I had it explained to me many years ago, maybe around 1980 and I fully
understood what was said. Time has however destroyed any details. All
I can remember from 1980 is who the prime minister was and I'd much
rather forget that :)
When I looked at a drawing recently that clearly said one system I had
convinced myself it was the exact opposite. Luckily there was an
isometric view otherwise I'd have made it upside down and inside out.
Conventionally, angle is measured anticlockwise from the horizontal.
Consider the horizontal plane and the vertical plane containing y & z
axes. The 'first angle' is in the first quadrant, top right. Projections
of an object suspended in this quadrant, the plan projected downwards,
and the left elevation to the right, are 'first angle'. The 'third
angle' is in the lower left quadrant, from 180 to 270 degrees.
Projections onto the planes of an object placed there, are upwards and
to the right. As far as I can see, you could perfectly well have second
or fourth angle projections, but, being unconventional, they would
probably be rather confusing.
On Wed, 15 Apr 2009 14:45:14 +0100, Charles Lamont
Thanks Charles, that clears some of the mist. It haddn't occured to me
to think in terms of the quadrants in that manner, though I'm aware of
the a-c/w convention.
However, I still can't see that the result of 1st and 3rd projections
as you describe would result in the layout we all use.
Taking your scheme for 1st angle so that the projection of the right
hand end is projected onto the Y-Z plane on the left, then yes OK,
But, in the lower left quadrant, you would project the left hand face
onto the Y-Z plane to the right - that's still a 1AP - Or are you
suggesting the view is always as seen from the positive X direction
(for projection onto the Y-Z plane obviously)? That would work, I
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.