Absolute Beginners Guide to Lathe Threading

Created by Randy Wilson ([email protected]) Public Domain – You may freely distribute, post, or publish.

Absolute Beginners Series Threading on the lathe Gunsmith Shop Notes Section 5


THREADING ON THE LATHE I have some good news for you! Once you learn to thread on the lathe the rest of your lathe education is all downhill. My friends watch me thread on my lathe and ask question after question. They are always amazed at how the process looks like a fine choreographed dance. The fact is threading is super easy once you understand how to do it. Learning to thread from a book however isn’t the best or easiest way to pick it up so you’re going to have to stick with me and pay attention. Many years ago when I first started out with my metal lathe I had no mentor and was trying to learn to cut threads on the lathe. I read a few good books and did everything they told me but my threads looked like crap. One day a seasoned elderly gunsmith happened to drop by my shop while I was busy ruining some threads and he showed me how to really do it. The surprising thing is that I had read everything he SHOWED me but until he showed me I just didn’t understand. I hope you are smarter than me! Let’s start real simple, get a piece of round stock and mount it up in your lathe chuck and dial it in true. Use a piece of round stock at least 1 inch in diameter for this.

Assumption: You have a modern lathe with a quick change gear set. You know what the parts of your lathe are called. You know how to dial in your work piece true or have a good three jaw chuck. Now I want you to set up your lathe to cut 28 threads per inch.

ALL CHANGES ARE MADE WITH THE LATHE POWERED OFF


Your lathe should have a diagram similar to this –

Your lathe may look very different but the principle will be the same, you have two handles, one controls the ALPHA digits and one controls the NUMRIC digits. Look closely on this picture and you can see to get 28 threads per inch you will need to set the handles at B and 7. Were you able to follow me on this? Look closely at the chart for 28 with a RED arrow pointing to it. Then you will see two green dots marking the 7 and the B. You simply look at the thread pitch number MARKED WITH RED ARROW and work up to the number (7) and across to the Letter (B) then set your handles to correspond, simple huh?


Next we are going to engage the lead screw you should have a leaver and picture similar to this –

The handle should be engaged as the red arrow demonstrates. This will engage the Lead Screw for threading, very simply stuff here.


Now insure your thread dial is engaging the lead screw, here is a picture –

Usually you just push the bottom of the dial and it pivots inward. This is going to be very important!

We are now going to set the speed (RPM) of the lathe as low as possible usually 60 to 90 RPM. Every lathe is different but there will be two leavers or dials for setting the RPM based on a chart on the front of your machine (if you have back gear use it).

Shop Tip: When you get good at cutting threads you will have the RPM set at around 200 as it makes them PERTY.


Now it is time to set your compound rest to exactly 29.5 degrees (or as close as the human eye can get it).

Ok that was easy cheesy! Now we are going to mount our 60 degree threading tool to cut outside threads.


You may have fancy insert tools or just the plain old HSS tools either will work if they are sharp and ground true. Best to buy them you are not ready to grind your own tools! I prefer using carbide tipped tooling for thread cutting.

So this tool is going to cut like this –

Pretty simple concept but we have to make sure your threading tool is in proper alignment with the work piece using a fish tail gauge.

Super easy, just place it against your round stock (flat) to be threaded and make sure the point of your cutting tool fits perfectly in the V slot. This sets you at 90 degrees, PERFECT! Now make sure everything is tightened down.


This is where you are going to have to pay attention! We are going to setup to take the first cut. Run the cross slide in until your tool point just touches your work piece and then set the dial on the cross slide to ZERO. Now do the same for the compound slide. You should have the cross slide and compound slide dials both set at ZERO and the bit is barley touching the work now. Now move your saddle to the right off of your work piece and using your COMPOUND SLIDE turn it in .010 that’s TEN THOUSANDTHS our first cut depth.

So now your lathe is setup to cut 28 threads per inch, it is in its slowest RPM setting, your compound is set at 29.5 degrees, and your tool is 90 degrees parallel to your work using the fish gauge. You have also zeroed your dials to the work piece.


On your lathe you have a lever that engages the Half Nut. Here is an example –

You are going to be using this to engage the forward threading motion along the axis of the round stock. OK it makes the tool move forward and cut until you disengage it, it’s not complicated at all. The trick here is you will want to engage this half nut at the exact same place every time. This is where the threading dial indicator comes into play. Remember this? –

Follow my instruction here carefully, we are just observing. Make sure everything is clear of your chuck and work piece and power on the lathe DO NOT ENGAGE THE HALF NUT YET. Now the lathe should be turning slowly. Look at the thread dial, it should be turning around slowly (if not you got issues).


So notice that the numbers reach the top over and over again. There should be a mark at the top they align with during rotation. We are going to practice catching a number. Turn your lathe off now. Move your saddle way back away from your work so we have plenty of room to practice. Now power up your lathe and practice engaging your half nut leaver on each marked number as it aligns with the top indicator. Engage it then disengage it, try to hit the same number over and over. You will FEEL it when you engage correctly. Always engage just before the number hits the top and you will feel the sweet spot. So now you are a PRO at engaging your half nut huh? Turn off your lathe, there is more to learn.

