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The Standardisation of Screw Threads

"A British Standard Screw"

Once upon a time in a far off distant land there was a war. It was 1855 and the British Government wanted supplies for an important fixture they had with the Ruskys in the Crimea. Due to a rather poor telephone line and some rather plumby old Etonian accents 120,000 left foot gumboots were sent to the front line.

“Gunboats I said, you blithering idiot, gunboats.”

“Oh I say, dreadfully sorry old chap.”

The start of manufacturing standardisation

The delay caused a little problem, they had only 90 days left to build 120 gunboats before kick off. Oh they could manage to build the boats alright but the engines were a different matter. Back in those days building an engine was nearly as difficult as finding an investment banker with a conscience (sorry I shouldn't exaggerate). It was an impossible task until some smartypants had an idea. The smartypants in question was a fellow called John Penn who just happened to have a couple of 60 horsepower engines of the type required lying around in the back of his potting shed. He had them taken apart and the parts were distributed to some of the best manufacturers in the country who were each asked to make up sets identical to the samples they were sent.

When the parts were brought together they fit, amazing, and 90 sets of engines were built within the 90 days. Such an achievement was unheard of at that time (the enemy were totally gobsmacked and complained to the International Association of War Referees). The secret to this feat lay in the fact that by then a whole system of measurement and quality standards (metrology) had been developed and adopted by manufacturing companies in Britain which meant that a screw in Aberdeen was just the same as a screw in Cardiff (apart from the accent).

Joseph Whitworth and the first standard screw threads

Let me explain. It all started with a clever chappie called Joseph Whitworth who devised the world's first standards for mass production. Back in 1841 Whitworth came up with the idea of standardising the design of screw threads which until that time were made as sets on a “one off” basis and, as a result they were monogamous, only one nut would fit any one bolt.

Whitworth determined fixed sizes, the crest angle of the thread was set at 55 degrees and the number of threads per inch (yes this was back in the days of those strange imperial measurements) was also fixed (20 threads per inch for a quarter inch thread) which determined the depth of the thread. It was all worked out to produce a screw thread that was an optimum compromise between being strong enough and having enough friction to not come loose.

Standard sizes of spanners and monkey wrenches

The sizes of hexagonal bolt heads were also fixed which meant that spanners could also be standardised which is why, to this day, you can go into a shop in Swaziland and buy a spanner that will remove the bolt through Frankenstein's neck.

Until fairly recently (relatively speaking) every bright eyed young lad starting his first day at an engineering apprenticeship in Britain would be handed a set of spanners (wrenches to you unsophisticated types who have a tendency to wrench, jerk or yank anything you get your hands on). They looked nondescript enough which belied the genius behind them. There were 5 in the set, increasing in size and with open ended jaws on each end. With only 5 spanners you could remove and install any nut and bolt between a quarter and one inch. All part of the system developed by Joseph Whitworth.

It should be noted that we are talking about real spanners here and not those “adjustable” things, an amateur handyman's excuse for a real spanner, infernal devices that any self respecting engineer would fall on his hacksaw if he was caught using, an evil contrivance that has a tendency to round off the hexagon head of any bolt it comes near while removing the skin from your knuckles, a fiendish contraption imposed on the world by one Richard Clyburn, a man of doubtful parentage who ran a joke shop in Lesser Frotterington. Here endeth the first rant.

British Standard Whitworth (BSW) Thread

Joseph Whitworth developed standard screw threads which became universally adopted as British Standard Whitworth (BSW) screw threads. He also developed a method of making very flat iron workbenches (known as surface tables) and manufactured screw cutting lathes which made it possible to measure and manufacture to very accurate tolerances. He was knighted for his efforts, it's a brave thing to do having some aging monarch waggling a sharp blade close to your jugular vein, beheading is, after all, in the family so to speak. “I dub thee....oh dear, the late Sir Joseph Whitworth”.

