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Seiran
31st Mar 2015, 09:26
I've recently been in awe at WW2 technology and the massive piston aircraft engines, it got me thinking how were things like centrifugal compressor wheels for turbos/superchargers made back then. All compressor wheels today seemed to be machined by CNC.

How did they machine such complex surfaces back then? I can't figure it out and it is annoying me.

Many thanks

joy ride
31st Mar 2015, 12:02
Skill and measuring in huge quantities!

First the scientists and technologists would decide exactly what was needed, and what shape and material the part should be, then huge ranks of highly skilled draughtspeople would prepare technical drawings and blueprints, then the engineers, foundry men and machinists would make parts as per the drawings, each one with huge quantities of skill and measurement!

You should never under-estimate human skills, but conversely you should never loose admiration for the incredible work done by previous generations, without modern facilities. In my opinion there is no better example of this than the Antikythera Mechanism.

Wander00
31st Mar 2015, 13:12
I am a non-engineer, but for a year, and what a year, 1969-70 I worked I production control at what was then called Marshall of Cambridge Engineering Ltd. Machine shop work went from thousands of small parts produced on capstan lathes, many of WW2 vintage, to nc work on the droop nose and visor for Concorde, other than the first aircraft. A small team at one end of the production office wrote the programmes for the punched paper tape, and designed the machine tool heads that were, ISTR, made by a firm called Marwins. One hole in the wrong place and 12.000 (at 1969 prices!) worth of alloy down the pan. We also did work on attenuation panels for the first generation RB211 (the one they tried to make with a carbon fan). It was all exciting stuff, and I sometimes wonder where I would have finished up if I had stayed there. Years later, as a sqn ldr, I was an usher at the BoB Memorial Service in Westminster Abbey, and as I was showing the then Sir Arthur Marshall to his seat in the Air Force Board seats, he looked at me and with a grin said "Didn't you used to work for me?". Good memory, as I was well down the food chain at MCE.

ian16th
31st Mar 2015, 14:44
Wander00

A little after your time at Marshals, around 1971-74, I was a maintenance engineer for a large American computer maker. One of my customers was Dowty-Rotol at Gloucester.

They used the computer to write the programs that were punched out into the paper tape, that were used on Milwaukeematic Machines. At this time they were producing components for Harrier undercarts.

To avoid the extremely large expense of the tools moving in the wrong direction, the cost of the damaged tool was likely to be more than the material, they always tested a new program on a block of wood!

Having learn't just enough about NC and in an earlier incarnation having worked on Beverley's, and being very aware of the reliability problems of the Bristol Centarus, this all made me wonder if today we could build the last of the big piston engines to a level that would give them the reliability of my Japanese motor car?

In particular I would love someone to build a Napier Sabre with modern CNC machine tools.

Kitbag
31st Mar 2015, 17:44
Antikythera Mechanism (http://en.wikipedia.org/wiki/Antikythera_mechanism)

There is always something to learn on PPRuNe

joy ride
31st Mar 2015, 18:24
Have a look, it is astonishing:


http://youtu.be/nZXjUqLMgxM

joy ride
31st Mar 2015, 18:25
Have a look, it is astonishing:



http://www.youtube.com/watch?v=nZXjUqLMgxM

Flybiker7000
31st Mar 2015, 19:19
At first the supercharger on the Merlin isn't such miniscule as the turbineparts in modern automotive turbochargers: http://upload.wikimedia.org/wikipedia/commons/6/6d/Bristol_Centaurus_centrifugal_supercharger.jpg
The parts for the Merlin supercharger aren't machined but casted. However, before casting a model has to be made, but this is done in simple materials wich is able to form easily. Additional, pattern makers are on the limit of artists/sculpturers and the modern CNC-milling of such parts have almost choked this fine craft to death.
Personally I find it very impressive when looking at the transmissionbars between the piston and wheels on steamlocomotives: All is handcrafted, but the fitting is perfect though rather complicated :-o
http://youtu.be/6HzRrmn_ZUc (at 0:45!)

ZeBedie
31st Mar 2015, 20:33
casting-modellers

Called "Pattern Makers" in the UK.

Seiran
31st Mar 2015, 21:27
I am in such awe of the people that could make a mold for something so complex as the supercharger, how would people even draw the blueprints for objects like that since they are so kind of 3 directional, it must've taken forever to sculpt the mold.

