Summer time by the river

Clever Designing 160 years ago

One drowsy, summer’s day in the World Heritage Site, Derwent Valley Mills, an elegantly curved weir makes the river swish and some ripe old engineering appears.

Long, wide weir in high Summer
Long, wide weir in high Summer

Beside it, through the green canopy, slightly mysterious stone structures appear.


As you approach it, machinery begins to take a clearer form; but more of that in the next post.


The stone structures and the machinery can contain and control the river flow even when it is thunderous in full winter spate. This place is where the mastery of water power brought industrial scale manufacture in its youth. The first factories here needed what, in the terms of the early to mid 19th Century, was a lot of power. When the river was angry in those winters long ago a lot of clever, state-of-the-art engineering design was needed to avoid their destruction, let alone power extraction and control. There can be more momentum thundering by here, far inland, than you will see other than at a gale bound sea coast.

The factory manager sometimes needed to control water flow rates into his mill wheels; at other times needed to allow water to bypass the weir. Then they raised or lowered massive gates. When closed and water depth was thus high on the upstream side, , these gates needed to be able to resist, without bursting, the high pressures of the water. Some things can resist pressure but flex a lot while doing it; balloons and car tyres are examples. But, these gates had to be rigid. If they were not, the flexing would jam them in their closely-fitting slide-ways and they could not be made to move up or down when that was needed.

An ideal material for the faces of the gates wetted by the water was then timber; for the strength of its fibrous structure, its integrity in large sizes and also because it expands in water to make a good seal.  Massive though the timber gate baulks were – even bigger than railway sleepers then or now – they would still deflect. So the design engineers set out to design a way of stiffening the timber beams: they used a design called a ‘composite deck’. This is like decking on some bridges where there is timber to walk on or drive a cart across and a steel structure of ties and struts below to stiffen it. Build this to the right size, turn it on its edge and, Olé, a water gate.

Now this was all being done by engineers, quite a long time ago. And here’s proof.


What? Ah yes, the gates themselves. Here’s one and we’re looking vertically down at the back of the gate at the stiffening structure.


Note here that the strut components, mounted at right angles on the backs of the timber baulks are cast iron [molten metal poured into shaped moulds] while, joining the top of the struts to each other and the to the gate frame at each end of the gate [out of shot in this image], the ties are wrought [forged at red heat by a blacksmith]. There are several identical [in this case], independent courses of struts and ties, above and below each other.


Each end of each tie is passed through to the water side of the gate and the nut is turned up tight to make the timber and the metal structure act as one.

I don’t know the name of the engineer who designed the gates and the stone structures. Whoever it was, he [it probably was a ‘he’ in those days] faced a similar conceptual struggle as does the design engineer today. Nowadays, she still has to look at a more or less blank page and produce ideas of a design that will fulfil the task specification. This designer did a good job with the technology to hand. Although the gates had to be of different sizes and so had to with stand different loads whilst doing the same job, he used a standard design concept. That concept used standard struts and ties in a determinate structure for each design.

Strut & Tie Patterns 02


With these simple designs of strut and ties, and depending on the depth of the water forcing the height of the gate, there are several levels of frame above each other. There are limits to the design in that those used here and shown above are determinate. By that I mean that, with these designs, it is relatively easy to calculate the loads that each component has to with stand. If you add any extra struts or ties and it is difficult to ensure that all are tight and if you do, then it is uncertain what load each is seeing. It is then called ‘indeterminate’.

This is not to say that this design did not have its shortcomings. For this, see the next post. Remember, few engineers of today will be credited with designs that are still doing a job after 160 years: I would be delighted if any of mine were.

How to get better! Boot-strapping.

A tweet by @Therese_LW on Sir Joseph Whitworth the Victorian engineer reminded The Lad [@isambardslad] of an interesting question. It sparked off an Twitter thread with @BillHarvey2

Whitworth was an engineer close to Brunel in any global ranking. He made enormous strides in improving the methods and accuracy of machine tools. In general, he greatly improved the efficiency of engineering manufacturing techniques. In this work he drove the Industrial Revolution into a higher gear.

Screw threads are an enormously important factor in engineering manufacture quite apart from their use in a multitude of types of fastener. In the form of lead screws they not only allow heavy masses to be moved but also, in many types of machine tools, allows the distance of movement to be very accurately controlled.

