Can a metric baby do imperial?

[noel shelleyParticipant @noelshelley55608]

I know I’m getting old and repeating myself BUT Metric whilst it may be more common is not the ONLY system in COMMON use, In hydraulics and pipework Imperial is normal world wide, JIS a hydraulic type has a form that looks like an American JIC but uses BSP, aluminium sections are often Imperial, Then there’s structural steel, WHY use sizes like 102, 203, 254, 305 Etc ?  In fact most european languages have words for Mile and Pound, Inch, the Dutch for inch is translated as thumb !

I tend to work in both, which ever is right for the job. I was lucky to come across a large quantity of NOS Mitutoyo instruments, both Imperial and metric, mikes up to 6″/ 150mm at £8 each, depth and verniers about £10, even a height gauge.

For Beeza650 I would remind him that old bikes and even some newer ones use 26 TPI Cycle thread in Imperial, but DO NOT confuse it with 26 TPI BSB, (British Standard Brass ).  Good Luck Noel.

[Howard LewisParticipant @howardlewis46836]

MOSTLY, I work in Imperial, because my my measuring instruments are Imperial, and both Mill and Lathe have Imperial graduations

I also have a calculator, and my digital callipers convert, either way, at the press of a button.

When working on Fuel Injection Equipment development, we worked in mm and microns, so I have a foot in both camps, and use which ever is most appropriate.

We should all work to the same motto as Radio Amateurs.

KISS

Keep It Simple, Stupid!

Adding complications for the sake of it is asking for trouble.

Howard

[John Haine]

On John Haine Said:

Just to add to the pedantry, maybe we could stop using the term “metric” and start using “SI”?  When I was at school we did woodwork in feet and inches but science in “metric” which at that time meant cgs – centimetre-gram-second.  Though there were fewer odd conversion constants it was still iffy. …

Agreed, though ‘Metric’ is handy shorthand.

MKS/MKSA/SI didn’t arrive in perfect working order on day one.   CGS was introduced by the British Association for the Advancement of Science in 1873 and it was flawed, mostly because units derived from centimetres and grams are often inconveniently large or inconveniently small!     A Dyne (unit of Force required to accelerate one gram at a rate of one centimetre per second squared”) is 0.00001 Newtons (0.225 lbf in Imperial), leading to gigantically inconvenient numbers when dealing with objects weighing tons!   CGS works, except where the decimal point ends up is often painful!)

Mostly fixed in 1943 by shifting the base units from centimetres to metres and grams to kilograms, but this meant much renaming of units and reprinting all the text-books!  The move from CGS to MKS was worthwhile because simplifying measure is important.

Curiously, although CGS had been in use for 70 years, there was no emotional reaction to ditching it.  Quite unlike Imperial, where many would rather die in a ditch than accept metric.   I think it’s because Imperial has heritage & cultural connotations, whereas CGS was understood to be just another tool, capable of improvement, hence no passion when dumped.   Also, most users don’t realise how bad the Imperial system is as a whole.  Pounds and Pints seem friendly, so no need to change.   That slide-rule man had to cope with an unholy mess matters not one jot when I’m all right Jack!

Retaining Imperial measure for historic, sentimental or lazy reasons isn’t engineering, is it?

Dave

 

[FulmenParticipant @fulmen]

Units will always be too small or too large for certain applications. The obvious solution for your example would be megadyne which is 10N or 1kg force. The pascal is even worse and it works just fine.

[SillyOldDuffer]

On SillyOldDuffer Said

Retaining Imperial measure for historic, sentimental or lazy reasons isn’t engineering, is it?

Dave

 

I find it is far better when reverse engineering an old item to work in what it was originally made to. I’ve had it a few times when doing CAD work for people, they measure as best they can an old part in metric and you end up with a bunch of odd figures. But if you convert those figure sto imperial you can see that they are close to fractional imperial sizes which is no doubt what the actual measurements were originally.

