Thread Cutting - Internal and External on a Myford ML10 Lathe




Introduction: Thread Cutting - Internal and External on a Myford ML10 Lathe

This Instructable shows my first effort at thread cutting. It seemed to be reasonably successful and I didn't want to forget what I did, so having a long-term record should be useful for me. I don't expect to do too much thread cutting in the future???

The Instructable gives details about making a pair of internal/external metric threads in 1" (25mm) diameter aluminium tube/bar. It covers the calculations, the preparation of the cutting tools and doing the whole job. I have a Myford ML10 with a metric leadscrew, however only a couple of steps will be specific to that lathe.

Step 3 explains exactly how to prepare the bar and tube with virtually no mathematics involved. One problem, I am a complete novice, so there are probably errors and examples of poor practice - be warned! I hope that comments from other people, will highlight these so I can correct them.


  • Choosing a pitch
  • Mathematics - deciding on the thread size and exactly how to move the cutting tools, etc
  • Thread cutting calculations without mathematics!
  • Preparing the bar for external threading
  • Grinding an external HSS threading tool
  • Setting up the lathe leadscrew and backgear
  • Cutting the external thread
  • Grinding an internal HSS threading tool
  • Preparing the tube for internal threading
  • Cutting the internal thread
  • Final fitting

Hope this might help someone else?

Step 1: Thread Cutting - Choosing a Pitch

I had some aluminium tube and some similar-sized aluminium bar.

The thread size comes in two dimensions, thread pitch and thread diameter. I decided to cut a thread with a diameter of M23.6 and a pitch of 1mm. This is why I chose 1mm pitch.

On a Myford ML10 with a metric leadscrew (like many lathes with metric leadscrews), some pitches are painful, because the thread indicator is virtually useless. I shan't detail the explanation as to why this is, but the table above, shows which threads can make some use of the thread indicator (highlighted in yellow) and which can't. The column labelled "Numerator" is the key to this. If the numerator has a value of "1" then that pitch is super-easy because you can engage the leadscrew at any time without worrying about the thread indicator at all. (Conversely, if the numerator is "4" or more, the thread indicator is so hard to interpret, that you have to keep the leadscrew engaged permanently and return the cutting tool back to the start by reversing the whole lathe).

The details about using the numerator column are given in the final step.

So, I looked at the table and saw that a pitch of 1mm was one of the super-easy pitches. In addition, the mathematics is nearly all based on the pitch, so a value of 1mm was brilliant from that point of view (less maths to do! - see next step).

A pitch of 1mm is decided!

Step 2: Mathematics of Thread Cutting

My 25mm external diameter tube had a reasonably thin wall (1.6mm) so I had to pick the thread diameter to suit that. The 25mm bar would always need cutting down, so that did not affect the choice of thread size. The internal thread is cut into the metal starting at the chosen diameter and cutting outward to a bigger diameter. Choosing a thread size which is too big, would mean a paper-thin (or worse) wall thickness.

The following maths seems unavoidable to me, once you stray away from the 'standard' threads. I have not seen a 'Machinists Handbook', perhaps they have tables for virtually every diameter? No doubt there are websites, which would do the maths for you.

Skip the rest of this step if you are bored by the maths - the next step does just the same, without any theory!


The pitch (1.0mm for me) dictates the 'main' thread triangle height (0.866 x the pitch for a 60 degree thread). For me this dimension is 0.866mm and most of the rest of the maths is based on that height.

I did not cut a radius/flat at the bottom of either the external or internal threads. In theory, a specific HSS cutting tool would be made for each thread pitch to achieve the proper profile - both my internal and external cutting tools were as sharp as I could make them, so the 60 degree angle went right to the bottom of the threads. This does not affect the maths.

In one respect, the thread diameter is not as obvious as it seems! Take an M10 thread as an example:

  • You might expect an M10 bolt to be 10.0mm across the tips of the triangles which form the thread. This is not true. It is 10.0mm across the flats which exist at the tips of the threads. The thread profile is not a triangle. It is a flat-topped triangle where the triangle has been chopped off by 1/8 of its height. (The width of the flat is therefore 1/8 of the thread pitch).
  • You might expect an M10 nut to 10.0mm to the base of the triangle which forms the thread. This is not true. It is 10.0mm across the flats which are (or should be) at the bottom of the threads. They should have a flat base, 1/8 of the thread height up from the point of the triangle.

