How to cut (external) threads in a lathe. Step-by-step.

Cutting threads is an indispensable part of machining.  The 60º external thread is the most common thread to be cut, and once you can cut it, no other thread (internal, external, whitworth, acme, square, etc.) is beyond reach.

Tools required:

1.  A bench grinder with course and fine stones
2.  A sharpening stone
3.  A 60º threading gage (‘fishtail gage’)
4.  A thread pitch gage.
5.  Lathe with threading gears or a quick-change gear box

Materials required:

1.Something to thread.

I am a self-taught machinist and many other self-taught machinists I have known have studiously avoided thread cutting.  Perhaps because it looked difficult to them (it isn’t) or perhaps due to a bad experience they may have had trying to cut a thread.  Let me tell you about my first attempt:  I put a piece of probably 3/8” diameter bar in the chuck – extending probably 8” and without any tailstock support, of course.  I hadn’t turned the outside, so it was only as concentric as the cheap chuck and machine I had at the time.  I didn’t know about setting the compound angle.  (If this doesn’t make sense to you, hopefully it will by the time you’ve read this article).  Using direct feed from my cross-slide I attempted the cut a thread.  In short, the only thing I got right was setting the thread pitch on the lathe!  Needless to say it didn’t work out well.  Fortune allowed me about three passes (and they were lookin’ ugly) before the forces caused the piece to roll over the top of the cutter and mangled the workpiece beyond recognition.  By that time I’d managed to stop the lathe.  I now had a gouged up ‘S’ shaped piece of metal with some horrifically rough thread-like cuts on about half its diameter.  Needless to say, it was time to hit the books!  After digesting the proper technique my very next thread was a success.  There are a few points I wanted to make by telling you this: 1. Threading is hard if you don’t know what to do. 2. Threading is easy once you do know the procedure. 3. Even if you messed up horribly before, you can learn.  If you’ve been afraid of threading, or had a bad attempt, I urge you to try now – I think I can make your first, or next, thread a success and from there it takes very little practice before you’ll be  cutting perfect threads.

Threading involves the use of a form tool.  A form tool is one that relies on the shape of the tool to make the shape of the cut.  Form tools generally require a greater amount of force and therefore greater rigidity than other tools.

Because threading tools are form tools, grinding them is, in some ways, a bit more demanding than grinding a normal lathe tool.  The thread form must be perfect or the thread will not be as strong and accurate.  On the other hand, they’re easier because no back or side rake (often the trickiest part to grind) is required.

I strongly recommend the use of a High Speed Steel (HSS) tool for beginners.  HSS tools often produce a superior finish and are appropriate to the slower speeds used by novice machinists.  It’s also nice to be able to re-sharpen a cutter rather than replacing a potentially-expensive insert.

There are an abundance of ready-made threading tools, both carbide and HSS available on the market, but for our purposes here we’ll grind our own.  The directions assume that the tool bit will be held horizontally, such as in a quick change tool post and not at an angle as in the older rocker / lantern tool posts.  If you use a rocker, you must adjust the angles accordingly.  We want the top of the tool flat and horizontal – use that as the basis for the other angles.

The goal is to grind a 60º point with enough side clearance to avoid fouling the thread.  To this end, tilt the tool rest on the grinder to ~5º and grind at 30º per side to make your tool.  Check the angle frequently with the fishtail gage to make sure you get a precise 60º – remember, the quality of the tool will partially determine the quality of the thread.

When grinding an external thread cutting tool I like to start by grinding a ~30º angle on one side (the right side, or side that will be toward the tailstock in use) of the tool bit for most of it’s width.   You can establish a 30º angle by putting layout fluid (or sharpie markings) on the top of the tool bit and using a scribe to scribe a line.  You can also scribe a line for the other face, but it’s just a guide – as you get close to the line you’ll want to adjust you angle to get a precise 60º.

Laying out an angle as a guide for initial grinding

By grinding the point off-center with most of the angle on the right, tailstock, or outboard side we can more readily thread to a shoulder.  If the bit were ground with the 60º centered we couldn’t thread right up to a shoulder.  The finer the thread pitch, the more off-center our tip needs to be.

Here’s a commercial bit, ground for extra clearance so it could be used threading close to a shoulder.

