Whitworth or UNC???

[tekno.mage]

Not sure if this is right place to ask about this - but here goes...

What is the difference between a Whitworth and a UNC thread? Say I have a tap which is 5/16" and 18tpi, (both measured, BTW) how do I know if it's Whitworth or UNC?

 

[CHJ]

Whitworth Treads have a 55 deg angle.

UNC is 60 deg.

For a given diameter you may be able to see that the UNC has a thinner core (root diameter)

I'm surprised that the tap does not have some other mark on it,

 

[paul-c]

zeus charts
they will give you the details of different threads.
i will check mine for you.
cheers paul-c

 

[paul-c]

chas beat me too it. =D>
cheers paul-c

 

[tekno.mage]

Whitworth Treads have a 55 deg angle.

UNC is 60 deg.

For a given diameter you may be able to see that the UNC has a thinner core (root diameter)

I'm surprised that the tap does not have some other mark on it,

Thanks, Chas - although how I'm going to measure the angle on something that small I'm not sure The tap is old and the markings obscured, I'm afraid.

 

[CHJ]

Do you have a 5/16 UNC nut?

Whitworth tap should not run through it without removing a smidgen of metal.


Conversely a Whitworth nut will often run on a UNC bolt.

 

[ash42]

P

 

[woodfarmer]

Whitworth threads have rounded tops and bottoms. UNC is uncut could be flat or pointed tops and bottoms.

 

[CHJ]

Whitworth threads have rounded tops and bottoms. UNC is uncut could be flat or pointed tops and bottoms.

There are also modified UNC-UNF threads with rounded valleys and tops, although the average UK DIY enthusiast is unlikely to see them.

Vitally important that anybody into Aircraft conservation, especially engines, is aware of the fact though and does not substitute a 'standard' version.

Because the standard UNC specification can lead to fatigue failure under heavy loads with its acceptance of sharp pointed Vee (root) terminations there are a series of Rolls Royce specification amendments in the ANSI system that have improved rounded valleys and crests for use on Aero Engines etc.

This I believe came about because of the experience of Packard when they replicated the British thread system (BSW-BSF) when building Rolls-Royce Merlins under license during World War II and subsequent sharing of Jet Engine technology requiring the finesse of rounded thread root for fatigue reasons.

 

[Spindle]

Hi

I think you'll also find that the rounded profile threads used in the aircraft industry and other high stress situations are formed by rolling as opposed to cutting in order to reduce stress raisers.

Regards Mick

 

[J-G]

Whitworth threads have rounded tops and bottoms. UNC is uncut could be flat or pointed tops and bottoms.

Because the standard UNC specification can lead to fatigue failure under heavy loads with its acceptance of sharp pointed Vee (root) terminations

To my knowledge there are no thread standards which specify a sharp Vee. The UNC/F/S etc. forms all specify a crest flat equal to .125 x pitch - A sharp V at the root is virtually impossible to achieve in reality anyway and a rounded root equal to .25 x pitch is preferred for external threads.

there are a series of Rolls Royce specification amendments in the ANSI system that have improved rounded valleys and crests for use on Aero Engines etc.
This I believe came about because of the experience of Packard when they replicated the British thread system (BSW-BSF) when building Rolls-Royce Merlins under license during World War II and subsequent sharing of Jet Engine technology requiring the finesse of rounded thread root for fatigue reasons.

I don't have any first hand knowledge of RR engines being made under license but I do know that Rolls Royce were one of the first companies to specify metric measurement - that was an interesting fact that I was made aware of very early in my apprenticeship back in 1957.

I think you'll also find that the rounded profile threads used in the aircraft industry and other high stress situations are formed by rolling as opposed to cutting in order to reduce stress raisers.

The major advantage of rolled threads is higher shear stress created due to the material grain structure following the contour of the thread form rather than being 'cut' - think of cutting threads in end grain timber.

 

[tekno.mage]

Wow - thanks for that wealth of knowledge on thread forms and the aero industry. Having inspected the tap again - it's Whitworth

 

[CHJ]

Wow - thanks for that wealth of knowledge on thread forms and the aero industry. Having inspected the tap again - it's Whitworth

There are many good references regarding threads and the choice of form on the web for those interested,

Not many people realise that the good old fine thread British Association (BA ) 0-12 BA being the usual range encountered, was a Metrically Sized system although defined in imperial units.

There is an easy to read synopsis of thread mix between UK and USA linked on my site Aircraft Engine Historical Society, Inc.

