Dimple without Drilling/Deburring Prepunched Kit

Several serial kit builder (some who are A&P mechanics) suggest that on the prepunched kit there is no need to drill and deburr - that the dimpling die sufficiently expand the hole without cracking.

Does anyone have experience or an opinion on this?

Opinion on general deburring.

During my tech counselor visits, the most common thing I find is insufficient deburring. 85-90% of all projects I visit have a lack of proper deburring.

After deburring you should not be able to feel a "rough" edge anywhere.

Almost every "crack" I have found on mature aircraft can be traced to insufficient deburring.

A few years ago, many people were over-deburring, and actually rounding edges and/or countersinking holes. We have now swung to the other side.

Please get a tech counselor inspection early on in the building process and learn proper deburring.

Punch creates a burr

montanaro said:

... there is no need to drill and deburr...
Does anyone have experience or an opinion on this?

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The verb "to punch" implies that you WILL have a burr on the side where the punch tool exits the sheet material.
I deburr all and every hole, regardless if they are going to be dimpled or not.
Even though, it might be a boring process, I am always happy that I only have to deburr and not to measure, to mark, to drill.... you get the point

I have been told by what I consider to be very reliable sources that both Vans and Synergy Air have tested drilled, deburred, dimpled, riveted joints versus simply dimpled and riveted and the non-deburred joints performed as well or better than the deburred. This is the way I am doing it.

Tim

My 48-year-old Cessna 150F has shavings showing around rivets on the main wing spar. Clearly they never deburred before riveting.

tjo said:

I have been told by what I consider to be very reliable sources that both Vans and Synergy Air have tested drilled, deburred, dimpled, riveted joints versus simply dimpled and riveted and the non-deburred joints performed as well or better than the deburred. This is the way I am doing it.

Tim

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I can reliably say that what you are saying is true but you have left out an important detail.

Dimpling without final drilling and deburring on prepunched holes for kits prior to the RV-12 and RV-14 is not recommended because the holes are punched slightly under sized. This means that the dimple die pilot is bigger than the hole, so it must be forced through which stretches the material even before it begins to form the dimple.
Doing this on a prepunched hole that has not been final drilled (cleans up the rough surface finish of the material edge inside the hole edge), or deburred, adds a much higher risk of radial cracks at the perimeter of the holes.

Case in point...
On the RV-12 and RV-14, which the plans specify dimpling without final drilling or deburring, the holes are already punched correct size for the rivets and the dimple die pilots are a slip fit. But on these two kits, on any holes larger than .128 (for 1/8" rivets) the holes are punch under size and specified to be final drilled to proper size, and then carefully deburred if it is intended to be dimple countersunk. The reason for this is that the bigger the dimple, the bigger the amount of stretch that occurs in the material at the edge of the holes while the dimple is formed.

So, Van's does not endorse (and we don't do it our selves) dimpling skins that have not been punched to final size, before they are final drilled and deburred (the deburring is because final drilling typically leaves more burr that the punching process does). This would mean that any kit other than the RV-12 and RV-14, the recommended procedure is to final drill before dimple countersinking.

Use a reamer

I have used drill on my tail and wing and begun to use a reamer when I do the fuselage. I really like it because the reamer make a perfect round hole and have less deburirng to do. I even always deburr all the hole...

Closterman said:

I have used drill on my tail and wing and begun to use a reamer when I do the fuselage. I really like it because the reamer make a perfect round hole and have less deburirng to do. I even always deburr all the hole...

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Martin, I'm with you. I like the reamer better than the drill bit for this job. A lot of the time I deburr with a maroon Scotchbrite pad instead of the deburring tool, especially if it's a line of holes like on a skin..

DaleB said:

Martin, I'm with you. I like the reamer better than the drill bit for this job. A lot of the time I deburr with a maroon Scotchbrite pad instead of the deburring tool, especially if it's a line of holes like on a skin..

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I'll third the Reamer for final hole sizing.