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New Builder Basic Dimpling Question.

Fewhairs

Member
I've taken the plunge and started an RV-12 tail kit. I'm building the vertical stab and have been doing some test dimples in material the same thickness as the skins. I have purchased a #8 dimple die for dimpling the forward stab skin for the countersunk screws as called out in the plans. The hole size is #19. When I dimple with either a c-frame or a squeezer, I noticed the the #19 holes become slightly enlarged so that the screws have a looser fit than the bare non-dimpled #19 holes. I'm trying to determine whether my dimpled holes that are slightly enlarged are unusable and to determine a solution before I dimple the actual skin. Any insight or wisdom is greatly appreciated.
 
Not sure what you mean by loose. try installing the forward skin on the vert stab and tighten the screws into the nutplates. They are #8 cs screws. They should tighten up nicely and sit flush with the skin. If the skin still moves around or the screws sit too high then that would be a problem.
 
I too have found that the process of dimpling for screws does indeed slightly elongate the holes ...

I would be more concerned with the edges of the holes showing some cracking after dimpling which I have seen occur once on one of my inspection cover plates after dimpling.

Happy building,
 
Hole enlargement is a normal byproduct of dimpling. Discussing risks/benefits of smaller hole size here will likely take on biblical (or even political) proportions.
 
I too have found that the process of dimpling for screws does indeed slightly elongate the holes ...

I would be more concerned with the edges of the holes showing some cracking after dimpling which I have seen occur once on one of my inspection cover plates after dimpling.

Happy building,

Ditto what John said. I've had trouble with cracking with #8 dimple dies on .032 Alclad. Anybody know a sure way to prevent this from happening?
 
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Thanks for the suggestion. I'll try it next time I need to dimple #8 holes.

Just keep in mind that using a #18 reamer will result in holes that are even more over-sized (the bigger the hole you start with, the bigger it will be after dimpling).

As already mentioned, it is not possible to dimple countersink without the hole getting larger, so the I.D. of dimple countersunk holes not being a tight fit to the screw is expected.
It would not be possible, but think about what would happen if the dimple process reformed the metal so that it bent downward 90 degrees instead of the 40 degrees that it does for a dimple. It would result in a hole that matched the diam where the bend took place.

The important thing for preventing cracks on larger dimples is nicely finished surfaces on the interior of the hole before dimpling and a properly sized hole so that the dimple die pilot is not stretching the material at all before the forming process begins (the pilot must slip into the hole with no force required).

This is why even though the RV-12 (and now RV-14) were designed to have most all holes punched to final size, any holes that are meant to be dimple countersunk for #8 screws are punched under size and then specified to be final drilled/reamed so that it cleans up the interior edge of the hole (always use a newer / sharp drill or ream). After proper deburring, this usually prevents cracking.
 
The important thing for preventing cracks on larger dimples is nicely finished surfaces on the interior of the hole before dimpling and a properly sized hole so that the dimple die pilot is not stretching the material at all before the forming process begins (the pilot must slip into the hole with no force required).

This is why even though the RV-12 (and now RV-14) were designed to have most all holes punched to final size, any holes that are meant to be dimple countersunk for #8 screws are punched under size and then specified to be final drilled/reamed so that it cleans up the interior edge of the hole (always use a newer / sharp drill or ream). After proper deburring, this usually prevents cracking.

Thanks for the additional information, Scott. There?s always more to learn, and you do an excellent job of keeping us on the right path. Your insite is always appreciated!
 
Thanks everyone for such instructive answers. Most of my test holes are probably within tolerances albeit slightly over-sized. I'm now ready to dimple the parts that I'd like to keep.

This forum is extremely helpful and informative. Thanks again for taking the time to answer new-be questions.
 
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