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  #11  
Old 11-21-2012, 01:18 PM
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clevtool clevtool is offline
 
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Quote:
Originally Posted by fred gassit View Post
... do you have an opinion on using a larger pilot hole than specified for those holes?
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.
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  #12  
Old 12-10-2012, 02:28 AM
fred gassit fred gassit is offline
 
Join Date: Nov 2008
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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.
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  #13  
Old 12-10-2012, 10:12 PM
chaskuss chaskuss is offline
 
Join Date: Mar 2005
Location: SE Florida
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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

Last edited by chaskuss : 12-10-2012 at 10:13 PM. Reason: clarified
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  #14  
Old 12-10-2012, 10:25 PM
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LifeofReiley LifeofReiley is offline
 
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Quote:
Originally Posted by clevtool View Post
Reaming the hole rather than drilling may help as well.
Amen... Amen...
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