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Doug Miller

In article , wrote:
(Doug Miller) wrote:
All this opens up a can of worms. Should the blind side of the rivet
be the same shape as the visible side (i.e. a barely discernible bump)

No.

or should 90% of the blind rivet be still un-flattened?

No. If that's what you're getting, then you're using the wrong size rivets.
Try getting some that are shorter. What size are you using now?

Looks like about 3/8 inch.

*Waaaaaay* too long for riveting two pieces of sheet metal.

The current
situation is mostly un-flattened but if this is the correct state of
affairs these have to be cut off so they don't tear the bags.

It's normal for there to be a protruding lump on the back side. It's not
normal for that lump to be much longer than the diameter of the rivet.

The web is annoyingly simplistic in all the how to's on the subject. I
have yet to find one that describes:

When to use aluminum and when to use steel.

When joining pieces of aluminum, or steel, respectively. Obviously.

Not obvious. What effect would there be to use aluminum (softer) in a
steel pipe in the absence of moisture

"Absence of moisture"? You live in an area where the relative humidity is
*zero* ??

and/or electrical conduction
questions?

Shear strength, if the joint is subjected to physical stress.

Even if corrosion resulted if it took ten years to manifest
itself it's unimportant given the application. OTOH if the aluminum
rivet will break after five days...that's another story.

Depends on how strong the joint needs to be. Use aluminum rivets on aluminum,
and steel rivets on steel. I repeat, this is obvious; if you don't find it so,
then perhaps you should consider that you're already in over your head.

Which # is first (or second). Is the first number the diameter of the
hole I need to drill

Yes.

and the second the thickness of the plate to be
joined?

Not quite. The second number is the *maximum* combined thickness of
materials that the rivet will fasten together.

In which case I should be looking at a 1/8 by (18 + 18ga) but
such a rivet doesn't exist.

1/8 x 1/4 or 1/8 x 3/16 should work just fine.

What do the HVAC people use?

Sheet metal screws.

Supply exists of things like 1/8 * 3/8
inches. Yikes! That implies two plates totaling 3/8 inch thick. A
massive piece of steel! Obviously I'm reading this all wrong.

3/8" is not "a massive piece of steel" by any stretch of the imagination.

Maybe my imagination stretches further than yours. 3/8 may not be
massive in the context of tank armor but a 3/8 thick duct pipe (or car
door, or refrigerator panel) would be huge.

Or maybe you just have an exaggerated view of what "massive" is.

A
more common use for a 3/8" long rivet would be attaching sheet metal to 1/4"
steel -- a 1/4" rivet isn't long enough.

Why would you ever use a pop rivet in 1/4 steel? Nut, bolt, screw,
tap... a lot more appropriate.

And a lot slower -- and hopefully, that makes the answer to your question
"Why"... obvious.

What size and type of gun do the people who use these frequently (not
in a factory environment) choose?

I just use a basic hand riveter that I bought at Sears many years ago for
about five bucks.