Take a picture like this, do you assume it is 36.65mm and redraw to that dimension. Or do you convert it back to imperial at 1.443″ and then use a bit of common sense and apply the likely imperial fraction of 1 7/16″. Had that calliper been reading in inches or even fractions the job of deciding on what the original was would be even quicker.

With 30 odd measuurements of that casting there would have been a lot of errors if I had been lazy and just assumed the metric were correct, many were taken from cast surfaces with draft to complicate issues. Like this, where do you measure and can a rough surface be trusted? I went with  1 1/4″ at the end.

[JasonBModerator @jasonb]

Was going to add this one but sofware will probably not show it.

Do you go with 92mm or 3 5/8″. Had the person taking the measurements been doing it in imperial I’m sure they would have moved the calliper to the correct imperial 3.625″ but you can’t trust these Europeans to know about imperial😉

Might not be Engineering but it is the practical approach and will bring a 130yr old engine back to life.

[peak4Participant @peak4]

Even though I diverted the topic slightly above with the photos of odd micrometers, I wonder how much the discussion here really helps Oliver.
Yes NASA apparently crashed a space lander due to a misunderstanding of units, but we mainly work in our own workshops.
Someone commercially producing items for third parties will be familiar with the spec required and work in the relevant units.
Apart from thread sizes, expressed as vulgar fractions in BSF/BSW/Unified, most other machining measurements these days are expressed in decimal (outside the US or old blueprints); sizes of bar stock might be an exception.
Yesterday I made some thumbscrews for a neighbour.
These are for attaching trays to the bottom of bird feeders. A quick read of several catalogues seemed to imply ½” BSP as the standard thread fitting for them, so I asked Steve to drop off some samples to measure, as I have a set of BSP taps & dies.
Well, the tap was a tad too large for the hole, and the broken plastic screw was a rattling fit in the die.
On measuring it, the plastic original screw was oval and about 30 thou under BSP, but seemed to be 14tpi.
On the other hand, it was close to 20mm 1.75 pitch. The thread under the diecast alloy feeder(s) was badly formed and hard to measure, as well as not being cut quite deep enough, regardless the thread pitch.
14tpi seemed a better fit than 1.75mm, both externally and internally, but they are almost identical when you hold up thread gauges to the light, so either would probably do OK.

Should I have made the screws 20mmx14tpi, or 1.75mm? or 0.780″ x 14tpi, or 1.75mm
I was screwcutting them regardless, as it wasn’t worth buying a one off die, and even a 20mm tap would have bottomed out, so I’d need one of those as well to suit the die cut thread.

All my lathes are imperial leadscrew, so I went for 14tpi x .780″ Second operation for the slot, to save damaging the thread, was in a 5C collet 25/32″ was the best fit, rather than 20mm.
i.e. picked the units to suit the machinery and the job in hand.

Bill

[FulmenParticipant @fulmen]

I agree that the metric/imperial debate isn’t that important for a home shop. But it is fun to discuss, especially with the ‘murcans.

[howardbParticipant @howardb]

All you need if you have the opportunity to buy some decent imperial mics is a calculator and the magic number 25.4.

[JasonB]

On JasonB Said:

On SillyOldDuffer Said

Retaining Imperial measure for historic, sentimental or lazy reasons isn’t engineering, is it?

Dave

 

I find it is far better when reverse engineering an old item to work in what it was originally made to….

 

Agree entirely!  “historic” was a poor choice of word on my part, given that not everyone reads or remembers what I said earlier!   Working mostly on Imperial plans or equipment is one of my good reasons for owning an Imperial workshop because it reduces the number of numeric conversions needed.

I meant historic in the sense that the inch is based on the width of a thumb or three barleycorns and has to be retained because it celebrates the glory days of the British Empire, national pride and my long lost youth!  Imperial measure is a workshop tool, not a cultural icon.

Though we can be sentimental about Imperial in hobby workshops, there’s very little call for it in new manufacturing, because we live in a globalised metric world.  Not good to thoughtlessly recommend it to newcomers for emotional reasons.