Commercial threading tool ground for extra clearance necessary to thread close to a shoulder.

Now grind the opposite angle, adjusting to hit 60º – checking frequently with the fishtail gage.

When you have 60º, stop!  To check the angle place the piece in the gage and hold it so there’s a white background behind it (such as a 3×5 card) in strong light.  Make sure you can see no light between the edges of the gage and the tool.

Checking the 60º threading tool with a fishtail gage.

The tool rest angle of  5º should not only have given us clearance on the sides, but some front clearance as well.  The square helps show the clearance produced by grinding the sides at an angle.

The front clearance produced simply by angling the tool rest when grinding the sides.

Stone the top of the cutter with a sharpening stone.  (I like a diamond hone for this, but an Arkansas stone is perfectly suitable.  Wet stones work, but you’ll be wearing away an expensive stone faster than normal). That’s all for grinding the cutter.  The rest of the work is at the lathe.

Lathe setup

Start the lathe setup by setting the compound at 29.5º.  We’ll use the compound to advance the cutter, so setting it at an angle slightly less than the angle of the thread causes most of the cut to be taken on one side of the cutter, reducing the force required if we were to just jam the cutter into the work.

Why the cutter is set at slightly less than half the desired angle

Most lathes have a graduation on the compound, but for some reason they usually reference the X axis, so 29.5º may say 59.5º.  What we want is the angle such that if zero is the compound pointing straight away from you, we want to move the front of the compound 29.5º toward the headstock.

The correct angle of the compound for cutting external threads.

Set up the workpiece in the lathe and center it.  If necessary, turn the diameter to the major diameter of the thread you are going to cut.  You can find the major diameter in any reference, such as Machinery’s Handbook.  Many threads are specified by their major diameter.  A thread such as 3/8”-16 is telling you that the major diameter is 3/8” and the thread pitch is 16 TPI (Threads Per Inch).  If the length to be threaded is more than two or three times the diameter, you should support the end with the tailstock.  If it’s extremely long, you may have to use a follow rest to reduce flexing of the part and ensure an accurate thread.  If this is a practice piece make it easy on yourself and pick a robust size like 7/8”, have it sticking 2” out of the chuck and we’ll thread it 7/8”-14.

If the piece allows for it, cut a thread relief.  A thread relief is a section that is cut as deep as the thread, but wider than a pitch or two.  This provides a place to stop the advance of the cutter.  Some designs allow for more thread relief than others – cut as large a thread relief as is practical for the piece you’re turning.  Beginners should cut 0.1” or more if possible.

Here we have turned a 7/8” diameter for a 7/8”-14 thread.  A generous thread relief has been cut behind the thread. Machinery’s Handbook lists the full depth of a 14TPI thread as about 0.06186” but you never really cut to full depth.  The same table shows depth of UN and UNR threads.  (UN and UNR mean are thread form designations, you can read all about them in Machinery’s Handbook, but the distinctions need not concern you for the vast majority of work you will do).  The UNR thread goes a little deeper and its depth is listed as 0.04253”  (That’s 0.043” for those of us in the real world).  So we cut our relief to 0.043” deep – that means reducing the diameter by 0.086”, twice the thread depth, in the relief area.

In the picture note that we are not threading to the shoulder – if we were the thread relief would be cut right at the shoulder.  This thread will stop in the thread relief.

A tenon, ready to thread with the thread relief cut.

Set the threading tool in the toolpost.  Set the height to right on center, or a tiny bit below.  Now, using the ‘fishtail gage’ to square the cutter to the work.  Note that the compound remains at the previously set angle, and only the toolholder is moved to set the cutter straight on.

Squaring the threading tool to the work to be threaded.

Advancing the cutter is done using the compound.  Since the advance will be on the compound, dial the compound back to make sure you have enough travel to get to the bottom of the thread.  (If you run out before the thread is done, all is not lost – see ‘picking up a thread’ further down). Run the cross-slide forward until the tool just touches the piece to be threaded.  Now set the cross-slide dial to zero.  This is ‘home’ – before each pass we’ll return the cross-slide to zero.   Zero the compound dial.

The next few steps involve controls that vary a bit by manufacturer.  You will have to consult your manual, info plates on the lathe, a friend, or figure out the specifics for your machine.

