin.

This is one of the few places that the plans will specify to not follow the recommendation listed in chapter 5

HiPlanesDrifter 12-12-2014 09:21 PM

The Plans are correct - once again!!!
 
Excellent info gentlemen! I appreciate it. Countersinking I shall do. Precisely per the plans.
Thank again!
Chuck

fl-mike 12-12-2014 09:27 PM

But!, make sure you have the hinge or other material behind the 0.032 you are countersinking, or the countersink will chatter or wallow in the hole!

Practice on some scrap first.

HiPlanesDrifter 12-13-2014 12:11 AM

Mike,
Fortunately I followed the plans to a T, and it called for having the hinge clecoed in place while countersinking. I wasnt sure exactly why, but you just explained it. It went great without a hitch.
Thank you!
Chuck

JonJay 12-13-2014 09:08 AM

Entire wings,HS, and VS have been skinned and countersunk instead of dimpling. If you own an Art Chard built machine, you probably have one. The wing was skinned entirely 032 which also got away from the .025 to .032 transition. However, it requires one to attain a high level of skill to master the technique.
Properly set up an experienced builder could skin a wing much faster as there are fewer operations to perform.
Sadly, rules changed and those builders quit producing airplanes.

Point being, don't be afraid to countersink .032. Practice, back your material, and make sure you have good tools.

az_gila 12-13-2014 09:11 AM

Quote:

Originally Posted by HiPlanesDrifter (Post 941300)
Mike,
Fortunately I followed the plans to a T, and it called for having the hinge clecoed in place while countersinking. I wasnt sure exactly why, but you just explained it. It went great without a hitch.
Thank you!
Chuck

This one job where you don't want to follow the advice of Tim Allen...

Real men don't use instructions, son. Besides, this is just the manufacturer's opinion on how to put this together.

TIM ALLEN, Home Improvement

rvbuilder2002 12-13-2014 11:30 AM

Quote:

Originally Posted by JonJay (Post 941336)
Sadly, rules changed and those builders quit producing airplanes.

Point being, don't be afraid to countersink .032. Practice, back your material, and make sure you have good tools.

Actually it is not a rule John, just a recommendation based on service experience.
Builders can still mach. C.S. .032 if they choose, but it has been found to not be as durable over the long term.

Couple Examples -
The red RV-6B prototype built largely by Art had the emp. stabilizer skins mach. C.S. After quite a few years there were smoking rivets in the rear spar that had to be repair numerous times until it was retired.
The fwd bottom skins on RV-6's used to be called for M.C'ing because it was .040. Even that thickness would develope smoking rivets over time.

Many times over, a dimpled rivet joint has proven to be more durable than a machine countersunk one (at least on RV's with the light skin thicknesses we use).

So from a static strength perspective mach. C.S. .032 is acceptable (other than small localized applications of a few rivets when necessary), but from a longevity standpoint it is not a good idea.

Tom Martin 12-13-2014 12:25 PM

I just went through this in another thread. If you were doing a repair to an aircraft you would need 1.44% more rivets using countersunk rivets to maintain the same strength as a dimpled joint. This information came from one of the older AN repair tech sheets. It makes sense as the two dimples "lock" together. I have also seen more smoking rivets with countersunk surfaces.


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