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Why are rulers measuring inches usually divided by multiples of eight?

Discussion in 'd.i.y.' started by Avon, Sep 16, 2022.

  1. Avon

    Avon pfm Member

    Another stupid question which probably has no answer: I need to measure 9.6 inches on a ruler. This would seem easier than measuring 243.8 mm. However, my ruler has subdivisions of 1/8, 1/16, 1/32 and 1/64 of an inch. So that’s 9 inches and 38.4 sixty fourths, which is even more ungainly than using mm. I see “decimal inch” rulers are available, but these don’t seem to be the norm. Why am I unusual in wanting to measure tenths of an inch; there must be an historical reason for this, but what could it be? The usual answers I see are "it makes good sense to keep dividing by two" or "inches are not a decimal system". PS. It’s the required pivot to stylus point distance of my tonearm. 9.6" looks sensible, until you realise you can't measure it easily.
  2. LPSpinner

    LPSpinner pfm Member

    Hi all,

    Its to do with easy divisions of fractions …1/2, 1/4, 1/8, 1/16, 1/32 & 1/64. Accurate measurements will revert to using Thousands of an Inch (decimal divisions). Before calculators were a thing people were taught to work with fractions so 1/2 1/4 etc makes sense. These days the metric system has changed that. Engineering workshop rulers will have graduations on 10'ths so go with those. Steel rulers are generally more accurate anyway because they will be machine graduated rather than printed.

    Nowadays, this is one of the reasons metric is so much more logical and easy, everything is in multiples of 10. As an engineer, when doing calculations my first step is to convert all units to metric. Things get even more messy when you start doing stress and Loading Calcs in imperial.

    Even in America, the last stronghold of imperial units, Nearly all Engineers will be doing their calculations in Metric.

    kensalriser likes this.
  3. Barrymagrec

    Barrymagrec pfm Member

    My various steel rules mostly have 1/8" divisions on one edge and 1/10" divisions on the other. Metric on the opposite face.

    One very old 2 foot rule is imperial only but still has 1/8" and 1/10".

    Rabone 64R, for instance.
  4. Avon

    Avon pfm Member

    Thanks for going to the effort, LPSpinner and Barry. Before 1586, decimal fractions did not exist in Europe. However, 11 divided by 10 would have been 1 remainder 1 (1 and 1/10), or 5 remainder 1 (5 and 1/2) in the case of division by 2. Both answers have integer / unit fractions though. But I suppose 2 just won out over 10, perhaps because you would cut a renaissance cake into 1/2 then 1/4 etc. So cakes provide the answer to my conundrum? Anyway, I need to buy a new ruler.
  5. bugbear

    bugbear pfm Member

    Measuring accurately is always tricky. I would recommend a metric ruler (for reasons made obvious by the answers in this thread) and measure 243.8 mm. Or 244mm, depending on the tolerance the measurement requires.
    a.palfreyman likes this.
  6. Barrymagrec

    Barrymagrec pfm Member

    Both measurement and absolute accuracy are problematic. I have just checked a Rabone against a Rolinx plastic rule and the Rolinx came up about 0.4mm short, I then checked the Rabone against a digital calliper and they were both in agreement. (all over 300mm).

    If the original measurement of 9.6" was critical it should have been expressed as 9.60" or 9.600".

    As an aside, I would call C3 a ruler, a Rabone 64R is a rule but that`s probably just me.
  7. a.palfreyman

    a.palfreyman pfm Member

    Bugbear beat me to it. Anyway, trouble is, you could measure it as 9 and 19/32 inches (9.594" near as dammit) but how you going to mark it? Pencil? Might get +/- 0.3mm if you are lucky. If you are measuring from an existing spindle you would measure from the edge of the spindle, minus the spindle radius, but you still got to mark the other end. If your rule slopes downhill from the spindle to the turntable surface, then the pirpendicular distance is shorter than what you measured. Then you have to measure your drilling points from this for the base, or just drill the centrehole so you have to align with an already dodgy marked point. Probably only get +/- 0.5mm if you're lucky. When I did tech drawing many years ago, you measured / marked everything with compasses.
  8. Richard Lines

    Richard Lines pfm Member

    Good Morning All,

    Pedant alert - a ruler is say someone like say King Charles III against say a rule which is a device for measuring...............


    narabdela likes this.
  9. a.palfreyman

    a.palfreyman pfm Member

    Noted. Post duly edited. ;)
    However, you should have opened with 'As a rule...'
    MikeMA and darrenyeats like this.
  10. Vinny

    Vinny pfm Member

    If anyone is designing anything outside of CAD etc. - back of a fag packet job - imperial is far easier to work with than metric.

