Answer: Divide the circumference and the pitch into the same number of equal parts (say 12) and join the points where the matching projectors meet; this gives the helix in the front view (the top view is just the circle).
Data: diameter = 60 mm, pitch = 70 mm (one complete turn rises 70 mm).
Steps:
- Top view: draw a circle of 60 mm diameter. Divide it into 12 equal parts (every 30°) and number the points 0, 1, 2, …, 11, 12 (12 coincides with 0).
- Front view: draw the rectangle of the cylinder, 60 mm wide and 70 mm high (one pitch) above XY.
- Divide the 70 mm height into 12 equal parts (each \(70/12 \approx 5.83\) mm) and draw horizontal lines through the division points, numbered 0 to 12 from the bottom.
- Project each point 0, 1, 2, … of the circle vertically up into the front view.
- Mark where the projector from point 1 meets horizontal line 1, from point 2 meets line 2, and so on up to 12.
- Join these points with a smooth curve. Show the part on the front half of the cylinder as a full line and the part on the back half as a dashed (hidden) line.
The curve is a helix: the point moves round the axis at a uniform rate while also moving along the axis at a uniform rate, rising one pitch (70 mm) per complete turn. In the top view the helix coincides with the 60 mm circle.