 

[woodfarmer]

It was Whitworth himself who experimented with thread forms and found the rounded thread which we now call Whitworth to be the strongest. The American UNC/UNF V threads are -much- cheaper to make and 60 degrees is also cheaper to produce. When cutting threads on a lathe, the sharp point is very easy to make but ideally they should as said be flat topped.

 

[J-G]

It was Whitworth himself who experimented with thread forms and found the rounded thread which we now call Whitworth to be the strongest. The American UNC/UNF V threads are -much- cheaper to make and 60 degrees is also cheaper to produce. When cutting threads on a lathe, the sharp point is very easy to make but ideally they should as said be flat topped.

Sir Joseph Whitworth was the first manufacturer to 'STANDARDIZE' the combination of diameter/tpi which eventually became BS84. Prior to his work, each manufacturer simply made whatever they thought would do the job in hand which made interchangeable components and after-market repairs quite difficult.

When I mentioned 'sharp Vee' I made a point of specifying 'in the root' - sharp point on the crest is, as you say, very easy! However, if you were to machine to a sharp point crest then the thread would be too thin and the effective dia. would be undersize.

 

[woodfarmer]

It was Whitworth himself who experimented with thread forms and found the rounded thread which we now call Whitworth to be the strongest. The American UNC/UNF V threads are -much- cheaper to make and 60 degrees is also cheaper to produce. When cutting threads on a lathe, the sharp point is very easy to make but ideally they should as said be flat topped.

Sir Joseph Whitworth was the first manufacturer to 'STANDARDIZE' the combination of diameter/tpi which eventually became BS84. Prior to his work, each manufacturer simply made whatever they thought would do the job in hand which made interchangeable components and after-market repairs quite difficult.

When I mentioned 'sharp Vee' I made a point of specifying 'in the root' - sharp point on the crest is, as you say, very easy! However, if you were to machine to a sharp point crest then the thread would be too thin and the effective dia. would be undersize.

I meant Whitworth experimented to find the strongest thread form, then set out "standard" sizes to make nuts and bolts interchangeable.
I had to cut a thread, about 3 inched diameter inside a stopped hole. it was a copy of a rusty thing so had to play it by ear as the diameters were worn. It is very easy to cut sharp points at both the tops and bottoms, neither of which is good.

 

[CHJ]

Very few 'home workshop' machinists have the facility to grind an accurate radius on a thread cutting HSS tool that would replicate a true standard specification of root radius, even using a modern pre-formed carbide tip will not produce the correct form across all diameters of thread (radius differs with each thread pitch) There's definite merit in roughing a thread on a lathe and finishing it with a "quality" Tap, Die or Thread Chaser if you need the strength.

If anybody wants to study the finer points of thread form, tolerances and definition of fits, there is hardly a better source than "The Machinery's Handbook" but it's the sort of book you need before you start your career (and can't afford it) and stays with you as the ultimate 'aid memoir'

 

[Phil Pascoe]

That one only seems to be expensive in hardback, the paperbacks are reasonable.

 

[J-G]

... even using a modern pre-formed carbide tip will not produce the correct form across all diameters of thread ...

Not true - the radius/flat width is dependent only upon the pitch, so as long as you cut to the full depth of the thread it will be accurate. Should you think of using (say) a 1.5mm chaser to cut a 1mm pitch by only cutting to 2/3 depth then the radius/flat width will be wrong.

... (radius differs with each thread pitch). ...

That bit is correct.

..."Machinery's Handbook" but it's the sort of book you need before you start your career (and can't afford it) and stays with you as the ultimate 'aid memoir'

I've taken the liberty of correcting the title (hammer) (there is no definite article - just like 'Messiah'). I was given mine - actually 'willed' - by my Grandfather in 1956. A 1923 edition which I abused as a teenage apprentice (not realizing its value when I was 16) until it fell into three parts. It still sits on my bookshelf and I used it virtually every day until I came across a free download of the 26th (2000) Edition and a short cut to that is on my desktop.

 

[CHJ]

That one only seems to be expensive in hardback, the paperbacks are reasonable.

The paperbacks are useful references but are basically just the day to day essentials, and being USA based there may well be UK equivalents more appropriate to a UK machinist.

The full Handbook has comprehensive info. on USA, UK, Metric (various) standards that are a goldmine to someone having to try and match up or compare standards from times gone by or need for material heat treatments etc. to meet specific criteria.