MYP Design Orthographic Projection and Dimensioning Notes

The Drawing a Manufacturer Actually Works From

1. An orthographic drawing is a set of flat views, each looking straight at one face of the object so nothing is foreshortened.
2. Because every view is square on, every length on it can be measured and every angle is true.
3. This is the drawing you hand to a workshop, a laser cutter technician or a supplier, and it has to answer their questions without you standing there.
4. It is drawn with instruments or in CAD, never freehand, because a 0.5 mm slip is a 0.5 mm error in the part.
5. A clear orthographic drawing is strong evidence for criterion C, Creating the solution, because it shows your plan was detailed enough to build from.

Three Views Cover Almost Every Shape

1. The front elevation is the view that carries the most information about the shape, which is not always the side the user calls the front.
2. The plan is the view looking straight down from above, and it shows length and width.
3. The end elevation is the view looking at one side, and it shows width and height.
4. The three views line up with each other, so a feature 30 mm from the left edge appears 30 mm from the left edge in both the front view and the plan.
5. Edges you could not see from that direction are drawn as short dashed hidden detail lines, which is how a hole inside a block gets shown.
6. A long dash short dash chain line marks a centre line through holes and axes of symmetry, and dimensions are often taken from it.

First and Third Angle Decide Where Each View Sits

1. In first angle projection each view is pushed through the object and drawn on the far side, so the plan goes below the front elevation.
2. In first angle the view you get by looking from the left is drawn on the right hand side, which catches people out constantly.
3. In third angle projection each view is drawn on the side you looked from, so the plan sits above the front elevation and the right hand view sits on the right.
4. First angle is the traditional European convention and third angle is standard in North America, so a drawing has to say which one it uses.
5. The convention is announced by a small symbol in the title block: a truncated cone drawn twice, once in profile as a trapezium and once end on as two circles.
6. The symbol follows the same rule as the views, so the circles sit to the right of the trapezium for first angle and to the left of it for third angle.

Dimension Lines Say How Big Without Cluttering the View

1. A dimension line is a thin line with an arrowhead at each end, carrying the size between two points.
2. It sits outside the outline wherever possible, joined to the object by thin projection lines that start about 2 mm clear of the edge they refer to.
3. The figure goes above the dimension line and is written so it reads from the bottom of the sheet or from the right hand side.
4. Put the small dimensions closest to the view and the overall size furthest out, so dimension lines never cross each other.
5. Every feature is dimensioned once and once only, because two dimensions for the same distance are two chances to disagree.
6. Use a diameter symbol (⌀) for a full circle and R for a radius, so ⌀8 is an 8 mm hole and R4 is a 4 mm rounded corner.

Leader Lines Point at Exactly What They Mean

1. A leader line is a thin sloping line from a note to the feature it describes, ending in a short horizontal tail under the text.
2. It ends in an arrowhead where it touches a line, and in a small dot where it stops inside a surface.
3. Leaders carry the information a dimension cannot, such as "4 holes ⌀5 equally spaced" or "3 mm clear acrylic, laser cut".
4. Keep leaders short and never let them run parallel to a nearby dimension line, since two thin lines side by side are hard to follow.

Tolerances Admit That Nothing Is Made Exactly

1. A tolerance is the amount a finished size is allowed to differ from the drawn size and still be accepted.
2. Written as 50 ± 0.5, it means any part measuring between 49.5 mm and 50.5 mm passes.
3. The same thing can be written as limits, an upper figure of 50.5 above a lower figure of 49.5.
4. Tight tolerances cost real money, so tighten only where parts must fit together and leave the rest loose.
5. A 20 mm dowel that has to slide needs a hole drilled around 20.2 mm, and stating that clearance on the drawing saves a part that jams.

The Title Block Is What Makes the Sheet Usable

1. A title block sits in the bottom right corner and carries the part name, your name, the date and a drawing number.
2. It also states the scale, such as 1:2, and the projection symbol, so nobody has to guess either.
3. Add the material and finish there too, because a part in 3 mm mild steel and the same part in 3 mm plywood are made in completely different ways.
4. Give each version a number and a date, so the workshop does not cut from a sheet you replaced last week.
5. Photograph or export the finished sheet into your ePortfolio, since a drawing that never reaches your design folder counts for nothing.