When planning the size of an internal thread, another size variation exists:

  • You might expect the triangle which forms an M10 nut to have a pointed peak, but is heavily flattened by 1/4 of the triangle height. So the actual height of the metal thread in a properly formed nut is 5/8 of the full triangle height (1/8 'lost' due to the flat at the bottom of the thread and 1/4 'lost' due to the flat-top of the thread).

What does all this mean for me? I settled on a thread size of M23.6 Doing the maths (see below) it meant that I had plenty of meat left in the tube walls.

Here is the maths:

1/8 of 0.866 = 0.108mm
5/8 of 0.866 = 0.541mm
7/8 of 0.866 = 0.758mm

Tube deepest cut (bottom of the triangle) = 23.6 + 0.108 + 0.108 = 23.82mm
Tube internal diameter (to the flat top) = 23.6 - 0.541 - 0.541 = 22.52mm

Bar biggest diameter (to the flat top) = 23.60mm
Bar smallest diameter (to the bottom of the triangle) = 23.6 - 0.758 - 0.758 = 22.08mm

The above are the sizes needed for preparing the bar and tube for threading. (The additions/subtractions are done twice because the diameter is made up of threads on BOTH sides of the tube/bar)

Finally. With all this geometry we know exactly how deep to cut our threads, which is useful for moving the tool in/out by the correct amount.

The top slide is angled by 29.5 degrees when cutting the threads, so that the tool only cuts on one of its edges (rather than plunging in). This enables the cut material to clear the tool more easily (and has less strain on the tool). Due to this angle being parallel to the triangle's sides, the distances to move the top slide are related to the length of the triangle side (1mm in my case) and not to the height of the triangle (0.866mm) which was used in the other calculations above.

When cutting the external thread in the bar, the top slide needs to move in by 7/8 of the 1mm pitch (ie 0.875mm). On the ML10, that is 44 divisions on the top slide dial.

When cutting the internal thread in the tube, the top slide needs to be moved out by 3/4 of the 1mm pitch (ie 0.75mm). On the ML 10, this is 38 divisions on the dial).

Step 3: Thread Cutting Calculations Without Mathematics!

Ignoring all the mathematics details. Choose a pitch and diameter, get a calculator and plug the two values into the formulae in the diagram above. To avoid stupid results, make sure you do the multiply bit first (eg 1.5155 x P) then do the addition/subtraction!

Step 4: Prepare the Bar for Threading

With the diameters known, turn the outer diameter. Leave a nice finish - it will form the 'flats' at the top of the thread.

I used a parting tool to form the lead-in and lead-out areas. I'm not sure what is the best practice for this. I know that some sort of chamfer is sometimes used - not sure about this.

Step 5: Grinding an External HSS Cutting Tool

I wanted to be able to have the tool cutting threads close to the chuck, so I set the 60 degree cutting triangle on a base which was sloping at 10 degrees.

To get an idea of what my threading tool should look like, I used a DTP program to mock-up the shape. The lines in the DTP file are rotated to 70 degrees and 130 degrees, then lined up to touch the rectangle.

After much mental torture, I worked out what angle the HSS tool steel had to be held at, on the grinder, to create the desired shape. Then I used the DTP software to produce lots of parallel lines at the two necessary angles (see the photographs). I stuck these onto the grinder tool rest and then worked hard to keep the tool lined up at all times. The tool rest is angled in a way to produce some relief, when the tool is held firmly on the (tiny) tool rest (5 to 10 degrees?). I must produce a bigger platform for the grinder!

The shape seemed ideal, with a skew which meant that I could set the tool post at the perfect angle to cut close to the chuck (see "Cutting the external thread" step).

Step 6: Setting Up the Lathe Leadscrew and Backgear


The ML10 manual shows the change gears needed on a metric leadscrew to get a pitch of 1mm.

60T and 40T on the first stud, 45T and any small spacer gear on the second stud, and 50T on the leadscrew.

The use of the small spacer gear, and the position of the leadscrew spacer can be seen in the photos.

I don't take the stud bolts off the banjo, just undo the screws on the studs and slide off the studs. I slacken off the stud bolts and fiddle about until everything is meshed properly. I make sure that there is a little slack in the meshing of all the gears (I guess this could be achieved by putting paper or foil between the gears when you tighten up the stud gears - I just make sure there is a little 'play'). Finally, I rotate the banjo to engage the 25T spindle gear with the 60T gear on stud 1 - then tighten up the banjo clamping gear.

