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  #21  
Old 05-28-2016, 12:19 PM
Marc Bourget Marc Bourget is offline
 
Join Date: Apr 2009
Location: Stockton, California
Posts: 296
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Mushrooming the head vs. swelling the shank.

From experience, I think you see mushrooming with increasing age/hardness of the rivets. Back in the 70's Fullerton Air Parts would get rivets shipments twice a week. A day, or less, from the mfg, you could drive them at 55# with a short burst. Perfect "donut" shop head, no swelling of the adjacent skin. After 4 days, it was 80#, more hand pressure and a longer burst. Sometimes you'd get localized deformation of the nearby skin.

Sometimes, when things don't quite "fit", you can pull the parts together by lightly driving the rivet, which swells the shank to the point you can use a bucking bar with a hole drilled to "draw" the parts together and there's enough drag to keep them tight so you can proceed to form the shop head.

Bought some surplus rivets. They had been around for a year and the shop head, actually the end of the rivet shank, would form like the bell mouth of a trumpet - not the "donut" that formed when fresh. No way to "draw" with the harder "trumpet" end rivets.

Conclusion: The "trumpet" shanks don't fill the hole as well as the "donut"

shop heads.


FWIW
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  #22  
Old 05-28-2016, 01:09 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 5,516
Default A follow-up.

Lots of solutions and diversions. Back to the OP - followup.

I have the Avery back set. 3/4" in dia at the plastic, 1/2" for the driver.

I took a small section of rib stiffener and measured, the rivet centerline is only .070" off center (.180 from the edge). The set is flat, so no wedging effect. There is another problem occurring here. The OP needs to adjust technique, equipment, pressures or other variables to fix the issue. Learning is part of the process. Knowing something can be done sets the goal - this can be done!

Now is a good time to practice by varying pressures and trigger time. Count the hits, hold the gun differently, quantify the results. I hold mine upside down with my palm over the center axis and pull the trigger with my middle finger. Develop your own technique, one that works for you consistently.

Alternatives are good, but won't overcome developed skill. A skill which must be improved to complete the airplane.
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Bill

RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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  #23  
Old 05-29-2016, 10:59 AM
Marc Bourget Marc Bourget is offline
 
Join Date: Apr 2009
Location: Stockton, California
Posts: 296
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Bill's advice is invaluable.

Years ago, I made a lot of extra work for myself, and achieved less than optimum results (but good enough prompt me not to re-do the task) because riveting conditions change almost day to day. Rivets get harder, even one production run of alu sheet from another, certainly different thicknesses.

I now make it a practice to "practice with test tabs" Excellent use of shear drops or trimmings that otherwise can't be used in the structure.

As a closing remark, these differing results are reduced the fresher (softer) the rivets.

mjb
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