According to Bernard Arnest :
Hi,
I was wondering if it would be possible to tap the interior of a ~4"
diameter pipe? I can't imagine a tap so large...?

I can. I've seen a web site showing the building of a 12"
diameter tap.

Can I tap it on the
lathe, by somehow getting the cutterhead inside (I'll need to go back
to the shop on Monday to assess if it can)...?

On a lathe, for making only one or a few, the thread should be
single-point cut, not made with a *very* expensive tap.

For making a *lot* of them, you might consider a collapsing tap,
mounted on a bed turret or on a lathe made from scratch as a turret
lathe. This is like a Geometric die head -- but inside out. It taps to
depth, then trips a release, and the chasers move to a reduced diameter
position, allowing you to withdraw the tap while the workpiece is still
spinning.

However -- for your purposes, I think that it is perhaps time to
learn to single-point cut threads -- first on the OD of something, and
once you have that right, then you try your hand at making them on the
ID. (First turn a "runout" groove at the end of the threads, to give
you someplace to stop the threading pass.

You don't say how long the workpiece is, but I suspect that at
4", you can't pass it through the spindle, so if it is short enough, you
can just hold it in the chuck, or when a bit longer, you'll need to set
up a steady rest to hold it concentric against the cutting forces.

If not, are there
places that will economically tap it for me?

Unless you happen to luck into a shop which already has a tap of
the desired thread, I consider *economical* tapping unlikely. But
economical internal threading is a different matter, and that should be
possible if the pipe is short enough -- or if the lathe is big enough.

Or, just rethink my
design to not require this; although it just makes the design better if
I can do it.

Secondly, is there a way to tap for acme-style threads;

Sure -- tap, or single-point thread on the lathe. Again, Acme
taps are expensive, and large ones are even more expensive, so
single-point threading makes more sense.

Note that part of this depends on the wall thickness of the
pipe, and the thread pitch desired. If the wall thickness is not
sufficient, you may have to select a smaller pitch. I threaded the OD of
some aluminum pipe to 20 TPI, because I did not need strength (these
threads were for screwing on endpieces to a waveguide antenna for
wireless internet frequencies), and because a more appropriate pitch
would come closer to cutting through the wall of the pipe.

are such
threads preferred for higher-load applications like bringing together
to vise jaws?

They are used for such applications -- in part because an Acme
thread can survive wear longer than a standard V thread.

Perhaps you should give a bit more detail about what you want to
do, and why (including dimensions like length) and you might get more
useful advice.

Good Luck,
DoN.

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