Engage the thread dial.  On some lathes the threading dial is always active, on others it is engaged either by rocking the dial into place or by advancing a screw.  Get your threading dial to engage the lead screw.  The picture shows the threading dial being advanced into the leadscrew by advancing a screw.  Verify that your leadscrew is completley engaged – if the threading dial doesn’t rotate consistently, your threads will not work!

Advancing the cutter along the X (long) axis is done by engaging the half-nut.  This nut engages the lead screw and ensures a precise ratio of turns of the spindle to distance advanced by the lathe apron.  Depending on your lathe it may be separate or the same as the engagement you make to cause automatic feed.  Locate your half-nut lever.

The half-nut lever (horizontal) on this lathe - yours may be different.

The threading dial may have singular marks or numbers, depending upon the lead screw in your lathe.  Using the manual, charts on your lathe, or trial and error you must determine where to start each pass, as indicated by the threading dial.  If you engage the half-nut at the wrong point you will split the thread – hacking your hard work to pieces.   Note the number or mark you need to start on for the thread you’re cutting.  In this case, I need to start on ‘5’.  There are multiple ‘5’s on my dial, all of which work, and probably multiple valid starting marks on yours as well – but your lathe may be as simple as odd and even marks.  The picture shows the engagement table that is particular to my lathe.

This chart explains which numbers to use for corresponding thread pitches.

Set the pitch by either changing the gears in your quadrant (the gears at the back of the lathe headstock) or using a quick change gear box.  Most lathes have a table showing the proper settings either inside the quadrant cover (for those requiring change gears) or on the headstock, for those with a quick change gearbox.  Most modern lathes have a quick-change gear box, so setting the pitch is  done by levers or knobs.  Set your lathe to the pitch you intend to thread.  Also, especially for beginners, set your lathe to the slowest speed you can – this may be limited by your thread pitch.

According to the chart on my lathe, I need to be at Red – B – 7 to thread 14TPI.  (There’s a 14TPI in the blue range as well, but that’s a faster speed…)  The chart has info for diametral pitch, metric threads, etc.  We only need to concern ourselves with the inch thread section for now.

Chart showing thread settings for the example lathe.

Now, just for practice, back up the cross-slide 0.200”.  We’ll ‘cut air’ a few times to get the motions down before cutting metal. Place your right hand on the half nut lever and the other on the cross-slide dial.  At the end of each pass, you’ll run the cross-slide dial back and simultaneously disengage the half nut.  Make note of where the cross-slide is (0.200”), and return it to that location after each pass.  Start the lathe and begin watching the threading dial.  When the dial is close to the mark you want to start on begin applying pressure to the half-nut lever.  (Don’t do this until the threading dial is past the mark in front of the one you want, or you’ll end up engaging the wrong spot which might wreck your thread if you were cutting).  When you engage the half-nut the apron should advance, but the threading dial should stop turning – it will not rotate again until the half-nut is released.  Allow the apron to advance, then disengage the half-nut and simultaneously back out the cutter.  Those are the motions needed to cut a thread.  When performing the disengage / back-out motion, stay relaxed – there’s more time than you think and panicked jerks will only serve to heighten any anxiety you may have about threading. Run the apron back toward the tailstock and continue  “cutting air” until you are comfortable with the motions and able to consistently engage your half-nut at the right mark.  When things are looking good, you’re ready to cut metal.

Now with the cross-slide returned to zero ( just touching the work) and the X axis away from the work by a little bit, start the lathe and begin watching the threading dial.  Don’t be in a hurry, if you pass the magic mark you can always wait until it comes around again.  As the mark arrives, engage the half-nut.  Watch the cutter advance until it reaches the end of the thread (your thread relief, if you have one) and simultaneously retract the cutter while disengaging the half nut.  Whew!  You’ve done it!  The rest is just more of the same.

Dial the apron (X axis) back to the beginning of the cut.  Advance the cross-slide back to zero.  Apply a light advance on the compound (0.005” is good in most cases).  Now check the pitch with a thread pitch gage and verify that it’s correct.  Assuming it is, we’ll just repeat this process until the thread is done.

Verifying that the 'scratch cut' produced the right pitch and that the lathe is correctly set up to cut the desired thread.

