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mdredmond

Well Known Member
The Standard Aircraft Handbook says in two places that holes to be form countersunk (dimpled) shall not be deburred. Not 'need not' but 'shall not'.

I'm uncertain how to reconcile this with Van's dire warnings about deburring... Also, since I am essentially a lazy person, I'd love to skip deburring 18 million holes if I can. Can someone clarify for me?
 
There is a definite difference between countersinking and dimpling.
Dimpling uses a set of dies to shape the dimple under pressure. You will need to deburr all of the holes you plan to dimple.
Countersinking utilizes a caged cutter, to actually cut out the depression. I guess you could say that the cutter actually deburrs the hole while the countersinking is being done. In the case of a countersunk hole, the underside of the metal ramains flat. Deburring the hole prior to countersinking could enlarge the hole allowing the cage to wallow in the hole and make a less than perfect countersink. (At least that's my opinion.)
There are also rules about the thickness of the metal that can be countersunk, versus dimpled, so you might want to check that out.
 
For Dimples it only makes sense if you are tense sensitive

In the Dimple case "...ed" is important in my opinion. I tried to debur all of my holes because it is important to get all of the little crack starters removed from the lip of the holes. However, if a hole is already dimpled and you try to debur it, well think about the interface angles of the tool with the sheetmetal hole edges and the surrounding sheetmetal face surfaces. If you dimple a string of holes without deburing them before hand, some will immediately contain visible cracks. Not deburring before dimpling is a mistake - again, in my opinion.

Countersunk holes are different. The integrity of the interface beween the manufactured head of the rivet and the adjacent sheetmetal depends on the countersunk surface and the hole diameter on the opposite surface of the particular piece of sheetmetal containing the countersink. When you apply the countersink you are deburring one surface of the sheetmetal at the drilled hole. If the material is thin and you aggressively debur on the side opposite the countersink you will enlarge the hole and reduce the fastener (rivet) contact surface with that piece of sheetmetal and weaken the joint. If you try to debur the countersunk side you will change the cone angle of the countersink and weaken the joint. Not good obviously.

Bob Axsom
 
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Bob, I don't have my Handbook here, but really, what is says is (I'm paraphrasing but am certain I'm getting the tense right here):

Drilled holes that are to be form countersunk shall not be deburred.

This is in chapter 4 somewhere. It appears twice.

I'm not saying deburring is a bad idea - but the book suggests it shouldn't be done. I have no idea why - perhaps the dimple dies squash the burrs back into place?
 
Matt you are too fast

You caught me in the act of adding the countersink paragraph in my earlier post. Sorry I fumbled and only considered the dimple situation before I posted originally.

Bob Axsom
 
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I'll try repeating as well...this is about dimpling, aka "form countersinking", NOT machine countersinking.

There have been discussions on this strange quote in various forums in the past (matronics I think) with no clear conclusion that I saw.

Personally, I will debur both sides of all layers of all holes that I possibly can. I am already noticing a few hairline cracks around certain dimples and I haven't even flown yet!

And to add to the confusion further, I noticed yesterday that the CherryMax brochure comes with an admonition to "not chamfer" the shop-head side of a hole to be used for CherryMax.
 
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