Some grease can be used on the gear teeth to quieten them a bit.


To make the machine cut more slowly, the backgear system is used. To make this happen, the following steps are done:

  • The 'special' short allen key is used to undo the screw on the bull wheel
  • The teeth which engage with the bronze gear are retracted and the screw is tightened again.
  • The backgear idler shaft nut is loosened and brought into engagement with the bull wheel and bronze gear - then tightened again
  • The backgear idler shaft is lubricated
  • The pulley is lubricated via its oil nipple (note this is OIL not grease)
  • Finally, the whole arrangement is tested by hand rotation, to make sure everything is right, before switching on.


Switching on the lathe will produce a different sound from normal. Lots more clinking from the sets of gears which are now in the drive train.

When the half-nut lever on the apron is engaged, the apron/cross slide will travel slowly towards the chuck.

Step 7: Cutting the External Thread

The top slide is rotated over to 29.5 degrees (see photos) and the cutting tool is positioned parallel to the work so that it cuts the threads symmetrically (hence, the tool is angled at 10 degrees for my 'skewed' cutting tool).

I can adjust the zero of my cross slide, so I zeroed the top slide, brought the cross slide in until the tip of the tool was touching the outside of the workpiece - and zeroed the cross slide dial.

Then it is fairly straightforward.

  • Move the apron towards the tailstock a bit
  • Make sure the cross slide is zeroed
  • Move the top slide in to the desired cutting depth
  • Engage the half-nut lever (mine did not engage fully sometimes, so I made sure I had plenty of distance before the cutting started, just to give me a chance to re-engage it if necessary). I did not have to worry about the thread indicator for this chosen pitch of 1mm.
  • Watch the tool as it cuts (should only cut on the left-hand edge due to the angled top slide)
  • When the tool stops cutting at the end of the threaded section, dis-engage the half nut lever.
  • Pull back the cross slide to clear the work, then wind the apron back to the tailstock end of the job (don't touch the top slide handle)
  • Repeat until finished

Being a total novice, I was surprised at how severe the cutting got, as the thread got deeper. This is pretty obvious due to the increased width of the cutting surface! I did not really adjust the depth of cut much, but realise that I could have started with a deeper cut and then reduced it considerably as I got near the end of the job. Somewhere I read that you should leave the last normal pass a thou or so short, then do a final plunge cut with the cross slide, to leave a nice finish.

I used the calculated dimension to know when to stop the top slide. I'm not convinced I got everything right, because the threads seemed very sharp with no real evidence of a flat top (I did clean the thread up later). However, the final job is nice and close-fitting so I think the basics are right?

Step 8: Grinding an Internal Thread-cutting Tool

I didn't have an internal thread cutting tool, so I adapted a home-made tool-holding bar (which was made for undercutting).

This tool-holder has a 3/16" hole through it, so I used a broken 3/16" drill bit to make a 60 degree cutting tool.

It ground down rather quickly, so I guessed that it was probably a high carbon drill rather than HSS. I'm not quite sure about all this, so I thought I would harden and temper the tip.

I heated it up to cherry-red and dunked it in water - no problem. To temper it, I wanted to judge the colour change with the tool starting from a high polish (I gather this is what you have to do - heat it up until the surface colour changes). However, my hardened cutting tool was pretty black and I was not sure how to make it shiny! I did not bother in the end, I just used it in its hardened state - the aluminium would not tax the cutting tool too much.

Step 9: Preparing the Tube for Threading

When I bought the lathe, it came with a weirdly shaped boring bar. This did the job.

I first bored out the tube to the correct dimension using the boring bar, then used it to 'relieve' the starting 3mm and also produce an internal groove between 15mm and 20mm inside.

I used an adapted digital tyre gauge (magnetic attachment) which made the job easy - especially as the dimensions were not critical:

  • Before starting, zero the gauge by eye when the boring bar tip is just at the edge of the workpiece


  • Use the top slide to bring the bar-tip to the required cutting depth
  • Move the apron to cut the 3mm for the 'start-groove'
  • Move the top slide so that it clears the workpiece
  • Move the apron until the digital gauge reads 15mm
  • Move the top slide back to the cutting depth
  • Move the apron along to 20mm
  • Move the top slide to clear the workpiece and wind the apron towards the tailstock until it is out of the workpiece
  • ...repeat until the desired depth has been reached

Step 10: Cut the Internal Thread

First I adjusted the angle of the top slide to 29.5 degrees (see photo) and mounted the threading tool (inserted into its bar) at the correct angle to get a symmetrical thread shape.

