Using Dies

I have used Tubal Cane's recommendation and also the handy method of "Diameter - pitch" for finding the correct drill size for tapping, successfully.

Using dies has given me a bit more of a problem. Even in the lathe, using tailstock barrel as a support, I have difficulty getting the die started on what I think should be the proper diameter for a particular size. I have often had to turn down the blank smaller than I expected to get the thread started, sometimes ending up with a flat-topped thread.

While the topic of tapping drill sizes is well catered for, the sizing for using dies is not. (At least I have not found it)

Is there a simple formula like the one mentioned above for determining the diameter of the blank for threading with a die. My threading is mostly in metric sizes.

In the opininion of the more advanced members, is it worth spending he extra on HSS taps and dies?

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

Firstly have you got the die in the right way round, theres usually more of a taper the one way and wont cut correct up against a shoulder. Also the actual size

Never had fantastic success with dies, they can rip even sizing after screwcutting. Assuming you have fed the die to the job either of two ways, feel for it cutting with tailstock while revolving the huck. Or just power the spindle slow and push the die and tailstock in. Plenty of cutting fluid.

The simple formula is size ie M25X1 turn to 25 o/d or for tap that would be 24mm i/d, simples.

It depends what you are working with, i always had HSS but found carbon cuts better.

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.

The starting diameter is not the nominal diameter of the thread such as 6mm for an M6. All threads have either rounded or flattened crests by design not sharp points. The reduction is typically 1/8 times depth of thread on each side of course so 1/4 depth off diameter. And depth is 0.866 times pitch for metric. So you can easily calculate the proper starting point. for each size not use some vague fixed amount. Also in many tables of thread data it is given as the 'major diameter'.

Edited By Bazyle on 14/03/2012 01:39:29

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

For anything over about 6mm I tend to cut a lot of the thread on a lathe then finish with the die.

This also means you have a straight thread

Taps seem to be a lot more user friendly up to 12mm, although I prefer 10mm max.

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

Thank you to all who replied. All those tips are very helpful. I'm encouraged by the news that I'm not the only one who had has trouble with dies!

My set is a new-ish split die set in carbon steel by CK, bought at a market tool-stall and of mixed quality. The 6mm die won't cut at all. I intend to buy a few quality dies and might stretch to HSS but they are costly. I was leading with the tapered side and using John F's method, taper and all but still had trouble.

It is interesting also that you all recommend the split dies over the die nuts. I accept that I should go ADY 1's way and cut threads on the lathe but I'm just not there yet.

Thanks everyone. Must appreciated.

John