    Anyone who has less than perfect sight will struggle to accurately mark 30.0cm on a work-piece using a rule or tape - it is as likely to be 29.9 or 30.1cm. Marking 12" is simple.

    Half or a third, or quarter, or eighth, or sixth of 12"? Even a fifth isn't too bad.
    Try that with 30cm.
  11. LPSpinner

    LPSpinner pfm Member

    Good call Richard, I also have corrected people for the same transgression… I consider myself duly reprimanded.

    Kings = Rulers

    Graduated Measuring Stick = Rule.

    Most good rules (see what I did there) are just as easily read on either Imperial or Metric graduations. My 6”, sorry 150mm Toledo Stainless Steel rule has graduations on both sides, 0.5 mm on one edge of the metric face, 1 mm on other edge of the same face. On the imperial face we get one edge with 1/20 for 2” then 1/50 for 1” with the rest is in 1/10. The opposite Edge in the Imperial face is 1/32, 1/64 then 1/16 for the same lengths. I use both side depending the situation.


    During my trade training as a toolmaker (before I went back to study Engineering), we had machines that had Metric feed screws with Metric Collars as well as machines with Imperial feed screws with collars using 0.001” graduations. We all had to learn to use both. These days most of the younger guys are all using Metric because Australia officially went metric in early seventies with only legacy imperial units still in usage such as automotive wheel rims and Rail Gauges. (fun fact; most tyres size use both units, my car uses 225 45 R17 That’s 17” diameter by 225 mm wide with a 45% profile. Lets mix units, metric, imperial and ratios !!!).

    MUTTY1 likes this.
  12. gavreid

    gavreid pfm Member

    Some of the tyre sizes and rims are mis-sold in the cycle world, I don't know about cars. The old 20" tyres 451 which were very common on raleigh bikes in the 70s (think Twenty) were replaced by 406 often used on BMX. These are completely different sizes but 406 is sold sometimes sold as 20 inch.
  13. kensalriser

    kensalriser pfm Member

    Perhaps the salient question here is the tolerance for this particular application. An exact 243.8mm sounds unreasonably and perhaps unfeasibly fine - there is no point in tying yourself up in knots over something that is not required.
    MikeMA likes this.
  14. LPSpinner

    LPSpinner pfm Member

    Indeed, a rule is generally good down to 0.5 mm, probably even 1 mm for someone who is not so experienced in "the shop". In my world if you only have a Rule than rounding the 243.8 up to 244 is perfectly legitimate and probably in line with what is achievable...

  15. a.palfreyman

    a.palfreyman pfm Member

    Yes, headshell slots give adjustment. I'd have thought <1mm was both achievable and satisfactory, less is better of course. It might affect the geometry slightly but won't be vast. Biggest change depends on inside diameter of disc. Last few mm of playing surface suffer biggest change of geometry.
  16. Jim Audiomisc

    Jim Audiomisc pfm Member

    I'm old enough to remember that decades ago primary schools taught in terms of fractions, not decimal. And the norm then with the UK measures was to divide inches in that way. So it is habit I guess.
  17. Jim Audiomisc

    Jim Audiomisc pfm Member

    I have a wooden stick with scratches on it that declares it is a "Scottish Ruler". It also lists all the Kings/Queens of Scotland up to last week. Now out of date, though. Useful at times as I went to school in London, so it is a handy reference when someone comments on ancient Scots history.
  18. Bob McC

    Bob McC Living the life of Riley

    As NASA discovered…
  19. mansr

    mansr Objectionist

    Several dictionaries say "ruler" has both
  20. Richard Lines

    Richard Lines pfm Member

    You mean they're dumbing down dictionaries now as well....... Dear God the country has truly gone to the dogs.......

    At least the Welsh managed to be flying the flag correctly today.........


    Barrymagrec likes this.

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