GarethBell said:

Would if I had kids :P Loooong way off that. Thanks though.

I had to have a few goes at the threading, because you can't "test the nut", which brings me to a point:

Is there a chart to say how much infeed you have to put in for standard metric threads?

Was a fun little project. :)

There's a few ways to deal with this. There are charts around for threads, but they don't often show the depth of the thread, more usually it can be derived from taking the 'Core Diameter' away from the OD and halving it. But, this applied to a perfect threadform, and unless the tooling is a specific threadform type it's going to be wrong in the point area of the tool. If you do it this way remember to allow for direct of indirect reading off your cross slide, if it's direct then the figure from taking the core diameter from the OD is what needs to be read off. Bear in mind that, predominantly in America, some folk angle the compound slide and advance that to make a cut, so some more calculations may be needed if you're doing it that way. I've yet to see any advantage in this method so have never used it, preferring to be able to dress both sides of the threads 'V'.

Realistically to get a well fitting thread there's a couple of options. First is to cut a test piece, simply form a thread on a piece of bar and record the feed in amount till it matches the nut, then apply that to the workpiece.

That's fine if you're using a stock nut, but another way would be to make the nut yourself. Form the spindle thread first then split it and make the nut to fit. 

It's like a Rite of Passage making one of these .

Having not actually done so I've just made this one, but just to introduce some variation this is a smaller thread than might normally be used, M6. The joint is half way along the thread.

There's a variation on this lathe trick out there somewhere, where the nut can only be turned in one direction. I was shown one years ago, a guy gave me a threaded bar with a long hex nut on it, I was instructed to screw it down, fine, then unscrew it, which it wouldn't. I'll see if I can dig this up on the net, it must be somewhere.

It's interesting, those not in the know always head to the ends to see if they've been added later, welded on is the usual explanation given. But today I took it in to work and gave if to a number of Engineers, they varied from 'well clearly it's been joined' but had difficulty finding that join, to some who sussed it straight away but still think it's a clever idea, so all in all it's been fun to watch people getting their heads around it . The best thing though is that I fiddle with this whilst on the phone rather than playing with my home made gyroscope which drives everyone nuts! So my team are happier now .

Thanks for the link, that's certainly the idea, I must keep looking though as that one will have one end of the bolt clearly turning, whereas the one I saw did not perceptively move. Thing is, this is a 30+ year old memory, and I was never shown how it was done .