Drawing Projections

[Nigel Graham 2Participant @nigelgraham2]

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 GilliganParticipant @michaelgilligan61133]

Posted by Nigel Graham 2 on 10/12/2020 22:40:29:

…
Perhaps it was a mis-quote of Square Rod.

.

Excellent deduction, Nigel

Confirmed by Ashford Allotment Society : **LINK**

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

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

Allotment Info

MichaelG.

[Martin KyteParticipant @martinkyte99762]

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 IoMParticipant @francesiom58905]

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 entwistleParticipant @royentwistle24699]

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

Roy

[Dave Smith 14Participant @davesmith14]

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 LongParticipant @keithlong89920]

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 2Participant @billdavies2]

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

Sorry, I've gone.

Bill

[Bill Davies 2Participant @billdavies2]

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 RenshawParticipant @rodrenshaw28584]

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 3Participant @nickclarke3]

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

[CirclipParticipant @circlip]

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

Dressmakers and Knitters

Regards Ian.

[Peter Jones 20Participant @peterjones20]

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 ?

[HOWARDTParticipant @howardt]

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.

[Terry KirkupParticipant @terrykirkup37827]

One of the biggest growth industries since lockdown started uses centimetres – bicycle frame manufacturers. But they are all lunatics anyway, or at least their marketing departments are. I'm sure anyone who knows that ten bob is the same as fifty pence (notwithstanding insidious inflation) is happy to use inches or millimetres as a norm. Sorry to jump in months later but couldn't resist!

[Darakhshan MuqaddasParticipant @darakhshanmuqaddas23860]

During my functioning life I just at any point met first point projection once. I needed to deliver a sketch changing a complicated spine and didn't understand the first drawing created by a German firm was first point projection. Luckily when the rib was set-up on the dance drill nothing checked out and I was called for.

Never expect a drawing is third point projection. In the event that it isn't you can deliver a dreadful wreck.

[Nigel Graham 2Participant @nigelgraham2]

"point" projection?

I've never seen that term – only ever first and third angle, projection. Does "point projection" differ from those?

Many drawings have a note naming the projection and giving a cone-and-circle diagram to prompt you.

[mannyroadParticipant @mannyroad]

I did my Technical drawing at school back.in the early 70’s. Back then I was taught that in the UK we only used first angle projection. This has clearly changed these days, though I still use it nowadays, as it just ‘feels’ wrong (to me) to use the former American projection.

The reason the truncated cone was used was because it is the simplest drawn form that is unambiguous in two elevations. I agree with this.

[HOWARDTParticipant @howardt]

School teaching curicculum is naturally behind the current real world practice, so first angle was taught but third angle was used in many places.  It is like the schools now using cm as the normal unit of measure, where as engineers always use the mm (in most of the world).

[Martin KyteParticipant @martinkyte99762]

Well that’s odd, when I did TD in the 70s at school we used 3rd angle projection.

[Martin ConnellyParticipant @martinconnelly55370]

I can’t remember what I was taught in the 70s but at work it was always 3rd angle.

Martin C

[mannyroad]

On mannyroad Said:

I did my Technical drawing at school back.in the early 70’s. Back then I was taught that in the UK we only used first angle projection. This has clearly changed these days, though I still use it nowadays, as it just ‘feels’ wrong (to me) to use the former American projection.

…

Over the years I realised that rather a lot of what was taught at my school was wrong!  Not all teachers keep up to date and many of mine were eccentrics!

In the late 60s I remember being taught the difference between 1st and 3rd angle projection, and why 3rd angle is advantageous.  Can’t remember which projection we drew in!

No need to worry about 3rd angle being an Americanism – projections are just geometry. Maths, not patriotism.

Arriving late to the Industrial Revolution, the US benefitted from European experience.  After noticing 3rd angle has fewer shortcomings they standardised on it.   Well done them – given the choice between two technical systems, engineers should always support the least wonky, whoever invented it!

In my limited experience 1st and 3rd angle drawings are usually equally clear.   Drawings that don’t say what the projection is can be confusing, but drawings that silently mix the two are truly evil!   I guess the draughtsman missed a lesson, or wasn’t paying attention.  Can’t throw stones: apart from having a few dodgy teachers, I spent many lessons thinking about girls and my many hobby interests…

Dave

[Author]

Posts