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Drawing Projections

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KWIL06/11/2016 10:44:34
3681 forum posts
70 photos

Micheal,

I note with interest the Pi Tape is 1/2" wide and 0.25mm thick.

JasonB06/11/2016 13:05:38
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25215 forum posts
3105 photos
1 articles
Posted by Ajohnw on 05/11/2016 23:17:48:

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

-

I hope you are not basing your kitchen on 666mm being "about 2ft" you may get away with thinking 600mm is 2ft but 666mm is just over 26"

Enough!06/11/2016 18:57:13
1719 forum posts
1 photos
Posted by Chris Evans 6 on 06/11/2016 08:32:41:

We had a draughtsman who regularly used both on the same drawing and could not see his errors.

Then he wasn't a draughtsman (though he may have been masquerading as one). No more than a toolroom person who uses his micrometer as a c-clamp can be called a toolmaker (yes, I've seen it and yes it was lightly clamped and quite temporary, but nonetheless ....).

Paul M10/12/2020 08:48:30
86 forum posts
4 photos

I have read through these posts and found them interesting.

I went to secondary school during the 60's and was taught 1st and 3rd angle orthographic projection. We worked primarily in imperial for all our measuring in all subjects. It was not until I was teaching during the 70's that metric was introduced and was taught. The transition was gradual and costly having to convert machines and all measuring equipment. Buying stock was interesting as metal remained in imperial sizes for quite a few years and again was gradually replaced by metric sizes.

I consider myself lucky in that I have no issues working in either metric or imperial apart from refusing to accept a centimetre as a valid way of expressing a measurement. I enjoy the mental mathematics when converting from one to the other. A bit like being married to a French lady - some conversation is in French some in English with the occasional use of a dictionary.

Peter Howell 110/12/2020 11:11:01
44 forum posts
4 photos

I agree with PaulM's comments. I must be a similar age to him and have no difficulty with either 1st or 3rd angle (providing you are told what the drawing is in) or metric / imperial. I sometimes use either. If I've measured something and it looks better (more rounded) in inches whereas it would be odd amount in metric I'll use inches. This particularly applies to DIY building jobs where accuracy in not so important so long as it fits.

I also avoid cm, never seen the point of them - they only add to the confusion.

My mantra for 3rd angle was "What you see - Where you see it".

old mart10/12/2020 20:59:36
4655 forum posts
304 photos

The drawings we used for Airbus, Westland, BAE usually had the projection symbols on them, but I cannot remember whether they were first or third angle.

Nigel Graham 210/12/2020 22:40:29
3293 forum posts
112 photos

What a drawing is dimensioned in is less the problem with me as I can work in both and I can convert if necessary. (I've a calculator, I also have one of those big Tracy Tools posters hanging in the workshop, using an old dress-hangar hooked over part of the overhead hoist - I knew I'd built that for a reason.)

More to the point is how it is dimensioned, and I do wish some of our drawings publishers would realise that machine tools are calibrated in either thousandths of inches or little bits of millimetres, not binary-fractions; and would think about how the part is made when deciding the datum face or corner.

I do agree the comments about the centimetre. I find them confusing. Tell me something is, oooh let's say, 10cm long and as a retired machine-shop worker and physics-lab assistant I have to convert it mentally to the proper 100mm. Then I'm happy. I can picture 100mm. It is four inches as near as dammit where real thous and proper bits of mm don't matter.

'

Placing the 1st / 3rd -angle note and symbol on a drawing, as Old Mart says, is normal trade practice.

'

I was intrigued to see in a local newspaper only the other day that the council is reviewing allotment rents, in .... the Rod!

Not sure how because the Rod is a lineal not areal unit, = 5.5 yards, about 5 metres. Perhaps it was a mis-quote of Square Rod.

As far as I recall, I was never taught the Rod, Pole or Perch even in Primary School way back in the early-1960s, where and when we had to learn such skills as Compound Multiplication to price commodities costing so-many £sd per Tons.cwt.qrs.

Michael Gilligan10/12/2020 23:44:59
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23121 forum posts
1360 photos
Posted by Nigel Graham 2 on 10/12/2020 22:40:29:


Perhaps it was a mis-quote of Square Rod.

.


Excellent deduction, Nigel yes

Confirmed by Ashford Allotment Society : **LINK**

http://www.ashfordallotmentsociety.org/how-allotments-are-measured/

Despite being overlooked by the National Allotment Society : **LINK**

https://www.nsalg.org.uk/allotment-info/

MichaelG.

Martin Kyte11/12/2020 11:02:20
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3445 forum posts
62 photos

Perhaps there is an argument for a new unit then. The BSA (British Standard Allotment).

All the Allotments round here are the same size and people either have one two or a half etc.

regards Martin

Frances IoM11/12/2020 12:42:53
1395 forum posts
30 photos
most Victorian areas of land are expressed in A R P - Acres, Rods(sq or otherwise) + Perches - 1A = 40R 1R = 4P
As it is likely that the council allotments were allocated during WW2 they would be simple fractions of an Acre - a typical garden for an agricultural labourer would be 0.5 Acre
roy entwistle11/12/2020 13:23:30
1716 forum posts

Frances IoM When I was at school, Rod, Pole and Perch were all 5 1/2 Yards

Roy

Dave Smith 1411/12/2020 13:51:52
222 forum posts
48 photos
Posted by Nigel Graham 2 on 10/12/2020 22:40:29:

More to the point is how it is dimensioned, and I do wish some of our drawings publishers would realise that machine tools are calibrated in either thousandths of inches or little bits of millimetres, not binary-fractions; and would think about how the part is made when deciding the datum face or corner.

