Using Dies

[FalcoParticipant @falco]

[FalcoParticipant @falco]

I would like to ask for the advice of "seasoned" members on how they arrive at the size of blank core before trying to cut a thread with a die. Most of my threading is metric, from 3mm. to 12mm. and I'm using ordinary carbon steel dies. Most of my work is on mild steel and I would normally cut a short lead-in taper before starting with the die.

I'm aware of the extrusion effects etc, but I rarely seem to get the blank diameter right to start with. Either the die just chews the end of the rod and wont start the thread, or the thread when cut is rough looking and too shallow.

Tubal Cain et al. go into great detail as to drill diameter sizes for using taps but give little more than a vague one-liner on the sizing for dies.

Is there any simple rule of thumb that you can offer that "works"?

[NobbyParticipant @nobby]

Hi
Turn job to nominal diameter put a 45 degree lead . open up the split die using centre screw .Then
bring the tailstock up and put pressure on die stock to keep it square with the job. Rotate keeping pressure on. cut thread check and adjust to get the size required . Use cutting oil
Nobby

[TerrydParticipant @terryd72465]

As Nobby says, cut the thread on nominal size bar (3mm for M3, 4mm for M4, 1/4" for 1/4 Whit etc the nominal sizes for all threads are available in standard tables), and prepare the bar as he suggests. Do not try to cut the thread in one go. Open up the with the centre screw in the die holder and use the lathe to guide the die as Nobby also suggests, then close up the die a little and take a second cut, and a third if necessary – with the work in the vice now if you prefer and keep testing after each cut until the female thread fits. However just a point, rotate chuck with the left hand while keeping tailstock pressure with the right don't try it under power! Occasionally take off the pressure and back off the die a small way to cut the swarf.

With a sharp die there should be no 'extrusion problem' and carbon dies are very sharp when new but tend to blunt faster than HSS but for our use they are more than adequate. I buy carbon steel for dies which are used mainly on brass or occasional use and HSS for frequently used taps and dies.

I'm not sure what you mean by 'core' diameter in this context. The core diameter is the theoretical minimum diameter of the thread so if you reduce a bar to that the die would just slip over it.

Regards

Terry

[JasonBModerator @jasonb]

Just one thing to add, when you start to slacken the centre screw nip up the outer two screws otherwise the die will just expand as it is run down the thread for the second time and not take any more off particularly if its a bit blunt, this may be why you are not reaching core dia.

J

PS use a threading & tapping fluid

[David Clark 13Participant @davidclark13]

Hi There

Whan cutting a thread with a die or diebox etc. I always left the work a couple of mm longer and faced and chamfered the thread to length afterwards.

regards David

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Specification of od for cutting external threads is exactly analogous to specifying tapping drill size for tapped holes .

Using nominal od gives 100 % thread form and any reduction in diameter gives progressively reduced thread form .

For most threads cut them as described by others above but for special threads work out the diameter reduction for specific ISO pitch to give about 85% thread form .

Purely as a matter of convenience when die cutting larger ISO threads reducing stock diameter a little will make cutting easier .

Die cutting of threads is always helped by use of special thread cutting grease . Ordinary grease and thick oil will work reasonably well .

Michael Williams

[NobbyParticipant @nobby]

Hi Guys & Falco
One of these would help when cutting thread in the lathe.this one is sprung loaded for safety
Regards Nobby

[AndyfParticipant @andyf]

Personally, unless for some reason I'm trying to achieve a 100% thread form, I usually start with stock which is a bit under nominal size, giving the die less work to do. So for M6x1 (thread depth 0.5mm), stock around 5.8mm will give a thread depth of 0.4mm. The missing 0.1mm is what would have been the crest of the thread, and that would not add much to its strength. The results are perfectly adequate for general "holding things together" purposes.

Where the job can be held in the lathe, one way to get a die started is to hold it in the diestock, screw it on an offcut with the same thread, grip the offcut in the tailstock drill chuck, butt its end up against the workpiece and then run the die from the offcut on to the workpiece. The diestock arms must be short enough to clear the lathe bed, of course.

If you have the patience, screwcutting the thread on the lathe until it's almost there and finishing it with the die produces nice threads.

Andy

[Peter G. ShawParticipant @peterg-shaw75338]

I can only give my experiences as a strictly amateur.

If using the die, I always have a lead in chamfer which can be removed after if necessary. This implies that the thread is cut slightly longer than necessary, although often this doesn't matter as it can also form a lead-in for the nut – as long as it is not too much.

I also find that using a die guide helps in getting it started, possibly because I can put more pressure on without fear of it going off centre. The downside is having to make the guides before hand, but on the other hand (sorry!) they do ensure a better chance of keeping the thread aligned correctly.

Sometimes if the work is already in the lathe, I will screwcut to start with and then finish off by die. Also, unless it's very long, I always use a mandrel handle. (Just try cutting say a 10mm long thread under power at 125rpm, my minimum speed. It's ok if you like living dangerously – I don't.) Or if it is a small enough thread, I may simply hold & turn the chuck or mandrel whilst keeping the die steady.

Tubal Cain, which you appear to have already read, does state that there is no benefit in trying to achieve 100% flank fitting with metric threads. I also noticed only last night that he recommends reducing from the nominal diameter by between 5% and 10& of the thread height.

Finally, I have to say that cutting full height does not work every time for me: sometimes I end up doing all sorts of things to get the nut to fit. Such as having to further reduce the already cut thread. Or even using a three-square file on the threads. Ok, ok, I know that has immediately made me into a metal mangler than an engineer, but it at least shows Falco that other amateurs also have problems. My personal belief is that forcing the die to cut full thread like this will not do the die any good – it's not meant to turn down the metal first. Fortunately, I do not do much screw & die cutting.

