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Posted by Carl Boyd on June 28, 2008, 10:10 pm
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I have had a problem on a couple of my recent projects. Both involved
1/2"-13 inside threads. Both were cut with a tap on a lathe.
My procedure was to drill the hole on the lathe, and then hold the tap in
the tailstock chuck to start threading until the tail stock chuck slipped,
then lossen and slide back the tailstock, put a tap wrench on the tap and
finish by had. Two out of three times the thread ended up significantly off
axis.
I had assumed that I would start the thread on axis with the tailstock chuck
and it would stay on axis when I finished by hand.
What is a better procedure? I have cut a few external threads, but am leary
to cut internals. I currently have no tools to cut internals threads but I
suppose I need some if that is the only way to do this.
CarlBoyd
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Posted by spaco on June 28, 2008, 1:20 pm
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Several good ideas in the other posts.
You probably know that there are 3 kinds of basic taps:
Taper
Plug
Bottoming.
Taper taps have as many as 6 to 10 tapered threads,
Plug taps, 3 to 5 or so and,
Bottoming taps, about 1 1/2.
Depending on which tap you are using, your tailstock chuck is going
to slip sooner, the taper tap being deeper in the hole and therefore
straighter before the friction goes 'way up.
If you are buying your taps at the hardware store, you are probably
getting "plug".
I like using the dead center in the tailstock to align with the hole
in the end of the tap or in the tap wrench. But, I also have a little
device that is a spring loaded "center" for the tailstock or mill
spindle that does the same thing while freeing you from having to crank
to keep the tap aligned.
Also, could it be that your tap(s) are worn out? Causes same problem.
Be sure to countersink the hole, too. Try a new GOOD one anyway. I
always keep one brand new tap of the most used sizes so I have a way to
check if I suspect that the tap I'm using might be dull. (A REAL
machinist would probably throw the things away on a regular basis).
Pete Stanaitis
-------------------------
Carl Boyd wrote:
> I have had a problem on a couple of my recent projects. Both involved
> 1/2"-13 inside threads. Both were cut with a tap on a lathe.
>
> My procedure was to drill the hole on the lathe, and then hold the tap in
> the tailstock chuck to start threading until the tail stock chuck slipped,
> then lossen and slide back the tailstock, put a tap wrench on the tap and
> finish by had. Two out of three times the thread ended up significantly off
> axis.
>
> I had assumed that I would start the thread on axis with the tailstock chuck
> and it would stay on axis when I finished by hand.
>
> What is a better procedure? I have cut a few external threads, but am leary
> to cut internals. I currently have no tools to cut internals threads but I
> suppose I need some if that is the only way to do this.
>
> CarlBoyd
>
>
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Posted by chipswarf on June 28, 2008, 10:28 pm
Please log in for more thread options Hi, Carl
Does your tap have a female center at the back? I think you'd be
better served, after starting the tap, to put a hard center in the
tailstock and use it to align the tap. Turn the tap with a wrench
while keeping tailstock pressure on the center. You can start the
thread this way also, and save your chuck jaws the beating they take
when the tap slips. Put a nice countersink in the hole before starting
the tap. I was taught this way when going through the USAF machinist
course in (gad!) '69.
You are backing off the tap every half-turn or so, to break the chips,
are you not? That greatly eases the job.
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.
Mark
> I have had a problem on a couple of my recent projects. =A0Both involved
> 1/2"-13 inside threads. =A0Both were cut with a tap on a lathe.
>
> My procedure was to drill the hole on the lathe, and then hold the tap in
> the tailstock chuck to start threading until the tail stock chuck slipped=
,
> then lossen and slide back the tailstock, put a tap wrench on the tap and
> finish by had. =A0Two out of three times the thread ended up significantl=
y off
> axis.
>
> I had assumed that I would start the thread on axis with the tailstock ch=
uck
> and it would stay on axis when I finished by hand.
>
> What is a better procedure? =A0I have cut a few external threads, but am =
leary
> to cut internals. =A0I currently have no tools to cut internals threads b=
ut I
> suppose I need some if that is the only way to do this.
>
> CarlBoyd
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Posted by Carl Boyd on June 28, 2008, 10:43 pm
Please log in for more thread options Mark
Thanks for the advice.
My taps do not have a female center in the back.
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 I am backing the, but it seems to cut fine for about 2 turns and then
get tight. Thats when I back it off.
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.
CarlBoyd
Hi, Carl
Does your tap have a female center at the back? I think you'd be
better served, after starting the tap, to put a hard center in the
tailstock and use it to align the tap. Turn the tap with a wrench
while keeping tailstock pressure on the center. You can start the
thread this way also, and save your chuck jaws the beating they take
when the tap slips. Put a nice countersink in the hole before starting
the tap. I was taught this way when going through the USAF machinist
course in (gad!) '69.
You are backing off the tap every half-turn or so, to break the chips,
are you not? That greatly eases the job.
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.
Mark
> I have had a problem on a couple of my recent projects. Both involved
> 1/2"-13 inside threads. Both were cut with a tap on a lathe.
>
> My procedure was to drill the hole on the lathe, and then hold the tap in
> the tailstock chuck to start threading until the tail stock chuck slipped,
> then lossen and slide back the tailstock, put a tap wrench on the tap and
> finish by had. Two out of three times the thread ended up significantly
> off
> axis.
>
> I had assumed that I would start the thread on axis with the tailstock
> chuck
> and it would stay on axis when I finished by hand.
>
> What is a better procedure? I have cut a few external threads, but am
> leary
> to cut internals. I currently have no tools to cut internals threads but I
> suppose I need some if that is the only way to do this.
>
> CarlBoyd
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Posted by DoN. Nichols on June 29, 2008, 10:21 pm
Please log in for more thread options > 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.
--
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
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> 1/2"-13 inside threads. Both were cut with a tap on a lathe.
>
> My procedure was to drill the hole on the lathe, and then hold the tap in
> the tailstock chuck to start threading until the tail stock chuck slipped,
> then lossen and slide back the tailstock, put a tap wrench on the tap and
> finish by had. Two out of three times the thread ended up significantly off
> axis.
>
> I had assumed that I would start the thread on axis with the tailstock chuck
> and it would stay on axis when I finished by hand.
>
> What is a better procedure? I have cut a few external threads, but am leary
> to cut internals. I currently have no tools to cut internals threads but I
> suppose I need some if that is the only way to do this.
>
> CarlBoyd
>
>