Dave

 

[SillyOldDuffer]

On SillyOldDuffer Said:

 

Though we can be sentimental about Imperial in hobby workshops, there’s very little call for it in new manufacturing, because we live in a globalised metric world.  Not good to thoughtlessly recommend it to newcomers for emotional reasons.

Dave

 

Unless you are manufacturing with a view to selling in the US where the market still wants imperial.

[Graham MeekParticipant @grahammeek88282]

I regret to say the Glory Days are long gone. For me they ended when Concorde was grounded. Some of my life went into the manufacture of that aircraft. Also this Imperial based, (at the time), country worked alongside the Metric based partner. No problem with interchangeability.

As regards dwelling on the past and to show how things have moved on. Should we not be working with slide rules, inside and outside callipers to preserve the authenticity? Micrometres and Vernier’s should be taboo by the same “Rule of Thumb”.

I see the calculator has been widely accepted in the posts above but not the dear old Millimetre.

Before the naysayers put pen to paper, I am trying to have a little smile about the “War of the Systems” that clearly still exists in the texts above, and to proffer how far do you take dwelling in the past.

Regards

Gray,

[Howard Lewis]

On Howard Lewis Said:

As already said, in our environment, no matter how good our measuring equipment, we are unlikely to be able to work to industrial standards of precision.

And we are discussing UNITS of measurement. It will not change the part whether we measure it as 1/4″ or 6.354 mm.

My advice would be;

If the component needs to be made to a Metric dimension, measure in mm; if it is Imperial, measure in Imperial.

M6 is 6 mm  x 1 mm, not 0.236220472 x 0.039370078″, any more than a 1/4 BSW thread is 6.354 x 1.27 mm

But a calculator can be useful, at times.

Horses for Courses!

Howard

On Speedy Builder5 Said:

But even imperial has its querquiness .  Will you work in fractions or decimal inches. My metric analogue vernier calipers have both metric and fraction scales, the fraction is to the nearest 1/128″.  Cheap digital callipers can have metric, decimal and fractional inch selectable readings.

Bob

So they can, but the each fractional measurement is an approximation give or take 1/128th which takes ‘that’ll do’ just a bit to far.

[duncan webster 1Participant @duncanwebster1]

Those dedicated to using fractions of inches might be interested in my proposed micrometer, which measures in 1024ths of an inch, that’s 1/2 raised to the power 10. It has a 32 tpi thread with 32 marks on the thimble. Easy to use? No, but it avoids all this decimal nonsense

[JasonBModerator @jasonb]

Will you put fractional handwheels on your machine too? My old Emcomat 8.6 had a leadscrew handwheeel with 62 1/2 divisions. It was fine for anything less than 1/16″ but hard to keep track of otherwise.

[SillyOldDufferModerator @sillyoldduffer]

Something in our brains prefers round numbers, and fractions fit that mindset.  Somehow 3⅓ feels ‘better’ than 3.333 recurring, perhaps we have a notion that there are never enough recurring digits for decimals to be accurate.

I design in millimetres, trying as far as possible to define everything in round numbers.  Doing so reduces production errors, 24.00 is safer than 24.01  So I found myself annoyed when my design required a part to be 23.67mm long, but relaxed psychologically when I realised that’s 23⅔mm.

My reaction to 23.67 was bonkers!  In the workshop,  I’d just machine it to 23.67 fully aware that my normal accuracy is ±0.02 mm.   Unless something special is required, this part made by me could be in the range 23.65 to 23.69mm.  Doesn’t matter unless that’s not “good enough”.

As Dave Halford says digital calipers with a fraction mode round to  ¹⁄₁₂₈”,  I hope no-one finds that a comfort,  The shortcoming matters too, so watch out!   ±0.02 (about a thou) is considerably more precise than ±¹⁄₁₂₈”.  Fraction mode isn’t good enough for other than crude metalwork and is too accurate for woodwork.    As the caliper gets comparably good accuracy in mm and thou, I never use calipers in fraction mode.

Dave

 

 

[JasonBModerator @jasonb]

So how did our forefathers manage with a steel rule and firm leg callipers if fractions were not good enough for accurate work?

