Mixing fractions and decimal units in an imperial drawing

Hopper. The fractions were generally measured with a rule even on lathe diameter work. That way they don't generate confusion. The main problem is that this takes some practice and just isn't on with the metric system unless things are made to 0.5mm. This is why imperial engineering rules have such a lot of different scales on them. Other than dia I still work that way. If metric I guess to 1/3, 1/4 or 1/5 of a mm.

These days if I need an new imperial rule I have to buy from the USA and hopefully get one with 1/100" on it but the way things were usually drawn 1/64" was fine - even for grinding allowances on turned parts.

My recollection of many model engineering drawings is that the people who did them have never worked for a company that had sensible standards. Some aspects can need construction notes because of the way they will be made. Eg a piston in a bore needs a certain fit. Within reason diameter doesn't matter. A shaft might be supported at each end. The hole size used at each end may be important also the fact that they closely align with each other but actual positioning may be less important. For some people these factors would be obvious just from looking at the drawings others may need constructional articles. The alternative would be to assume people had jig borers, hole micrometers and a number of other things. Ideally thing need to be thought out so that they need minimal equipment. Say a lathe and a vertical slide.

Mmm. The dividing head probably needs a T slotted cross slide as well. Many more complicated model engineering tooling designs did.

John

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Edited By Ajohnw on 06/11/2016 10:01:02

John, imperial scale rules are still available, RS and Buck and Hackman sell them to name just two.

Martin

For those that don't use or have not seen fractions on a drawing how do you specify threads?

0.375BSP or 0.25BSF just does not look right.

And what about SWG do you use that or just the decimal thickness?

I've usually bought Starrett from the USA Martin via ebay but will take a look at what those people offer as I could do with a better 12" really. Above that I have a rather tarnished 30" vernier. Think I have used it twice but it was more or less given too me.

John

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I have an old book called " The Boilermakers Assistant" published in 1904 and this has measurements such as 7 3/8 -1/16 and this is consistant throughout, it is always -1/16. A very easy system to use in fractions.

To answer Jason regarding sheet metal thicknesses, I've always been used to SWG or the American equivalent and this was working on aircraft parts.

I've also worked on Airbus parts in metric but cannot remember how sheet metal was specified but I suspect metric.

Click the Machine DRO ad at right, they have plenty of imperial measuring equipment including M&W rules.

Neil

My old book (ICS Geometrical Drawing Mechanical Drawing Sketching Projections Development of Surfaces, London 1907) gives the following advice, which is consistent with earlier posts.

This drawing (part of a commutator) is an example of mixed fractional and decimal dimensioning:

Of interest is that decimal dimensions omit zeros before the decimal point, i.e. .063" rather than 0.063"

Here's the text (from 122 onwards) :

Fans of imperial fractions may note the very last sentence. It addresses a disadvantage of that system. Extra calculations are needed to convert thousandths into 64ths and use of a table is recommended to avoid doing them.

I have a 1947 book that pretty much repeats the same advice about dimensioning. Does anyone know when fractions in drawings were phased out in favour of decimal inches?

Dave

That reeks of --- " I borrowed it & never returned it !!!!!!!"

Which we have all suffered from in some form or other over the years

Only thing I would like to do that with would be the mother in law - but it never seems to work-- Like a damned pigeon !!!

No. Some one begged me to have it as I kept refusing. The price was a tenner and it took some persuasion to get me to buy it. Must admit the couple of times I've used it a rule would have been a bit dodgy.

I need to mark out a 3 4 5 triangle on the floor. Should I use mm or m ? Think I'll use feet - the type on a tape measure.

John

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This is a toolroom style 6" rule Neil - not those stupid things with mixed graduations along their length.

~kAAOSwcUBYHtXG">http://www.ebay.com/itm/Starrett-C304SRE-6-Semi-Flexible-Steel-Rule-with-4R-Inch-Graduations-/232135140790?hash=item360c5525b6:g~kAAOSwcUBYHtXG

I like to have 2. One rigid and one flexible.

John

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Make sure you use a calibrated one, they vary in length by quite a bit. Try comparing 2 different makes of 5 metre tape.

When BS 5740 ( think that was the No.)was first introduced there was a lot of fuss over calibrated tapes in the building industry - now long forgotten. But certainly the old carpenters boxwood 3 ft rules varied by over 3/16 inch as we used to compare them in the joinery shop when arguing about fit of various components.

If you buy a measuring wheel from Screwfix & measure the same 30 metres twice you will often find it varies by up to 1.5 metres each time you run the 30 metres

John you can get the same thing from MSC by either Mitutoyo or the SPI EZ-View for half the price and no need to import, I have a couple of the flexi EZ-View ones 6" and 12"

True in part. Look on 'a well-known auction site' for fraction calipers.

