Sizing double flush rivets?

[digidocs]

How does one determine the correct length of a double flush rivet? Do the usual rules apply?

If it makes a difference, both surfaces are parallel and I'll be squeezing them.

Thanks,
-DC

[terrykohler]

Try using the same length rivet you would use in a normal stack-up of the same thickness. The shop head should contain about the same amount of material as the factory-formed head, i.e., about 1.5 diameters.
Terry, CFI
RV9A N323TP

[rvbuilder2002]

Originally Posted by terrykohler

Try using the same length rivet you would use in a normal stack-up of the same thickness. The shop head should contain about the same amount of material as the factory-formed head, i.e., about 1.5 diameters.
Terry, CFI
RV9A N323TP

This would be way too long.

You can generally use the same 1.5 D rule but use a length that protrudes 1.5 D passed the bottom of the countersink on the shop head side.

Using the 1.5 guideline for double flush actually is a little long on rivet length and it wont generally squeeze or drive flush with the countersink. Choosing a size that is just shy of 1.5 is a better fit if you need it to be totally flush with the material surface.
One mistake people often make is assuming that the shop head will totally fill the countersink. It won't, unless you are using special application rivets and then shaving them flush after driving.

[Tom Hunter]

You can determine the correct length by setting up a test strip first and experimenting with your squeezer. Also, you must be careful to keep pressure on the squeezer so that the rivet remains tighly seated in the "top" countersink while you are filling the "bottom" countersink.

If you are back riveting if possible clamp the parts to the backing bar. This will insure that the rivet doesn't back out of the countersink.

On some occassions, you may find that none of the rivets you have on hand are the correct length. In that case you need to have a rivet cutting tool so you can cut a rivet to the exact length you need. If you are building an RV-12 which makes use mostly of POP rivets, the investment in a rivet cutting tool is probably not worth the money.

You can make a simple rivet cutting tool with a pair of pliers. You open them slightly, then in your drill press, drill a #40 hole on one side and a #30 on the other.

Then when you insert the rivet and close the pliers, the rivet is cut cleanly. I once had a pair of pliers that I modified to do this. Note: not all pliers are tight enough to give you a clean shear cut.

"If" you have a steady hand, you can cut a rivet to a shorter length with a dremmel cut off wheel. Wear safety glasses. This is only practical if you have but a few rivets to shorten.

Tom Hunter

[Mel]

I have an old Sears crimping tool with bolt cutter for #4 through #8 screws. It works quite well as a rivet cutter.

[gasman]

Originally Posted by rvbuilder2002

One mistake people often make is assuming that the shop head will totally fill the countersink. It won't, unless you are using special application rivets and then shaving them flush after driving.

Depending on the location, this might be a good place to use soft rivets?

[terrykohler]

absolutely not.

[Andrew M]

double flush making the shop head countersink smaller than a 426 head. A 1097 head or smaller works well. The driving issue is by the time the shop head is smashed down enough to fill the countersink, it has work hardened to the point it begins to deform the base metal (.032 or thinner) Thicker applications it works well to start upsetting the shop head with a bar and finnish by back riveting. Last thing to consider is location. How far down the stress path is the fastener, thus how close to standard practice does it need to be. Rivits are very forgiving as long as it fills the hole.

Andrew
-4 started and sold
dreaming again
20 years bending tin