Imperial fractions on drawings.

[steveteeParticipant @stevetee]

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 .

[jason udallParticipant @jasonudall57142]

Steve tee..as the poster of the ” 0.250 or 0.25000..? Moment “.It apears I need to clarify what I mean.. ( yeah yeah again)..
I get drawings for parts..
With say a 1/4″ hole
3/32 holes and reamed hole of 0.093″..-0.0 + 0.5 thou..ok sofar..
Now we allknow 3/32 =0.09375..so could drill all the 3/32 holes 0.091 and ream the required holes?..so look at the tolerances for the 3/32..aha fractional dimentions +/-1/64″..bingo..so off we go..but oh no..the 3/32 have go to be 3/32 …
See what i mean….
So another process another gauge..for a hole that to the drawing has a 1/64 tolerance..
End rant…..

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

[MuzzerParticipant @muzzer]

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

[Bill PudneyParticipant @billpudney37759]

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

[Martin KyteParticipant @martinkyte99762]

_"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

[Gordon WParticipant @gordonw]

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.

[Neil WyattModerator @neilwyatt]

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

Neil

[Martin KyteParticipant @martinkyte99762]

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

[Roderick JenkinsParticipant @roderickjenkins93242]

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

[roy entwistleParticipant @royentwistle24699]

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

[Gordon WParticipant @gordonw]

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!

[Neil WyattModerator @neilwyatt]

FACEPALM!

Someone should proof read my posts…

Neil

[Martin KyteParticipant @martinkyte99762]

So you have proved my point then Gordon.

I does depend on how the dimension is written.

best Martin

[Keith LongParticipant @keithlong89920]

Posted by Roderick Jenkins on 09/04/2014 12:18:41:

By the way, are equivalents of the constant pitch screw series (26, 32 and 40 tpi) available in the metric system?

Rod

According to my 1968 copy of Kempe's Engineers Year Book – yes there are, but not quite the same as the imperial set.

Kempe's shows 0.5mm pitch as being available in 4 to 5.5 mm dia – (actually Tracy Tools list taps and dies from 3-6mm), Kempe's shows 0.75mm pitch from 6 – 11 mm (Tracy from 4 – 18 with gaps) and 1mm pitch Kempe's shows 8-30 (Tracy 5 – 27mm).

So it looks as though you could work with all metric if you wanted.

[Michael GilliganParticipant @michaelgilligan61133]

Thinking back to the original post …

This is probably the tidiest conversion chart I have yet seen.

MichaelG.

[Bill PudneyParticipant @billpudney37759]

Mention has been made about tolerances being implied by the use of fractional or decimal dimensions. In my twenty five years employed as a draughtsman, in a variety of companies, it was always stated on the drawing, either in the title block or as a note that "Unless stated otherwise tolerances are……". Clearly before the globalisation of everything there would have been company standards that would have been unique to that organisation, maybe even different standards within organisations. However any organisation that isn't a "man in a shed", that requires local or tribal knowledge to produce a good part has probably gone the way of the dodo by now.

My boat's still floating, sorry

cheers

Bill

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

removed

Edited By MICHAEL WILLIAMS on 10/04/2014 08:42:23

[BazyleParticipant @bazyle]

Back to start. There are still lots of lathes etc with no dials as was normal and an extra even for Myfords back a while. It is very easy to estimate 1/8 of a turn, pretty easy to estimate 1/16 of a turn and below that "a tad". These machines may also have had 8 or 16 tpi screws so very easily align to fractions. I have a dial free machine with a 10tpi screw so am often working in 160ths. Only the last couple of cuts of a tad and then a smidgin need care to fit the mating item.

Industrially though a part might go from CAD to CNC without drawings when a container load get delivered to the ISO9001 customer a % go to their metrology or QA lab. They ask for the drawing and tolerances to inspect against and the whole delivery goes into quarantine until that is provided bringing the production lines to a halt.

Last year we had happily bought a million plug/socket pairs from an Americal company hence supplied as 1/4 inch. No need for spec 'don't care just make your plug fit your socket." as we were buing the whole not a part. Then some bright spark wants to try a different plug supplier and a modern British spec has to be in mm. So I had to spec 6.3mm with a big enough tolerance to cover the 1/4 in part they would actually supply as a metric part is a special and the whole world uses the 1/4 inch which is made in billions.

[NobbyParticipant @nobby]

HI
Most modern camera's still use 1/4" whit thread tripod mount
Nobby

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Nobby on 10/04/2014 13:03:02:

HI
Most modern camera's still use 1/4" whit thread tripod mount
Nobby

.

That's what I thought, Nobby

But I was proved wrong on a recent thread.

… Apparently, the Tripod Mount got a nasty dose of DIN Standardization.

The specified thread is now a sloppy-fit 1/4" 20 UNC … selected to retain "compatibility" with the proper Whitworth ones. [Sir Joseph must be turning in his Grave]

MichaelG.

Edited By Michael Gilligan on 10/04/2014 22:15:45

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