Imperial fractions on drawings.

Basically true, but errors can still occur.

A few years ago, I was a member of a small team who had to design and build a special wheelchair. One member, who was very experienced with Autocad, accepted the job of creating the drawings. We should have heeded the warning signals when the first drawings appeared with dimensions in 1/10000", but we continued until one day we got a drawing where two important parts simply wouldn't fit. A dimension was exactly 1" out, to 1/10000"!. He had committed the cardinal sin of modifying a part without updating the dimensions.

Gary

Edited By Gary Wooding on 06/04/2014 17:55:00

I have expressed my thoughts about the metric system elsewhere on this forum. In 1841 Joseph Whitworth developed the worlds first standardised screwthread system, it uses fractional sizes. In 1969 the Americans went to the moon, in imperial measurements. That'll do me. Like the earlier poster I was expected to be able to convert fractions to decimal sizes from memory, it's a useful skill. Oh and by the way .250 and .250000000000000000 are exactly the same thing ......... it's like saying " Ooh look I have 25.00000p in my pocket". Nought is nought to however many decimal places you like.

.

But 0.0001 is not the same thing as 0.0000

... even if using three decimal places would appear to show that it is.

MichaelG.

Edited By Michael Gilligan on 08/04/2014 07:40:19

But I think you will find when they went to the moon they did not use fractional measurements.

Today even the Americans are modernising as many of their bigger car manufacturers are moving over to use metric, so it can't be long now before they catch up with the rest of the world

Jo

One tenthousandth and zero are never going to be the same , how could they?

Here is a link to New Scientist which carries alink criticising NASA for still using imperial measurements in 2009. One sentence reads "But NASA still can't make the jump to metric.".

This would tend to suggest that it would be unlikely that the Apollo program 40 years earlier used the metric system.

.

Agreed ... So what point were you making with the reminder assertion that:

"Oh and by the way .250 and .250000000000000000 are exactly the same thing"

I'm not being funny ... it's a genuine question.

MichaelG.

Edited By Michael Gilligan on 08/04/2014 17:42:52

Regarding the USA and Canada and metric -

1) the "kids" doing 3D printing over here are using metric, and seem to have no problems. That includes kids in the USA.

2) had a discussion with a retired construction guy in my Canadian province who stated "all construction plans are in inch", unfortunately, in the province of Ontario, the ton of construction site plans in the office where my wife works, are all in metric. When asked, the engineers said that it's been that way for at least a decade.

So, the old "inch to my grave" guys are out of touch with reality, and will be more so in the coming years.

I don't care one way or the other, but, gosh, the world is metric, for better or worse.

Another John.

It was an observation on an earlier poster who stated : "..I hate fractions used in measuring...it gives me that do they mean 0.250 or 0.25000..? Moment "

and then went on from there. Perhaps for too long .

Edited By jason udall on 09/04/2014 01:52:24

Further to John AS's comments, as a Brit living in Canada, I was almost shocked that find that there is more metrication in general here in Canada than in the UK. It's true that metric is the default in the UK (and indeed almost anywhere outside of the US) but there is so much imperial stuff hanging on outside of the engineering world.

In the UK the vehicles work in miles, beer is in pints, distances are often measured in inches and feet and you can still buy food and drink measured in pounds (albeit also measured in kg). In Canada, vehicles use km and litres. Food is generally in litres and kg. Engineers work in SI units, unless they are working within a US-centric business.

Where it all falls down is the local availability of metric tools, fasteners etc. Mostly the supply of these parts piggy backs on the back of the US supply chain, so I've ended up having to buy a variety of imperial drills and threading tools.

Of course, the most shocking thing is the almost complete lack of pubs in the accepted meaning of the word. And when you finally find one and order a pint, they give you something that is almost 20% short. That's hard to live with as a Scottish-blooded Yorkshireman....

Murray

There are a couple of things that float my boat.

1/ When I earned a crust as a draughtsman I hated those firms that required drawings dimensioned in fractions. It's very difficult and time consuming to get the actual characters on the drawing sheet looking as if they were not done by a drunken spider. I had been trained to take a pride in the presentation of my drawings

2/ When a drawing is presented with dimensions to three or four places of decimals, e.g. 1.2500, when clearly the feature only requires one or two places. Even worse is a feature that has been drawn and dimensioned to 4 places, e.g 4.3456 when it might be a clearance hole for a fastener, or some other non critical feature. Simply because the CAD operator couldn't be bothered to look at it critically, or set parameters etc etc. The practical measuring of a feature to four places of decimals in metric is close to impossible without incredibly expensive facilities anyway.

Most times, when my boat gets floated, I just bite my tongue and carry on. Sometimes I just cannot help responding!

cheers

Bill

_"Oh and by the way .250 and .250000000000000000 are exactly the same thing"_

Wrong. Mathematically it is but engineering assumes tolerances are 'built' into the specified measurement.

Hence

.250 is the same as 0.25001 or 0.24999 but

.250000000000000000 is definitely not.

Martin

I get fed up with this metric v inch stuff, crops up every year. A thing is a thing, and the size is the same whatever units are used. The posts about how many decimal places in an imperial measure ( these are metric btw )are just assumptions made , drawings should state tolerances and no assumptions made. If I am asked to make a 1/4" hole that's what they get, made with a 1/4" drill.

Of course, 1/8, being a ratio of two integers, is 0.25000r which is more precise than either 250 or .250000000000000000

Neil

Exactly Gordon. But if you were asked to make a 0.2500 hole you would probably bring it to size with a reamer.I agree with you it's all a bit academic as most of the stuff we make only has to fit other stuff we make.

Martin

I agree that that implied tolerances are assumed in model engineering drawings. In this case though, .250 surely implies .250 +/- 0.0005 i.e .2495 to .2555.

Reading this thread, I've come to the conclusion that fractions do have a place in imperial drawings. 1/8" implies 1/8" +/- 1/64". That really tells you more than the decimal alternative of 0.12" +/- 005. Often the drawing implies the use of stock materials which you will have bought as 1/8", not 0.12". And if you need a hole for for a 0.125" shaft you will get a 1/8" reamer from the draw.

I'm 58. My whole technical education was in the metric system (CGS, MKS, SI). In order to become a model engineer I've had to learn the imperial system. It's not rocket science (well, unless you're in the US).

Gordon's point reinforces the need, in MODEL ENGINEERING for text on the drawing that says drill, ream, bore etc.  This in either imperial or metric (as in using the metre).

I'm all for the metric system, but while imperial drawings exist, I think the use of fractions is more logical. By the way, are equivalents of the constant pitch screw series (26, 32 and 40 tpi) available in the metric system?

Rod

Edited By Roderick Jenkins on 09/04/2014 12:21:31

Edited By Roderick Jenkins on 09/04/2014 12:25:49

Neil Of course, 1/8, being a ratio of two integers, is 0.25000r which is more precise than either 250 or .250000000000000000

If I remember rightly 1/8 is 0.125

Roy

Martin- no ,if I was asked for a 0.2500 " dia. hole I would refuse the job, not having tooling or measuring equipment to check it. But that would depend on the required tolerance!

FACEPALM!

Someone should proof read my posts...

Neil

So you have proved my point then Gordon.

I does depend on how the dimension is written.

best Martin