The easiest way is to use a tap on the lathe. There is a handy little
gadget called a tap guide, it is spring loaded to keep contact with the
tap as you advance and comes with a point on one end and a cone on the
other. The cone end is for the smaller taps. You drill your hole in
the bar in the lathe, chamfer the hole(optional but it does help the tap
and looks a lot better) then insert the tap guide into the drill chuck
and with the tap placed up against the hole, bring the tap guide up and
center the tap with the end and then move the unit forward with the
tailstock handwheel. Be careful not to go to far or you could chip the
teeth where it is touching the work. Tap into the work and
occasionally move the quill up to keep the tension on the tap guide.
There should be one in every toolbox, only about $6 from KBC or similar
outlets. If you are not sure of what they look like, see
and goto page 201, middle of the page.
Walter Harley wrote:
Newbie question - not a big deal, just wondering about something. I need to
tap a 1/2"-20 thread through the center of a 0.75" long mild steel cylinder
In principle, I can do this either with a tap or with a lathe, correct? Is
one preferable to the other? What's the smallest hole you can thread with a
A tap is generally preferable, but you can do it on a lathe if you want
to. The tap is a lot faster. The lathe may be less expensive if you only
need to do a few and don't care to buy the tap to do them faster, and/or
will enjoy the challenge of cutting them. If you're willing to grind the
tools and willing to work within the limits of strength of the tools you
grind, you should be able to go pretty small, but I can't say I've
really experimented with how small a hole you can thread, since a tap is
faster unless you cannot get a tap. Certainly 1/2" should be doable.
Tap is preferable if you have a suitable tap on hand.
Lathe works when you don't, and perhaps one isn't even available.
Smallest hole that can be single-point threaded on a lathe depends
on the machinist, the material and the threaded depth. I know I've
threaded holes about 1/4" dia, and I think I've done smaller. Made
the cutting tools by grinding them from broken drillbits. I have one
boring bar with a 1/16" shank.
"Walter Harley" snipped-for-privacy@cafewalterNOSPAM.com
As another suggests, you can combine the two and use a tap pushed by the
tailstock of a lathe. This will give a concentric thread, which will be harder
to produce hand tapping in the vise. If your tap has a small 60 deg hole
centered on the back end, you can fit a dead center into that (with the tap
held in a tap wrench as well) to aid hand turning, and alternate torquing the
ts handwheel and tap wrench ( always renewing and keeping pressure from the
dead center on the tap) as you go. This can also be done under power, jogging
the motor so you tap just a bit at a time.
Sometimes you may want to single point some or most of the thread first (cut it
with a boring bar in the lathe), then finish with a hand tap. Even if done in
the vise, the tap will then follow the started single-point cut, so remain
concentric to the bore.
The smallest internal thread you can do on the lathe is the same as the
smallest internal thread you can do with a tap--multi point vs single point is
the only difference. For me it's about 3/4 10.
Threading on a manual lathe is easy enough in general. I do it
frequently -- both OD and ID. I go to my smaller CNC machine usually
for metric threads, though, because they are a pain on a machine built
for inch threading -- even if you have the conversion gear set.
1/2-20 should not be a real problem, as there is enough room for
sufficient meat to allow threading a 0.750" long through hole. If it
were 1/4-20, I could see problems with either route.
1) For a lathe, there would be little clearance for the chips, and
for backing the cutting tool out, especially with a hole that
2) For a tap -- I think that 0.750" length of thread would be a
problem because only part of the length of thread is full thread
form. The end of the tap has a taper to make it easier to
start, and I'm not sure that a 1/4-20 would have sufficient full
diameter thread -- so you would have to turn it around and pass
the tap from the other end to enlarge the threads to full size.
However, since you are at 1/2", not the 1/4" which I first
thought that I saw, you are fine either way. The 1/2" taps usually have
a smaller shank than the thread root, so you can push it on through to
complete the thread, even if it is not long enough in the flutes to
complete the task.
I would normally use a tap for 1/2" and down, simply because the
smaller tools are a bit harder to find (or make) -- especially since you
need a bit longer than your 0.750" to get the cutting tip clear of the
workpiece to get the whole length. (And if it were a blind hole, it is
even more difficult.
I *do* have an insert threading tool which will work down to a
10-32 thread, but that I probably would not use except in the CNC
machine, as there is very little clearance for what you need to do.
However, here, I would suggest that the threading on the lathe
could be considered a learning experience, even if you would not do it
that way most of the time. Sometimes you will wind up needing to cut an
uncommon thread, and can't wait for a special tap to arrive from whoever
you mail-order from. Or -- the special tap may be more expensive than
it is worth for a single threaded hole.
