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In a Model Engineer's Workshop

I had my first workshop at about the age of fourteen. It was only a draughty, unheated Victorian conservatory, five feet square with a crude bench, but it was my space to make models, electronics and generally star accumulating bits and pieces. It seemed natural enough, after all my father and grandfather both had workshops. Sadly when I went to college at 18, I had to make do with tables and odd corners for about fifteen years, but eventually I got a proper workshop again.

This section of the website will gradually accumulate some of the tools and techniques I've got to grips with over the years. Some of these may have previously appeared in print in a slightly different form.

Making Workshop Tools


The Right Temperature for Hardening Silver Steel

Making a Fly Cutter

Making a Micro Adjustable Surface Gauge

Accessories and Machines

E. Bourdon's Patent Compound Gauge

Workshop Data

BA Fixings Data

Workshop Ramblings

Dr Schlesinger's Limits

Are BA Fixings Metric or Imperial?


See all articles tagged WORKSHOP


From time to time I get telephone calls from Mrs Mandrel, whom, in the midst of a car boot sale has found something that ‘might interest me’. When I visit such events I usually discover rust-riddled-reamers and chucks so bell mouthed you could ring them! My wife often has more luck. One day, I encouraged her to search me out a cheap aneroid barometer to calibrate an electronic barometer I was testing. She rang that day with a tale of various ‘pressure meters’, I said go for it, and it turned out that she did rather well. In a series of tins and boxes she presented me with two working barometers, a third with no needle and a whole host of barometer spares, all virtually given away with that wonderful British sentiment ‘they would go to someone who wants them’. There was also the, very grubby, internal mechanism from a large pressure gauge, probably 4” in diameter. Initially I thought this may have been from a full size steam engine, but it appears to read only to about 20psi, so who knows what its original purpose was?

Along with all these rather tatty but useful bits and pieces, was another gauge that had been polished up and mounted on a stand made from a piece of hardwood. Advised by a bystander that it was rather old and a good deal, my wife parted with £5 for this object.

It took me a double-take to be sure exactly what this object was. At a glance it was a large pressure gauge, on inspection it was compound gauge, measuring vacuum and pressure. Better still, it was a genuine “Bourdon Compound Gauge” – a fine instrument and possibly one made in the days of Monsieur Bourdon himself!

Eugene Bourdon's Patent Compound Gauge

Eugene Bourdon's Patent Compound Gauge

Patented by Eugene Bourdon in 1849, the Bourdon Gauge is a familiar instrument to model engineers, most usually encountered as a means of measuring steam or air pressure. The working principle is simple – a curved pipe, usually of a thin, springy material such as hard brass or beryllium copper, unrolls slightly when pressurised (like the feathered toys found in Christmas crackers!) This movement is amplified by a rod and lever, and, with better quality gauges, passed to the indicator needle via a pinion and a sector of a gear. Bourdon gauges can also be arranged to read vacuum relative to ambient pressure, and, if the mid point of the scale is carefully chosen pressures above or below ambient may be measured.

Such a gauge, measuring both pressure and vacuum, is a ‘compound gauge’, and mine was an example of this, reading +/- 4 pounds (presumably per square inch), subdivided to tenths of an inch. 4 psi is 276 millibars or rather more than a quarter of an atmosphere.

The gauge is housed in a heavy, circular brass case, 5” in diameter and 2” deep with a flange 6 1/2” across. A bezel holds in place a glass that is clear and without blemishes, but lightly rippled, suggesting that it is original old glass predating modern float glass. At the bottom of the circular case is a large thread, on which a nutted union fits, this appears to be original and contains a rather worn fabric washer. The union joins to a bent brass tube with a soft-soldered brass piece on the end, which appears to have provided a spigot for attaching a rubber tube.

The dial of the gauge is beautifully hand-figured in red (vacuum) and black (pressure) ink. In impeccable hand lettering the dial is also marked “E. BOURDON’S PATENT COMPOUND GAUGE”. Beneath this is stamped the maker’s mark “E. BOURDON Inventor & Manufac.or PARIS”. This mark is repeated, with only minor differences as a deep stamping in the rim of the brass case. Adjacent to this mark is the stamped number 10, probably not a serial number, it may relate to a calibration record. The case itself, and its bezel, appear to have been turned from brass castings. The finish is either very fine turned or finished with emery – not a polished appearance.

Dial of Eugene Bourdon's Patent Compound Gauge

Markings on the dial

On removing the gauge from its plinth a black steel backplate is revealed (which may have been repainted), along with the serial number 3614 stamped into the case. On removing the backplate the mechanism is revealed, together with, scratched into the back of the dial, the serial number 3614 once again and the initials AM. These are both in a bold and, to my eye, continental hand. There are also the figures 11 and 2 marked in a more casual hand. The mechanism itself is pinned and screwed together. Interestingly the lever is made adjustable in length. Presumably the screw allowed calibration to be carried out before the two parts of the lever were finally pinned together. Aside from steel pivots and the copper(?) Bourdon Tube itself, all parts are brass and not highly finished.

