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Imperial fractions on drawings.

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John Bromley02/04/2014 20:25:16
84 forum posts

Probably been covered before but searching for this is like looking for the proverbial needle.

I have been working from Tubal Cain's books on simple steam engines. The dimensions are given in Inches and fractions of an inch. My initial thoughts are that measurements in imperial decimals would be better.

My Boxford lathe is metric, working with thousandths is not so difficult. But having to convert from fractional Imperial to decimal Metric is not easy, especially as I was taught Metric at school.

I appreciate I'm a beginner and have a lot to learn but it just seems a strange way of working.

So any tips from old hands? Are dimensions in this format the norm for model engineering plans?

Jens Eirik Skogstad02/04/2014 20:42:49
395 forum posts
22 photos

Very easy to read imperial fractions!

Edited By Jens Eirik Skogstad on 02/04/2014 20:46:34

Michael Gilligan02/04/2014 20:43:55
17011 forum posts
756 photos


Imperial fractions were commonly used; especially in America, where many items were designed in 1/128ths.

Regarding Tubal Cain's book [etc.] .... If you are consistent throughout the model, it would do no harm to work in metric on the pretence that the conversion factor is 25.6mm to the inch instead of 25.4


jason udall02/04/2014 20:45:09
2029 forum posts
41 photos
Don't know about old hand. But
Are dimensions in this format the norm..
Sorry . There is no norm.

I too find fractions a pain.. ( not the maths but the umteen umteenth bit)
What I do is go over the drawing and mark up in units as required... inches mm cubits whatever but as decimals
.then in the case of some older drawings also convert the drill number sizes. .gauge etc.

Your next problems will be fits .. again almost never marked on drawings but refer to the text for guidance....

Good luck
jason udall02/04/2014 20:46:02
2029 forum posts
41 photos
Oh like the 25.6 idea...
Jens Eirik Skogstad02/04/2014 20:50:52
395 forum posts
22 photos

How to read in the caliper..



Stephen Benson02/04/2014 21:04:27
203 forum posts
69 photos

When I was making the SS50 from Stewarts it was driving me daft so I wrote this and put it on my clock site



It works XP, Vista and Win7

Edited By Stephen Benson on 02/04/2014 21:06:01

John Bromley02/04/2014 21:06:35
84 forum posts

Thanks for the replies.

The taking of decimal measurements is not a problem in either case, Imp or Met.

It is the application of measurements when say, laying out or taking a cut. I does seem that some planning in the early stages by going through drawings and converting to Metric might prove the best approach.

Sorry my maths is not great. Why would you use 25.6mm to the 1" as opposed to 25.4mm.


John Bromley02/04/2014 21:09:49
84 forum posts

Many thanks Stephen, that looks very useful. I'll have a play with that sometime.


Michael Gilligan02/04/2014 21:14:03
17011 forum posts
756 photos
Posted by John Bromley on 02/04/2014 21:06:35:

Sorry my maths is not great. Why would you use 25.6mm to the 1" as opposed to 25.4mm.


Because it makes converting from

1/256, 1/128, 1/64, 1/32, 1/16, 1/8, 1/4, 1/2

an absolute doddle.


John Bromley02/04/2014 21:37:16
84 forum posts

Yes Michael that makes much more sense. Took me more than a while to see why! Must brush up on my maths,

Are new drawings tending to be in Metric or is this fractions "problem" ongoing.


Clive Hartland02/04/2014 21:41:06
2630 forum posts
40 photos

I have a Vernier which reads 128ths, a bit confusing at first but its very good when doing woodwork and setting the blade on the saw bench which is Imperial.


John Bromley02/04/2014 22:00:42
84 forum posts

Clive it baffles me why you would want to measure in 128ths. Does that mean you also need a rule marked in 128ths to do lay out work or do you have to convert it all into some usable unit. Seems much simpler to measure in metric decimals and use the same units for all machines and instruments.

Probably flogging a very dead horse here but ..........Imperial Argghhh! Hateful system. Well in the fractional sense anyhow.

I jest of course, I love a bit of tradition and the fact that model engineering is a stronghold for the Imperial system, but it ain't easy for newbies like myself.

Thanks for all the advice guys.


Nicholas Farr02/04/2014 22:20:12
2545 forum posts
1205 photos

Hi John, fractions are not exclusive to the imperial system. e. g. you can get 1/2 m and 1/2 mm for instance. You can also have a situation where measurement b is 13/16 (or any proportional amount) of a, where a is any amount you like, be it metres, mm, grams, feet ect.

Regards Nick.

FMES02/04/2014 22:21:03
606 forum posts
2 photos

John, have you tried a set of Zeus tables, you can directly read the imperial fraction / imperial decimal and nearest metric equivalent all on one line.

