On 2008-06-29, Carl Boyd wrote:
Mark
Thanks for the advice.
My taps do not have a female center in the back.
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.
The spring-loaded center gadget which was mentioned before has a
replaceable follower -- one for the female center taps, and one for the
male center taps.
I did not countersink, the one that came out straight had a fairly long 1/2
inch counter bore that I am sure helped to keep the tap straight. I'll
remember the counter sink idea.
Yes -- the counterbore will certainly help to guide the tap in
straight. Starting with a taper (starting) tap will help too.
Tapping in the lathe tailstock chuck could be helped by using
gun taps (spiral point taps), which chase the chips ahead of the tap
instead of collecting them in the flutes where they can jam if not
cleared by frequent backing. But you can't go all the way to the bottom
of a blind hole with those, because the buildup of chips gets too big.
Use the gun tap to start the threads, and switch to a plug tap to
continue down to the bottom -- or finish up with a bottoming tap if you
need maximum thread length.
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.
Note that there are releasing tap holders made (designed to
mount in a turret on the lathe, but you can adapt them to the tailstock
or the carriage). These feed in with the tap until you reach a stop
(part of the turret -- if you are doing carriage work, you'll have to
set up a bed stop). Once you reach the stop, the tap self-feeds for
about another turn after which it disengages a dog clutch in the tapping
head, so the tap is free to spin. Then you reverse the spindle and pull
back on the tap holder to back the tap out of the hole. This (of
course) needs a gun tap or a thread-forming tap, since it offers no
provisions for backing out every half turn or so.
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.
I have ground my own external threading tools, the nuts fit the threads
fine, I have no idea if they meet spec or not. Grinding for an inside
thread seems harder, maybe, I'll give it a try.
A 1-2" internal thread is a rather tricky one to start with (too
small). Learn using something larger first, like a 1" ID. Among other
things, the direction to crank the cross-feed to clear for backing out
is different from external threading, and it is easy to make a mistake
with old habits. And a larger hole lets you see things like how close
to the bottom of the hole you are getting. For small holes, run your
tool in without cutting (spindle not turning) until it hits the bottom
of the hole. Then back it out a little and put some bright-colored tape
around the shank lined up with the end of the workpiece to tell you when
to disengage the half nuts. Then, start the spindle, crank the tool out
(into the workpiece walls) to cut a groove to full thread diameter, and
move it slowly by hand towards the bottom of the hole to produce a
runout groove at the bottom of the threads, so you don't have to be
totally precise as to when you disengage the half-nuts.
And one particularly important factor in the single-point
internal threading on your lathe is what is the minimum spindle speed.
Especially when first cutting internal threads, you want it slow enough
so that your reflexes are a lot faster than what you need. My 12x24"
Clausing will go all the way down to 35 RPM at the slowest belt speed
plus back gear. This is probably where you want to be for your first
internal threads. Later you can speed up to whatever you are
comfortable with.
Some of the small import lathes have a minimum speed which is
frightening when considering internal tapping. :-)
[ ... ]
If you supply yourself, or make, internal threading tools, you'll
enjoy the results. If you doubt your grinding, you can start by
screwcutting with a pretty-good tool, and finish with the tap. That'll
provide better alignment, too. Have fun, I sorta envy where you are on
the learning curve. I had fun learning all this stuff.
Note that the relief angles will be reversed for internal
threading.
Good Luck,
DoN.
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