A lead screw on a simple lathe is an important example of this. Here a cutting tool is usually mounted on a saddle whose position is governed by the lead screw. Such a machine can, using the lead-screw as a controller and via gearbox in the headstock, cut another screw thread.

diagram lathe
The parts of a simple lathe


The point here though is that a new screw cut on such a lathe cannot have a better accuracy than the lead-screw that controls the cut. It can only be, at best, as accurate as the lead-screw or, to some degree, worse. This seems to be a completely general feature of machining.

So, this is the question. How can you get from a screw thread that looks, in a wine-press of the early 18thC, like this….

Wine Press of 1702


….to one that looks, and is, much, much more accurate like, for example, this lead-screw?

Acme thread
This is an Acme thread Lead-screw


The Lad and @BillHarvey2 had an interesting, if rather compressed discussion on Twitter on the topic. He made the point that, in the early days, the toolmakers would have improved the shape and accuracy of a thread like the wine press or somewhat smaller, manually with hand tools like files and scrapers. This could then be a master screw to make others similar.

The Lad believed that there would be a limit to what could be done this way as the threads got smaller in diameter and the required accuracy increased. As was said above, you cannot expect to make an identical screw in a lathe that is better than the lead-screw you are using. It can only be as accurate or worse.

Then Bill Harvey gave what seems to be a breakthrough idea. A lathe is not limited to making screw threads that are the same dimensions as the lead-screw. So, let us fit our best, hand-improved screw as the lead-screw in a lathe. Then we set the lathe gearing suitably to cut a smaller diameter and finer [more threads per inch] screw. This way, the inaccuracies in the existing lead-screw will still be there but reduced in absolute terms in the new lead-screw. Fitting this new, finer and smaller screw back as a lead-screw in the lathe [probably a specially designed lathe for the job], we can then use it, with another gear change, to make a large screw again. This new one could be improved even more by hand.

Then this process could be repeated, with an improvement in accuracy each time, over several cycles of large to small to large threads. This is a boot-strap type process that seems to make possible an improved thread. There are other aspects perhaps such as a wide-span follower on the lead-screw as part of the improvement process.

All this so far though is deduction and an element of supposition based upon engineering judgement. The Lad must therefore check it if possible. First port of call will be “Sir Joseph Whitworth – The Worlds’s Greatest Mechanician” by Atkinson. See if that supports our deductions.

Thanks to @BillHarvey2 for the original discussion and the large/small thread idea and to @Therese_LW for raising the matter of Whitworth that sparked us off. Bill Harvey also posted on this subject.

The engineer’s new coin

It’s 1968 and the Decimal Currency Board is planning decimalisation. This group of the great and good wanted to get rid of the Ten Shilling note. Change it, they decided, to a new coin that had not previously existed and worth what is now 50p: a value greater than the then biggest coin. Now a bigger value usually means a bigger coin. Not a good idea this time though: for, to fit into the purse or pocket, it needed to be not too large or too damagingly heavy.

What’s to be done?

Through the ages, for every State and its Mint, resistance to the counterfeiter has been vital. But modern coins need another feature too. They need to operate vending machines and be suitable for bank counting machines.

Therefore, their weight and thickness have to have the right relation to other value coins. But the major criterion, by far, is that when you pop it into the machine, however a coin is oriented, the machine recognises its value because the coin width is constant.

Ergo, the coin has to be circular. Right?

One of the great and the good was Sir Hugh Conway. He was a top notch engineer, having spent most of his career designing aero-engines and was a recent past President of the Institution of Mechanical Engineers.

Sir Hugh Conway.

We mustn’t hold it against him that the company portrait gave him a strong resemblance to a Stage leading man of the 1950’s. Sorry, Sir Hugh.

Anyway, Conway was familiar with the processes used by his production engineers. They are the Company team who carefully engineer the methods for making amazingly-accurate engineering devices.

There are hundreds of different machining processes. Grinding is one way of making some highly accurate pieces. There are sub-types of grinding: some for flat items, others for cylinders.

