Very interesting John, I think I have vaguely heard about this. Also seen new old stock half nuts for a Colchester that were in one piece as they had been machined and were split just before fitting. If you think about that of course although the half nuts would be identical, only those machined together in one piece would have a matching thread on both halves as there would be know way of telling where the thread would start. Can't see the point of not splitting a die though, unless they got a lot of breakages in packing or transit.

Phil

Just a thought. Some manufacturers produce both adjustable (split) dies and solid (unsplit) dies – the advantage of the latter being, I suppose, that they cut accurately to the final size.

If I were such a manufacturer, I would probably make a single product and simply omit the final slitting operation. The dimples will be there on both as they are needed to prevent rotation in the die-holder. As for the v-slot, it might simply be an artefact of when they separate the two versions in the manufacturing process.

I know that many people are dead set against solid dies but I think they have their uses. You can get a nice, accurate thread by first cutting with a split die opened up a bit, then finishing with a solid die. Or screw-cut the thread on the lathe and finish with the solid die.

I think split means adjustable for cutting a thread and unsplit is used for cleaning up a damaged thread.

Solid dies are by their nature dead size, which makes them extremely useful for finishing and cresting a screw cut thread when you don't have a full form threading tip. They should really be reserved for sizing operations, as using them to cut a full thread will only blunt them making them useless for their intended use. A round split does give a very small amount of adjustment, which can be beneficial when cutting a thread in hard steel, as the first cut can be made with the die as open as it will go using the central screw, then finished off by loosening the central screw and tightening the side screws while sizing the thread using a standard full nut as a gauge.