Fishtailed
31st Mar 2015, 22:42
Wander00, I worked on Marwin Maxitrace machines at Strand Road in 1970, they were 1/4 in. magnetic tape controled then. I was at Marshalls in 2008 and noticed they were still using the identical machines. Strand Road dissappeared years ago:{

Wander00
1st Apr 2015, 06:08
Marshalls always seemed to make machinery last - extracted every ounce of value out of a piece of kit

Flybiker7000
1st Apr 2015, 09:17
Thanks for correcting!
I have edited the text to 'pattern maker' now and You might wish to delete Your answer for not to look foolish ;-)

Flybiker7000
1st Apr 2015, 09:25
I've worked where the welding-robot had been modernized with a modern harddrive instead of the antique 1,44Mb floppy-disc. However, the processor couldn't handle all that information and the harddrive had to be sectioned in 1,44Mb parts :-o
Its not only in the homeoffice-corner the electronics evolve too fast and some day short in the future will CNC-milling be just as difficult to comprehend as the mold/cast, because 3D-printing have taken over!

joy ride
1st Apr 2015, 10:21
Pattern Makers often used Jelutong, a dimensionally stable soft wood with very close grain which allows it to be worked to a reasonable degree of accuracy.

Seiran
1st Apr 2015, 11:57
Does anyone know how they manage to measure and make things like the compressor wheel. It is a part with so many curves it seems impossible to not only draw but impossible for someone to get such complex curves correct or so many blades. I would be forever grateful if anyone could explain how engineers and machinists/pattern makers etc dealed with such complex surfaces?

Flybiker7000
1st Apr 2015, 22:26
Does anyone know how they manage to measure and make things like the compressor wheel. It is a part with so many curves it seems impossible to not only draw but impossible for someone to get such complex curves correct or so many blades.

Such mechanics was mostly created during experiments and not calculated to seventh decimal like expected by digital design today.
It could be quite interresting to have the effiency valuated against a modern CAD design with same measurements!

To me, the model might have been made by two dishes with curved ribs mounted evenly around but mirrored and thereafter layed together.
The major issue will be the balance but that should be the easiest part to weight out - On the model as well as after the cast!

reynoldsno1
1st Apr 2015, 22:47
so many curves it seems impossible to not only draw

The beauty of the french curve maybe - but how did they draw a french curve without a french curve?

http://www.isomars.com/productpic/d5ff9cad-bb4b-4134-8a71-701a5a42e558.jpg

India Four Two
2nd Apr 2015, 02:50
The beauty of the french curve maybe - but how did they draw a french curve without a french curve?

In a similar vein, how do you make a more accurate machine tool using one that is less accurate?

Seiran
2nd Apr 2015, 05:40
How were the propellers machine back then? Metal ones obviously

Evening Star
2nd Apr 2015, 10:33
In a similar vein, how do you make a more accurate machine tool using one that is less accurate?

As a young lad doing my O-level Metalwork, that question did my head in ... how on earth did Henry Maudsley perfect his first precision screw cutting lathe from less accurate tools?

Ian Titcombe
2nd Apr 2015, 16:32
I was a Vickers-Armstrong apprentice at South Marston, Swindon starting in 1961 and remember being told that many of the machine tools were WW2 era, (when it was originally Short Brothers). Some of the great British manufacturers of that time were on show, Ward, Dean Smith and Grace and Colchester lathes, Alfred Herbert and Cincinnatti milling machines plus the many other odd machines that made up the two large machine shops turning out at that time, very high quality aeroplane parts. Saw the early implementation of CNC with MilwaukeeMatic and Marwin machines and it was obvious to me that it was going to be that way for the future of production engineering, but never envisaged for one moment the effect computers were going to have in later years.

henry_crun
2nd Apr 2015, 18:38
Ah, yes, machining back then was an art.

One company made the housing for an equipment out of pieces of balsa wood glued together to establish the best shape before using that to generate a precision casting.

Many parts were cast by the lost wax method and then finished by hand.

I can recall making a replacement part for our home ice-cream scoop (quite complex with gear teeth on it) by filing it by hand.

