Metric or Imperial, Fractions or Decimals

Edited By Terryd on 23/01/2011 00:47:40

By the way, try making a set of hidden mitre dovetails with an axe.

Edited By Terryd on 23/01/2011 11:14:23

The practice cited in the OP, of both fractions and decimals of inches, was also once common in industry.

I would advocate that consistency and how the dimensions are given, are what really matter though.

If using inches, you could use 3-place decimals for machined parts but 2 where less critical, but given that it's probably much more common now to use machine-tools for most of our workshop operations, all-decimal anyway would make sense.

I would advocate too, thinking how the part is to be made before dimensioning it, to avoid the builder having to re-calculate datum faces and the like. Not everyone has digital read-outs on their machines, let alone NC machines, but even with these it is still worth thoughtful datum locations to minimise movements and reversals.

'

As for Pi... Mathematical Pi is still published!

With regards to machines, with the advent of cheap DROs it really doesn't matter. I normally work in inches because I make models of 18th & 19th century items and my 'newest' machine is a metric Senior Major ELT but it came fitted with Mitutoyo DRO so no problems.

Brian

Using either type of decimal measurements is fine, but try to avoid fractions.

Guys,

Thank you for your input. You've resurrected my very first forum post from 10 years ago

Cheers and stay well,

Rod

When dimensioning drawings, in what ever units you choose, it is better to use co ordinate dimensions rather than "point to point", since any build up of tolerances or errors should not then be cumulative.

If you consistently make an error, the effect is likely to be less than dimensioning from one point to another, (which will make the error cumulative ). It is amazing how the odd thou , each time, can build up into an unacceptable discrepancy.

Co ordinates is the way that Industry has done things for many years.

Howard

So do I. If I got a first angle GA of the Titanic, it seems crazy I gotta hike to the bow if I wanna look at the propellers...

My hero!

Ian P

My Myford is graduated in 1/1000" with additional graduations every 5/1000" and 1/100" the 1/100" marks being stamped in 10's. Not a decimal point in sight. What's that if not fractions.

regards Martin

My metric mill has dials which are in fractions and graduated in 1/1000 metres but like imperial machines with graduations in 1/1000" it is using a denominator with 10 as the base number. (decimal system).

I started my career as an engineering draftsman and remember calculating lengths in fractions before the invention of calculators. A real pain in the butt ! (Try adding up: 3' 5. 37/64" + 1' 2. 17/32" + 4' 11. 9/16", )

Paul

Trust the late John S to sum it up in a couple of sentences!!

cheers

Bill

Perhaps we should invent a new measurement which converts imperial/metric with perfect precision

widgets

Just returning to Rod’s original question ... I recall posting this in 2013

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Just an aside ...

It was quite common, until at least the mid-1960s, for American draughtsmen to work in 1/128ths of an inch.

I had hours of fun trying to understand all the strange dimensions on the drawings for a Polaroid 5x4 film holder ... until I realised that it had been designed in 1/128ths and then later "modernised" to decimals. Once I had reverse-engineered the "rounding", the design made sense.
___________
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The moral [if any] being that perhaps drawings should be dimensioned, or at least co-dimensioned in the system of units appropriate to the original design. ...

MichaelG.

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Ref: https://www.model-engineer.co.uk/forums/postings.asp?th=90017

Edited By Michael Gilligan on 23/05/2021 08:08:01

Or , as my old woodwork teacher used to say , " Engineers have to work to within ' a thou ', carpenters have to

be exact."

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We did ... when we declared the inch to be exactly 25.4mm

MichaelG.

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http://metricationmatters.com/docs/WhichInch.pdf

Edited By Michael Gilligan on 23/05/2021 09:57:52

Martin -

No-one said decimals are not fractions, but the thread's colloquial language somehow suggests that fractions exist only in the vulgar form!

'

No, of course there isn't a decimal point on a machine handwheel that counts entire thousandths or an inch, or fiftieths of a millimetre... but I've still to decipher the faint lines on my Denbigh H4 milling-machine's dial, on a feed-screw of one-sixth inch lead! Very few regular decimal dimensions match sexagecimals closely.

'

I've worked with Imperial drawings dimensioned in both in vuglar binary and decimal fractions. The tolerances were given as +/- 1/64", and 0.0xx" , respectively; but any machining and the inspection was all decimal.

'

Michael -

1/128" ... Strange limits. I wonder why they didn't go straight to thous or at least hundredths. I once had a steel rule on which one of the inches was graduated in one-hundred-and-twenty-eigths - as if anyone could be expected to use it sensibly. I still have one of those transparent plastic rules intended for drawing-office and laboratory use, with a similar-triangles scale that "magnifies" titchy bits of inches to legibility. Never had to use it!

In line with your comment about matching system to original, I am used to Imperial and metric but am building my engine to one-third scale, in its ancestors' feet and inches. The drawings I make reflect this but with decimal equivalents (TurboCAD, presumably any CAD, uses only 10s-base, Imperial or metric). Even so I often use mm when measuring its physical steelwork to verify alignments or match non-machined parts directly rather than via drawings.

I used all-inches to design and build my workshop's travelling-hoist, but due to its metric fastenings, metric steel-stock and increasing complexity, I wished I'd gone all-metric. It would have been easier!