On Mon, 30 Jun 2008 20:49:33 -0400, "Carl Boyd"
wrote:


A 1/2-13 without a center hole? That sounds like cheap taps.
Smaller taps 10-32 and smaller) may have a male center instead of a
female center. I don't think that I have ever seen a 1/2-13 with that,
however.

I am using a Craftsman Tap, I've gotten a lot from the set, although I am
sure
they are not industrial quality, I don't consider them cheap (i.e. Harbor
Freight is cheap).
A good set bought new would cost me more than my lathe. Budget has been
tight but
may loosen up some next year. I picked up 3 Union Butterfield, 3 tap sets
last month at
reasonable prices. I'll continue to keep my eyes open.


I forget whether you mentioned the workpiece material. A thread
forming (rolling) tap instead of a thread cutting tap can deal with
such holes without generating chips if your workpiece is not too hard --
but it needs a different staring hold diameter. The standard will jam
it and break it off in the holes.

The workpiece material has varied from CRS to an old lawn mower crankshaft.


Yes I am backing the, but it seems to cut fine for about 2 turns and then
get tight. Thats when I back it off.

Hmm -- you should back more frequently than that -- before it
gets tight -- unless you have a gun tap or a thread forming tap.

2 turns is pretty much where it starts to get tight. When just starting I
do
tend to back off quicker, but when the tap is well started I turn until I
feel the resistance increase. A 4 flute 1/2-13 tap has a fair amount of
room for chips before they start to bind up. With some taps and some
materials I back up every 1/4 turn. Why is 1/2 turn the magic
number?


Some of the small import lathes have a minimum speed which is
frightening when considering internal tapping. :-)

My 16" LeBlond goes pretty slow. I cant remember of the top of my
head but I believe it is under 20 RPM


Note that the relief angles will be reversed for internal
threading.

My external tools I ground with symetrical relief angles and pretty
much just eye balled them, but I have not done any very coarse theads.


Good Luck,
DoN.

Thanks, I need it.

CarlBoyd



Greetings Carl,
I (almost) never have trouble with tapping in the lathe. When tapping
the tap is gripped hard enough in the drill chuck so that it doesn't
slip until the tap starts to bottom. Of course, large taps or really
tough materials may make the tap slip in the chuck. Tap sets often
have taps made for hand tapping. These taps will have 4 flutes instead
of two or three. And the chips do pack into the flutes. Stringy
materials like mild steel can pack up real tight because the chip
tends to be continuous. This why the tap is backed off every so often.
I generally use spiral point taps or roll taps. The spiral point taps
push the chips ahead of the tap and so don't tend to fill the flutes.
If you examine the end of a spiral point tap you can see how it will
push the chips forward. Also, the tap thread is actually ground with a
decreasing pitch diameter at the end of the tap. This makes them cut
easier. Hand taps just have the first threads ground away at an angle.
Spiral point taps do pack chips into the bottom of the hole though. So
blind holes can be a problem. I try to use roll taps whenever I can
because they don't make chips and they make better threads. Roll taps
require a larger hole because the metal is only displaced during
tapping. The holes also need to be closer to the exact size because of
the metal being displaced. A slightly oversize hole will result in the
minor diameter being too large and a slightly smaller hole will
require much more torque and may break the tap. Roll (AKA form) taps
work best in ductile materials. This means that wrought aluminum,
copper and most of its alloys, mild steel, most stainless steels and
zinc are good candidates for roll tapping. Some cast aluminums and
some cast irons do not tap well with roll taps. The metal fractures
from the displacement and this weakens the threads. The weak thread
may not be obvious until it fails under pressure.
Cheers,
Eric