One of our first apprentice exercises was to file a rod of arne (very hard steel which work-hardened much harder) by hand to an exact inch in length and exactly square. Amazing just how accurate that had to be so that a set-square placed on end and side did not show the faintest glimmer of light. I was first out of our course to finish, exactly right, dead square, absolutely perfect, but, as Mr Derek Wilkinson was quick to point out, exactly 1.100 inches long. I spent hours getting that down to the required 1.000 inch.

There were a lot of cunning techniques for making better and better machines. In fact, one of our tasks was to make on a really grotty old lathe jobs which the better lathes could knock out in seconds. That sorted out the men from the boys!

One of the most spectacular jobs I had to do was to machine a great ingot down on an Ormerod shaper. This had a to-and-fro action, and, if you came in carefully from the side you could work up to enormous cuts taking great swathes of metal off, which when dosed liberally with cutting oil, left great smoke trails across the model shop. The next lad took over from me and enquired what cuts to take. I told him I'd been doing half an inch by fifty thou. Didn't expect him to start right in the middle of the workpiece. The shaper drove the tool clean into the piece, the Ormerod stopped dead with a broken carriage, and took the workshop floor up like a good-strength quake. :E

Flybiker7000
2nd Apr 2015, 20:08
Continuing the thought long back one ends up with the tool version of the hen and or Egg paradox: The hammer or the anvil! ;-)

Jetex_Jim
2nd Apr 2015, 20:42
What a great thread. And great stories of handworking to great precision.

However, I hope it's not too churlish of me to bring up Sir Stanley Hooker's story about how the RR Merlin drawings had to be totally redrawn to Packard's standards - as used in motor car production.

Could it be that British manufacturers, with far cheaper labour costs, were actually too dependent on the hand skills of its labour force. Later, when labour costs rose, the companies concerned were slow to change the manufacturing culture and so many jobs went away.

Flybiker7000
2nd Apr 2015, 21:15
The world was bigger at that time and actual it's as late as after CAD/CAM and the web that drawing spec's have been finally internationalised. The Packard/Merlin story shows only that Packards management managed to favour their methods of drawing and producing - Maybee it was reasoned well (by american state of art series production) or maybee it was a matter of hard negosiations wich Packard might have been better suited for or maybee it was a pure political decision :-/

I recall from 'Guy Martins Spitfire' that the production of the plane first came into beat after being supervised by the former Morris Boss, this even though the production of the plane was crucial for the RAF and with that, the country!
Planning and leadership is nonplusultra and lack of that might have been the fate of the later british industry :-/

ian16th
2nd Apr 2015, 21:44
Isn't it true that before the Packard Merlin, there was a Ford plant in the UK that had to build Merlin's to tighter tolerances than RR used?

This was because Ford used unskilled and semiskilled staff where RR used 'skilled' fitters who made things 'fit'.

barit1
2nd Apr 2015, 22:50
I once bought a 12" steel rule at a surplus goods store.

Very handy for some layout work I was doing - until some of my projects didn't fit together just right.

Then I happened to read the fine print on the scale - it was a patternmaker's scale, 1/4" to the foot LONGER than standard, to compensate for metal shinkage as the casting cooled! :ugh: :{

onetrack
3rd Apr 2015, 00:01
I can recall where the Americans had to convert Merlin blueprints to decimal fractions.
Despite both countries using the inch measurement system, the Americans had long ago changed to decimal fractions for ease and simplicity in engineering work - whereas the British still stuck with fractions.
So a Merlin blueprint would state [1 1/16"] whereas the Americans needed it to state 1.0625".

As far as the machining of early turbines for superchargers went, many lathes and hobbing and milling machines were set up with additional jig-holding drives, that moved the component as it was machined, thus enabling complex curves to be machined.

Gear cutters and hobbers are the classic machines for producing involute curves on gear teeth. Once you see a machinist who has gear-cutting skills in action, you'll see real skills with machine tools and calculations.

Wikipedia - Gear Hobbing (http://en.wikipedia.org/wiki/Hobbing)

FlightlessParrot
3rd Apr 2015, 08:40
In a similar vein, how do you make a more accurate machine tool using one that is less accurate?

I think the principle with screw cutting is that you start with a lead screw that is fairly good, and use a compliant follower (say, cork) that averages over several turns to cut another lead screw that is better than the first.