Nigel you are so right!

Some comments on dimensions. A drawing is the design definition for a part or assembly and should have sufficient data for it to be manufactured and controls how the part operates or interacts with its environment or mating parts. Sometimes the best way to dimension a part for manufacture is at odds to the functional dimensions required to control the design definition and in these cases the functional dimensions prevail. This can be got around by production engineers using stage drawings during the manufacture process. With CNC machining from a 3D model the issue goes away providing the functional dimension tolerances are met at inspection.

However that does not excuse the questionable dimensioning that occurs model engineering drawings. To get around the issue on my Don Young Aspinall build I have a full 3D model and I produce drawing to suit me from that.

Dave

Keith Long11/12/2020 14:01:01
883 forum posts
11 photos

I think you'll find that in land measurement the " A R P " refers to Acres, Roods (not rods) and Perches, the ration being 40 Perches = 1 Rood, 4 Roods = 1 Acre.

Where I used to live in the West Midlands (used to be part of Worcestershire) there was an location Rood End, I'm not sure whether it was named after the land measure or had ecclesiastical connections as in rood screen.

Bill Davies 211/12/2020 14:23:45
357 forum posts
13 photos

Keith, perhaps naughty stuff went on (or came off).

Sorry, I've gone.

Bill

Bill Davies 211/12/2020 14:31:54
357 forum posts
13 photos

And further to Nigel's point about units, I haven't yet come across a trade that uses centimetres. As a school technician in recent years, I've used centimetres, but my engineering training from the late 60's informed me that SI units went in steps of 1000 (10 to the power of 3), so km, metres, mm, etc. Other units were depricated.

No doubt I will be corrected, perhaps workers in textiles use cm.

Bill

Rod Renshaw11/12/2020 16:29:50
438 forum posts
2 photos

I don't use the cm much either.

When I went into a school last year I saw young children doing some sort of practical work with card and scissors, and they had been issued with plastic rules calibrated only in whole cm. I asked a teacher about this and she said they were not allowed to use inches any more as they were regarded as obsolete, and the children could not understand anything as small as a mm. I suppose it was a solution to a perceived problem but it seemed rather a poor solution to me.

Rod

Edited By Rod Renshaw on 11/12/2020 16:30:30

Nick Clarke 311/12/2020 18:50:56
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1607 forum posts
69 photos
Posted by Rod Renshaw on 11/12/2020 16:29:50:

I don't use the cm much either.

When I went into a school last year I saw young children doing some sort of practical work with card and scissors, and they had been issued with plastic rules calibrated only in whole cm. I asked a teacher about this and she said they were not allowed to use inches any more as they were regarded as obsolete, and the children could not understand anything as small as a mm. I suppose it was a solution to a perceived problem but it seemed rather a poor solution to me.

Rod

Edited By Rod Renshaw on 11/12/2020 16:30:30

Our secondary age blind students, in the absence of any other condition, are expected to learn how to measure to the nearest 5mm or 1/2 cm with a ruler.

Had a quick check on a major educational supplier and the cm only rulers are listed as for younger children or early learning - ie Nursery or Reception classes in general, 3-5 year olds

Circlip11/12/2020 18:58:21
1723 forum posts

" I haven't yet come across a trade that uses centimetres. "

Dressmakers and Knitters

Regards Ian.

Peter Jones 2011/12/2020 19:20:22
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63 forum posts
9 photos
Posted by Nick Clarke 3 on 11/12/2020 18:50:56:
Posted by Rod Renshaw on 11/12/2020 16:29:50:

I don't use the cm much either.

When I went into a school last year I saw young children doing some sort of practical work with card and scissors, and they had been issued with plastic rules calibrated only in whole cm. I asked a teacher about this and she said they were not allowed to use inches any more as they were regarded as obsolete, and the children could not understand anything as small as a mm. I suppose it was a solution to a perceived problem but it seemed rather a poor solution to me.

Rod

Edited By Rod Renshaw on 11/12/2020 16:30:30

Our secondary age blind students, in the absence of any other condition, are expected to learn how to measure to the nearest 5mm or 1/2 cm with a ruler.

Had a quick check on a major educational supplier and the cm only rulers are listed as for younger children or early learning - ie Nursery or Reception classes in general, 3-5 year olds

Seems pretty sad state of affairs to me.

Back in the 1960's I remember being taught 1/16" measurements before I was 7 years old.

Seems various governments over the years have dumbed down education ?

HOWARDT11/12/2020 19:36:49
1081 forum posts
39 photos

I went three years engineering education in imperial to the fourth entirely in metric. But my job in a contract drawing office was still imperial but had to use the projection required by the customer, mostly third but occasional first angle projection. Over the years and different employers it became mostly third angle and metric, but only mostly, I still did work with some customers being American based and being first angle imperial. The one thing over the years was the move to entirely metric threads, even American companies used them, if reluctantly. During all those years the one thing I don’t recall using is BA threads, everything else but not them.

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