George

Edited By Georgineer on 06/11/2016 18:44:30

When emeshed in these fractions / decimal / metric controversies its easy to forget that the underlying philosophies of the systems are different. The imperial fractional system is based on integer (whole number) divisions of what ever base unit is considered suitable. In the real world outside of machining levels of precision that's a very useful way of working. If you need more than one base unit to span the distance in sensible numbers you "and" them together. Such as 5 miles and 387 yards and 2 feet and 8 inches and 11/16 inches. Apart from being immediately and obviously absurd, feet and yards should never be mixed, its clear which component defines the accuracy. The 11/16 which implies ± 1/64 accuracy, two steps down on the divisor chain. In the real imperial world you should never use more than two base units anyway. Its miles and yards, feet and inches (with fractions if need be).

The beauty of the basic imperial system is that its relies on sensible size baseline units, thats why there are so many of them, with minimal faffing about to produce stuff that fits. Don't forget that imperial was developed for and used by essentially innumerate folk. For example you can set a roof on an imperial house pretty much without calculation. Heck if you are an American working to 4 ft module you can build the whole darn thing without maths. Setting a roof in metric can give even the pros pause for thought. The guy who helped me build my workshop got into heaps of trouble until he gave up being modern & metric.

Thou's and the like came in to align inch divisors with base 10 maths. Which has its points but inevitably leads to irrational numbers and incomplete divisions. When adding up you have to have enough decimals which soon leads to absurdity with fairly precise work generating a conflict between design figures and tolerances. Fractions are exact by design. Strictly you should and out the different divisors too 11/16 being 1/2" and 3/8" and 1/16" but folk familier with the working do it automatically in their head. As ever textbooks make the easy hard.

Metric of course is an additive and subtractive system working up and down from a baseline unit with suitable mutlipliers to keep the number of digits in bounds. If you don't keep your brain engaged it rapidly becomes absurd. Being aligned with the base 10 number system can make maths easy but it becomes hard to spot errors. I'd like pound for everytime I've had to explain the absurbity of the metric equivalent of that 5 miles example.

Clive.

I would suggest that you can just as easily build a house on a 1 metre module than a 4 foot one & your friend building your workshop probably got into trouble because he was possibly trying to think in imperial as well as metric. Clearly not a tradesman experienced in metric units & not a reason to reject the metric unit. I have used both & have difficulty with imperial units. As a joinery manufacturer I often had to go onto my shop floor & do some setting out or to explain something. Messing about in 1.8's. 32.s etc was a nightmare & millimetres was far more accurate for setting out. My older machinery was all converted to metric scales until renewed with new plant to avoid errors. My joiners & machinists understood it easily as well.

In my early years on site I used to use a steel square to set out roofs ( A roofing square) for the carpenters & mine was in metric configuration. This was far easier than our old contract managers imperial one as one could simply multiply by 10 when scaling rafter lengths etc.

I can assure you that the mix of feet & yards has caused lots of problems as the units are so close. We had a lorry load of DPC that lasted years because the site ordered yards instead of feet ( then the secretary typed it as rolls but that is another story) & motorway curbs that were a yard long not a foot. The list went on.

That is why the industry (construction) chose millimetres & metres. There can be little confusion between the 2 being such a big difference in size

The problem comes when people try to think between the two systems. Once they are tuned to think metric & forget imperial they find it easy. Do mnot think of a room in feet , think of it in metres. Unfortunately in the construction industry problems arise because in schools kids are taught to think in centimetres as well & carry that forward in future life.

Edited By Sam Longley 1 on 06/11/2016 19:53:13

That's exactly what my grandfather told me, thirty years ago - work in what you're given, and never try to convert. While I think in metric, imperial measurements aren't a problem.

Clive, I'm afraid you're going to have to explain the absurdity of your 5 miles example again, as I can't see how 11.45km is even remotely complicated? Whereas converting the mixed imperial units into one number to get that 11.45km was a pain.

To their amazement the ancient Greeks came unstuck with that notion when they tried to solve this simple example. Given a right-angled triangle with two sides of length 1, what is the length of the diagonal expressed as a fraction?

Fractions are an excellent tool but they also have serious limitations. I'm not suggesting decimals are perfect but they are considerably more general.

Dave

Edited By SillyOldDuffer on 06/11/2016 20:56:38

Should that be 15/16" being 1/2" and 3/8" and 1/16"

You greatly confuse me at times, you post said you could do with another 12" rule...

But you mean like this Mitutoyu one: www.machine-dro.co.uk/mitutoyo-fully-flexible-steel-rule-150mm-6.html

Neil