Zeroed both the cross slide and top slide to touch the interior surface of the tube.


  • Make sure the apron was well clear of the workpiece - towards the tailstock
  • Zero the cross slide
  • Adjust the top slide to give the required cutting depth
  • Engage the half-nut lever - making sure it was properly engaged
  • Watch the depth gauge go from a negative value, through 0mm and start thread cutting after 3mm. The leading edge of the cutting tool should be making the cut.
  • Keep cutting until 15mm has been reached - disengage the half nuts
  • Wind the cross slide in to make sure the cutting tool clears the workpiece
  • Move the apron back towards the start

Keep going until the calculated depth of cut has been reached on the top slide - this should coincide with touching the 'bottom' in the first 3mm of the tube.

Again, the later cuts were much more severe, due to the increased width of cut as the threading tool cut deeper into the workpiece. I probably should have reduced the depth of cut at this later stage more than I did.

Step 11: Final Fitting

First fitting of the threaded bar into the tube did not work! I thought there were two possible faults:

  • Measuring across the external threads gave me a reading which was too large by 0.1mm or so. The threads also looked to be without a flat top. Not quite sure how this happened! I re-chucked the bar (not good) and took the threads down by about 0.1mm.
  • I had messed up engaging the half nuts on a couple of occasions which caused some grief at the start of the internal thread. To help the fitting, I parted off the 3mm relief cut from the start of the internal thread - and with it, the poor thread-start which the half-nut errors caused.

This did enough to enable me to screw the parts together. Not particularly smoothly, but well enough.

In general, I was not overjoyed with the quality of the finish. I didn't take a tiny 'polishing cut' at the end of the thread cutting, and some of my depths of cut were too big, I'm sure. This meant that the aluminium had some burring and a poorer finish than I would have liked.

To improve the finish, I 'lubricated' the thread with Brasso (metal polish - slight abrasive) and screwed/unscrewed the threads a few times.

Cleaning and oiling the threads then left a silky-smooth fit on the threads.

In general, I think the basics are sound, but (as ever) several learning points for my next attempt!


Final note about using the 'numerator column' - see Step 1:

Details of using the Numerator Column are given in the page extract image. This is from the "Myford ML10 and Speed 10 Lathes notes on Installation Operation Maintenance" (Booklet number 743L).

As previously noted. If the numerator is 1, then you can engage the half-nuts anywhere you like.

If the numerator is 2, then you set the thread indicator dial to zero during the first thread-cutting pass. At the end of the pass, you can disengage the half-nuts and wind the carriage back by hand. To do subsequent passes, you can engage the half-nuts at any EVEN line on the thread indicator dial.

If the numerator is 3, then ditto, except the line must be 0,3,6,9,12,or 15.

For other numerators, see the page extract - and the best of luck! This seems so tricky to me that I would think it would be better to keep the half-nuts engaged at all times and run the lathe backwards to return the carriage to the start. For short threads, the whole operation could be done by disengaging the drive completely and using a Spindle Handle (see another of my Instructables for a guide to making one) to cut the thread and return the carriage.

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    7 years ago

    Well done. I've yet to try thread cutting myself, but your explanations are very clear and I particularly like your comments about what was actually happening at each moment.


    Reply 7 years ago on Introduction

    Many thanks for your comment. Have a go! If you buy a really cheap, plastic cooking spoon from a discount store, you end up with 30cm of polythene rod which is over 1cm in diameter. Cut a short length, chuck it up and get threading. This is brilliant for playing with external thread cutting - you can't damage anything because the material is soft, but it holds a thread nicely.

    Best of luck.


    7 years ago on Introduction

    hi guys I am doing my level 1 apprentice as a toolmaker so I want to know how can I make M12 can you guys help me with all the calculations and preparation. this is a turning between centres workpiece.


    Reply 7 years ago on Introduction

    Hi there. Use the info in Step 3 to work out the dimensions you need.

    D = 12mm and P=1.75mm


    8 years ago on Step 5

    You can't rotate the tool holder on your lathe?


    Reply 8 years ago on Introduction

    Hi there. Thanks for your interest.