But have a 1/2-20 tap on hand anyway -- since the first time or
two you might have problems hitting the thread size right, and you can
finish the task with the tap much easier than you can re-chuck and
re-align the workpiece to open the threads up a bit if too tight. (If
you have something to test the thread before removing it from the lathe
chuck, that is a different matter.
If you want CONCENTRIC STRAIGHT threads, then use the lathe. You will
find it difficult to hold a 3/4 in piece of steel in anything where
the outside doesn't slip and scar when running a 1/2 inch tap. Even
if you finish the tapped hole with a tap, starting the tapping
procedure with a lathe will remove material without the gigantic
stress load of the tap. AND use a SHARP GOOD QUALITY (meaning not
Harbor freight Chinese grade) tap in any case...especially if you
don't pre-thread on the lathe. But for concentric, straight threads,
use the lathe. For a higher probability of correct thread form use the
In my mind, the most despicable of all men are those that try to
discourage others from gaining a skill they don't have, BUT CAN
DEFINITELY LEARN. IN this particular instance, nothing was mentioned
about precision threading, and for 1/2"-20, I would also do it in the
lathe with a tap. However, were I lacking the tap, I would do as I
have for others, including down to 1/4"-32, that is single point it.
Threading isn't something for old germans, it used to be something, as
little as 20 years ago, that an apprentice was expected to master
It would be interesting to hear how the one that made these obviously
ignorant statements would handle a 1 1/2"-2.5 double lead internal
left hand acme thread, and I would also love to be his tool supplier.
Ignorance is expensive.
Ummm, yes. I've cut that thread, and didn't take a week to do it
either, more like an hour. The next four pieces went a lot quicker.
The first always takes longer.
Hmmm... Yes, no, and maybe! I have a 1/2-20 tap.... that is what I'd
use in the lathe.
If I had a bunch of these to do, I'd BUY a tap....
But if I only had one to do and had no tap, I would consider chasing
the thread in the lathe.
There really isn't one too small, though grinding the tool might
become prohibitive for very small (common) threads (just buy a tap and
get it over with)..... as Larry the Cable Guy would say, "Get 'er
IMHO, the decision on how to accomplish this task would be built
around these questions:
-how many holes do I need to thread?
-do I have the tooling, now?
-how much will tooling cost?
-how long will it take to get the tooling?
-how quickly do I have to get this done?
FWIW, threading is taught in the first quarter of the lathe class at the
technical college where I'm taking evening machining classes; you have to
cut an internal and an external thread before you pass the class (basically
the class consists of making a big bolt and matching nut).
Only reasons I don't know how to do it, at least at a rudimentary level, are
(1) I'm taking the milling class rather than the lathe class, and (2) my
home lathe is a Sherline without a threading attachment.
You seem to have confused "chasing" a thread on a lathe with cutting a
thread on a lathe. Two different processes. The former is indeed a
high-skill and largely out of date (e.g., for old Germans, Swiss,
Norwegians, and Americans). The latter is a basic, not very difficult skill
to master in Lathework 101.
In cutting a lathe thread on a lathe you must have a screw-cutting
capability -- that means, a lead screw and appropriate gearing -- whether a
built-in "quick change" gear box or separate gears that you have to change
for each pitch. Alternatively, a lathe with some kind of CNC control for
cutting threads. The key is that the tool is a single-point cutting tool.
"Chasing" a thread is an altogether different (and largely obsolete)
skill. In chasing, you use a multi-point tool. Sort of looks like a lathe
tool with saw teeth on the end. The spacing of the teeth determines the
pitch of the screw. You place the chasing tool against the work while the
work is turning, at the same time moving the tool toward the headstock. If
you do it just right, you'll find yourself cutting a screw thread. That's a
shallow, starting thread. Then you have to push the tool in successively
farther until you get the thread depth you want. Since it takes several
passes, you have to "chase" the thread with the tool to line it up for the
next cut. With luck, you should be able to get a decent thread after a few
hundred tries. Now for an inside thread (eg., a nut) you do the same, only
this time you can't really see the thread you're trying to cut ..a few years
and a few thousand attempts should bring it home. Any one who is serious
about machining should try (and succeed in) chasing an external thread at
least once in his life -- makes you appreciate tools like taps and dies.
