Drawing Projections

I must admit that I also was wondering what those strange marks were on plans. And after seeing the explanation on the other thread I'm afraid I'm non the wiser

"Hi,

The 'two circles' and 'trapezoid' are the standard symbols for indicating the drawing layout of the different views, i.e. first angle or third angle. Imagine the 'trapezoid' is a tapered cork and that you are looking at it. That is how the drawn views are oriented. It can make a great difference as to how the drawing is read.

First angle Ortho (as previopusly used in the UK) -

Third Angle Ortho as is normally used now (aka 'American' layout) -

"

They look the same to me??

Edited By Wolfie on 16/09/2012 11:05:55

Bollocks, spent 10 minutes posting a reply only for it to disappear.

this has got to be the crappies web site known to man.

OK ignore the 3D views.

Take to top example and using the side view, that's the one on the right, 'roll' the cone to the left and you will get two circles, both visible. One roll so that's First angle projection.

Now take the second example and to get the side view to resemble the end view you have to 'roll' it 3 times, hence third angle projection.

Typical yankee operation trying to make out because 3 is bigger than 1 it's better.

That's why they don't have ground floors and their ground floor is the first floor so building are higher. same as staing vehicle weights as 80,000 pounds is far bigger than 35.7 tonnes.

Not quite, the concentric circles are at opposite ends of the side view for first and third angle, that's the key.

Regards,

Andrew

Hi I am just as baffled as Woolfie! three pictures duplicated one above the other but the top two are the opposite way round to the bottom two .If you put them all upside down they are still the same six pictures.I know that Iam past my sell buy date but I also have never been taught the difference and I still cannot see it .help please.

Maurice B.

During my working life I only ever met first angle projection once. I had to produce a sketch modifying a complex flange and did not realise the original drawing produced by a German firm was first angle projection. Fortunately when the flange was set-up on the jig borer nothing made sense and I was called for.

Never assume a drawing is third angle projection. If it is not you can produce an awful mess.

Oh well, let me try my definition.

We have two drawings of the same thingie, one to the left, the other to the right.

First angle: the picture on the right side shows the item as seen looking from left.

Third angle: the picture on the right side shows the item as seen looking from right.

In the case of that truncated cone it does not make much difference, but this also serves only to show which type of drawing it is. As was already told, in drawings of more complex nature it can make all the difference.

By the way, all drawings I get in my semi-professional work are in first angle.

Greetings, Hansrudolf

It hasn't been made clear that the PURPOSE of the symbols is to show how the different views are arranged.

It isn't hard to imagine a drawing with a front view, plan and two side views, where it isn't possible to be sure which of the two side views goes on the right and the left of the object.

If you know the projection used, then it is easy to work out which is which.

With third angle each side view is next to the end of the elevation it represents and the plan is above the top of the object. You can imagine folding them back around a box to make a 3-d model of thE object.

With first angle the views are adjacent to the side opposite the one they represnt. As has been mentioned you can imagine turning the object through 90-degrees to one side, and drawing the view that faces upwards.

The little sketches are just an unambiguious way of showing which projection has been used.

Neil

Neil

Thanks for that description it is succinct to the point and certainly, for me that is, makes it easy to understand what is meant. Now all I have to do is to remember 1st or 3rd and the appropriate symbol but the viewing direction will make sense. Yet again living and learning!

Cheers

Martin

Wikipedia has a good page on it.

Glad it's not just me struggling 

Chris

Edited By chris j on 16/09/2012 14:56:23

See here. its all explained. **LINK**

Bri

What do you make of this? on a drawing received (from the UK) a while ago.

Hi Bri,

Either a beer glass or of one of my drawings

Neil

Michael, Are we back to school days (70 years ago for me) when it was Plan, Side View and End View ? Bill

Never been thoughly convinced by this symbol

hollow cup or truncated cone or lampshade or fez...

I guess thats the point...

cup ..... would show wall on one end and an id on other

fez .... likewisewith wall at other end

lampshade ..id seen at both ends

thus we are left with truncated cone..

one system shows "side elevation" the side you would see it from

the other shows the "projected" view on the far side

As to first ,second ,third and fourth these refer to the quadrant represented by the drawing of a 3d object projected onto three othogonal planes ; both 2nd and 4th are "valid" but unused ( we seem to have enough fun with the other two )

1st angle uses plan under elevation and side elevation the farside you are looking from,

(projection "puhes" )

3rd angle uses plan over elevation(eek) and side elevation the side you are looking from

(views from)

THATS WHY IT IS SPECIFIED...It has wide effects on the interpretation of the drawing.

Why not in words? well the symbol is quicker ( pre cad) to draw than letter" 1ST ANGLE " and provides a cue.

Hth

Edited By jason udall on 17/09/2012 00:01:05

Most (probably all) drawing standards require hidden lines (or further views) to be used if the design intent is unclear without. In this case, with no hidden lines, the legitimate assumption is that it's a solid, truncated cone (frustrum).

Hi,

The cone is a solid, when I served my apprenticeship, the old guy that taught us used a glass bowl for his examples. The bowl was upside down for first angle and right way up for third angle.

Even now some thirty years on the first thing we do is take a marker pen and our blocks of metal and you can look around the shop and see others mouthing the words `up the bowl’ or `over the bowl’ as they mark out the features.

Third angle is the most common now, I would suggest that you get your material and get use to physically making the move from plan view to end view by making the move up the edge of the (imaginary) bowl. As John said it’s a rolling movement through 90 degrees.

If you are machining features that are up at compound angles it really helps to have visual aids.

As JA stated, old school drawings made on a board followed the rules. Drawings made on CAD by young lads that have not served time in a tool room or even seen a machine could be anywhere………..does that sound cynical?

That's possible with many 2D CAD systems which in many ways are really just manual extensions of the old drawing boards onto the screen. In that case if the user isn't "fluent" in projection systems misteaks can happen. (Although young or not, anyone producing drawings professionally should at least be able to get the projection right .... and where's the checker anyway? {rhetorical]).

Modern 3D CAD systems operate differently however and once the configuration is set to the projection system of interest (1st or 3rd), drawing view production and placement is handled by the program and will be correct.

With respect, Dave, the kind of gyrations you describe, (while fairly typical of toolrooms and machine shops) are more of a sign of people who are not completely comfortable with drawing projection systems than anything else. Experienced Engineers and Draftsmen do none of that. They simply look at the drawing and "see it".

An analogy is parallel parking a car between two oher vehicles at the kerbside. Novice drivers have have all sorts of rules .... line up this corner of the car with whatever .... now turn the wheel until something else lines up .... now turn the wheel back etc. The experienced driver just drives the car into the space, backwards.

Edited By Sid Herbage on 17/09/2012 02:03:16

.

Again with respect Sid but how do they become experienced ?

Not knocking Wolfie but it's obvious from his posts that he's just starting out in this game and the original question was a valid one in his search to become one of the experienced ones.

Now to try and get the truck into the yard in one attempt

John S.

Wierd,

I read this thread when Wolfie posted and didn't know what he was getting at - not struck the problem I suppose.

Just recieved a new copy of ME, and there on the bottom of some plans were these symbols.

now I have to go back and see if i understand them. - this board keeps finding more interesting things I don't understand (yet)

Dave

At last ... I have found this properly presented explanation of Orthographic Projection ... theme and variations.

[Wolfie, and others] this is highly recommended reading.

MichaelG.

If the graphic on p38 doesn't clarify the difference between First and Third Angle Projections, then nothing will.

Edited By Michael Gilligan on 17/09/2012 08:46:29