Somewhere I have seen what must be a simple explanation of this, but I can't find it on Wikipedia under Maudslay or Whitworth.

seafury45
3rd Apr 2015, 09:42
centrifugal compressor wheels.
I am not familiar with machining these items but they were probably done as Onetrack suggested on specialist lathes and other machines. I am sure I have seen photos of early ones where individual blades were set in slots in the backing disc.

More accurate machines
One way to make more accurate machine tools is to hand-make more accurate components and fit them together into a new, more accurate, machine. Then you are able to use that prototype to build your better production machines.

Templates were often used to check progress. So you machine a bit, check against the template, then machine a bit more. Accurate and relatively fast.

Seiran, have a look at the work of Model Engineer enthusiasts. A lot of traditional skills are maintained by these hobbyists and many of them are very proficient.

mostlylurking
3rd Apr 2015, 13:25
I ran a medium sized machining co. From the late 80's until the early 2000's. We had around 60 machine tools.
The co. had been in operation from the late 40's and had examples of the history machining from 1920's capstan lathes through to the (then) latest CNC's
Man has always been ingenious.
I would think that from pre WW2 to the advent of NC (C omputer came later) the technique for machining intricate shapes would would have been hydraulic copy - a stylus followed a 2D model of the desired shape and controlled a hydraulic ram (no lead screw). It was accurate enough to produce tolerances suitable for bearing fit. We still occasionally used hydraulic copy up to the time I left. It was 100% reliable, unlike CNC.

Flybiker7000
4th Apr 2015, 20:42
The precision of modern tools isn't due to the digital programming as much as the invention of extreme fine electronic measuring devices to control the tooling!

PAXfips
4th Apr 2015, 21:17
The precision of modern tools isn't due to the digital programming as much as the invention of extreme fine electronic measuring devices to control the tooling!
Indeed, plus the ability to do time-based critical movements - which have been back then just not possible on pure mechanical ones (say, pre NC).

When I did my degree in mechanical engineering (mainly about surfaces) we've been put into a non-(C)NC workshop for three months to learn that three axis exist even outside computers :p (early 90s). Students later on
didnt had that and they really had struggles about it - full 3D visualization
wasnt available yet.

CNC saves time, not clue. :cool:

onetrack
5th Apr 2015, 01:34
The greatest advances in modern machining are to do with cutting-tool technology.
In the old days, carbide tools were cutting edge, but they were limited in speed and loading - and many materials had to be ground because they couldn't be machined.

Todays tooling technology is absolutely amazing, with astonishing speed and loading abilities, that enable fast machining of items that formerly had to be ground.

alisoncc
5th Apr 2015, 02:59
I would be forever grateful if anyone could explain how engineers and machinists/pattern makers etc dealed with such complex surfaces? NURBS - Non-Uniform Rational 'B' Splines. Or that was what was used in the 1980's when I was an area manager for Computervision.

Peter-RB
5th Apr 2015, 14:24
What a cracking Thread,

to add just a little, Ford Motor Company manufactured Merlins at Trafford park during ww2, and it seems they were easily serviced when compared with RR built Merlins.

ian16th
5th Apr 2015, 15:39
See my earlier post #27.

it seems they were easily serviced when compared with RR built Merlins. This was because the parts were interchangeable. The RR ones were individually 'fitted' by craftsmen.

Compass Call
5th Apr 2015, 21:43
henry crun
Your post reminds me of two of my apprenticeship jobs.

We were given a fist sized lump of steel from the foundry and had to carve it to a one inch cube, scraped flat - no machines allowed, only hand tools.

When we had completed that to the training managers satisfaction, we were given an even bigger fist sized lump of steel from the foundry. This time we had to carve it to a one inch thick by four inch square plate, scraped flat, again no machines. Then we had to cut a one inch square hole dead centre of the plate - we were allowed to drill a half inch hole in the plate, using a hand drill. Then the previously made cube had to fit the square hole we had cut in the plate - every whichway!

We learned how to use hand tools and measuring the hard way.:)

joy ride
6th Apr 2015, 07:05
Compass Call, great post, it reminded me of the old test which was given to itinerant shipbuilders wanting to work in the Tudor royal docks. The test for an adze (a curved horizontal axe held in both hands brought down from over the head to ground level) was to cut and point a small stick about the size of a match in as few blows as possible, then drive it unbroken into a plank with one highly accurate blow with the back of the adze.

Skills accumulate and mature over centuries, but get wiped out over a generation by bean counters and politicians!