Brtish Standard Fine (BSF) and British Standard Pipe (BSP) threads

BSW was only the start, a finer thread was developed for more delicate jobs known as BSF (British Standard Fine) and BSP (British Standard Pipe threads) which is still used as the standard for pipe threads in many parts of the world (including Indonesia). You will find that plumbing fittings still widely use British Standard Pipe threads which come in parallel and tapered variety (what?). Next time a plumber comes to your house and removes a tap or a shower have a look at the screw thread. It may be parallel (in which case it will have a rubber or fibre washer to seal it) or it will be tapered (the diameter of the thread decreases towards the end). Tapered threads are designed so that when you screw them up they will tighten and seal the pipe, they don't need a washer. If you've lost the thread and don't know what I'm talking about don't worry about it.

American USS and AF threads

In 1864 an American, William Sellers, proposed the introduction of standard screw threads in America. He modified Whitworth's standard (they do have a habit of going their own way don't they) changing the 55 degree thread angle to 60 degrees and coming up with a standard that became the United States Standard thread (USS thread) and these evolved into the American National Coarse, National Fine and National Taper threads. The second world war brought standardisation between the US, UK and Canada in the form of unified threads although Britain tended to stay with the established British Standards.

Metric Threads

Europe developed metric threads which also have 60 degree thread angles. The standard specifications for metric screw threads were one of the first world standards established by the International standards office or ISO in 1947 these days known as ISO 68-1. Britain went metric changing from Whitworth to metric threads in the 1970's. These have since been adopted across most of the world including America.

All good stuff but it is unfortunate, however, that one of the lesser appreciated aspects of Whitworth's genius was missed.

Whitworth's Genius and preferred bolt sizes

You see old Joseph was clever enough to select which sizes would be commonly used, which ones would be “preferred sizes”. When Britain joined just about all the rest of the world and went metric they did not state preferred sizes. They didn't say “I say chaps let's not bother using a 13 mm thread, it's only a flea's ear'ole bigger than 12mm and 14mm isn't that much bigger either”.

At first it may seem that this is not of much consequence but anyone who spent years of their lives trying to keep some old banger going in the North of England will understand what I mean. When you are lying on your back on the garage floor with a set of spanners on a freezing winters night, your fingers sticking to the icy steel and knocking the skin off your knuckles when the spanner slips, the last thing in the world you want is to be changing spanners every 20 seconds.

My cynical side might suggest we've all been screwed, it's all a ploy (nay a plot) to sell more spanners. These days the set of five open ended spanners young apprentices received in days of yore has grown to about three thousand four hundred and sixty two to cover the same size range. You can't keep your spanners in your overall pocket anymore, you need a tool chest on wheels that you have to drag around to keep them in and the sizes are so similar you end up trying 26 before you manage to find the right one.

Multiply this by the millions of people all over the world taking things apart and putting them back together again all busy swearing and cursing because they selected the 18mm instead of the 17 and we have to wonder why such a simple thing was missed.

Did I hear something? It must have been old Sir Joseph turning in his grave. Here endeth the second rant.

These days there are many different sizes of screw threads for all sorts of purposes such as bicycle threads, watchmakers thread, meccano bolts, small machine nuts and bolts used by electronic engineers and, of course, those little gold threaded bolts we have in our navels to stop our bottoms falling off. There are also manufacturers that purposely design non standard nuts and bolts just to make sure you buy original spare parts from them.

Standardised Metric Nut and Bolt Thread Sizes

In spite of this you will find that if you go and buy nuts and bolts they will generally come in metric sizes. They start with the letter M for metric followed by a number which is the diameter of the bolt across the outside of the thread. M10 is a metric bolt with a 10 millimetre diameter thread. An M10 nut fits an M10 bolt (surprise surprise).

Standard metric bolts have coarse threads for general purpose work but fine and even superfine metric threads are also available.

These days there are in fact preferred sizes for metric bolts which are: 1, 1.2, 1.6, 2, 2.5, 3, 4, 5, 6, 8,10, 12, 16, 20, 24, 30, 36, 42, 48, 56 and 64 millimetres. Unfortunately these preferred sizes are often ignored and even so there are still rather a lot aren't there?

There are some very good nut and bolt shops hidden away in Bali, you'll find a couple of them on Jalan Gatot Subroto but don't expect the extensive range you'll find in western countries.

As a final comment if you are buying nuts and bolts it is a good idea to make sure they fit together before you leave the shop or you might get home with something as useful as a left footed gumboot.

Copyright © Phil Wilson 2011
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