W

Wendell

Member
I have been looking at my RV-8 fuselage project for a couple of years now (looks like an upside down boat on a table) I have the longerons in place, skins cleco?d to the bulkheads, now am ready to dimple the skins and rivet to the longerons.

On my practice pieces I always have a gap between the skins and the aluminum angle, be it ever so slight, I can still see light between the skin and aluminum angle.

This is driving me crazy because the skin is not perfectly flush with the longerons. I bought a new set of dimple dies, I have used both a C frame and DRT-2 to create the dimple. I have even made the counter sink into the aluminum angle deeper, still have gap.

Does anyone have any suggestions what I could possibly be doing wrong?
 
The machine countersink into the angle has to accomodate the diameter of the rivet and the thickness of the skin in order for the skin to sit flush on the angle. Check to see your countersink is deep enough to accomodate the skin dimple. Stated another way, the diameter of the machine countersink must be as large as the diameter of the dimple on the inside of the skin.
 
CV 580 reference

I see this a lot and it's not a big deal. Smashing a rivet in will close it up a lot, and I have even seen a Structural Repair Manual reference to this issue. (gap allowance limits) Making the underlying countersink a little deeper will help, provided the material is there to remove. In dimpled sheets it's not going to nest perfectly, nor does it need to to provide a joint superior to un-dimpled joints.
 
You do NOT want to countersink so deep that the skin sits flush before riveting. The problem is that the edge of the countersink is sharp while the dimples have a radius at the edge. If you countersink enough to accommodate this radius, the remainder of the dimple is unsupported. Instead, setting the rivet will pull the dimple into the countersink, effectively letting it act as a female die. Confession: I had some problems with this on the -6A; I typically set my #3 rivets with 20-25 pounds on the gun. I solved it by going up to about 40# on the longerons. The lower pressure was more easily controlled but took longer and didn't add enough force to close small gapping like that. With practice, I can now set rivets in thin skin and structure at higher pressure and don't have this issue anymore.
 
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