As much as possible I do all my threading and tapping on lathe --
using taps and dies. For tapping you use a spring-mounted point tool to
hold the tap against the work with the right pressure. Assures a perfect
allignment almost always. For threading, I made up similar, sprint-loaded,
Over the years, I've accumulated a very big assortment of weird
taps and dies but every once in a while, especially when helping a friend
restore an antique instrument, you come across threads that follow no
recognizable standard. Then you have to do it on the lathe. For threading,
it isn't much of a hassle. But if you have to tap a weird thread in a small
hole and can't just fake it by using a heli-coil, then it is worth making
your own tap. Not that big a deal.
Although I have a bunch of very big taps and dies (up to 1.5 x 8) I
usually start by cutting big threads on the lathe, and then finish them off
and clean up using the tap or die as appropriate. It's a lot easier doing
it that way than getting a die lined up correctly .. Also a lot less brute
I doubt if there's more than a dozen denizens of this news group who
can consistently chase decent threads -- I'm not one of them. As for
threading on a lathe without using taps and dies, I'd guess that its a basic
skill mastered by more than 95% of this NG.
Boris Beizer Ph.D. Seminars and Consulting
1232 Glenbrook Road on Software Testing and
Huntingdon Valley, PA 19006 Quality Assurance
Email bsquare "at" sprintmail.com
I am not slamming those that don't know how, for many that have access
only to lathes that do not have threading capability, it's totally
understandable. I am slamming the attitude that it is something that
is too difficult to be learned. Threading, like any other machining
operation, is a series of simple steps, that can only be taken one at
a time. It's a process, not a magical incantation, there's no
wizardry involved, and anyone that can do basic turning and boring can
learn it easily.
Whoever posts under "larson" or what ever name this hacker uses is not
a machinist, not even a good apprentice, but a butcher of metal.
During all of my years in machining, over forty, every competent man
that I met or worked with would learn all he could, if for no other
reason than it was something else he knew and nobody could ever take
that away from him. To discourage someone from trying anything that
is well within the possible, and not even in the difficult, is
Why wouldn't you just put the tap in the tailstock chuck, turn the
lathe on slow and feed the tap in by pushing the tailstock forward
without clamping it? Been doing that for over 40 years and I guess
that's why I never understood why anyone would buy tap guide. Leigh at
"Boris Beizer" email@example.com
wrt chasing a thread
It may have been easier in wrought iron.
I can't see even trying it on modern steel.
The method does survive for turning wood threads.
I have a few of the old hand tools, but have not had the urge to press them
against substance yet.
You have a point there. I urged people to try this technique just for the
My first attempt was on a 3/8 aluminum rod. After butchering a foot or two
of this material, I switched to 1/4" brass. Never did try steel.
Obviously, you do this at the slowest speed, using the back gear. I once
saw a 90 year old something watchmaker who could zip out watch screws about
as fast as it would take using a die .. note, mostly left-handed threads.
This makes we wonder if the prevalence of left-hand threads in Germany and
Switzerland hs to do with the fact that it much easier to chase a left-hand
thread on a screw than a right-hand thread.
Chasing survived in the US late into the last century, by hobbyists involved
in optics. The special, large-diameter, fine-pitch threads used in making
telescopes are all but impossible to make any other way on a home-shop
lathe. And you probably will wind up taking your outside-threading chaser
and bending the end over to make your internal threading tool. It can be
hard to get the pitch exactly right on two different tools, although the
real experts seem to do it Ok.
Several old guys who were members of an astromony club nearby were experts
at it. My uncle, who taught shop in the school in that town and who knew all
those old guys, picked it up and became quite good at it himself. He taught
me, and I was able to chase good threads after relatively few tries. A key
to it is to have a very smooth top surface on your follow rest (which looks
like the follow rest on a woodworking lathe, except that it clamps onto your
compound). I polished mine and stoned the burrs off with an Arkansas stone.
Since I never picked up much interest in making telescopes, I lost the skill
over the years. But it's really not as difficult as it sounds. While you're
learning, you do tend to make double-start threads at twice the pitch you
intended , but you get past that.
BTW, most of those optical-equipment threads were cut in brass, but I've
done it in steel with no problem.
Walter .....(My thoughts on driving a 1/2" tap using a Sherline with
only a 1/4" tailstock chuck)
You could lock the lathe spindle (or hold it), hold the part in
the chuck then keep the tap on line by pushing on it with a dead centre
in the tailstock and turn the tap with an open end wrench or visegrips.
Even then I don't know if the lathe has the strength to keep the tap in
place while you'er trying to turn the tap by hand.
Bill D (O: