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Drawing Projections

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Terryd20/09/2012 13:01:36
1926 forum posts
179 photos

Qute:- "These problems are interesting but they are asking for the side view and not where to place it so are immaterial to the original question.

I have to say that I would have thought it was blindingly obvious to put the left hand side projection on the left, and the right hand side on the right, the top at the top and so on. Who would do it any other way??"

 

Hi Wolfie,

Firstly they are very relevant and your second statement seems to suggest that you have not understood that relevance, it is not simply about how views are located, it is about how they are placed relative to a hypothetical viewer of the component being drawn.  It is that which can lead to confusion between the two systems.

Your drawing is close, but not close enough. Notice the vertical lines on the elevations (solid and dashed) that should give you more of a clue, every line in an engineering drawing is important and ignored at ones peril. A person who can read engineering drawings does not really need a plan view here as it would not reveal any extra detail. and there is nothing that my drawings do not show, except the projection system (and of course dimensions), which is crucial to its accurate interpretation.  If extra detail is needed I would have drawn the plan view.

My Chief draftsman always insisted on economy as he viewed unnecessary drawing as a waste of time. As that great pioneering engineer Henry Maudslay said -

"Avoid complexities. Make everything as simple as possible." and

"Get rid of every pound of material you can do without; put to yourself the question, ‘What business has this to be there?"

A few designers in the pages of ME and MEW could do no better than to have this advice pinned over their workbenches.

By the way I will post a 3D sketch when you have had time to consider the above.

Best Regards

Terry

Edited By Terryd on 20/09/2012 13:16:12

Wolfie20/09/2012 13:30:58
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502 forum posts

OK lets try this then (Ignore the lack of drawing skills lol )

Edited By Wolfie on 20/09/2012 13:31:26

Terryd20/09/2012 14:26:00
1926 forum posts
179 photos

Hi Wolfie,

Better, and half right. (drawing skills no problem, as long as you can put over an idea accurately it is inconsequential). Actually that is one possibility, however if you gave that to an engineer he would either wonder what the line down the centre of the top represents or conclude that it is two seperate components.

These are the two possible solutions (there are others but more of that later) :-

3d examples of projs..jpg

If the drawing was in first angle proj. it would look like the example on the left (as you drew it, but rotated 180) the sketch on the right is what the component would look like in 3rd angle. Of course this is a simple example and would be explained with a plan view, but not all confusions are this simple and can lead to some real problems if the conventions are not understood and observed.

The cut out could also be curved (e.g. quadrant) and that would also need a plan. I have though seen drawings in ME which have confused me until I realised that one side view was in first angle projection and the other in third angle crook. It is useful to understand the differences and be able to interpret drawings correctly to prevent expensive errors

I was recently able to buy for pennies a rather nice piece of steel, about 20mm thick by 300mm wide and 450 mm long which had been accurately ground on both sides. The unfortunate machine operator had read the drawings wrongly (1st/3rd angle confusion) and made a very expensive piece of scrap - great for me, not so for him I believe.

It's not just a pedantic exercise, believe me, when some use 1st angle and others 3rd angle projection systems. It would be good if we could all agree on one system but its a bit like the metric v imperial debate, but let's not go there - its Tiger country! wink

Best regards

Terry

Edited By Terryd on 20/09/2012 14:27:57

Wolfie20/09/2012 17:28:23
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502 forum posts

"Actually that is one possibility, however if you gave that to an engineer he would either wonder what the line down the centre of the top represents or conclude that it is two seperate components."

The line is down the middle cos I couldn't work out how to rub just half of it out lol teeth 2

Edited By Wolfie on 20/09/2012 17:29:00

Wolfie20/09/2012 17:34:25
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502 forum posts

Actually I've just noticed summat. If you rotate those two solids 180º they swap projections. I drew your left hand one but I thought it was 3rd angle cos the end view was on the right.

fizzy22/09/2012 16:28:34
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1614 forum posts
109 photos

Now im convinced thats not right....have a look at this http://www.youtube.com/watch?v=_wDpN6Zi1hE

Ed Duffner26/11/2012 12:40:33
730 forum posts
61 photos

I've wondered about these projections myself and to be honest can't see the point of having or using them. If a drawing is made clear enough with enough views of the part, why introduce a layer of confusion.

The examples of the square inside another where you have to show the side view would never come up in real life surely? it would be an incomplete drawing yes?

It's the same with off-side and near-side on a car, what happened to left and right?

blowlamp26/11/2012 12:51:59
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1202 forum posts
82 photos
Posted by Ed Duffner on 26/11/2012 12:40:33:

I've wondered about these projections myself and to be honest can't see the point of having or using them. If a drawing is made clear enough with enough views of the part, why introduce a layer of confusion.

The examples of the square inside another where you have to show the side view would never come up in real life surely? it would be an incomplete drawing yes?

It's the same with off-side and near-side on a car, what happened to left and right?

Left and right change, depending on if you're looking from the front or the rear. indecision

Martin.

Gordon W26/11/2012 16:44:28
2011 forum posts

Or ,of course left and right hand drive.

Mark C26/11/2012 18:21:53
707 forum posts
1 photos

I thought I might add a bit to this - I have read the whole thread (but not all the links) and cannot find any mention to the link between CAD and 3rd angle.

CAD is generally drawn 3rd angle (and I think there is mention of it being preferred in the old BS308) today unless there is a specific reason (locally) to do otherwise. It is done mainly due to a desire to make drawings un-ambiguous as has been stated before - disambiguation is the key to preventing scrap parts and wasted time/money along with recrimination about "who was to fault". As the parts get more complex, the greater the importance of the drawing layout - I have attached a drawing for a part that I drew to give an idea of how a modern CAD drawn part would be produced. It is not complete in the sense that there are a number of extra "sheets" to fully detail it along with a file containing the 3D solid geometry for direct import into the machine center (saved as "as cast" geometry including shrinkage and machining allowances). You will notice the projection information (little cone) and the magic words "if in doubt, ask" which roughley translate to "on your bike, I am making it like this!"

Mark

snap hook assy.jpg

hawkeyefxr17/05/2013 06:31:07
11 forum posts

When i was doing my apprieticship in the 60's i had a spell in the drawing office, i was told one was the American view the other English view, Which was what i can't remeber now.

Alan Jackson17/05/2013 11:58:42
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166 forum posts
67 photos
Posted by hawkeyefxr on 17/05/2013 06:31:07:

When i was doing my apprieticship in the 60's i had a spell in the drawing office, i was told one was the American view the other English view, Which was what i can't remeber now.

I think 3rd angle projection is american and mostly used. If you ever got a ist angle (english) drawing it became misleading and you had to read it with care to avoid misinterpretation. The comment "If in doubt ask" was often cynically restated " If in doubt don't ask" you got no help from nobody.

Alan

M0BND17/05/2013 21:00:04
81 forum posts
9 photos

I see a lot of German drawings - all 1st angle! They can catch you out because your eyes and brain get used to 3rd and the drawings look very weird.

I haven't followed or read all the posts on this thread but I was always taught 1st angle is to take the part and just roll it to the new view(to the left, right, top or bottom). For 3rd angle, pick up and place to the left, right, top or bottom. Hope this helps!

David Jupp18/05/2013 07:29:04
695 forum posts
17 photos

The 3D CAD system that I use assumes by default 3rd angle for inch based templates (US paper sizes), and 1st angle for Metric (ISO paper sizes). This is simply down to the way the provided 2D templates are set up.

This can catch out the unwary, especially as the projection method can only simply be changed before adding views to the drawing.

For this (and some other reasons of style/preference) I recommend that users set up their own drawing templates, customised to give their desired 'look' to their drawings.

Ady118/05/2013 08:37:08
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3463 forum posts
513 photos

Maybe this is why so many government development projects went miles overbudget

in the case of Concorde Denis Healy said "Eventually we simply stopped counting how much it cost"

thomas oliver 221/11/2013 19:27:20
104 forum posts

Putting it all as simply as possible:-

1st Angle Looking at the front elevation - What is seen from the left is projected over on the right.

