Some at least of the answers to your questions are to be found in Martin Cleeve's "Screwcutting in the Lathe" (no 3 in the Workshop Practice Series).

You don't say what your job is but I think most amateurs aim to get the screw to fit the nut rather than to produce a theoretically correct thread form; by "fit" I mean on the flanks of the thread – as long as the crests and roots clear each other their precise dimension and form are less important.

It would also be helpful to know something about your lathe and tooling: single point threading techniques are different from those applicable to insert tooling.

If you are a beginner then some practice on scrap before embarking on the real thing is a good idea.

Visual comparison with an M6 machine screw is a useful tool too. The thread form is the same but yours is just 6mm greater in diameter so if the M6 thread is measured as 5.9mm diameter, cut your 12mm master 11.9mm. You can also put the M6 in your chuck, advance the threading tool up to it so it sits fully into the existing thread, note the slide position reading and then add 3mm to the reading as your finished depth position when cutting the bigger version. Another useful tip is use a jewellers magnifying loupe to look closely at the thread to check it looks like your M6 version. Coincidentally, I've been cutting 12mm x1mm threads this week.

But unfortunately incorrect.

The major diameter for an external metric thread is nominally the diameter of the thread minus a fundamental deviation, and also a machining tolerance, both dependent upon the class of fit. You do not take off the truncation of the thread form, that is already built in. Incidentally the height of the crest truncation is H/8, where H is the theoretical sharp thread depth.

For example take a 12×1.25 thread. The nominal OD is 12mm and the fundamental deviation for a 6g fit is -0.028 giving a maximum major diameter of 11.972mm. There is then a machining tolerance of a maximum of -0.212mm. A 12×1 thread is not standard, so you'll have to work out the numbers yourself.

That's the theory. In practice I start with the work at the nominal diameter or very slightly above, in this case 12mm. I then cut to the calculated thread depth and check final fit with a mating thread. I find that one needs to cut a thou or two deeper than the theory in most cases. For standard thread forms I use full form threading inserts. That way all the crest and root shapes are taken care of.

Andrew

Look again at the book and you will see as Andrew points out the "Major Diameter" is to the crest not the projected point that is "taken off". The 0.108P or H/8 is over and above the Major Diameter.

If using aground tool or partial profile you may need to run a file down the crests as you get close to remove burrs so that they do not contact the nut first and give a false sense of a good fit. Also depth of cut will depend on the type of cutter as a pointed one will need to go in further than one with the correct radius on the end.

Edited By JasonB on 25/01/2019 08:58:37

Gosh people working to 1/10000 of a mm. I feel like a real wood butcher

That;s the best way: see how it goes. Try a few test pieces first and get the feel of things before attacking a real job. And you can always use a die to finish off your thread if you feel so inclined. Usually gives a good finish with minimal time spent on final finishing.

A 12×1 metric thread is not standard in the sense that it isn't in the coarse or fine series. It appears in the constant pitch series, for which hardware, and taps and dies may be available, or not, as the case may be.

I normally use partial thread engagement for threading. However you only need the reduction on one half the combination. If it's on both then the thread engagement is a lot less. Since it is simple to produce a full depth external thread, but difficult for internal threads (especially when using taps) that's what I do. I cut external threads close to full depth and drill for internal threads to give the required engagement.

I don't work to micron accuracy, but I do like to understand what I'm trying to do. Understanding the theory to a reasonable degree of accuracy eliminates it as a source of error. Once you have numbers they can be adjusted to whatever tolerance is needed, or accepted. Misunderstanding the basics and making incorrect approximations just leads to sloppy work – possibly in more than one sense.

Andrew

What's meant by 'fairly important' is the elephant in the room. It decides whether Andrew's answer is more appropriate than Hopper's.

In my workshop, I'm usually with Hopper. I mostly make threads where the only requirement is they have to fit together. The threads don't have to maximise strength or to minimise backlash. I don't bother reducing the rod slightly below nominal size, I use an ordinary 60 degree cutter, and I don't worry about getting the thread form perfect. I cut down towards theoretical depth and then test with a nut, only going deeper if the nut won't go on or jams. I usually run a file over the crests to flatten them and remove any excess diameter that may have squeezed up during cutting. Rough work in terms of precision and accuracy but plenty good enough for 95% of what I do.

Occasionally I need a better fit. My preferred approach is to rough out about 60% of the thread with a lathe and then finish off with a die. The die puts a more accurate finish and profile on the thread. If I don't have a die of the right size, and the thread needs to be a good fit for strength or backlash, then I use carbide index thread tools and take much more care with dimensions and finish. This is much more like Andrew's approach. As I'm not skilful and have poor concentration, about 1 in 4 of my 'best effort' carbide cut threads are rejects due to small flaws.

Where thread strength and fit are both paramount – like the bolts that hold aircraft wings on – it's unlikely that a high-end fastener would be made by a cutting process. Rolled threads are considerably stronger that cut threads. Most commercial fasteners are rolled for cheapness, but the posh ones are made more carefully from better metal, finished by grinding, and inspected.

If 'fairly important' only means it has to fit together with moderate strength (60-70% of theortetical), Hopper is plenty good enough. But if the thread must be strong and/or a tight smooth fit best proceed as Andrew, or compromise with a die. Finally, if the part is safety critical – essential to holding a car's brakes together – then I'd recommend buying the right commercial part.

If it's important because the thread has to be cut on an existing part that can't be replaced, practice on something else until you get good results!

Dave

Edited By SillyOldDuffer on 25/01/2019 10:42:50

Last time I bought a m12 x 1 nut for a ballscrew it was left hand thread. Glad you didn't find that out after making one.

When you get to the stage where you can just about make out the lines on a 6" rule,precision takes on a whole new meaning !!

When it comes to screwcutting,I just turn the bar to the full size and thread away,have a nut to hand to check the fit.

Nothing fancy,but it works [most times ]