Drawing Projections

As someone who was at school before metric became standard I have never really mastered cms, despite doing O-Level early enough to encounter CGS. For ordinary life I tend to think of things either in imperial, or, after a career in engineering, in mm. However, I did measure something in cm today, so there is hope yet.

Although it is a very sensible system there is a problem with the metric multiplier abbreviations. It is all very well laughing at the press when they write mg for micrograms, or use the everyday m for million where M might have been better, but really, who thought a system with three multipliers with vastly different values, all, abbreviated to various forms of 'm' was practical?

The trapezoid shape is representative of a frustrum of a cone and is the side elevation or main view of the drawing

The rule I was taught when in the drawing office is very simple,

For first angle projection view the side elevation from the right hand side and draw the resulting end view on the left hand side of the side elevation( i.e view through the object)

For third angle projection view the side elevation from the right hand side as before and draw the resulting end view on the right hand side of the side elevation.

The same rule applies for the plan view.

Paul

Quote; "W J Hall

I'm glad it's not just me, I don't understand cms either and I have to convert them to mm 's before they make any sense.

Only dressmakers use centimetres, bless 'em!

Duffer - Not just dressmakers, respiratory therapy people too - I recently had some tests done and the pressure in the therapy was expressed as "centimetres of water column" in a manometer. JD

Furniture Stores use centimetres.

I have then to convert them to imperial measurements for the wife who does not do metric!

Edited By V8Eng on 05/11/2016 18:53:43

My stepson is a kitchen fitter. Oh how we have laughed when his insistence on millimetres collides with my DIY centimetres...

Neil

A Model Engineer using centimetres?

.

To my surprise ... That lovely Swiss pantograph that I'm tinkering with is calibrated in centimetres.

MichaelG.

.

Edited By Michael Gilligan on 05/11/2016 19:24:23

Many areas in Europe use cm's and other divisions and take no notice what so ever of what ISO say. There reason is simple - how for instance can something be about 150mm long as mm are rather small. About 15 cms makes more sense but some might say about 1 1/2 what ever the 10cm unit is - I can't remember. Prior to ISO's stupidity I would have had no problem remembering what they were at all. ccs went the same way but are still widely used.

John

-

> It took me a while to work out that it was graduated in Decimetres, the only example that I have uncounted of this multiplier being used.

Quite so. But for further examples of the use of multipliers which are not powers of 1000, look no further than meteorology. I expect most of us are used to the millibar, but this is now a deprecated term and the replacement is exactly the same size, but with a brief nod to the SI unit of pressure (the pascal). It is the hectopascal (hPa). This has even infiltrated into aviation where one might hear on the radio that the QNH is 990 hectopascals. (QNH is a shortform code for the pressure at sea-level).

As if that weren't enough, a chart of the upper air will have height contours labelled in decametres (abbreviation: dam).

Here's a chart with both together!

Trevor

PS In case you are wondering what 'Thickness' means: in this case it is the 1000-500 hPa thickness which is the vertical distance between where the pressure is 1000 hPa and where it is 500 hPa. It represents an average temperature in that layer of the atmosphere.

I'll always remember the apologetic mention of sorry folk we now have to talk tonnes per m^2 and can't use tonnes per cm^2 any more. Frown - ISO say so.

The problem is down to visualisation again. Easy to look at sensible things and thing tons per inch^2. Not too bad to think in terms of tonnes per cm^2 but 1m^2 is just too big, May as well use hectares or square kilometers.

Doesn't matter didley squat when calculations are done but does when it comes down to rough visualisations.

John

-

OK, I was completely wrong about only dressmakers using centimetres.

Everyone knows that dressmakers use inches. Of which there are 30 to the metre:

Dave

Where did you get the tape, Dave?

... Slimmers' World ?

MichaelG.

Busted again!

I still refuse to believe that one-thousand-four-hundred-and-thirty-five-point-one millimetres is easier than four foot eight and a half. And a one-thousand-two-hundred-and-ninteen-point-two by two-thousand-four-hundred-and-thirty-eight-point-four millimetre sheet of plywood is easier than a four by eight. Especially if you need to cut it into four equal widths of shelving.

And how mad is a fifteen-thousand-two-hundred-and-forty-millimetre long room, rather than a nice even fifty feet. I see this on architectural drawings. Can anyone really visualize fifteen-thousand-two-hundred-and-forty-millimetres. Fifty feet is easy. Sixteen and a half yards / fifteen and a quarter metres even.

And don't get me started on Americans and 'millimeters' et al.

Ian

Off-topic, I know ... but here is an interesting tape measure: **LINK**

http://www.pitape.co.uk/aboutus.asp

MichaelG.

The answer to about 666mm problem is to always follow it with a tolerance. About 67cm isn't much better.About 6 1/2 of the units I can't remember is probably the best one. I chose 666 as it's the number of the devil. It's also about 2ft.

John

-

You would not, you would use millimetres & metres,( which are the correct units to use in the building industry,) hence for the last one you would say 15.24 metres

As for your sheet of ply you will notice that it is not 3/4" thick etc it is 18mm etc

I would further suggest that you try working out the area of 4 ft 12 15/32" multiplied by 2 ft 6 3/16"

Then perhaps you might welcome the introduction of the metric unit.

Fortunately my training started just as metrication in the building industry was introduced because it drove my father mad trying to teach me duo decimals

I am late to this thread re 1st and 3rd angle projection. We had a draughtsman who regularly used both on the same drawing and could not see his errors. We got used to it in the toolroom and didn't bother trying to correct him as there was no hope.

Sam is 4ft 12 15/32" 5ft 15/32" in my book ?