    I'm not quite sure what your question refers to. The top slide rotates to any angle I want (with respect to the cross slide). So, as you can probably see in the photos, I can rotate it to 29.5 degrees in both directions (there is a graduated scale of degrees engraved into the lathe, which shows me what angle I have set).

    Also, I can rotate the tool holder to any angle (with respect to the top slide), just by slackening off the large clamping nut in the centre of the four-way toolpost.

    To make everything work, I have to do both rotations. First I set the 29.5 degree angle (so that when I advance the top slide, it only cuts on one side of the thread); then I adjust the angle of the four-way toolpost to bring the cutting tool back to be 90 degrees to the work.

    Does this answer your question?

    Best wishes


    Reply 8 years ago on Introduction

    After posting that and reading the rest of the instructable, I can see that you can rotate the toolholder. I was thinking the offset angle on the tool was to compensate for the 29.5 deg on the compound (didn't do the math to see that 10 != 29.5..)
    I guess my question is, why the angle offset on the tool? Why not just a straight on 60 deg (30 each way) cutter?


    Reply 8 years ago on Introduction

    Ah - I understand the question. The reason for offsetting the 60 degree tool by an extra 10 degrees as I ground it, was so that I could keep the workpiece very close in to the chuck without any danger of the top slide crashing into the rotating chuck. To explain: on my small lathe, I try to work very close to the chuck for maximum rigidity, this means that I might be working the cutting tool as close a 2mm or 3mm from the chuck. When I clamp the cutting tool into my 4-way toolpost, there is a little bit of overhang, where the toolpost is slightly bigger than the tool and overhangs the tool in the direction towards the chuck. If the 4-way toolpost and tool are exactly at 90 degrees to the axis, and I try to drive the tool very close to the chuck, then the (oversized) toolpost can foul the chuck. If however, I grind the tool to have a 10 degree offset (as described), then the toolpost has to be rotated by 10 degrees as well (so that the tool is now at 90 degrees to the axis). This means that the head of the cutting tool is now the first thing which would hit the chuck - ie I can work the cutting tool right up to the chuck and not worry that the tool post (or the top slide) might foul the chuck.

    My lathe was once owned by an educational establishment and the top slide shows much evidence of chuck-crashing!!!

    I hope all of the above makes sense - sorry to be so wordy. If you need more, let me know - I will try to get a photo to show the benefit of the 10 degree offset.


    Reply 8 years ago on Introduction

    Yes, that makes sense. Didn't realize quite how close you were to the chuck and that your toolpost was a bit big.


    8 years ago on Introduction

    If I were to add anything to your instructable I would say that you should consider a Positive Rake Cutting tool. This will help "cut" the threads instead of smashing/ forming them (like in a zero rake tool), especially in aluminum. When making your HSS threading tool you will also want to "hone" a slight radius on the cutting edge with a fine grinding stone. This will allow you to achieve a sharper cutting tool, thus a better finish.
    Also, don't forget to use some sort of cutting oil (way oil, synthetic, etc) during the cutting process.

    **The reason a 10mm bolt must have a smaller major diameter is because of Thread Class Engagement. This means that the best strength-to-fit ratio is around 75%. If the threads fit better than this -say 90%- you would not be able to spin the two together and, if the threads were around 20% they could easily pull out.

    Now when cutting threads again, always check them with a hardware store nut/ bolt.

    And YES, it is possible to re-chuck a piece of work to "chase" the threads after it has been taken out of the chuck.

    ALWAYS trust your math!!


    Reply 8 years ago on Introduction

    Many thanks for the comments. The finish I achieved was not brilliant. I have not really got the 'hang' of honing my HSS tools; and in addition, I don't have a particularly fine grindwheel on my grinder. Hence I think your thoughts are probably spot on!

    When I try honing, I am never convinced that I am actually IMPROVING the finish. There are virtually no YouTube videos on honing lathe tools (a good number on honing knife blades, but they are quite different). I don't really know how to judge whether the edge is sharper or when to stop honing. I don't know what a typical number of honing strokes people use (1, 5, 10, 20???). I have a proper 'fine' oil stone and three (fairly cheap)diamond impregnated stones (medium, fine and very fine) but I'm not really sure which would be best to use. Finally, I'm not very sure about the best way to hold the cutting tool, or the best order for honing the edges. Apart from that, I think I understand it fully!

