F

fred gassit

Member
Gday All

I have been careful (and lucky) until now in avoiding cracks in these #8 screw dimples but I have got some now in one of my F-820 fuse skins where it attaches to the gear weldment.

I am wondering what repair options are available, the cracks are not overly long but some are deep and can be seen from both sides.

Any repairs may need to incorporate the countersunk reinforcement plate and 4130 plate underneath.

Any and all advice is welcome!

Thanks in advance.

November14201203
 
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Cracks

How did you form the dimples , squeezer , DRDT or C frame ? I have only used a pneumatic sqeezer and the DRDT to form dimples , but I noticed the fuel tank instruction spell out to use the C frame and hammer to form the dimples . I have not seen any cracks in the dimples , but have not used the C frame .
Tom
 
g zero said:
How did you form the dimples , squeezer , DRDT or C frame ? I have only used a pneumatic sqeezer and the DRDT to form dimples , but I noticed the fuel tank instruction spell out to use the C frame and hammer to form the dimples . I have not seen any cracks in the dimples , but have not used the C frame .
Tom
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Have you dimpled the tank skins for the #8 screws?
 
8 dimples

g zero said:
Not yet .What ya thinking ?
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Drill and dimple a test patch using the dies and technique which will be used on the tank skins. I did this and found most if not all the dimples were cracked. I increased the hole size one numbered drill bit at a time until I didn't see any cracks.

Like most others, I am using good quality crisp dies. I have a suspicion that a cheap set of dies might not cause as much cracking but I haven't done any testing.

I probably wouldn't be overly worried if I had a crack in the tank skin but I wouldn't be real excited to have a known crack in the 820 skin. I would probably replace the skin.
 
While it won't help with the current situation, this thread may help avoid the issue in the future. http://www.vansairforce.com/community/showthread.php?t=68593

Pasted from the thread above is something I noted a while back.
Also, please note that you really want to make sure those #8 and and #10 screw holes are smooth and de-burred well before dimpling. Don't over-do it...but just get it right. There's a lot of bending and stretching going on and the chances for cracks around the dimple edge is higher than the smaller holes.
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-Jim
 
I think in my case I've been more lucky than good until now. My fuel tanks worked out ok as did one of the F-820 skins.

I used a hand squeezer on the skin, not sure of my thinking now, probably along the lines of being able to control the rate of distortion.

I've been aware of the propensity for cracking and been careful but the one thing I haven't done is use a larger pilot hole.

It is possible my hand squeezer causes cracks more often but it is also used more in those awkward, heavier gauge situations where they are more likely to occur

If I replace that skin (looking likely) I'll certainly be following what has been suggested here and on other threads and opening the holes up a little more.
 
New skin

Fred,

I see shipping might be an issue for you. If you have access to material use the old skin as a pattern and make a new one. Common technique in the world of restoration.
 
I'll be doing that as well thanks, just before I get one of your die sets for #8 screws.
Incidentally, do you have an opinion on using a larger pilot hole than specified for those holes?
 
fred gassit said:
... do you have an opinion on using a larger pilot hole than specified for those holes?
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Opinion yes, but not much to stand behind it. We recommend using the industry standards for all fasteners. Designed by people far smarter than I. We did experiment with dimpling with a smaller pilot at #21 to allow use of a standard cleko in #8 screw holes. We assumed that the 0.007" would open up when dimpled and allow for a tighter fitting screw hole with the advantage of clekoing without the clekos popping out. We found that these consistently produced cracks. So it does not surprise me that a #19 is right on the edge of acceptable. The larger the dimple, the more material you are stretching down that funnel, and the larger your chance for a crack.
 
Gday all,

The skins have been replaced and although what I've got to share is likely sheetmetal 101 and rudimentary for most of you it might be worth relating.

I test drilled a heap of practice holes beforehand and noticed that where the sheet is allowed to ride up the drill by even the smallest amount the holes take on a slight heart shape.

These were the only holes I found to crack, I think when the skin rides up the flute it leaves a shear mark inside the hole which can open up when dimpled.

Because the sheet has tilted on the bit the hole is also oversized so running the bit in the hole after doesn't completely remove the nick. The solution was to clamp the sheet down as firmly as possible so there was no movement.

So summarising,

I clamped the sheet (close around the hole to drill) against a piece of wood,

Turned the drill moderately slow-not electric drill slow but not air-drill flat out either.

Deburred with a fine Roloc disc both sides of the hole until they were absolutely flush with skin, (takes only seconds)

Used the C frame with a ginger tap-tap-tap rather than one huge thump.

All the dimples are perfect, I inspected them all with a magnifier to satisfy myself.

The (fairly obvious in hindsight) conclusion I've reached is that hole concentricity is at least as important as deburring and #19 holes should be as big as you need to go as long as you can keep 'em round, I would have used a reamer but didn't have one..


Thanks for all the advice given.
 
If possible, I would file out the cracks using Jewelers Files. FYI, this sort of cracking is commonplace with #8 or larger dimples. Install a doubler if desired. My RV guru, Jody Edwards, [professional aircraft mechanic, A&P IA] told me that the best way to avoid those cracks was to use a drill, two sizes larger than recommended [ie for a #8 screw, drill a #17 hole, rather than a #19] I've since followed that advice and have had no more cracking issues when dimpling large holes.
Charlie
 
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