Trying to avoid the cry, ‘Too much detail!’, The Lad resists the nerdy approach. One sub-type, see image below, squeezes the ‘work piece’ between two rotating wheels. The ‘moving’ wheel moves towards the other removing the metal; the other ‘stationary’ wheel rotates the work piece. As the machine removes material, the work piece will remain circular, won’t it?

This is how centre-less grinding works.
This is how centre-less grinding works.

No, not always. The work piece certainly does stay a constant width. So that’s circular, then? Not necessarily. If the engineers don’t set up the machine properly, the work piece will become – not circular but a lobed, non-circular shape. Certainly it is a strange shape but one which has a constant width. To the production engineers demanding a perfectly accurate, circular cylinder, this was nothing less than a pain in the butt.

To Conway though on the Decimal Currency Board, it was an opportunity. Make the 50p piece that shape and it will still work a vending machine while yet looking and feeling different to people.

Try this video to see a spritely take on the shapes.

To see this strange, lobed shape in real life, just look at a 50 p piece from your purse. Another thing is to put the coin on edge on a table. Then roll it, very carefully to stop it falling over, between a flat rule and the table. You will feel NO BUMPS at all. I guarantee you will think that your eyes are deceiving you.

In October 1969 the 50p coin was introduced, with the 10s note withdrawn on 20 November 1970.Now, in 2013 in the UK, the 50p is not the only coin this shape; the 20p coin is another. It was an announcement a couple of weeks ago of a new coin design that reminded The Lad of this story.

Not all think it was a good thing. Here is a ‘mint’ example of a truly ridiculous, bonkers, journo screed lauding romance apparently – by someone who does not calculate very much – his partner probably does it for him.

Engineering influences our daily lives in some ways that you may not expect. So, there it is: a new coinage by the engineer – both literally and metaphorically.



Systems and their Safety

Almost any piece of engineering is more complicated than it appears to the untutored eye. That is as true of a single component as it is of a complex system.

Whereas the engineer may discover ways for a component not to perform as she designed it, it is also vital that a system be studied with its many components interacting. The engineer may find that, in a system, every component may behave perfectly in itself yet still those components interacting may give problems.

System engineering is is an important subject and many engineers wrestle with and study the performance of complete or partial systems. It is very difficult too.

It is a fact of simple statistics that many components interacting offer many, many possible combinations of ways to go wrong or just not perform as planned. That also means that it is so very easy to imagine what it is that is giving the problem. Beware of getting fixed on such an idea: for it can just as easy for it to be wrong.

The Lad’s car was losing water, but only a certain volume – not all of it. Then there was the strange bubbling in the coolant tank. On hearing this, the garage mechanic said that’s probably a leaking header tank; that model is prone to it. OK, some things now seemed to hang together. The garage ordered a new header tank. Took a few days as it had to come from abroad. That with the labour costs as well took it to about £150. Drove off. System still leaked.

Then we looked – instead of making such an easy, unexamined assumption. It wasn’t the header tank. It was the radiator. Ooooohhh Noooo. Not one our finest moments – not the way an engineer should do it. The seemingly significant point that not all the coolant disappeared – but only a certain amount – was not significant at all. Only a little disappeared because we were checking frequently – it would all go eventually. Radiator replaced and problem solved!

Comet 1 nose
Comet 1 – The face of triumph and tragedy.


On a much more serious level but many years ago, was the Comet crash problem. When one crashed losing many lives, they held an enquiry. Wreckage was inaccessible at the bottom of the Mediterranean. The designers developed several ideas for the possible cause and designed ways to put each right. The Abell Committee concluded that:

“Although no definite reason for the accident has been established, modifications are being embodied to cover every possibility that imagination has suggested as a likely cause of the disaster. When these modifications are completed and have been satisfactorily flight tested, the Board sees no reason why passenger services should not be resumed.'”

The Air Safety Board concluded that:

It realises that no cause has yet been found that would satisfactorily account for the Elba disaster…… Nevertheless, the Board realises that everything humanly possible has been done to ensure that the desired standard of safety shall be maintained. This being so, the Board sees no justification for imposing special restrictions on Comet aircraft.

Sixteen days later another Comet crashed in the Mediterranean taking with it all the passengers and crew. There was fatigue failure between two openings in the top of the fuselage that had not been considered.