Fitter2
6th Apr 2015, 08:57
Hi Compass Call

At the Railway Museum in Darlington there used to be (don't know if still there) similar early 20th C apprentice test pieces made with hacksaw, cold chisel, file and scraper only, but the square was instead a 5 pointed star.

Having just about mastered the 1" cube, I applaud their skills.

ian16th
6th Apr 2015, 10:54
Compass Call

Over the years variations of this have been and are still used.

It was used by the RAF for Boy Entrant and Apprentice trainees in the aircraft trades.

I was pleasantly surprised that when my son was training at Witwatersrand Technicon for his National Certificate in Jewellery Manufacture and Design, they used a very similar, albeit much smaller, exercise.

The common thread is that you are never have the 2 pieces in your hand until you try to fit them together.

Peter-RB
6th Apr 2015, 11:30
At my old School in the late fifties, our metal/wood work masters asked us to plan, draw and make a working model of a single cylinder engine from wood, with gear driven cams to work side valves, in the cylinder head box, they promised if we could do it within term time , the next term we could make one from steel, we all had a portion that needed to be done with correct measurements so they mated with the other sections and we overcame alsorts of problems, not only on the size and bore but in working out how to make the gears with angled teeth so as to operate smoothly, it really tested us, but we just managed to finish it in our Autumn term, we all received a headmaster award for that wooden engine, but when asked not many of us wanted to then go on to a steel replica, then we found out the lathes were not big enough to do what was needed and so sadly the project in steel was abandoned, our Wood and metal instructors were ex military and ex RR , so they then gave us lumps of mild steel and a measurement to make a cube to fit into a hole we had to make in a flat piece of 1/4"thick steel plate, now at 14/15yrs old that really tested us.. but that taught us much about files and filing.. I don't think they do that now..!

Flybiker7000
6th Apr 2015, 20:41
Skills accumulate and mature over centuries, but get wiped out over a generation by bean counters and politicians!

. . . And often plain knowledge too :-(
Some swedish indland lake-fishers got a very old wooden box with rusty items in their net - and the archeologists went crazy: It showed to be a craftsmans toolbox from the first millenium-change.
In Scandinavia is amateur experimental archeology a widespread branch and when copying the hammers in the box they found that they had so much punch and precision :-o
My brother in law is carpenter and lets no chance of testing hammers he come past get spilled, hence he knows that it's not obvious that modern hammers do have punch and precision as his hands experience it and the one wich do never must be unattended!
The viking carpenter had probably had his words with the blacksmith about the tools, where my inlaw must chose between houndreds of seria-produced 'items'. Development?

reynoldsno1
8th Apr 2015, 00:41
exactly right, dead square, absolutely perfect, but, as Mr Derek Wilkinson was quick to point out, exactly 1.100 inches long. I spent hours getting that down to the required 1.000 inch.
an able demonstration of the difference between accuracy and precision ....

barit1
8th Apr 2015, 02:01
Plus propeller operation details: https://www.youtube.com/watch?v=65sdTFdj5VU

Art Smass
8th Apr 2015, 02:31
I did my apprenticeship with BAe in the 70's - as a rule the best became toolmakers (pattern makers).

Plenty of guys there then who were 25 year + veterans of their trade - great skill sets, great days

AS

Windy Militant
8th Apr 2015, 22:10
Nowadays a Bastard File is one that disappears into cyberspace just as you've almost finished working on it.

As an apprentice I helped on a couple of projects which involved making spiral and involute flutes on shafts these were done by linking either a rotary table or dividing head to a milling machines feed mechanism so that they rotated as the bed of the mill moved along. Working out the ratio between them to get the correct end result was a complete nightmare but we eventually got the required locus, I think that was the posh word for it. I also learned to chisel keyways in machine shafts. ;)

Seiran
9th Apr 2015, 13:35
Yeah machining the propeller seems too easy these days but how did they do it before CNC/NC?

teeteringhead
9th Apr 2015, 14:33
The Halton Apprentice test piece was so famous, that a model of it became their "memorial".

Sounds similar to some mentioned (I wasn't a Halton Brat, so don't know exactly what had to be done (I'm sure someone will tell us!!))

http://rafhalton77thentry.org.uk/wp-content/uploads/2013/05/sculpture1-150x150.jpg

ian16th
9th Apr 2015, 15:07
During the later part of my Boy Entrant training, some of us fairies had to share a billet with some oily Riggers!