Inside Eugene Bourdon's Patent Compound Gauge

Inside the case

An interesting touch that can just be seen in the photograph is that the end teeth of the gear sector that turns the pinion have been bent inwards. A crude but effective way of ensuring that over or under pressure can’t run the pinion off the end of the sector.

My limited lungpower can drop the vacuum to the full 4”, but only raise 1” of pressure. The reading appears accurate compared to a good quality but less sensitive modern gauge. It only reads correctly when the gauge is vertical; lying it down introduces an error of 1/2” of pressure, which illustrates how sensitive an instrument it is. Whilst ideal for demonstrating that a model engine can run on the merest whiff of air pressure, the gauge is far too sensitive for use on a working steam engine. It seems to me that this is a laboratory instrument of some class. Perhaps it was rescued from a school or university laboratory during the days when any useful, educational or interesting equipment was liable to end up in a skip, and mounted on its wooden plinth by appreciative hands?

I was surprised by how difficult it is to find out more about Monsieur E. Bourdon and his invention, despite the fact that there are now millions in use across the globe. Eventually, I was directed to a 1887 Negretti and Zambra catalogue which showed the same maker's stamp and embossing on the gauge.'Purchasers are desired to examine and compare M. Bourdon's Gauges. They will find the works to be constructed and finished like a watch, whilst the majority of imitations are put together ROUGH FROM THE CASTINGS, consequently liable to adhere and give erroneous indications'.

Bourdon's Double Gauge

Bourdon's Patent Double Gauge in an 1887 catalogue

A second source gave images showing the mark in an 1865 issue of The English Mechanic. It seems that the gauge would have come out of M. Bourdon's workshop some time between 1850 and 1890, if you can help with dating from the serial number, please get in touch.

The English Mechanic, March 31 1865

The English Mechanic, March 31 1865

The Maker's Mark in the text

The Maker's Mark in the text...

... and on the case

... and on the case

I contacted the Science Museum, whose expert stated he can's remember seeing one in such good condition. They already hold another gauge presented by M. Bourdon himself in their Motive Power collection (inv. 1877-450).

So, it seems this gauge is rather special but best of all at over 150 years old, it still has a job of work to do from time to time. I hope this article makes it easier for anyone else with a  similar gauge to find out its history.

Georg Schlesinger was born in Berlin in 1874. After a short but successful career in induistry in 1904 he became professor of Machine Tools at the Technische Hochschule Charlottenburg. During the First World War he was the head of the gun plants at the Spandau works, but also worked on arm and leg prostheses for injured soldiers.

Georg SclesingerGeorg Schlesinger 1874 - 1949

As a Jew, he faced difficult times with the rise of the Nazis and he was charged with embezzlement, industrial espionage and treason. Following a brief time at the ETH Zurich, he gained a professorship at the Free University in Brussels. Schlesinger headed a laboratory at Loughborough from 1939 to 1944, and died at the age of 75 in Wembley.

He was a highly inventive and successful engineer, but his greatest contribution was his work on testing machine tools. The 'limits' he devised are still used to set the standards to which production and toolroom machinery are made today, although some modern machines work to standards he could only of dreamt of. The clever thing about his standards is that they are designed so that any errors, wear or flex in the tool will either be in the direction where least problems are caused (e.g facing slightly concave, not convex) or to reduce any residual errors before they make it worse. A common misunderstanding is that these errors should be 'built in'. This is not so, they are 'tolerance bands'.

Testing Machine Tools

Be aware that to properly test machine tools you need measuring equipment of greater accuracy and a lot of skill and care. Lathe owners can become despondent, reject their machine or try all sorts of unusual 'fixes' if their machine fails the tests. Often the reason for failure is not the quality of the machine, but a loose spindle, a misaligned bed, dirt or swarf on mating faces or flexibility in the mounting of a dial indicator.

Equally I have seen a quality milling machine described as 'built to Dr Schlesinger's limits' - this is misleading, as there are different sets of figures depending on the use of the machine: whether for the toolroom or the factory floor, for example. And a temperature controlled toolroom, as clean as an operating theatre, is far from the typical hobby workshop!

using a dial indicator to align a tailstock

Testing alignment of a tailstock barrel

In truth, there is only one valid test of a machine tool - will it allow you to produce work to the standard you need? So turn or mill some test pieces, and if they aren't up to scratch put some effort and thought into carefully adjusting the machine , rather than assuming it is faulty. Any fool can make bad work with the best machine, but a skilled machinist can turn out excellent work on a poor machine.

With that caveat, here is a copy of Dr Schlesinger's testing machine tools, should you wish to spend your days chasing illusory tenths of a thou...

Testing Machine Tools by Dr Georg Shlesinger

Testing Machine Tools by Dr Georg Shlesinger

Micro-Adjustable Surface Gauge

Micro-Adjustable Surface Gauge

A surface gauge is an essential aid to accurate marking out. What is needed is a stable, flat bottomed base with a pillar and clamp to hold a scriber, allowing lines to be made truly parallel with the top of a surface plate. Such a device has many other uses, such as helping set work in a four-jaw chuck or any other task where the ability to ‘mark’ a point in space with the end of a scriber is needed.

