Why is the world of model engineering still imperial?

Imperial lathe and mill for me so may not apply in some cases. Though with a DRO on the manual mill I seldom actually look at the handwheel dials. lathe is easy enough to convert with the digi callipers or a calculator

CNC mill will just cut whatever it's told in my case metric

Just to keep the fractions brigade happy I'm thinking of making a range of micrometers which read out in 1024 ths. This is 2 raised to power 10. You can't have it both ways, if you want fractions, you're cheating if you suddenly go to decimal inches when you need some precision. This argument is futile anyway, no young person understands Imperial, even in the USA science is conducted in SI, and I believe the space programme is in SI after an unfortunate mis understanding.

I'm old, I have an imperial milling machine with DRO, I habitually use it in metric, it's easier

OK, here is my twopence worth.

Generally the prototype of the model was designed in imperial so scaling to imperial is easier.

BA fasteners were designed for models and instruments with suitable size heads to represent whitworth fasteners.

Part of the enjoyment for me at least is walking a similar road to the machinists and designers who worked on the prototype and using the same measurement system helps with that.

I have a clock design which I shall build at some stage and it’s a conversion to metric design and looks a complete mess in terms of dimensioning.

All that said I don’t have any issue with stuff designed in metric from the get go.

Regards Martin

Edited By Martin Kyte on 03/03/2023 15:14:02

I can relate to this the other way around, a popular scale for model ships is 1/2" to the foot, a person in scale is 2 3/4" to 3" tall and 120' tugs neatly end up 60" long!

Here's the debate ender... Fits n tolerances!

You can pretty perfectly gauge an amount of slop in a slip, or an amount of welly on an interference in a handful of thou. Tell me you can do that so simply in metric so I can call you a liar 😉

J

From what some of our European members have said model engineering is very limited in Europe, so there's not much demand for machines nor designs.

The world is not as metric as those who prefer this system of measurement would have you believe ! I have worked in both systems and am happy with both. BUT structural steel beams often use imperial expressed in metric, steel/ iron pipe work all over the world is very often imperial and the majority of hydraulic work is imperial. If my memory serves me right the Brithish Metrication Board retreated from the task of forcing a wholly metric world on the UK realising it was an impossible task ! The metric system started as a scientific system in europe that eventually spread, largely due to war ! The so called imperial system was based on practical usage and was used all over europe and the world. I could go on - but I won't ! Noel.

Nearly all of my micrometers were bought about 60 years ago so I am very happy to see imperial dimensions as it saves me from having to convert from mm. When I bought a mini mill, some time ago, I got the "imperial" version ...Big mistake; it has metric leadscrews with nonsense dials so I added a DRO. I am happy to work in metric or imperial anyway it's just that I already have imperial measuring tools. Colin

Many years ago when I made all the steel & aluminium panels and skins for vintage R/R and Bentley I used imperial for measurements plus my eyes, and BA and imperial fixings but for the last 20+ years I have been using metric, now I have an imperial lathe I am glad I have digital micrometer & callipers, so until I get imperial back in my head I can cross reference, luckily my mill has dro..

Dell

Which is ironic because metric is far more logical and consistent than Imperial. Imperial is only friendly when most of it is ignored. In sawn-off form Imperial works pretty well for simple workshop jobs, but the system behaves badly as soon as anything complicated is attempted.

The prosecution says witnesses have no right to extol Imperial unless they know how many:

• hands in a yard
• yards in a rod
• rods in furlong
• links in a chain
• chains in a furlong
• gills in a quart
• gallons in an anker
• gallons in a barrel
• puncheons in a tun
• firkins in a kilderkin
• grains in a dram
• pounds in a quarter
• pounds in a cental
• scruples in a drachm
• cubic inches in a cubic foot
• drachms in a fluid ounce
• bags in a chaldron
• weys in a last
• grains in a carat
• reams in a bundle
• acres in a rood
• acres in a square mile
• etc etc

