"Richard J Kinch" wrote in message
. ..
Ed Huntress writes:

'Sounds reasonable, but why do they plunge straight in with *external*
Acme threads?

You're correct in your earlier posts that this doubles the chip load, but
the cutting forces oppose each other and cancel out, and moreso with the
narrow Acme angle versus the 60 degree thread form. This cancellation
means that the tool rigidity can handle a lot more pressure than if you
were making just a one-sided cut. It is like drilling a center hole,
where
two cutting edges oppose each other and allow you to use a lot more
pressure and feed than if you were boring a center hole with one cutting
edge.

That could be it; the resistance of the tool and toolholding pieces are a
lot more rigid when resisting loads perpendicular to the bed.

I've never doubted that there must be some reason for it. The thing that's
always bugged me is that I don't recall ever reading or hearing an
explanation. The reason that's curious is that I immersed myself in old
machining books when I was an editor at _American Machinist_. And we had
what probably was the best collection of them in the world.

sigh It will have to remain one of those mysteries of the universe. g

--
Ed Huntress