Thread cutting with dies

[Richard BrickwoodParticipant @richardbrickwood17062]

I can find loads of information on drill sizes to use when tapping a thread – but nothing on the diameter to reduce a bar to use to cut a matching thread ie for a 1/4″ BSW thread do I use a 1/4″ round bar – or reduce that slightly to “start” the tap?

[JasonBModerator @jasonb]

Generally you use the nominal size stock for the thread so your 1/4″ Whit can be cut on 1/4″ dia stock. Having said that most thread data sheets include the “Major diameter” which is what you need to look up if unsure such as here

There are a few exceptions that you will need to look up such as pipe threads and also ones that use numbers rather than diameter such as small UNC size sand BA.

[Nigel Graham 2Participant @nigelgraham2]

Do measure the bar though. If good quality from a reputable supplier it should be fine but I have encountered some evidently drawn through worn dies, so oversize and ribbed.

For my use of that particular steel, simply a static stay-rod, it was not important and a touch with the lathe sorted it, but it’s not what you want for machine parts and the like.

[SillyOldDufferModerator @sillyoldduffer]

There’s a rule of thumb thing.  To cut a full depth M4 thread, feed a 4.0mm diameter rod into the die, job done.   However, as this is hard work, wears the die quickly, a blunt die might bend the rod, and not much strength is lost by cutting a reduced depth thread, I often turn a little off the recommended rod diameter to make the job easier.  It’s allowed!

Tubal Cain’s “Model Engineer’s Handbook” covers thread depth and is well worth the money for other reasons.  (Link is to Waterstones.  Many other sellers available!)

Dave

[FulmenParticipant @fulmen]

SODs advice is especially useful in gummy material like stainless. For taps I often drill .1 over the nominal for the same reason.

 

[Richard BrickwoodParticipant @richardbrickwood17062]

Many thanks for all your help – its pouring with rain – ideal time for an afternoon in the workshop for a change 😉

Richard

[Howard LewisParticipant @howardlewis46836]

FWIW, I always use a lubricant when using either a Tap or a Die.

If possible, I tryn to find a commercial bolt or stud to which the Die can be set.

At a pinch, bacon fat will work (Not too different from the tallow that was used MANY years ago.

Howard

[John PurdyParticipant @johnpurdy78347]

One of my thread cutting references suggests that the major dia. should be reduced by 12% of the pitch when cutting an external thread to reduce the chance of the thread crests interfering with the root of the mating female thread.  I have made a habit of doing this and it works well, makes a noticeable difference in the force required to turn the die. I also do it when screw cutting a thread. For your 1/4 BSW (1/4-20) the suggested major dia. would be .244.

John

[Neil WyattModerator @neilwyatt]

It also depends on the material. Aluminium alloys, which tend to deform and extrude into the profile, are best turned noticeably undersize. Brasses which chip freely can be left at nominal size. Steel typically extrudes a little.

[Mick B1Participant @mickb1]

BDMS would usually be a couple of thou undersize anyway, though check that of course. It’s probably also useful to file or turn a bit of a taper to help centre the die, providing you’re holding the 1/4″ stock reasonably concentrically.

[Martin CargillParticipant @martincargill50290]

One thing you can do (assuming you can get the part to be threaded into a lathe and that there is surplus length on the material to be threaded) is to reduce a length of the material, equal to the width of the die, down to the minor diameter of the thread to be cut. This makes it very easy to start cutting the thread straight.  The reduced portion can be cut off after threading has been completed.

[Nigel Graham 2Participant @nigelgraham2]

Martin –

Yes, a technique I use when screw-cutting too, as a guide to depth.

Sometimes the spigot is left as a feature on finished items, usually where a split-pin or safety-wire is added above a lock-nut, and it can aid assembly in confined places.

[Russell EberhardtParticipant @russelleberhardt48058]

The best lubricant I have found for threading tricky materials or small diameters is Rocol RTD Compound, not sure if it’s still available.

Russell

[Nigel Graham 2Participant @nigelgraham2]

Yes – looking up simply “Rocol RTD Compound” on-line produced immediate ads for it.

[FulmenParticipant @fulmen]

You can make a pretty decent replacement by vegetable oil with 5-10% sulfur. Smells a bit funky though.