Might be argued that many of the above have been abandoned by even the most ardent Imperial fanboys, and that the simplified remains are all that's needed in engineering. Sadly not, once an engineer gets to work in Imperial, he finds himself in a mathematical minefield. Velocity: feet per second or miles per hour? Force - poundals, or foot-poundals ? Work foot pounds, or inch ounces? Power is measured in foot-poundals per second; what's that in Horse Power and Watts. Pressure - pounds per square inch or tons per square foot. Mass - ounces, pounds or tons? Density - hundredweight per cubic foot, pounds per cubit foot or pounds per cubit inch? Watt-hours, Calories or British Thermal Units? All these units are valid, but they generate blizzards of cross-conversions. Not too bad if the engineer can stick to applying formula out of a book, but Imperial is error-prone and awkward as soon as anything non-trivial is needed.

Scientists were the first to notice Imperial has serious shortcomings, and they enthusiastically developed metric as a rational alternative. The most important simplification is that metric units relate logically, for example:

1Watt = 1Joule/s = 1Newton.m/s = 1kg.m ².s^-3 = V.A = 1A².ohms

Note that metric brings power, time, work, length, mass, volts, amperes and ohms together neatly without any conversion factors. Imperial doesn't have the same consistency and engineering maths is full of magic numbers as a result. The magic numbers are needed to manage the complicated relationships between Imperial units.

The Imperial system is complicated because the units are derived from the commerce of yesteryear, before it was understood that the units were related. In consequence, the Imperial system is a mess from the foundations up and it matters!

Imperial is superficially user friendly, but it turns nasty as problems scale up. Not an issue making a steam loco on a Myford from a drawing, but the inconsistency of Imperial is best avoided when designing spacecraft. How much fuel is needed to propel a one ton satellite to Mars and back? Having a ruler graduated in sixteenths or understanding Turns per Inch doesn't help!

Three reasons for preferring Imperial:

• Facilitates simple jobs
• You've already know Imperial, think it's lovely or patriotic, and learning Metric is an unwelcome challenge
• There's a need to work to Imperial plans or mend old equipment

Only the last reason is valid.

In my opinion the UK made a serious error by not railroading metrication through ruthlessly from day one. Instead, governments allowed Imperial and Metric to coexist, which created the worst possible muddle. Children are required to learn an obsolete irrational system, and a different rational system, and also to know how to convert between the two. It's a mess, and one of the factors that brought traditional manufacturing in the UK to it's knees. No one abroad wants products full of Whitworth nuts and bolts, or cares a hoot about tradition!

Dave

I didn't know that children, except for those in their 60's, 70's, 80's and 90's ever needed to learn the imperial system.

I would have thought the come back "bog off grandpa" would have sufficed at any attempt to engage a 14 year old in the world of ibs, feet and shillings. The UK mile is of course a complete anomaly doubly so when my cycle computer is calibrated in miles along the flat, but in metres of climb, Duh! :0)

I promise, no more silly posts from me, unless i am provoked.

Edited By Paul McDonough on 03/03/2023 16:24:21

As others have said I think we still have so much imperial measurement in the hobby because of the legacy from the wealth of models designed before metrication of the UK. Being 66 years old I straddled the change to metric and feel comfortable in both systems. My mill is metric and my lathe is half metric. A DRO fitted to a machine should make working with either system fairly straightforward but as the hobby should be filling with metric trained people by now then imperial will become more of an anachronism. In another 20 years there will be few people left in the hobby who have not been brought up totally within the metric environment. Will people who have only used the metric system go to the trouble of familiarisation with imperial to build an old design or will they just opt to build a metric design? I get the feeling that even the USA are using more metric in their designs although I don’t think they have made an official transition. I have quite a bit of tooling for both disciplines which is obviously an additional cost but as I said I am as happy with a metric mic as I am with an imperial version. Strangely my wife who is a year younger than me does not relate to metric measurements at all but she is not an engineer and seems to have escaped the conversion to a metric world.