What is seen from the right is projected on the left.

What is seen from above is projected below.

3rd Angle Looking at the front elevation- What is seen from the left is projected on the left.

What is seen from the right is projectied on the riight.

What is seen from above is projected above.

The spacing of views on the paper is first worked out and normally the FE is drawn, then one side elevation. By projection lines directly from these views, and via a 45 degree line, the plan view can be drawn without measurement.

Russ B06/03/2014 00:26:28
547 forum posts
21 photos
Posted by Wolfie on 20/09/2012 12:34:04:

 

I have to say that I would have thought it was blindingly obvious to put the left hand side projection on the left, and the right hand side on the right, the top at the top and so on. Who would do it any other way???

 

3rd angle is only used America and Australia, the rest of the world uses 1st angle ortho - so not really that obvious, and not "as previously used in the UK" either - it really depends on your way of thinking.

The easiest way to understand the difference is by imagining that your "object" never leaves the page, - now this is a fairly lose analagy from my brain - for a proper insight, as I think previously linked see here - but you might get bored quick, even though the page contains less words than this post....... http://www3.ul.ie/~rynnet/orthographic_projection_fyp/webpages/third_angle.html

1st angle, the object sits ON TOP of the page, and so rolling to the left, displays the right side of the object (assuming its a cube or simple shape)

3rd angle, imagine the object is under the page, rolling it to the right, displays the right side.

now if you think about 3rd angle from a 1st angle persons perspective, you can see where it gets its name from, as from a 1st angle persons way of working, you'd have to rotate the part 3 times to get an adjacent view.

If you look back to the cups, it should make sense now, page under...... page over etc. =) - the iso view is for folk who can't read 1st & 3rd angle and don't know the difference between a hidden line and a normal line as 1st or 3rd angle - doesnt matter, same information, different arrangement on the page!

Sry I'm editing like a bugger ignore what I just said, imagine a drawing of a dice with a printed symbol on each face. Lines and fetures wont position each face so unless you knew if you where look at a 1st or 3rd angle, you couldn't possibly arrange the symbols correctly as the draughtsman intended (maybe its loaded..... or why would it matter lol, loose example)

Also, yes 3rd angle is also acceptable in the UK, but as my boss and mentor would say if I turned out a 3rd angle drawing, "we don't work in imperial any more, its EN, not BS from here on"

Edited By Russ B on 06/03/2014 00:35:07

Edited By Russ B on 06/03/2014 00:43:36

TJ5506/03/2014 11:04:54
1 forum posts

Hi Everyone,

My first post!! although I have been keeping up with all subjects I have not found the need to throw in my tuppence worth, until now! The subject of 1st/3rd angle has had me puzzled (especially how to explain it) for quite a ling time. So I read through all the posts until I found some explanations that helped me understand. I don't profess to have the answer but a combination of 2 posts provided what I think is a very good way to look at the subject (then go away and think about it).

I took the diagram supplied in Coalburner's post 17/9/2012 12:16:19 and combined it with the explanation in Martin Kyte's post 17/9/2012 12:31:31 and this seems to explain it really well.

I understand everyone has a different way of learning but this combination did it for me.

Just my twopence worth, Thanks to Coalburner and Martin Kyte

Mark C06/03/2014 11:57:26
707 forum posts
1 photos

Quote:

"3rd angle is only used America and Australia, the rest of the world uses 1st angle ortho - so not really that obvious, and not "as previously used in the UK" either - it really depends on your way of thinking."

Simply wrong, industry uses both (still, and accommodated in the standards) but predominantly 3rd angle is the default in mainstream use.

Mark

IanT06/03/2014 13:58:35
1324 forum posts
136 photos

As my 2D CAD is generally just for my own use Mike, I'm afraid to admit that I never worry too much about which 'projection' I'm using. I just draw things as I see them in my 'minds eye'. Maybe I should try a bit harder and adopt one system (or the other) as my 'standard'. That list of things I should be doing is getting longer by the day...

wink

IanT

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