Engage the half-nut at the appropriate mark, watch the cutter until it’s close to the end, then retract while disengaging the half-nut.  Return to the beginning of the thread, return the cross-slide to zero, advance the compound and you’re ready for the next pass.  Note the flat crests at the top of the partially completed thread.

Partly threaded tenon. Notice the flat tops of the threads - this is a good indication that we're not done.

The top, or outside, of the thread (the ‘crest’) is the thing to check between passes.  As long as it has a visible flat, the thread is not done (unless the mating piece is loose or excessively worn, in which case some flat may be present in a finished thread).  As the flat gets narrower, be watching and start checking the thread with the piece it’s going to mate to.  Your first few times you’ll probably be quite a bit premature in checking, but you’ll get an eye for it soon enough.  If you don’t have a mating piece you can also check with your pitch gage, a special thread measuring tool, or the 3-wire method (look it up).  Make sure the thread is clean when checking – a few chips could make it seem tighter than it is. Take a spring pass every other cut as you get close – a spring pass is a pass without advancing the compound in order to compensate for any spring in the workpiece.  When the piece can thread on for a few full turns, stop.  The thread is ‘done’.

Now there may be a little roughness in the thread – if it’s a lot your advance was too heavy, if it’s a tiny bit, you can polish it away with 3M Scotchbrite pads, Cratex rubberized abrasive or steel wool.

Now, right were the thread starts there’s a partial thread – it doesn’t hurt the function, but sometimes, especially with a fine-pitch it gets to be a wispy piece of steel that is just about ready to come off on its own.  Dressing the end is optional and you may not want to do it, but there are several ways.  One is to cut a chamfer at the end, another is to cut a ‘nose’ (shown here) to help get the thread started.  You may also lightly file the end in the lathe, or carefully file the first thread so it goes from its minor diameter and tapers up to major diameter.  These are all just nice touches that may, or may not, be applicable to your work. Remember to take your lathe out of threading ‘mode’ before resuming normal turning.

Congratulations.  Hopefully you now have a successful thread.  If not, analyze what went wrong and try again.  Once you’ve cut a few threads successfully you’ll wonder why it’s such a big deal to some.

Picking Up A Thread

If you move the piece in the chuck, or you’re threading between centers and remove the piece, or you need to take the piece to the mating work to check it out, your previous settings are lost.  Don’t despair, getting them back is easy, and is called “picking up a thread”.  To pick up a thread simply install the piece back in the lathe.  The threading dial should be engaged and the lathe set to the appropriate TPI. If you just took the piece out, none of that should have changed.  Now, with the cutter back so it won’t touch the work, start the lathe and engage the half-nut at the appropriate mark.  As the cutter advances, stop the lathe without disengaging the half-nut, somewhere along the thread.  Now advance the cross-slide close to the thread and advance the compound and cross-slide until the bit steers neatly into the existing thread.  At this point zero the cross-slide and compound and you’re back where you started, albeit with a different setting on the compound.  Back out the cross-slide, return to the beginning of the thread and you’re ready to resume!  Don’t let this process scare you – it’s super easy.

Metric threads

On most lathes (even imports) cutting metric threads calls for a special technique.  Do everything as you normally would, but never disengage the half-nut.  That’s it.  When you make your first (scratch) pass, rather than disengaging the half nut you simply back the cutter out as you stop the advance.  Then run the lathe in reverse to return the cutter to the start of the thread.  Run the cross-slide back in to zero and start cutting again.  (No need to watch the threading dial – with the half-nut constantly engaged it should never move.  Your carriage will advance as soon as the lathe is started.  If you do absentmindedly dis-engage the half-nut, just pick up the thread and resume.

Sharpening the threading tool

Sharpening your threading tool is simplicity itself – just hone the top until it’s smooth and shiny, check to make sure your angle is still perfect and you’re good to go.  If you have an adjustable height tool post you’ll be able to thread in your sleep long before your threading tool wears out.  Some folks grind the top to sharpen – this is just wasteful in my opinion and results in premature need to make a new tool.

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3 Responses to How to cut (external) threads in a lathe. Step-by-step.

  1. Is it okay to post part of this on my page if I post a reference to this webpage?

  2. Rakesh kumar says:

    We have an external thread in one compoent having a specification of M85X2.5 6g. Kindle let me knoe how to check the thread runout of the same

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