Mike

Whoa there. The question was why is Model Engineering still imperial and really that only relates to linear measurement. So not much to learn really, everything is pretty much expressed in decimal inches and simple fractions. Small hole sizes are generally produced by metric drills and sometimes specified as such. 25.4 is pretty much the only constant you need to remember and with DRO’s and digital callipers not even that. If you want to start doing sums on boiler energy or any sort of equipment physics you immediately drop into SI units.

regards Martin

I'm not sure about imperial prototypes better suiting the imperial system when scaling and that is why they are common, just look at any loco and it's a bodge to suit a gauge rather than a scale that works out nicely at 1/12th, 1/16th, 1/24th, 1/32 etc. at least the traction engine and stationary engine boys get it right in that respect

Can't argue with Dave's comment! My love of imperial measurement ends when the workshop apron comes off, pretty much everything else in the world I'm fully metric.

I do love this bi-annual dust up though LOL

J

Edited By UncouthJ on 03/03/2023 17:10:40

Edited By UncouthJ on 03/03/2023 17:11:49

DROs mean you can buy a good old imperial unit and do metric

Paul's question was a good one.

However most of the postings have moved to the old Imperial - v-Metric discussion. This is an arguement I do not understand (and I am old).

JA

I get it, a key driver is that so many models which are still being made were drawn in imperial, although i am suprised that stock material is seemingly still available in imperial.

I really do not have an axe to grind over this except for my irritation that I may need to buy several different sets of taps and dies and that really isnt the end of the world.

Hi, I guess the reason is that most of the older drawings and all the castings that are available, are all suited to the imperial system, so the have all of them converted to metric would probably be a costly business, and then you may have to have stocks for both, although the castings could probably be cut to metric sizes in most cases. There are plenty of conversion calculators around for those who don't understand imperial measurements and want to do in metric. I know both and can read drawings in either, and measure out material in both systems, and have had to all my working life, I've also had to use both together, usually when joining old with new.

Regards Nick.

We are still largely using Imperial plan-sets and machines even if we convert the dimensions and fastenings.

In the end it does not greatly matter but near-matches need watching carefully. For example, 1/16" and 1/32" are not as was stated further up the thread, but are 1.6 and 0.8mm, respectively.

BA threads are a metric system with a geometrical diameter progression; but the BSI always quoted them in inch dimensions. They do not match the ISO-M range much though, and then only in diameter. OBA is more or less M6, but that's about all.

One objection to normal ISO-M fastenings is that most of those commercially available look rather unsightly in places needing close prototypical appearance. This is valid in fine-scale work but less important where the fasteners are difficult to see or are painted over: I have used socket button-head screws to resemble rivets, tapped into the underside part, and with the paint filling the sockets.

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It is what we choose to use; but if we have a fully-metric workshop building a model to drawings published 50 years ago will need us do a lot of converting if we don't have a DRO.

I use either depending on what I am making; and my steam-wagon is accumulating BA, ISO-M and BS threads and fasteners on a primarily inch-based project that has already taken many years to build. Whereas for other projects I might use all-metric.

Quoting units like Rods, already obsolete over a century ago, does not help; but the SI is more for physics and critical engineering designing than practical, everyday use. It was designed simply for mathematical neatness, and like all measuring systems its base units are still arbitrary inventions.

The power unit, Watt, has been that for many years, and the conversion 1HP = 750W is as damn' near accurate as any of us are going to need. (Some European car manufacturers still quote engine powers in PS - a old German, though metric, unit).

Arcanities like Newtons are relevant to designing boilers or very highly-stressed shafts and beams; but do not cut metal and are not necessary for building something to a long-established design.

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Inch-sized stock materials are still common presumably because it would cost money to convert the machinery and people are still buying those sizes!

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Schools have taught only metric units for years now, but they insist on clinging to the fashion-trade's non-preferred centimetre. If I am quoted a length in cm I have to change it mentally to millimetres to picture the item. (I might then equate the mm to their nearest inches though!)