They did not put a number to the fit, just went by feel depending if they wanted a full or shy fraction. Same can be done for metric.

[FulmenParticipant @fulmen]

Because you could read a newspaper through their definition of accurate?

[JasonBModerator @jasonb]

But it all worked.

[JasonB]

On JasonB Said:

But it all worked.

Still does for lots of hobby stuff made in a shed.

Doesn’t really cut it for production though.

High end home equipment was not, and still is not that far behind common industrial practice.

[Chris GunnParticipant @chrisgunn36534]

On 14 February 2025 at 12:03 JasonB Said:
But it all worked.

They made a hell of a lot of fixed gauges back in the day, and used them to maintain commonality and make sure everything went together. The actual dimension did not matter as much as the parts fitting together.

Chris Gunn

[duncan webster 1Participant @duncanwebster1]

Doesn’t matter too much to model engineers, bit difficult to make it to fit when the machine you’re trying to make a spare part for is at the opposite end of the country. You either send a man to make up pin gauges etc or leave excess metal in appropriate places and send a man with files/scrapers to fit it.

Even for model engineers the ability to measure accurately means you can machine to final size much more quickly.

When Ford were contemplating making RR Merlin engines they reckoned they couldn’t work to RR tolerances. Smiles turned to frowns when Ford said they needed to be tightened up so that for instance any piston could be fitted in any bore, not the selective assembly and hand fitting that RR were used to.

[JasonB]

On JasonB Said:

But it all worked.

Yes, and it’s the method I use in my workshop.  Hopeless in manufacturing though because the methods are skill and labour intensive and parts so made that way aren’t interchangeable, resulting in high maintenance costs.

Having a bunch of smart toolmakers produce jigs, fixtures, and gauges at 10x accuracy allowed factories to dump large numbers of skilled fitters with an unskilled or semi-skilled workforce knocking out precision parts at a fraction of the cost.    Only works for mass production because tooling up is expensive, so limited runs produced by fitting can be cheaper.

In the 19th Century the British gun and clock trades both self-destructed after refusing to modernise.  Rather than adopt the American System of Manufacture they burbled nonsense about quality and went bust instead.   Problem wasn’t that they made rubbish, it was that their fitted products cost more than customers wanted to pay.   Firms using fitting were doomed once a competitor started selling mass-produced American System equivalents.

Still true today: failure to keep manufacturing methods up-to-date is financial suicide.  Old ways are the worst!

Doesn’t matter in a home workshop were labour is cheap and we don’t work in teams. Forr us old-fashioned methods are just the ticket.  They fail when a workshop has to profit in competition with an efficient firm though.   Many examples in industry where production efficiency is all, ideally unskilled, or churned out by unsupervised machines. Skilled filing is outpaced by semi-skilled manual milling and manual milling is outpaced by automatics supervised by unskilled operators…   Output is important, not traditional skills.

Professional machinists should take a crash course in economics!   Hobbyists can do what they like.

Dave

 

[SillyOldDuffer]

On SillyOldDuffer Said:

  Output is important, not traditional skills.

Professional machinists should take a crash course in economics!   Hobbyists can do what they like.

 

I still find the hobbyist approach of not caring how long something takes to be very odd. Wouldn’t actually using the Ultrafabwonderwidget be better than spending yet more hours trying to make working parts?

[Peter Cook 6Participant @petercook6]

Depends why you are doing it. If the (primary) goal is the output, and the methodology of getting there unimportant, then you may be right. If however the primary goal is the pleasure of the making then as Dave said hobbyists can do what they like, and typically will do what maximises pleasure.

If my pleasure came from using the Ultrafabwonderwidget, I would probably buy one rather than make one. In economic terms (even excluding my time) the cost of the materials, consumables, heat light and power and amortisation of the capital equipment and workshops space would probably exceed the cost of purchase and I would get gratification far quicker.

If however the pleasure comes from the making and the sense of gratification when it comes together then that is a hobyist!

[Author]

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