Old rule divisions twelfs etc

some of my older steel rules, have eigths,sixteenths on one edge and the opposite edge is divided into twelfs ,twenty forths and 48 ths, one of these rules is dated 1959 ( ex WD) what was measured in twelfs etc I have never seen a dwg using these fractions what industries used these fractions.printing is a possibility.

Architectural drawings - or indeed any drawings produced at 1" to the foot ratio (???)

It's known as a scale rule.

Russell

Not necessarily: the image below shows a scale rule with a 1/8inch scale at the top and 1/4inch scale at the bottom. For comparison the lower part of the image shows a rule (an old school ruler) showing Twelfths.

John

Just an ordinary ruler.
One of those odd coincidences that this afternoon I printed a picture of my milling machine adjusted to be at 1in = 1ft to help check some clearances in a workshop layout. Needed the 12th to draw and measure the inches.

Twelve is a useful number [divisible by 2, 3, 4, 6] and also begets dozens, and hours

… All of which makes the old School Ruler a handy aid to familiarisation.

Twelfth Scale is also popular for Doll’s House and other models … being a convenient ‘one inch to the foot’

What’s not to like ?

MichaelG.

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Crossed with Bazyle’s post

Edited By Michael Gilligan on 28/09/2021 21:41:51

Ah, I thought I remembered twelfths from primary school rulers in the 50s, but wondered if I'd just made it up!

Thanks, folks.

I find it odd that you haven't got at least one and haven't seen one for decades. I have a wooden one here on my desk, a plastic one on the other side of the room, a couple in my workshop as well as the steel rules, and one in my desk at work. I do notice at work people keep wanting to borrow mine as they don't have their own. In my mind it's just one of the things every home has.

A useful scale for representing things made in feet and inches; and printing font "point" sizes are or were based on twelfths of inches.

My Denbigh H4 horizontal mill has 6tpi screws, corresponding neither to binary fractions nor regular 5s-based multiples of thousandths of inches, nor to millimetres; and I have wondered if the machine was originally to some special order, perhaps connected with the printing trade.

(1/6 " = 0.16666... " = 4.23333... mm)

I don't know if they still teach times tables in schools but those of us who are old enough will certainly have had to learn their 12 times table due to there being 12 pennies in a shilling and 12 inches in a foot. As a result twelfths on a school ruler made sense as another example of the use of something divided into twelfths. Additionally as pointed out by MichaelG if you are asked to draw a line that is, for example, a third of 8" it is easier with a ruler with twelfths on it than with tenths or sixteenths as it is divisible by factors that don't go into the other two divisions.

I vaguely recall that the requirement to learn times tables was dropped for a period as calculators made them unnecessary! It then became apparent that people who couldn't do times tables were handicapped because they did not recognise when there was finger trouble in their calculator inputs and outputs.

Martin C

When I think back to the late thirties and forties when I was at school and compare it with what I'm told is being taught today. I wonder where we found the time. I always thought that boys should have been taught the basics of cooking and both boys and girls a smattering of first aid. Incidentally does anybody remember decimetres ?

Roy

I don't believe twelfths were associated with a particular trade, rather it's just a handy scale for rough measuring as might be required in carpentry, dress making, or any other rough work. For this system to work it's necessary for rules to provide several different scales: ⅛, ¹⁄₁₂, ¹⁄₁₆, ¹⁄₃₂, ¹⁄₄₈, ¹⁄₆₄" are common. Late model imperial rules sometimes do ⅒, ¹⁄₂₀ and ¹⁄₁₀₀"

Fractional rules aren't a precision system, and they date back to simpler times when most practical problems could be calculated with fractions to the extent that decimals weren't taught to many Victorian school children. They had little use for advanced maths!

However, progress changed all that. Fractions are clumsy in many situations, especially science and technology. As a system, decimals are much more general purpose, simplifying complex sums, and avoiding awkwardnesses like is ¹³⁄₃₂" bigger or smaller than ⁵⁄₁₂". Another major advantage of decimal arithmetic is accuracy is managed simply by calculating to more or less decimal places. Though fractions have many useful applications, decimals have replaced them for most purposes. Metric measure doesn't use fractions at all and no-one complains...

Dave

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In your role as a Moderator … Would you please correct that abuse of the word ‘precision’

The system of units has no effect upon the precision of a scale !!

[ Rant over ]

MichaelG.

Just wait till USA goes metric, bet they will use fractions just to be different.

Peter

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With the Pantomime season fast-approaching … Permit me to reply:

<< Oh yes they do !! >>

MichaelG.

Why do todays youfs need to no how to calculate when there's a picture of the goods on the till keys?

Regards Ian.

Edited By Circlip on 29/09/2021 10:55:12

Although it is true that in all the drawing conventions I've seen, dimensions expressed as fractions have a looser general tolerance than those expressed as decimals...

Yes. There was only ever one of 'em on a school ruler.

I wondered if they were ever meant as a metric replacement for the horsicultural 'hand' ?

A Google search for ‘precision’ … [selecting images] will display numerous illustrations of the distinction between accuracy and precision.

MichaelG.

Um, I plead 'not guilty'

I was referring to the precision of a fractional rule in use, not the scale. One definition of precision: The ability of a measurement to be consistently reproduced. Rules aren't much cop for that.

On the ⅛" scale, what's the diameter of this 5p coin? How precise is the measurement in the sense it could be reproduced?

I'd claim no better than about ¹¹⁄₁₆", which is roughly 17.5mm. Official size is 18.0mm; what's that as an inch fraction?

Dave

Edited By SillyOldDuffer on 29/09/2021 11:45:40