Hi Falco, For external threads you should use the nominal diameter i.e. 4; 6; 8 mm etc but I agree starting the die can be a little difficult.

You should always put a chamfer on the end of the bar to the depth of the thread and when starting in the lathe there are several things you can do. I usually start the thread by using the tailstock but lock it on the bed and apply a gentle pressure as you start the thread, you have to make the die "bite" into the work and it will not do this unless you apply some pressure to feed it in. It is a matter of "feel" when doing this.

Another approach is to put a taper on the work or reduce the diameter for a short length [as you have already found out] then remove this when finished.

On large threads it is helpful to truncate the thread diameter by a small ammount, as a guide use 1.5 percent of the diameter per 0.100" e.g. a half inch thread reduce by 0.0075" This is not definative but it works well, to be correct you would need to consult the relevant BS tables but for ME work it is OK. Not necessary when screwcutting them for metric threads but you do need to truncate imperial and unified threads unless you are useing a full form cutter. [sorry digressed a little there]

HSS vs CARBON, generally HSS will be a better quality and the best are ground thread dies & taps but they are considerably more expensive. Carbon will cover most of you needs but I usually byt HSS ground thread to H6 tolerance.

Hope this helps a little.

Regards John

I've noticed the same thing and I agree that it seems an omission.

Similar sentiments were discussed in another thread (sorry for the pun) here I'm not sure that it resolved much but it might be worth a read.

I always have trouble with die threading. Some cheap dies are not split, these will only be of use for chasing or cleaning old threads. Start the die opened as far as possible, on a taper, seems to me that carbon steel dies do not like being bent open. I now turn down the o/d, roughly in accordance with Bazyles method, works much better. A good starter is to measure a commercial thread, material being threaded also varies the dia. as in tapping. NB I only do largish threads, M5 being the smallest

Are you using split dies, or the glorified die nuts that they sell as dies these days, these can be split using a dremmel tool and thin grinding wheel, you then stand half a chance of cutting a thread, although some of the dies I'v seen don't even seem to have a taper lead in. Ian S C