The authorities did not know what the cause of the first accident was but let the aircraft fly again with passengers. Hindsight is a marvellous thing but The Lad believes that the authorities should only allow an aircraft to fly after a crash


Is this too strong? Do the current Regulators follow this principle?



Madeleine trumped by Ruby Loftus

Trefolex and Tufnol ride again in Mechanical Engineering 101.

Famously, the hero of Marcel Proust’s multi-volume novel, “A la Recherche du Temps Perdu”, found that memories of a period many years before were triggered by a smell and taste of tea and a dipped madeleine.

Immediately the old gray house on the street ….. rose up … and the entire town, with its people and houses, gardens, church, and surroundings, taking shape and solidity, sprang into being”

Risking the bathetic after such a graceful idea and prose, The Lad found the opposite the other day: smells from his youth were triggered by this striking picture. It had been featured in a recent exhibition, Women War Artists (now closed), at the Imperial War Museum, London – website .

Reduced Ruby
IWM2850 Dame Laura Knight - 'Ruby Loftus screwing a Breech-ring'. 1943, Oil on canvas


Instantly, the workshop with its high roof appeared; it’s the Fifties. Machine tools are ranked in bays each controlled now by an apprentice in a white boiler suit. Each bay patrolled by its white coated Instructor. There was the low rumbling of the milling machines, the hum of the lathes just like the one in the picture, the hiss of the grinding machines. Equally pervasive was the smell of cutting oil and often the raw, burning smell of machined Tufnol – a composite material of linen and resin – still going It seems, [ ].

But if it is a really vile smell you are after then it is in the fitting bay in the corner of the machine shop. Here are the benches and vices where there was hammering and a continuous swish of filing. There, on the bench were the pots of Trefolex, a thread-tapping paste, green coloured and with a vile stench. You can still get it today [ ] though they may have got rid of the smell by now.

The Training School entertained both Trade Apprentices and Technical Apprentices. The Trade Apprentices were being aimed at careers as skilled machinists; whereas the Technical Apprentices were intended for the draughtsman’s career. Notice the suffix ‘man’. No women or girls then in the Fifties: they were all long gone after the Second World War. Not even an office girl. “Disturb the youths, you know”.

But, contrary to the popular vision, the Training School does not represent the milieu of the professional engineer. It is Mechanical Engineering 101 where, then, he or nowadays often she discovers the basics before moving on. The mechanicals move on to design or work on new products for the machines to make, the production guys to introduce better machine tools or ways of dealing with difficult materials.

It was here, in the Training School, that The Lad first tried cutting an external screw thread on a lathe with a single point tool. He found it difficult while the Trade Apprentices seemed always only to thrive on the challenge. Then they moved on to using the same technique to cut an internal thread. This is what Ruby Loftus in the picture is doing: under intense pressure in War time using the high skill of cutting what would be a buttress thread for a breech block of an artillery piece.

This magnificent picture though is much more interesting than just for the memories of The Lad. The picture is, most importantly, but also so powerful, painted in 1943 the middle of the War, shows only a one male, a single small figure in the distant background. The thunder on the Home Front had changed everything. The painting of this picture shows that the changes were so radical that they had to be recorded. It was a peak in women’s employment never reached again even today. However, almost 70 years later, girls and women do take their proper place as apprentices and professional engineers.

The dignity of Ruby and her concentration on the complex machine and work piece stands four-square with the haughty gaze of Henry VIII in Holbein’s painting.  The painter was Dame Laura Knight who had, pre-war, specialised in painting dancers and circus performers. . Though less important, The Lad believes this powerful painting is also the most accurate, realistic canvas of a rare subject – a snippet of Production Engineering. Unless, that is, someone else knows better.

There is the contained poise and tension of the lean of Ruby. It is that of a dancer yet with stillness. Countering the figure of Ruby and closely observed, are the dark, gleaming masses of the lathe: the tool post, the carriage and the tail stock. The tools are there too: a scraper to remove burrs, an internal calliper for measurement, parting–off tools and tool holders and a ring spanner for the bolts on the carriage.

