Why is the world of model engineering still imperial?

I still think that the whole "metric/imperial stuck in the dark ages" argument is a bit of a red herring. It's easier with a DRO and its metric/imperial switch, but even decimal imperial dimensions aren't that bad with a calculator if you are using metric graduated handwheels or vice versa. Well, apart from the Myford 125 division leadscrew handwheel anyway - a pain for dimensions greater than one revolution.

What really fouls it up for me is the use of fractional dimensions on the still-used drawings from our usual suppliers. Pretty much anything I use in the workshop, including working copies of the original drawings, will end up with scribbled decimal equivalents because, curiously, none of my machines are calibrated in fractions...

Totally agree regarding the Myford lead screw handwheel but you have to admit that it is graduated in 1/8”

I also have a saw table that’s good at cutting things in half.

regards Martin

I am going to help make this thread go to fo. 11! Such an emotive subject just runs and runs. What I really hate about Imperial length measurement, is like an old article reproduced in the latest MEW. The offending item has 3 diff. dias., so each one has its length shown just above, 1 1/8", 7/16", 3/4". Nothing wrong in that, other than in order to ascertain how long a piece of bar to cut off, I have to add 3 dimensions to obtain the total, where each dimension has a different denominator in the fraction. Fiddling about with those could so easily cause an error. If it was in metric, 29mm, 11mm, 19mm, it would be much quicker to add up to 59mm with no risk of error, at least not due to all those different fractions. Yes I know the design was decades ago and the world has moved on as they say, but I'm only using that as a convenient example.

One very useful chart on my workshop wall, well, actually a framed cork noticeboard on the door, lists in Imperial decimal order, sizes of all the number, letter, fraction and metric drills. So useful in quickly deciding which best to use for an appropriate cutting allowance for a reamer! It's also a help in deciding what tapping size drill to use for a desired % depth of engagement. Only other items on the door are a row of screwed hooks for the exterior padlocks and an old tee shirt being recycled as a dirty hand wipe before I go indoors!

Edited By DMB on 10/03/2023 22:05:59

DMB the issue really isn’t about the system of units it’s about bad drawing practice. Admittedly the imperial dimensions compounds the difficulty but you should be able to read every dimension you need straight from the drawing.

regards Martin

I just add the highest denominator, so 2 plus 7 plus 24

Mental arithmetic

It's just a habit if you do it all the time, but issues do arise if you are an occasional dabbler

...or should that be 12...

Edited By Ady1 on 10/03/2023 23:07:13

Hi DMB, easy enough on the right sort of calculator, and no messing around with imperial decimals.

Only used the two calculators to save changing the mode on the left hand one. The hardest part is learning how to use the left hand calculator, but it didn't take too long.

Regards Nick.

I vaguely remember being drilled, and drilled, and drilled, on "mental arithmetic" at school, which included adding and subtracting fractions and the like. So in those days every schoolboy and girl must have been able to instantly recognise how many 16ths in the 1/8 and the 3/4 and add them to the 7/16 without any drama. I struggle to do it now though. I blame years of working in metric for that.

Hi Hopper, yes it's harder to remember these days, but there is no need too, unless you enjoy mental arithmetic.

Here's an improper fraction you may or may not recognise, 127/5, but easy to work out.

Regards Nick.

Sort of shot in foot moment, the first figure was 1 1/8 which is 18/16. Bit of an easy example, try it with some 64ths mixed in.

JA look at this thread for my post on 8 Jan 23 Special characters

Martin C

Keith & Martin

Thanks for the details on super and subscripts.

Martin, I have just taken a look at your link. How am I going to remember that at my age? Perhaps I could if I removed Imperial Units from my brain.

JA

You can also create a post in Word or some other suitable word processor then copy and paste into here like this:

x² x³ x⁴ x⁵

x^-1 x^-2 x^-3 x^-4 x^-5

√()

Or even just copy from this post and paste into your own text

Martin C

Edited By Martin Connelly on 11/03/2023 13:50:44

In the dark satanic mills of Clydeside in the sixties where I served my time (as an apprentice marine engine fitter) most of the drawing dimensions were in fractions - possibly just as well, as there were very few micrometers outside of the tool room.

