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  #1  
Old 12-03-2017, 05:22 PM
StressedOut StressedOut is offline
 
Join Date: Jun 2017
Location: Fullerton, CA
Posts: 89
Default Dimple resting in a countersunk hole

I've got a problem and I'm not sure how to proceed. I'm working on the VS rear spar and the plans call for dimpling the rear spar and countersinking the doubler. It's a #30 hole for an AN426AD4 rivet.

Before I do something irreversible to my doubler, I decided to do some fit checks on scrap. I cut a small coupon out of 0.032 in. material, which is the same thickness as the rear spar, and did some test countersinks on a thick piece.

Here's a picture: https://ibb.co/fojrnw

So the problem I'm seeing is that the dimple will not allow the sheet to lay flat against the countersunk part. I tried several different depths of countersink to get it flat, but when the sheet does lay flat the dimple will wiggle from side to side.

I measured the size of the dimple and it came out to about 0.350 in. I then countersunk another hole to 0.360 in. C/S diameter and it still didn't fit perfectly flat, however it was closer. There was still a gap between the sheets, but no wiggle though.

The dimple set is a 426-4 from Cleaveland Tools and the C/S bit is a #30 100 degree also from Cleaveland.

So to my questions:

(1) Is there a recommended diameter countersink for AD3 and AD4 rivets for a dimple resting in a countersink? Of course by diameter I mean the diameter of the C/S on the surface.

(2) What is the acceptable gap tolerance between parts?
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Art Jackson
RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
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Working on: Empennage (Elevator)
Construction log - mykitlog.com/ajackson
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  #2  
Old 12-04-2017, 09:10 AM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,617
Default

Have you seen this thread?http://www.vansairforce.com/communit...d.php?t=155478

Post # 7 links to a previous post I made regarding countersinking for dimples........

http://www.vansairforce.com/communit...82&postcount=5

To specifically answer your questions -

- There is a specific diam. for what Van's considers a proper countersink to accept a dimple but because of the challenges in measuring it accurately a depth is specified instead.
This is detailed in Section 5 of the construction manual.

- There is no published gap tolerance because even if there were, it would be very difficult to measure accurately because of many different variables (level of dimple quality, which dimple dies were used, etc.
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  #3  
Old 12-04-2017, 09:48 AM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Los Angeles
Posts: 470
Default Just my opinion

Just my opinion, but to improve the fit between a countersink and a skin dimple, I think one needs to break the edge of the countersink. In my view there is no way to get the dimple with a crisp enough edge to exactly match the countersink. But I think even if the coubtersink edge is not broken it will be ok anyways because we are allowed 0.002” gap between dimpled skin and spar. I think it is prodent not to exceed the counter sink depth specificed by Vans in order to improve fit. Saying all this, I am going to start hitting the countersinks with 220 grit to break the edges. JMHO
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  #4  
Old 12-04-2017, 10:25 AM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 2,905
Default

Make another dimple test patch for the fit test. Put it in your C-frame, and whack it like you're driving a 20 penny nail. (That's a big nail, if that helps.) See the faint ring embossed in the metal; same diameter as the male die? That means you did a proper dimple. If you don't see the ring, you didn't hit it hard enough. Now, try the fit on the countersunk piece.

The test tab in the pic looks significantly under-dimpled (which is very common on RVs, including the -4 I currently own). Drop a rivet in it, and see if you can catch your fingernail on the edge. If you can, it's under-dimpled.

Under-dimpled holes leave a rounded corner on both top and bottom of the skin, which causes the rivet to stand proud of the surface, and prevents the dimple from nesting properly in the substructure. It also tends to leave the skin...less than flat. Rounding (breaking) the corner of the substructure countersink is compensating for the actual problem.

Charlie
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  #5  
Old 12-04-2017, 01:41 PM
StressedOut StressedOut is offline
 
Join Date: Jun 2017
Location: Fullerton, CA
Posts: 89
Default

I re-read Section 5.5 on machine countersinking and there's my answer. C/S to flush and go 7 mils deeper. If the dimple still rides higher than 2 mils I'll touch the edge of the C/S with 220 grit to break the corner.

I have a DRDT-2 dimple press and I made sure I adjusted the business end to so that near the end of the stroke where the mechanical advantage kicks in the dimples dies touch each other. I doubt the dimple in the picture is under dimpled for a couple of reasons. The rivet lays perfectly flat and even with the surface, and it doesn't have a distorted reflection around the hole.

Thanks to all who replied! I really appreciate this community!

P.S. I'm still in perfectionist mode since I just started. I'm sure I'll grow out of it soon.
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Art Jackson
RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
Scrapped: Rudder
Working on: Empennage (Elevator)
Construction log - mykitlog.com/ajackson
Dues paid on 2 November 2017
Member of EAA Chapter 92 (KCNO)
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  #6  
Old 12-04-2017, 05:27 PM
DavidAhrens DavidAhrens is offline
 
Join Date: Feb 2007
Location: Bakersfield, CA
Posts: 45
Default dimple nesting

hi Art, another thing to consider, sometimes the DRDT doesn't make as clean a dimple as a simple C-frame tool. Can you borrow a c-frame tool and make some samples?
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  #7  
Old 12-04-2017, 06:38 PM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Los Angeles
Posts: 470
Default DRDT

With the DRDT you have to adjust per the instructions. The frame has to flex in order to get the high force to form the dimple. I also had to shim the reciever to match the top die. It takes a little time to setup , but then it holds the setttings forever and allows me to crank it out.
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John S

WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

Extra dues paid 2018, worth every penny

RV9A- Status: tail feathers done less tips
Wings done less tips and avionics
Fuselage started
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  #8  
Old 12-04-2017, 11:47 PM
StressedOut StressedOut is offline
 
Join Date: Jun 2017
Location: Fullerton, CA
Posts: 89
Default DRDT adjustment

I adjusted the ram slightly to get a bit of flex in the frame and that seemed to do the trick. The reflection around the dimple went from nearly flat to absolutely flat with no distortion whatsoever. The gap is gone in my test countersink. I couldn’t discern any difference in the flushness of the rivet though.

I still need to drill a countersink using the method in section 5.5 and test out the fit.
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Art Jackson
RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
Scrapped: Rudder
Working on: Empennage (Elevator)
Construction log - mykitlog.com/ajackson
Dues paid on 2 November 2017
Member of EAA Chapter 92 (KCNO)
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