Ruby is gazing intently at the spot lit tool. It is cutting the thread inside the breech ring during the very brief period as the tool passes from the side of the work piece nearest her out of the side within the rotating chuck. Her hands are not just supporting her but a part of her vital control of the machine. She rests the fingers of her left hand on the tool post to check and confirm the faint, continuous, humming vibration showing that all is well with the single point cutting tool. Her right hand performs the same check on the gearbox driving the feed screw and confirms that the carriage is moving. That same right hand was also well placed to switch off the drive to carriage and work piece in an instant when it left the inside surface of the work piece before reversing it back out in the same helical groove of the buttress thread but with a slightly deeper cut.

Now, in the Twenty First Century, a CNC Turning centre does it all more quickly, just as accurately but with less involvement of a human operator – male or female.


Engineering is one of the three drivers in the advancement of the human race. This blog aims to give to career seekers and also to the general public a taste of how this might be so. They are not well served by the current media. It is an engineer posting: not a ‘scientist’. It describes real professional engineering as it is in the real world usually in the present and occasionally as it was in the recent past.

The One Hoss Shay

To add to the joys of the New Year, it seems a good time to repeat one of The Lad’s favourite texts. It’s called ‘The Deacon’s Masterpiece’. Why is a favourite of The Lad as an engineer? Firstly, although it is not one of Shakespeares’s Sonnets, the subject is quite fundamental. Optimum design is the target of most engineering design. Not many components, apart from fuses and quill shafts, are not designed with the Deacon’s principle in mind. Secondly, it’s funny.

Oliver Wendell Holmes wrote it and, even now, those subtle guys – the accountants – use the sub-title of ‘The One Hoss Shay’ as a technical term for one model of depreciation.

Anyway, enough of all that, here it is.

HAVE you heard of the wonderful one-hoss-shay,
That was built in such a logical way
It ran a hundred years to a day,
And then, of a sudden, it–ah, but stay
I’ll tell you what happened without delay,
Scaring the parson into fits,
Frightening people out of their wits,– Have you ever heard of that, I say?
Seventeen hundred and fifty-five, Georgius Secundus was then alive,— Snuffy old drone from the German hive,
That was the year when Lisbon-town Saw the earth open and gulp her down, And Braddock’s army was done so brown,
Left without a scalp to its crown.
It was on the terrible earthquake-day
That the Deacon finished the one-hoss-shay.

Now in building of chaises, I tell you what,
There is always somewhere a weakest spot,
— In hub, tire, felloe, in spring or thill,
In panel, or crossbar, or floor, or sill,
In screw, bolt, thoroughbrace,–lurking still,
Find it somewhere you must and will,
— Above or below, or within or without,
— And that’s the reason, beyond a doubt,
A chaise breaks down, but doesn’t wear out.

But the Deacon swore (as Deacons do,
With an “I dew vum,” or an “I tell yeou,”
He would build one shay to beat the taown ‘n’ the keounty ‘n’ all the kentry raoun’;
It should be so built that it couldn’ break daown!
–“Fur,” said the Deacon, “t‘s mighty plain
That the weakest place mus’ stan’ the strain;
‘n’ the way t’ fix it, uz I maintain,
Is only jest I make that place uz strong uz the rest.

So the Deacon inquired of the village folk
Where he could find the strongest oak,
That couldn’t be split nor bent nor broke,–
That was for spokes and floor and sills;
He sent for lancewood to make the thills;
The crossbars were ash, from the straightest trees,
The panels of whitewood, that cuts like cheese,
But lasts like iron for things like these;
The hubs of logs from the “Settler’s ellum,”
Last of its timber,–they couldn’t sell ‘em,
Never an axe had seen their chips,
And the wedges flew from between their lips
Their blunt ends frizzled like celery-tips;
Step and prop-iron, bolt and screw,
Spring, tire, axle, and linchpin too,
Steel of the finest, bright and blue;
Thoroughbrace bison-skin, thick and wide;
Boot, top, dasher, from tough old hide
Found in the pit when the tanner died.

That was the way he “put her through.”
“There!” said the Deacon, “naow she’ll dew.”
Do! I tell you, I rather guess
She was a wonder, and nothing less!
Colts grew horses, beards turned gray,
Deacon and deaconess dropped away,
Children and grandchildren–where were they?
But there stood the stout old one-hoss-shay
As fresh as on Lisbon-earthquake-day!