Clive Foster02/04/2014 23:14:56
2530 forum posts
82 photos

Fractions on Model Engineers drawings date back to the days when marking out would have been done with a surface gauge and deep engraved ruler in a vertical holder on whatever surface plate subsitute could be found or afforded. Usually thick plate glass. Clicking a scriber point correctly into the various imperial fractions isn't too difficult, OK a magnifying glass helps for the 64 ths. Actual measurements in 128 ths was always pretty rare although rulers so calibrated were available, I have one somewhere, and more to do with implied tolerancing. It should be immediately obvious that imperial fractions are a named increment digital system so the inherent tolerance is ± 1/4 the increment as being unabiguously closer to the true value than to the next one up or down. So working in 32 nds implies ± 1/128, in 64 ths ± 1/256 and in 128 ths ± 1/512 call it ± 10, 5 and 2 in more familiar thous or ± 0.2, 0.1 and 0.05 in mm. As ever the effects of the errors inherent in practical implementation are a whole 'nother thing.

Surface gauge and rule work to any sort of accuracy with a metric rule is not a pleasant prospect. Far too easy to mis-count.

In these days of relatively affordable measurement equipment its hard to understand the different mindset needed with simpler gear. Heck I have a full sets of micrometers up to 12" and 300 mm, internal and depth micrometers to 8" and 200 mm, verniers out to 2 ft, dial calipers in 6", 8", 150mm and 200 mm, 4 height gauges 2 vernier two Microball. Which, logically, is ridiculous for home shop guy. Probably around £1,000 carefuly spent when right thing at right price came up. Not trivial but people fork out that kind of money for a years golf club subs. There is another £1,000 odd worth out there which we won't mention. Big change from 1974 when it took a deal of hard saving and not a few missed lunches before I could afford my first micrometer, a second hand and somewhat iffy import. When Tubal Cain was writing that sort of equipment level was beyond dreaming for a home shop. Even the local departmental workshops at the MoD establishment I used to work for got along with much less.

Except where you need continuous measurements aligned to the mathematical notation imperial style named increment systems are much more practical out in the real world for jobs which don't need machine precision. If you use the appropriate unit conversion issues are rarely a worry and its usually easy to see if there is a serious error or someone is being silly. How many people who talk of tenth thou precision actually know what a tenth thou really looks like. Decimal point migration can be a major problem with continuous systems. Someone starts talking about 1/10,240 th of an inch increments you start thinking "Hang on a bit ..." and should consider whether another unit is appropriate. But tenth thou goes through on the nod. Flying at 40,000 ft sounds fairly innocuous even to Ms Nervous Flyer. Translate to 7 and 1/2 miles ......


John Bromley02/04/2014 23:31:14
84 forum posts

Lofty, I must pick up a Zeus booklet next time I visit my local tool shop. Lots of useful stuff in those.

Danny M2Z03/04/2014 02:08:34
915 forum posts
292 photos

G'day. I have an ancient calculator on the workbench, my first ever, it dates back to 1975, it's a Sharp Elsimate.

Just punch the fraction in and voila, decimal units Eg; 3/8 = 0.375. Work to the first 4 places, as fractions such as 1/3 will give you an infinite number (0.3333333333 nausuem). (In this case, 1/3 is more accurate imho)

I would be lost without the battered old Sharp actually. I have noted some recent digital calipers that offer a readout in fractions, but this seems to be more of a gimmick.

Regards * Danny M *

Russell Furzer03/04/2014 02:56:00
49 forum posts
8 photos

I have a poster (that I made up) with the fractions, thousands and metric conversion in a 3 column spreadsheet. Makes it a no-brainer.

Although my school vintage makes me metric as a first language (meh oui!), my hodge-podge of rules, calipers, mikes and machine scales means that I have learnt to cope.

In fact for things that I am making up from scrap-bin obtanium, I will choose to use metric or imperial based on the size of the scrap/stock, the size of the finished item or the machine i'll use the most for the job.

Eg If I want a cylinder "about an inch", I don't bother with 25.4mm. If I want a cylinder "about 20mm", I don't bother with 3/4" - UNLESS it matters.

Also the fact that 40 thou is pretty close to 1mm is helpful. My milling machine has thous on the knee and mm on the quill !!


Clive Hartland03/04/2014 07:45:44
2630 forum posts
40 photos

John, re. the 128th Vernier. I find it useful on the wood working as I set the fence against the blade, it is much better than using a hard to see ruler. The Vernier was bought in Germany, Kanon by name and I did not notice the 128ths until I was home ! At least I found a use for it.


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