48610 Chap.1 Orthogonal Projection and Third Angle Views
Orthogonal Projection and Third Angle Views
Week 1 sets the success test for the whole drawing block: can somebody you will never meet build your part from your sheet alone. A photograph fails that test because it cannot be measured, and a single pictorial view fails it because faces are foreshortened and the far side is invisible. What passes is a set of flat views taken square on to each face, arranged so that any engineer can read them.
The subject names three jobs a drawing does: views communicate shape, dimensions communicate size and location, and notes communicate everything non graphical such as scale, units, material, process and finish.
The Australian Standard AS 1100, based on the international recommendations, supplies the grammar.
Six line types carry meaning through thickness and dash pattern, from continuous thick visible edges to chain thin centrelines and chain thick cutting planes, and a drawing uses one line group throughout. The arrangement is third angle, in which each view sits on the side of the object nearest to it in the adjacent view, so the right side view is drawn on the right and the top view above.
The drawing material is blunt about alignment: views that do not line up score zero for that part, because a drawing that cannot be cross read cannot be measured.
What this chapter covers
- 01
Three purposes of a drawing: views, dimensions and notes
- 02
The AS 1100 line alphabet: continuous thick, continuous thin, dashed thick, dashed thin, chain thin, chain thick
- 03
Line groups, and using one set of thicknesses for the whole sheet
- 04
Projection families: perspective excluded, parallel orthographic used
- 05
Orthogonal projection with lines of sight perpendicular to the plane
- 06
Third angle placement against first angle placement
- 07
Alignment rails: width shared with the top view, height with the side view
- 08
Depth appearing twice, so the third view checks the other two
- 09
Reading a view set: a face becomes an area in one view and a line in another
What a step and a hole look like in each of the three views
- +1Locate the step relative to the viewing direction. It is cut from the rear corner, so from the front you look at an untouched face and the step lies behind it. Its two vertical edges are therefore hidden and are drawn dashed, 20 mm in from the right edge, running the full 40 mm height.
- +1Decide what the hole contributes to the front view. Its axis runs away from you, so you look down the bore and see a true circle of 16 mm diameter, drawn as a continuous thick visible outline with a chain thin centreline cross through it.
- +1Swap to the top view and the roles reverse. Seen from above, nothing hides the step, so it becomes a visible notch in continuous thick line. The hole is now seen across its bore, so it becomes a pair of dashed lines 16 mm apart spanning the full 30 mm depth, again with a centreline.
- +1Check the alignment. The two dashed step edges in the front view must sit directly below the two visible notch edges in the top view. If they do not, one of the two views is wrong, and the rule tells you which pair of edges to re-measure.
Key terms
- Orthogonal projection
- A drawing in which the lines of sight meet the plane of projection at right angles, so each view shows one face as if looked at square on. It produces a complete and accurate description of shape rather than a realistic picture.
- Third angle
- The view arrangement required by the Australian Standard, in which each view goes on the side of the object that the view itself shows, as seen in the neighbouring view. The alternative, first angle, places each view on the remote side.
- Hidden edge
- An edge that exists on the object but is obscured in a particular view, drawn dashed. A drawing may use the thick dashed or the thin dashed form, but not both on one sheet.
- Centreline
- A chain thin line, long dash short dash, marking an axis of symmetry, the axis of a cylindrical feature or a pitch line. It is not an edge, and every hole carries one.
- Line group
- One of four paired sets of thick and thin line widths defined by the standard, running from 1.0 and 0.5 mm down to 0.35 and 0.18 mm. A drawing picks one group and stays inside it.
- Alignment
- The requirement that the top view share its width with the front view and the side view share its height, so that measurements can be carried between views. Unaligned views cannot be cross read.
- Detail drawing
- A drawing that includes everything needed to manufacture the object, meaning views, dimensions, tolerances, material, scale and units, as distinct from an assembly drawing which shows how parts fit together.
Orthogonal Projection and Third Angle Views FAQ
Why three views rather than one clear picture?
Because a pictorial view cannot be measured reliably. Turning an object so that all three axes are inclined foreshortens every face, so a distance measured across the drawing is not the distance on the part. Three orthogonal views each look square on to one face, so every edge in the view that shows it appears at true length and can be dimensioned honestly.
The third view is not redundant either: depth appears in both the top and side views, so it lets you check the other two against each other.
How do I tell third angle from first angle at a glance?
Look at where the side view sits. In third angle the view showing the right hand side of the object is drawn on the right of the front view, and the top view is drawn above it, because each view goes on the side nearest to it in the adjacent view. First angle reverses both, putting the right side view on the left and the top view below.
Drawings usually also carry a projection symbol to declare which convention is in use, and the Australian Standard calls for third angle.
Does line thickness really matter if the shapes are right?
Yes, because thickness carries meaning rather than style. A continuous thick line means a visible edge; a continuous thin line means a dimension line, projection line, leader or hatching. If a marker cannot tell those apart at arm's length, the drawing has failed to say which lines are the object and which are the annotation. Pick one line group and keep the ratio between thick and thin visible across the whole sheet.
What does it mean that a face becomes a line in another view?
Nothing on the object disappears when you change view; it only changes from an area to a line or back. A face you are looking straight at appears as an area, while the same face seen edge on appears as a single line.
The Week 1 exercises are mostly reading exercises for exactly this reason: matching lettered faces on a pictorial view to numbered regions on the orthogonal views forces the realisation that every face is present in every view.
Assessment move
Draw rather than read. Take one simple object with a step and a hole, and produce the three views by hand three times over the week, leaving the construction lines and alignment rails on every attempt. Between attempts, do the reverse exercise: given two views, work out the third before looking.
Keep a card with the six line types drawn at their relative thicknesses next to your board until you stop having to think about which is which. Finally, check every finished sheet against three rules before you submit it: views aligned, third angle, hidden detail and centrelines present.
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