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Spins in the dihedral system
Spinning is moving an element around an axis until it sits in a convenient position: parallel to a projection plane, edge-on, or simply somewhere else. In dihedral the axis is placed perpendicular to the horizontal or the vertical plane, because that way the movement of each point looks like a circle in one projection and a line in the other.
What you need to know
- Spin a point with a vertical axis and with a pointing axis.
- Spin a line by spinning two of its points.
- Spin a plane until it is edge-on or parallel.
- Choose the axis and the angle so the result ends up where you want.
How it is solved
- Fix the axis: perpendicular to the horizontal plane (vertical) or to the vertical plane (pointing).
- With a vertical axis, the point keeps its height and moves on a circle in the plan.
- With a pointing axis, it keeps its distance and moves on a circle in the elevation.
- Spin the points you need and join them again to rebuild the line or the plane.
How it appears in the exam
They ask to spin a line until it is parallel to a projection plane (to see it in true length) or to spin a plane until it is projecting. The typical mistake is spinning the projection instead of the point: the height or distance that does not touch the axis does not change.
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