When threading there is a lot going on at the same time so let’s break it down – PLEASE DON’T START THREADING YET 1. You will engage the half nut leaver using the threading dial indicator. 2. You will quickly disengage the half nut lever when the thread tool reaches the end of your thread. 3. You will turn the cross slide dial outward about half a turn to clear the thread. 4. You will manually reverse the saddle back to the starting place (anywhere in front of the work is fine but not too far away). 5. You will reset your cross slide to ZERO. 6. You will turn your compound slide in the desired amount and start the process all over again. Make sure no one is around to bug you while you do this, it takes concentration. Friends trust me when I tell you threads are not cut in ONE pass. It takes a few passes to form or cut a thread and there is a system to it. You will never cut more than .010 and usually only that much on the first cut.


There is lots of technical information on threads and thread cutting you can read, but I think you just want to get started so I am leaving all the science to others. Cutting or forming a thread is done in progressively lighter cuts. You will take the deepest cut first and slowly take less and less. How do you know when you have cut deep enough? Well there are fancy wire gauges and calculations but I usually just try to screw on the mating piece and if it works I am done. I know I am not very scientific but I have had true machinist tell me my threads look perfect.

It does help to figure out a target depth of a thread and since we are cutting 28 threads per inch how deep should we cut it?

http://www.americanmachinist.com/Calculators/ThreadCutting.aspx I just use a thread depth calculator! Use the one linked above or Google for another. So anyway our basic depth for 28TPI is going to be around .0225 so we are going to cut it as follows – 1. Take 2. Take 3. Take 4. Take 5. Take 6. Take

a a a a a 3

.010 cut .005 cut .004 cut .002 cut .001 cut clean up cuts not increasing depth at all

Step 6 will amaze you as more material will be cut even though you didn’t adjust it to, it’s called a cleanup cut, I like to take them between each real cut.


You are going to read lots of theories and get multiple opinions from other people about threading. Some people will tell you to cut as slow as possible, some as fast as you can control. Every book says use a lubricant but I know machinist who cut threads with no lubricant at all perfectly! You will develop your own style and what works best for you. I will tell you a faster RPM makes for nicer looking threads but it is more difficult to catch the number and stop the process in time, so for now go slow. Before we start a read cut let’s talk about the thread dial indicator a little more and how it’s used.

As a general rule when you are cutting EVEN NUMBER THREADS like 8, 16, 24, or 28TPI you can pick up your cut on any number. What I am saying is when you engage your half nut it each time it doesn’t matter whether you pick it up on 1 or 7 as long as you pick up a number. This is not the case for ODD NUMBER THREADS, you will have to pick up the same number for them each time. Guys I am throwing a lot of information at you here. It sounds difficult but it really isn’t. You need to read all of this until it makes sense to you. Only then should you attempt to make threads. You’re going to screw up but it is ok that’s why you are practicing. Try not to get frustrated, walk away when you need to. We are NOT going to use coolant or oil for this lesson so you can clearly see all that is going on. Lubricant often can obscure your view and create smoke. Understand however I do use a thick pipe thread cutting oil with sulfur when I am cutting threads, I buy it from Lowes.


You might want to watch a few Youtube.com videos on thread cutting to see how it works now that you have the foundation. The more you learn the easier you will pick it all up. So let’s make a cut! Crank up your lathe and engage your half nut on a number. The tool will move across the work piece making a small line like this – (The lathe was already setup to take a .010 cut)

Immediately disengage to half nut when you reach the end of the thread cut. Back out your cross slide ½ a turn. Move your carriage back to the start. Return the cross slide back to ZERO. Increase the COMPOUND by .005 Engage you half nut again on a number taking another cut. With each progression you will see a thread forming as pictured

Keep repeating this sequence like we discussed above 1. Take a .010 cut 2. Take a .005 cut 3. Take a .004 cut 4. Take a .002 cut 5. Take a .001 cut 6. Take 3 clean up cuts not increasing depth at all


So after 8 passes you should have a very nice 28TPI thread. Congratulations! You just cut your first thread on a lathe (hopefully).

I am going to share a couple tricks to make your threads look a little better. When you finish cutting your thread you should take a stiff wire brush a burnish the threads while spinning the lathe on a high RPM like 500. Basically you are pushing the wire brush into the threads to remove the particles and burrs.

Next take a fine metal file and GENTLY touch the threads while the lathe is rotating to de-burr the tip of the thread.


I also like to finish up by using the file to chamfer the start edge of the thread, it looks professional and it helps the thread start easier.

This document is a DRAFT, I am going to actually take some great pictures and add to it as I have time but so many people are requesting this tutorial I wanted to get something out as quick as possible. Keep an eye out I will revise this soon. I would like to thank Greg Branton who taught me to thread on the lathe. Greg is one hell of a machinist and custom shotgun builder. If you ever have the chance to meet Greg you will be better for it.

Absolute Beginners Guide to Lathe Threading

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