EIGHTEEN HUNIDRED came and found
The Deacon’s Masterpiece strong and sound.
Eighteen hundred increased by ten;
— “Hahnsum kerridge” they called it then.
Eighteen hundred and twenty came:
— Running as usual; much the same.
Thirty and forty at last arrive,
And then come fifty, and FIFTY-FIVE.
Little of all we value here
Wakes on the morn of its hundredth year
Without both feeling and looking queer.
In fact, there’s nothing that keeps its youth
So far as I know, but a tree and truth.
(This is a moral that runs at large;
Take it.–You ‘re welcome.–No extra charge.)
FIRST OF NOVEMBER,–the Earthquake-day.
— There are traces of age in the one-hoss-shay
— A general flavor of mild decay,

But nothing local, as one may say.
There couldn’t be,–for the Deacon’s art
Had made it so like in every part
That there wasn’t a chance for one to start.
 For the wheels were just as strong as the thills,
And the floor was just as strong as the sills,
And the panels just as strong as the floor,
And the whippletree neither less nor more,
And the back—crossbar as strong as the fore,
And spring and axle and hub encore,
And yet, as a whole, it is past a doubt
In another hour it will be worn out!

First of November, ‘Fifty-five!
This morning the parson takes a drive.
Now, small boys, get out of the way!
Here comes the wonderful one-hoss-shay,
 Drawn by a rat-tailed, ewe-necked bay.
“Huddup!” said the parson. —Off went they.
The parson was working his Sunday’s text,
— Had got to fifthly, and stopped perplexed
At what the– Moses–was coming next.
All at once the horse stood still,
Close by the meet’n’-house on the hill
–First a shiver, and then a thrill,
Then something decidedly like a spill,
— And the parson was sitting upon a rock,
At half-past nine by the meet’n’-house clock,
— Just the hour of the Earthquake shock!
—-What do you think the parson found,
When he got up and stared around?
The poor old chaise in a heap or mound,
As if it had been to the mill and ground!
You see, of course, if you‘re not a dunce,
How it went to pieces all at once,
— All at once, and nothing first,
— Just as bubbles do when they burst.
End of the wonderful one-hoss-shay.

Logic is logic. That’s all I say. 

First of November, ‘Fifty-five!
This morning the parson takes a drive.
Now, small boys, get out of the way!
Here comes the wonderful one-hoss-shay,
 Drawn by a rat-tailed, ewe-necked bay.
“Huddup!” said the parson. —Off went they.
The parson was working his Sunday’s text,
— Had got to fifthly, and stopped perplexed
At what the– Moses–was coming next.
All at once the horse stood still,
Close by the meet’n’-house on the hill
–First a shiver, and then a thrill,
Then something decidedly like a spill,
— And the parson was sitting upon a rock,
At half-past nine by the meet’n’-house clock,
— Just the hour of the Earthquake shock!
—-What do you think the parson found,
When he got up and stared around?
The poor old chaise in a heap or mound,
As if it had been to the mill and ground!
You see, of course, if you‘re not a dunce,
How it went to pieces all at once,
— All at once, and nothing first,
— Just as bubbles do when they burst.
End of the wonderful one-hoss-shay.


Logic is logic. That’s all I say.


Just roll those carriage builders’ technical terms around your tongue.

A great old engineer, HL Cox, who guided The Lad at one time introduced him to it. He was a specialist in the design of optimum structures.

Happy New year

Peterson and the danger at every corner.

“Where’s Peterson?”

It was a query from someone at the Stress Office door to no one in particular. There were ten or so Stress Engineers at their desks; some were poring over their stress pads covering them with calculations using their Otis King cylindrical calculator; others were staring thoughtfully out of the windows and ignoring their Fowler’s circular calculator [both calculators are more accurate than a slide rule]. A copy of Chambers’s [a thick book of seven figure logarithms much more accurate than a slide rule] stood ignored on a shelf: not used since someone’s Detail Office days. The Stress Office product is calculation. “Try the Design Office”.

The search was not for a person but for a book.

A small number of books are so influential within a group, that the author’s name is enough to identify it exactly. So it was for several books among The Lad’s group of ’60s and 70’s design engineers.  ‘Peterson’, ‘Kempe’s’, ‘Rollason’ were some. It is probably the same for any close-knit, professional group; not just the engineers. This piece is about the first-named; the others will come later.