[Phil BoylandParticipant @philboyland37326]

Don’t forget a nice big chamfer on the end. Certainly helps you start the thread off and can always trim it back a bit when the thread is complete.

[David George 1Participant @davidgeorge1]

I am a retired toolmaker, Service manager and time served apprentice trained engineer. I would not recomend reducing the thread outside diamiter to make it easier to cut a thread. Although it depends on the loading that is put on the actual item. I saw a wax die mould which had a side plate held in place by swing bolts and thumb nuts made of EN16 high tensile steel strip the thread under load as the outside diamiter had been reduced to make it easier to die down. The die split opened and molten wax was released but was contained by guards on the moulding machine. What is the thread holding perhaps steam pressure or holding on a chuck on is it worth reducing the efectiveness of a standard safety item. I always use the correct chart to check thread dimentions both outside diamiter and hole size for threads.

David

[HuubParticipant @huub]

I am with John, you need to reduce the major diameter of the bar to get some play between the nut and the bolt. The amount of play depends on the thread tolerance you need. For a general metric thread without a thread tolerance a 6g tolerance is standard. That results in a play of about 10% of the thread height.

For a 1/4 BSW, I would reduce the diameter of the bar to 0.234″. I have found different formula’s to calculate this diameter (I am a metric guy) so you better stick to the value of John.

If you would cut this thread using a threading tool, you still would need the theoretical depth of cut, compensated for the tool nose radius of the threading tool.

[Brian Baker 2Participant @brianbaker2]

Greetings, I think I am with David here, and would be against reducing the diameter at all.

I have found that if a tread is difficult to cut with a die, ie 1/4 BSW, then it is much better to screwcut most of the material and use the die to form the finished thread, which would be stronger and run better.  Srewcutting  does need a little practise, but will produce an accurate thread, noy always true with a die.

Have a try.

Regards

Brian B

[Anonymous]

This is an interesting thread (pun intended)! I have produced hundreds, if not thousands, of threads using Coventry dieheads or by screwcutting, and sometimes with circular dies. I have never reduced the diameter prior to threading. I just use stock as supplied (as noted it is normally slightly under size) or turn to nominal diameter.

I have just measured a few home made 1/4″ BSF studs and bolts with threads cut using a Coventry diehead. The OD of the threaded portion is 0.245″, ie, 5 thou under nominal.

Andrew

[Martin KyteParticipant @martinkyte99762]

If you are threading in the lathe using a tailstock die then could I suggest that you screw cut the thread first as practice. It’s a really good way of building confidence in your single point threading abilities. Finishing off with a die has the advantage of always producing a good result when you are learning and pre cutting will ensure you get a more accurate result. As your confidence builds you will soon feel like cutting threads using single point tools alone.

regards Martin

[Nigel Graham 2Participant @nigelgraham2]

Finishing with a die, die-box or chaser has the advantage of giving correct thread-form a normal single-point tool won’t produce.

[Richard BrickwoodParticipant @richardbrickwood17062]

….thank you all for so many helpful responses – screw cutting is definately on my “to learn” list – meanwhile I wanted to move on over Christmas so reduced the thread I wanted on a mandrel  I intended to use to turn on awkward shape in the lathe to 2BA (having broken my only 1/4″ die) and that did the job. For now.

[Howard LewisParticipant @howardlewis46836]

With regard to calculating the Diameters of stock for screwcutting.

Whitworth, BA, Unified and Metric threads specify a reduction of stock diameter (proportional to thread pitch) to ensure that freedom from root / crest interference.

At Rolls Royce, the truncation for Unified threads was given as 0.108253P for a screw and 0.144338P, for a nut, where P is the pitch.

Since Metric threads are also 60 degree form, I would expect the same factors to apply.

Whitworth (55 degree) form threads have rounded crests for the same reason.

For these the truncated diameter of the screw is given as Nominal Diameter – (0.1478/tpi)

For the nut the truncated diameter is given as Nominal Diameter + (1/tpi)

The radius of the rounding is given as 0.3137329P

The truncation is then 0.160083P

For 47.5 degree BA threads, also with rounded crests, Radius = 2 x (P/11) the truncation is given as 0.27P resulting in the truncated diameter of the screw being Nominal Diameter – (0.2172/tpi), and the nut Nominal Diameter – (0.9/tpi))

HTH

Howard

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