As it happens, I am now gradually moving towards always slightly reducing the nominal diameter before I make the thread, but as I say, I don't do much so only time will tell how succssfull this will be.

Regards,

Peter G. Shaw

[Ian S CParticipant @iansc]

I'm proberbly going to say something you already know, make sure you have the die the right way round, one side has a taper lead in, and while you can use it back to front whe cutting a thread to a shoulder, it's not ment to be used that way cut the bulk of the thread. Ian S C

[Sub MandrelParticipant @submandrel]

I know M-series taps have a diameter greater than the nominal diameter, to allow for the flat at the top of the thread. I checked with Wikipedia which isn't too helpful, but the nominal diameter is the maximum for screws so any tolerance (and there must be some) will all be for reductions in diameter.

I just measured a small selection of M6 and M5 screws (from different batches/sources). One was 5.92mm, the others were all at least 0.2mm undersize, two M5 screws were 4.75mm.

This may reflect different fit classes.

Neil

[FalcoParticipant @falco]

I want to thank all those who replied patiently and shared their advice.

I will experiment with 5% and 10% off the full nominal size to see what works best for me. I had been doing many of the things that you all advised eg, Rocol lubricant, supporting the die with the square face of the tailstock barrel and I was leading with the tapered side of the die! It just wasn't working out. It looks like I was not getting the size of the blank right and also I must look at the sharpness of some of the dies.

I'll work through all the tips and no doubt, practice makes perfect. I hadnt been adjusting the die properly as advised by Jason and I hadnt thought of the threaded guide for the die before and will try that as well.

Thanks for the sympathetic reply Peter. I don't feel quite so bad having read your post!

Regards all

John

[Peter G. ShawParticipant @peterg-shaw75338]

John,

I have just had another thought. What about the specification of the mild steel you are using.

When I first became involved in this hobby, I used to use all sorts of steel scrounged from anywhere and the results of machining were at best, variable.

Slowly, I began to understand that there were differences in steels and so I started using free-cutting stuff. The transformation was unbelievable, smooth and easy to turn. From that experience, I also realised some of the scrap steel I have acquired is actually free-cutting, whilst some is, well, shall we say tough! As a result, it makes me wonder about your quality of steel. In addition to everything else, that is.

Regards,

Peter G. Shaw

[blowlampParticipant @blowlamp]

The quality of the die holder (and die come, to think about it) can also affect the results you get and I'm talking about the positioning of the three clamping screws/dimples here.

I once had a problem with a job where the threads were coming out with a very rough finish, which I eventually tracked down to the die being kind of twisted out of shape by a badly placed hole for one of the clamping screws in the holder. The cone point of the screw was also off centre and that didn't help either.

Martin.

[jim bothParticipant @jimboth37830]

major dia is ALWAYS smaller than nominal.

biggest cause of dies not working is cheap dies!!!

[Sub MandrelParticipant @submandrel]

Don't forget metric threads to teh full profile are flat-rtopped – doesn't look pretty, but it is right!

Neil

[JasonBModerator @jasonb]

What tables are you looking at to come to that comclusion John??

All these have major the same as nomimal

BA      Metric     BSF etc

J

Edited By JasonB on 29/07/2012 17:00:54

[jim bothParticipant @jimboth37830]

**LINK**

all the thread info you will ever need!!!

[FalcoParticipant @falco]

Peter, you might very well have a point. Nearly all my metal has been "useful" bits that I have salvaged. Living miles from suppliers and having a magpie instinct is my excuse. Indeed I can remember several tussles in the past with shock absorber centre bars.

Neil, if not looking pretty is one of the tests of a good metric thread, mine are well on the way to perfection .

Jason and Jim, thanks for the links. Lots to learn.

Investing in a few decent quality dies would eliminate some of the variables. With so many suppliers out there selling different brands, would anybody like to recommend where they have had good quality at a decent price for taps/dies?

John

[Ian S CParticipant @iansc]

Jim, good thread charts, they missed one relevent one, the Whitworth extra fine, or model engineers thread. I see they have the United extra fine. Ian S C

[S.D.L.Participant @s-d-l]

The top diameter is statement is a bit misleading.

As with everything threads are made to a tolerance and as the male thread on hole based (normally on metric threads) if you look at M6x1, 6g both tolerance limits will be negative just like a shaft. so for M6x 6g we end up with a maximum diameter of 5.97 to 5.79.

See here to calculae it and other combinations.

Now where this matters is that many modern taps and dies are made to produce to a specific tolerance so i would ain to turm the shaft to the mid position so uin this case say 5.9.

Steve Larner

[speelwerkParticipant @speelwerk]

When making long threaded lengths with a die it is important to keep the die square to the shaft. That can be difficult with a handheld die holder, you can use the tailstock but íf you do that with too great pressure the die is forced to much which gives a ugly thread. A tailstock die holder keeps its square and eliminates that problem, but if do not have a tailstock holder you can use a handheld one with a special made guide. The guide starts with a hole that slides accurate over the shaft, than follows the die and this is than fitted in a normal handheld holder, the guide centres on the not yet threaded part of the shaft and keeps it very accurate square.

[NobbyParticipant @nobby]

HI Guys
One of these Niko refers to

[jim bothParticipant @jimboth37830]

I've probably tried 15 different makes of M6 dies ( a regular job in stainless), only ones that last are Titex ones, get around 750 threads (15mm long) out of a die.

Cheapo dies won't even do one part!!

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