Peterson front cover
The front cover of Peterson © 2010.

A slim volume, it is a maroon, hard-back with its title in gold, embossed lettering on the front cover. Called “Stress Concentration Factors”, by R E Peterson [Manager of Mechanics Department, Westinghouse Research laboratories], it was published in 1953 by John Wiley and Sons, Inc and Chapman Hall. There was no title on the spine due to its unique binding. Almost every page of the book was a full-page graph that had to be scalable: each page therefore had to lie completely flat leading to the spiral wire binding protruding through the back.

Peterson spiral bound spine
Peterson spiral bound spine © 2010

One of the most important measures in the engineer’s design tools is ‘Stress’. It is true that his task is to wrestle with forces but most forces can be withstood if the component is robust enough. The usual design measure therefore is not ‘force’ but ‘force per unit of component area’. Just as the ‘price’ of an item is not always the most important number to the canny shopper; he will often compare goods or large or small packs with the ‘price per kilogram’. So it is with ‘stress’: we define the tensile strength of metals shown by experiments on plain test specimens as the stress [pounds per square inch in Peterson’s day – newtons per square metre nowadays] that they can reach in a single loading before they begin to fail.

However, and this is where Peterson came in, real components are often not of a ‘plain’ shape and the vicious menace of metal fatigue is always waiting around every corner. In the 40’s and 50’s many propellors fell off ships and in 1954 two Comet jet passenger aircraft, the pride of the UK, exploded and crashed into the sea killing all aboard.

One of the features of the Mark One Comet had been its large rectangular windows. Unfortunately no allowance, or at least not enough allowance, had been made for the increased stress at these corners. The result of repeated pressurisation led to apparently low stresses in the fuselage greatly increased by unrecognised stress concentrations leading in turn to metal fatigue. The result was the aircraft at over 30 000ft split open explosively and plunged it and its passengers into the sea.

It had been known before these and other accidents that the tensile strength measured from single loading was not the only measure of strength for metals. A component could fail at a much lower load if there were many cycles off and on. This lower stress is called the fatigue strength and is smaller, down to a lower limit, the more loading cycles that the component sees. The problem was that stresses in real components with holes or with more or less sharp corners or, worse – sharp scratches, were much higher close to the holes, corners or scratches than in the ‘plain’ sections.

The purpose of Peterson was to provide the engineers with factors to multiply the simple stresses to give the higher stresses at changes in sections in components of every sort under every type of loading such as tension, bending and twist. The book is made up almost entirely of full-page charts: there are 125 of them culled from 174 references from the world’s literature at that time. A mammoth task. They were drawn by the hand of that (long gone) craft of the tracer using a template for the lettering. The few other pages show a digest of the relevant theory.

Slotted bar chart
Chart of the possible stress concentration in a slotted bar

The sample chart shown deals with the simplest of problems. A bar in tension with a notch in it. The vertical axis on the left gives the stress multiplication factor for use with fatigue calculations. You can see that it can easily reach two and a half times the simple stress and more. The amount depends upon the radius of the slot, the bottom axis, and the parameters of the widths of the bar and the ‘throat’ under the slot.

Tragically for the Comet passengers, the fuselage had been designed before the advent of Peterson. But we can be assured that the book did prevent many more fatigue failures and probably many deaths.

The Lad has a small collection of textbooks and professional reference books in editions contemporary with those that he used including Peterson. He had never seen another hard back binding like it. The Lad’s copy of Peterson came to him damaged with masses of ancient, cellophane tape like toffee over the wire binding. Ian Pell, the bookbinder of Derby, very skillfully recovered it for him by re-backing it to bring it nearer to its original condition. Thank you, Ian.

There was, The Lad remembers, a wry comment then by the Stress Engineers about the trials of their job. Typically each engineer is wrestling to analyse the stresses in some elaborate structure as complex as a large bridge. She would say,

” I spent three months calculating the areas. Then I spent four months calculating the loads. When I had done that, they had changed the area.”

The same complaint would probably be still expressed today were it not for the fact that, as a civil engineer friend of mine said, nowadays the cost of computation is effectively free. Along with that, the modern Finite Element Methods of computation allow today’s engineer to calculate stresses on a component using its actual shape. In those days instead, they used an idealised shape and then a factor to account for the real shape.

