Saturday, August 10, 2013

A lesson in dimpling

I have come to realize that a huge part of the quality of a set flush head rivet is the quality of the dimple that it is put in.  That was very apparent while working on the wing top skins.  Well today I decided to put to "paper" what I have learned so that I can refer back to it in the future and maybe somebody will stumble upon this article and it will help them.  I will most likely post this on as well so that I can get feedback and suggestions for things that I missed.

Anyway here is the story.  When I start dimpling I always put the male dimple in the bottom of the C-Frame and the female in the top.  That helps make sure the dimple dies are lined up with the hole, and not some random spot on the skin.  This is not always possible but when it is I do it.  Next, I always hold the female die down against spring pressure onto the male die.  This prevents the spring from bouncing around and allows me to be much more precise with my dimples.  Then I use multiple soft blows with the hammer to set the dimple.  The purpose of this method is really two fold.  You CAN hit the dimple too hard and harden the skin around the dimple which could lead to future cracks.  It also lets me watch the reflection of the C-Frame arm in the skin to see when the dimple is properly formed and I can quit whacking it.  I will try to explain with pictures...

See the reflection of the C-Frame arm?  Thats my reference.  You can also see in the reflection my hand holding the female dimple down firmly on the skin over the male dimple.
Next I start tapping and you can see my reference line (arm) start to reflect a small bow inward.  That means the dimple is not yet properly formed.

Here you can see a small "volcano" affect in the reflection.  This means the dimple still needs a little more whacking.  Put the female die back down and give it another whack or two.  The last whack or two do seem to take a little more effort but its also easy to over-do it at this point.

Speaking of over-doing it, see the slight "cup" shape in the reflection?  That is where I have set it a little too much.  This one is not bad but they can get pretty "cupped".  If you reach this point simply use a pop-rivet dimple die and re-dimple this hole and it straightens up nicely.  

This is what the reflection should look like if the dimple is properly set.  No cups or volcano's.