At 02:48 PM 5/21/2011, Jim Felder wrote:
>Great job of describing thread restoration and handling taps.

Thanks.

>  I have just one thing to add that came my way as an army
> machinist, and I'll bet most folks don't know it. When tapping, is
> is best not to just keep turning the tap (or die) in. It should be
> turned in about a quarter turn, then backed out to break the chip,
> then forward again, then back until the job is done.

Yeah, I didn't want to get into that, though maybe I should
have.  For thread-chasing mostly don't need it.  For actual tapping I
think every turn or so is enough, but if you wait too long you'll
break teeth on the tap as sure as little green apples.  For
through-holes there's something called a spiral-flute tap (commonly
called a gun tap) that pushes the chips ahead of it and can go
straight through without reversing, but you won't find them at Pep
Boys.  And then there are rolled-thread taps, a whole 'nother animal
that need a larger hole.  Also, drill-size tables for tapping usually
are based on 75% depth-of-thread.  That's fine for aluminum but for
tougher/harder materials it's probably a reasonable idea to use a
slightly larger drill to get maybe 60% depth or even less.  Tapping
will be much easier and the strength isn't much less.  Here's a chart
giving tap and clearance drill sizes for 75% and 50% thread depth.
http://littlemachineshop.com/Reference/TapDrillSizes.pdf

For dies one side is larger than the other; that's the side you start
on.  The teeth are ground to cut in that direction and the taper is
to help get started.  Also helps a lot if you grind a taper on the
first part of the thing you're threading (doesn't count for thread-chasing).

For tapping fluid I usually use Tap Magic which comes in two flavors,
regular and aluminum (which smells somewhat like cinnamon and I find
tiresome after about a minute).  But Home Depot sells sulfurated pipe
tapping fluid which is stinky but no doubt works ok.

I can't emphasize enough both the importance and the difficulty of
starting the tap (or die) straight.  If you can possibly clamp the
part on a drill press table and start the tap by hand in the press,
do it.  I have a tap handle that fits into a flared base with a
couple vee notches in it so it will stand perpendicular on a flat
surface or a rod/tube.  It's a huge help if there's room for it.

For cleaning up male threads in place it's also possible to get a
thread-chasing file that has eight pitches on it.
http://www.metric-tools.com/page/page/4362146.htm  I got mine from
MSC and I don't think they (two inch and a metric) cost nearly that
much, but it was a long time ago.

Yrs,
David