Today, Peterson sleeps.

Making its Mark


Whole block
The whole scribing block-© 2010

Engineering was a craft before the Industrial Revolution but then much changed. Is this a “statement of the bleedin’ obvious” as Basil Fawlty would have said. No. I am not referring to the invention of the Steam Engine and the various weaving machines in themselves.

Simply put, it was the knock on effect, of interchangeable spares for those and other machines. Before the Industrial Revolution each machine was made by hand. Each of its parts was made to fit sufficiently accurately and smoothly with only the other parts of that particular machine. As more machines went into service and, with time, they needed replacement parts. It was expensive and a pain to have to go to the machine in question and make a new part to fit. Not only a pain: possibly in a working assembled machine it would be impossible to ‘find the sizes’.

Someone had the brilliant idea  to make all the parts sufficiently accurately that any part would fit into any machine. The phrase “sufficiently accurately” is the secret here. All sorts of machine tools and techniques had to be developed to do the job but The Lad describes here only one.

Much increased accuracy was needed compared to woodwork. Before, wood could accomodate some inaccuracy due to its flexibility, lesser rigidity, and its usual static not dynamic function. Before you can make something accurately to size, you have to know what that size is [See my later post on the micrometer]. Then you have to mark the material accurately to that size. One component of the marking out process was actually a pair of items. The surface table and the scribing block.

The material that you are trying to make into an accurate component must be marked acurately, much more highly accurately than had been done previously. They need very fine lines permanently marked to a very accurate position. Then you can remove material to the marks. So: you stand the component on a very flat table usually made of cast iron, sometimes granite. If it is already machined, you may coat the surface with blue dye or copper sulphate to give a blue or coppery surface to make any scribed marks easy to see. A hard point mounted on a heavy block with a flat surface can then be set to a given, accurate height. As it smoooooooothly glides, steel on cast iron, over the table’s flat surface, it scribes a fine line on the material.  Turn the component through a right angle, reset the scriber to a different height perhaps and scribe another line crossing the first. There you have it. At the intersection, is the place to drill a hole accurately in position.

The surface table is too big to form part of the current collection of The Lad. But the scribing block is not. Here it is in pictures.

lower pivot
The left thumb screw provides fine adjustment of the pillar and the right locks.© 2010

As well as the heavy base, note the heavy sections that give us a rigid structure. Under the small scribing load, this rigidity leads to almost zero deflection and thus mark-up accuracy.

Pins & vee
The pins have been pushed down through the base. The vee is for a cylinder mount.© 2010

Sometimes the engineer is scribing several parallel fine lines on a horizontal surface rather than a vertical one. If so, she can use the machined straight edge of the table using the pins pushed down as shown to run along the edge. Even a cylindrical guide to the scribing can be used if she finds it necessary by mounting the vee in the base on the cylinder.

Scriber mount
This shows the rigidity of the scriber clamp to the post. © 2010
Pins up and rainbow
A handsome protective finish on the base. The pins are up here. © 2010

The whole unit weighs 1.1kg. The makers say that this one was made around 1965. The surface finish shows a slightly raddled beauty: it must have been very striking when it was new. I asked the Wolfson Heat Treatment Centre what the effect was. Derek Close and his colleagues suggest that it was made by steam blueing, a process where a component at greater than 600 degC is immersed in dry steam.

The Centre is at which operates within the Surface Engineering Association at

It certainly has some corrosion resistant properties. I say this because all but the, ground, corroded, lower surface of the base is steam blued and is not corroded.

This is one of the artefacts that The Lad collects. The essence of it is that it and the others in his collection are all things that The Lad has used albeit briefly. A scribing block has been around for a long time and its pedigree is tightly intertwined with engineering history.

This tool was called a “surface gauge” in the Buck and Hickman 1958 catalogue at £2.60. This catalogue is a weighty gem from The Lad’s 1960’s collection of engineering books. The Lad has always called it a scribing block so he will stick to that. Moore and Wright still exist by the way in Sheffield and Hampshire but are owned by Bowers Metrology Group []. As an indication of a classic tool, the identical design of scribing block can still be bought new from Buck and Hickman that is also still thriving in 2010, .  The current price is instructive though.