Question 4 of 10: Dimensioning an Orthogonal Projection

Question 4: Dimensioning an Orthogonal Projection (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A circular flange with four equally-spaced (90° apart) bossed mounting holes and a central bore, drawn full scale (so a scale rule against the drawing gives the true millimetre sizes), plus a stepped side (sectional) view. No numeric values are printed on the source figure — dimensioning is the entire task, so the sizes below are read directly off the full-scale drawing to the nearest clean 2 mm increment, exactly as a candidate would with a scale rule in the exam.

Find. A fully and non-redundantly dimensioned drawing: every size needed to manufacture the part is given exactly once, with correct extension lines, arrowheads, leader lines, and the diameter symbol (⌀), plus centrelines/centre marks on every circular feature.

Approach. Apply CSA/ANSI dimensioning rules: (1) dimension circular features with the diameter symbol and place the dimension on the view where the circle is a true circle, not the view where it is a rectangle; (2) use a bolt-circle (centreline, dash-dot) plus a single "N× ⌀d EQ SP" callout for repeated equally-spaced holes rather than dimensioning each hole separately (avoids redundancy); (3) put overall size dimensions outside the smaller detail dimensions so extension/dimension lines never cross; (4) every circular feature gets a centre mark (crossed centrelines) even where no diameter is called out on that view.

1. Add centrelines/centre marks. A crossed centreline is placed at the flange centre and at each of the four boss centres — mandatory on every circular feature per CSA/ANSI convention, independent of whether a size is toleranced there.
2. Dimension the bore. ⌀110 mm placed above the flange, with extension lines from the true-size bore circle — the largest inner functional diameter.
3. Dimension the overall (outside) diameter. ⌀190 mm placed below the flange, kept clear of the bore dimension so the lines do not cross ("smallest dimension closest to the view, largest farthest").
4. Dimension the bolt circle and holes with one combined callout. "⌀118 mm B.C., 4× ⌀18 THRU, EQ SP" — the bolt-circle diameter is carried on its own dash-dot centreline circle (not a solid outline), and "EQ SP" replaces four separate 90° angular dimensions since equal spacing is implied and stated once.
5. Dimension the side view. Overall length and the step/counterbore depth are added on the profile view, each dimension line kept parallel to the feature it measures and terminated with closed filled arrowheads per CSA/ANSI practice.
6. Audit for redundancy. Each size appears exactly once across both views (rubric penalizes both "−1 per unnecessary dimension" and "−1 per missing dimension") — the bore, overall OD, bolt circle + hole size/count, and the two length dimensions together fully define the part with none doubled.