The tolerance on Fractional sizes were meant to be to the nearest similar mark on a steel rule, ie a 16th inch size was to the nearest sixteenth on a steel rule.

Decimal sizes usually had an actual tolerance specified.

We all had learnt our tables up to 12 times at school, now we had to learn our 1/8ths times table - 1/8th equals 0.125", etc. And 3/16" was just half of 3/8. 17/64" was difficult.

As an aside to another discussion here, a 6" rule was also used as a teaspoon. Hence "how much sugar do you take in your tea?" - answer "an inch and three quarters"

Dick

If using a proper word processor (Word, Libre office or similar) you can use Unicode to get special characters/effects. For instance if you type 221A then alt X you get the square root sign. It doesn't work in this forum editor, but if you're typing anything complicated it pays to use something offline as it doesn't get lost as easily.

One thing I have learned to dislike about the imperial system is that 1/16"s are at the hanging loop end of my 6" engineers rule and not the other end. Those dam 1/20"s keep catching me out.

That is all!

Agreed, whgatever the unit used additive dimensioning is bad practice, inviting trounle when twom or more components have to be jojned ntogether.As an Apprentice, we were taught that the correct way to dimension a drawing, to avoid tolerance build ups, was to have a common origin for each axis.

In that way, tghe tolerance on the dimension for hole 12 was the same as for hole 1, so that there not 11 tolerances displacing hole 12 from the ideal postion. Who wants to find hole 12 is 0.55 out of postion, rather than 0.005?

That would be a problem whatever units of length were used.

Howard

That is the way I was taught, oh-so-many years ago in the Marconi DO School, as an electronic engineering apprentice. It also translates well into useful 2D drawings for use in the workshop when you are using a DRO-equipped mill, for example. Set up your DRO datum to match the drawing and away you go.

However, those trained this way are the worst possible students when it comes to 3D CAD modelling! The usual way to build up a 3D model is to start with a "sketch" - which is much more than a sketch and does contain accurate outlines and dimensions. Then you extrude that sketch into a 3D shape which can then be further modified. However, the idea is that the sketch represents "design intent" rather than the dimensions used in a traditional engineering drawing. You would mark two holes as being some distance apart and equi-spaced about the centre of the work, for example, rather than dimensioning each separately from the datum. Why? Well, later design changes - like making the piece a bit longer or shorter - will leave the holes the same distance apart and still equi-spaced about the centre. Or just change the hole centre spacing and they remain symmetric about the work centre. That's a simple example and the real world can get much more complicated but the same principle holds. The model expresses "design intent" and forget engineering drawing conventions. Those are handled later in the process. However, this approach so strongly contradicts the fundamental principles of traditional draughting that it takes longer to get those habits unlearnt than it does to get the new approach on board! And some never make the transition and never really get to grips with 3D CAD.

The 3D CAD packages that I have used do allow entry of fractions, though - and even calculations in the dimensions used in sketches. Useful when you are building a 3D model from old 2D engineering drawings.

Spent 50 years in the Construction Industry (Structural Engineer) and remember as the drawing office junior in 1971 one of my tasks was converting imperial dimension to metric and putting the Letraset “Key” on drawings to show they were metric.
Structural Steelwork is still rolled in imperial sizes merely converted to metric and rounded, ie 203x102 UB right up to 914x305 UB. Similarly masonry is largely imperial converted to metric and rounded, perhaps a little more so as it is hardly a precise material.

Having been taught imperial at school and metric in further education I find switching between the two for larger dimensions easy but must admit to a “feeling” for a couple of thou that I don’t have for 2 decimal places of a millimetre.

Guess I join the Old Gits Club 😜

This thread just runs and runs.

But this guy settled all this some time ago.

Grand Unified Scale - https://www.youtube.com/watch?v=KX6FpLBDOFs

Also I may have posted this before. (But I can't search it out.)

Regards.

Frank C.

A work of comic genius

Hopefully he also has one scaled in 1/128 of an inch, with the convenient metric numbering

MichaelG.

Edited By Michael Gilligan on 06/04/2023 19:47:47