A basic ‘scriber on a post’ works well, but sometimes a more robust device is needed, or perhaps the ability to make a fine adjustment to the height of the scriber is useful. A design that has been made (with many detail variations) by hundreds of apprentices over the years is the answer, and it relies on the fact that the pillar does not need to be vertical. As long as the base of the body is flat, and there is no play anywhere, the whole thing is inherently accurate (at least in Euclidean space – and if your workshop is on the edge of a black hole you may have other worries!) The design in the drawings uses a tilting bar in a heavy base. No dimensions are given, make the parts to suit what offcuts you have.

Plan for the Surface Gauge

Plan for the Surface Gauge


The pillar is mounted in a tilting bar held between two ball bearings. Silver steel of 1/4” (or 6mm) diameter is fine for the pillar, do not harden it, as this may cause distortion. Ground mild steel would do just as well for the pillar. I tidied up a piece of steel bar by skimming the ends in the mill and draw filing. The hole for the pillar is best made in the lathe, as is the cross hole for the pivot, but the threaded hole for the adjuster is not critical. Thread both hole and pillar to match: 1/4” BSW, 0BA or M6 will do fine. Carefully open out the upper part of the hole, so that some of the full diameter of the pillar enters the hole, holding it in line. You can also use a force fit, a retaining adhesive or even a set screw to fix the pillar in place.

Both ends of the pivot hole need to be countersunk, using a centre drill of the same diameter as the ball bearings for the pivots, make a countersunk hole to the full diameter of the drill, but don’t drill past the tapered section of the drill. Follow up with a drill the same size as the tip of the centre drill. Now turn over the bar and carefully align the hole with the drill before clamping the bar in place. Now swap back to the centre drill and create another countersink.

Setting a Surface Guage

Setting the height of the gauge against a rule

The base is a section of mild steel bar, skimmed on all sides (on lathe or mill) to get it square. The V-groove in the base can be milled with a suitable cutter, holding the base in a pair of v-blocks as in the photograph. This allows the base to sit on a curved surface (or the inverted-v bed of my lathe), but if all you want is to ensure the block sits fairly on a flat surface, then a simple recess is sufficient.

The groove on top of the base should be milled out to be a sliding fit for the tilting bar, but drill the cross-hole for the pivots first, as it is essential that these are in line. These pivot holes need to be a close fit of the ball bearing pivots. For the record, I used two 1/8” ball bearings and tapped the holes M5 for some flat-tipped grub-screws. It should now be possible to mount the tilting bar between two ball bearings held in the tapped holes using a pair of setscrews. The bar should move freely, even when the bearings are held quite firmly.

Locate the correct position for a stepped hole to take the return spring under the pillar. The return spring should be reasonably strong, but it does not need to be excessively stiff – in use the action of scribing pushes back against the adjusting screw and assists the spring. Make a suitably decorative pair of knurled nuts, one for the clamp, and one to secure to the end of the adjusting screw for the tilting bar. Take a little time to set the bearings just right, and you will be rewarded with a silky-smooth action to equal any bought piece of equipment.

Adjusting screw with knurled nut

There's knothing kwite like a krisply knurled knut :-)

The simplest and best clamp for such a device is shown in the drawings. A central shaft is threaded at one end and has a hole for the scriber at the other. This is threaded through the body of the clamp, a sleeve fitted around the scriber end and the scriber inserted through both sleeve and shaft. Provided the clamp body is good fit on the pillar, then a simple knurled nut on the thread will pull the parts together with ample force to secure everything in place. Brass is a good material for the clamp body (which is best finished by draw filing), while the other parts of the clamp can be mild steel.

Clamp for Surface Gauge

Details of the Clamp

The scriber should be made from silver steel (3/32” or 2.5mm is good size), cut it to length and carefully turn a taper on each end, finishing with a tiny 60° point. If you use files to do this, take great care, making sure they have secure handles and that you wear safety goggles – if the file contacts the chuck jaws it can be thrown into your face. Bend the scriber about 30° near one end – this will help when setting it low down. Hardening such a long, thin piece can be difficult. It may be easier to heat it to a bright red along its length using a gas cooker or a camping stove, rather than a blowtorch, picking it up at the centre with a pair of pliers and plunging it rapidly into a bowl of water. An alternative is to heat and quench each end separately, but this will leave the centre of the scriber very soft, and it may bend in use.

Similarly there are two approaches to annealing the scriber. Heating the centre until the ends turn a dark straw colour will leave the ends hard and the centre less so, but again my fear is that the centre will be easily bent. I cook my scribers at gas mark 5-6 which gives a good colour and even temper along the whole length. Remember though, gas ovens are not precision instruments! I know my oven runs hot, so I suggest you put the work in at a low number, and then ‘tweak’ it up every ten or fifteen minutes, until you get the right ‘bronzy’ dark straw colour.


This is a repository for all sorts of workshop data and background information.