    Best wishes and thanks for the advice on lubrication, etc


    Reply 8 years ago on Introduction

    I hope you know that we are just being picky!! As long as your threads fit they are fine. As you "hone" your threading skills you will get more comfortable and familiar with how your lathe reacts to the tools that you present and eventually you can worry about the smaller things like thread measuring and centricity.


    8 years ago on Step 10

    Have you shaved off some of the topslide casting? If so, by how much. I was thinking of doing this in order to use my existing quick change tooling. Matt


    Reply 8 years ago on Introduction

    No, I put my 4-way toolpost in the 4-jaw chuck and took a carefully calculated amount off the bottom of it. This bought the top face of the HSS tools to their correct place (just below the centre line of the lathe). I took the bottom of the toolpost down to about 3.5mm thick. This is as thin as you would want but I think it was calculated to suit 9mm (or10mm??) tooling to fit. I currently use 5/16" tools which need packing underneath them, which is ideal.

    The 3.5mm thickness is not as bad as it seems, remember that the TOP of the toolpost is very thick (10mm+) and its screws press the HSS tools, the packing beneath them and the bottom of the toolpost VERY firmly onto the top slide. There is not a hint of instability or movement in the setup.

    Hope this helps.


    Reply 8 years ago on Introduction

    I don't have a brilliant photo of the toolpost, but you can see how it fits in the top bit of this shot.


    One little trick if you don't like running the tool towards the chuck and would rather run away from it. Flip the tool over and run the lathe in reverse. This will allow you to cut right hand threads from left to right. Be sure to adjust the tool so it is ABOVE center, not below, as everything is upside down now.

    You don't have to feed in with the compound slide (top slide as you put it). You can simplify things and feed in with the cross slide. The only difference is you will be cutting on both sides of the thread instead of one side. If your tooling is sharp, this won't be a problem. Saves a lot of time for production work.

    Try and learn to know what good threads will look like. You mentioned your threads were pointier than you liked. You don't have to feed in the calculated amount, as long as the threads are cut correctly. Many variables go into how deep you need to go. The calculated depth is more of a reference for about how far you need to go. You might consider buying a magnifying loop to inspect the threads while cutting. It's amazing how much more you can see when things are magnified.


    Reply 8 years ago on Introduction

    Many thanks for your comments. It would be good to cut away from the chuck! Some machines like mine have screwed-on chucks, so cutting anything with the machine in reverse is a big no-no for me (the chuck can/will unwind and fall off!!). Looks like a real possibility for other machines??

    I'm sure you are right about the depth of cut. In many cases, people have a 'target' nut or bolt which they test when they are near the end. Then they know when to stop.

    Thanks for your interest.


    8 years ago on Introduction

    Nice instructable! I would like to give you one pointer regarding the tool chatter as you get deeper. This is because the screw cutting tool is cutting on both faces. For the best results keep your compound slide parallel to the the axis of the thread. As you get deeper in the cut, wind the compound slide forwards a bit. If you follow me, the thread is then opened up, and the thread cutting tool is only cutting on one face. You get a nice shiny thread (especially with cutting oil). As for the little dial, I never use that, I just leave the screw engaged and reverse back to my start point. Another somewhat scarier tip is to do the thread with a faster spindle speed, usually your reactions are the limit, when you see a CNC doing a small screw thread it is actually quite horrifying! :)


    Reply 8 years ago on Introduction

    Thanks for that. I think the offset of 29.5 degrees forces you to cut on one side of the thread only???

    I found that (as a novice) I had a very high 'workload' during the whole screw cutting process (operating levers, turning handles, and thinking!!). The prospect of having a faster spindle speed frightens me!!


    Reply 8 years ago on Introduction

    I know the feeling. I started out the same.
    It becomes like riding a bike. it all just falls into place, the more you do it.
    You are correct the 29.5 degree angle, cuts on one edge.
    Can I ask what speed you were using to cut the thread. I thread to 125 on my M300. Any faster I would have a nervous breakdown. It is hands and eye coordination and brain trying to keep everything in going correctly. The more you do, the easier it gets.
    Doing work on Rifles I thread imperial. My Lathe is metric. I need to keep the lead screw engaged whilst doing imperial. I did this for so many years. I cut my first Metric thread and thought this is so much easier disengaging and re engaging. It is all a learning curve. The more you stand at the lathe cutting metal the faster the learning curve is.