These guys all seemed to have kept their test piece and from memory they had been given a piece if 1 inch thick mild steel plate. They had to make it a 3" square, then put a 1" square hole in the centre.

A separate piece of mild steel had to be made into a 1" Cube.

The 1" cube had to fit into the 1" hole, any possible way, with not more than a 1 thou gap.

We fairies, played around with 'dominoes' of brass, ebonite and paxoline. All soft materials where it was dead easy to take too much off with one swipe of a file.

Flybiker7000
9th Apr 2015, 16:14
@Seiran: Yeah machining the propeller seems too easy these days but how did they do it before CNC/NC?


Actual pretty simple and just like this wooden shoes: http://youtu.be/gUDHPiJXkyU - 1:1 after a model/pattern!

Seiran
9th Apr 2015, 20:49
Amazing, thank you so much!

onetrack
9th Apr 2015, 23:43
But how did they make the 1st propellor perfectly, to enable it to be used as the master on the copy lathe? :suspect: :confused:

Art Smass
10th Apr 2015, 02:26
But how did they make the 1st propellor perfectly, to enable it to be used as the master on the copy lathe? :suspect: :confused:

Skill ....same as those 17th century carpenters who produced immaculate woodwork which would be difficult to replicate today, or the stonemasons carving perfect statues hundreds of years ago.

Some people are just incredibly F#$king talented

Wander00
10th Apr 2015, 09:09
AS- I'll go with that, but IMHO in UK we are very poor at recognising that talent, and more importantly, ranking it with academic achievement

joy ride
10th Apr 2015, 10:20
The British attitude towards engineering and manual skills was amusingly highlighted to me a few years back at an orchestra concert in London, using instruments I had designed and made. After the concert the conductress brought the borough's Mayor and his wife to meet me and see the new instruments.

They were polite and enthusiastic and said it was "absolutely mwarvellous, mwarvellous " to see a new design in use. After a few question the Mayor's wife asked who made them, I replied that I did. Then she said "Yes, but who actually does all the metalwork for you?" "I do". Her expression changed to disgust, she said "Oh, so it's manual labour" and they turned promptly and left. Clearly I had instantly been demoted from "Designer" to "Grease Monkey" in her and her husband's opinion. I could hardly control my laughter.

teeteringhead
10th Apr 2015, 10:26
Thanks ian16th

I just knew someone here would come up with the answer! :ok:

Flybiker7000
10th Apr 2015, 17:08
@ onetrack: But how did they make the 1st propellor perfectly, to enable it to be used as the master on the copy lathe?


A propellarblade is an array of wing-sections, differing in thickness and incline as the speed through air is higher at the end than close to the hub.
The cross section is rather easy to calculate for each 1/4" and likely ist possible to cut out the same sections in 1/4" wooden plate and stack them all together, whereafter the 'digital' surface is smoothened to 'analog state' with sandpaper!

Actually I think the RC-guys makes wings in the same manner, just with styrofoam :-/

Windy Militant
10th Apr 2015, 17:56
@ onetrack: But how did they make the 1st propellor perfectly, to enable it to be used as the master on the copy lathe?

It's the same as shipbuilding there they made a drawing of half a hull then reversed the profiles to get the same shape on the other side.

There are if you look for them Profiles of aerofoils designed and tested by the RAE in the UK and NACA as was in America.
Somebody would sit down and work out the theoretical profile that should work and then they would draw that profile using standard drawing methods usually third angle projection. From this sections would be be taken at stations along the propellor and a jig made up with negative profiles which would allow for the twist in the chord to be included and a master blade would be made which would fit into this jig.This blade would then be used as a pattern to carve the next set of blades. These then would be tested to see if the actual performance matched the theoretical figures, the blade would then be tweaked until the desired performance was obtained and this would then be used on the aircraft.
Tis an arcane art but they've pretty much got it knocked these days. Not like the days when Sir Stanley Hooker joined RR and improved the performance of the superchargers by a large amount with a simple change to the shape of the rotors.

fastjet45
11th Apr 2015, 16:15
Here is an interesting link to the mass production of the Merlin engine in Derby and how they went about it.

https://www.youtube.com/watch?v=-fo7SmNuUU4