Explain the purpose of orthographic projection in engineering drawing, and explain why dimensions and tolerances must be stated on the drawing rather than m...

Orthographic projection is a technical drawing method used to communicate a part accurately for manufacture.

1. It represents an object using separate views taken at right angles to its faces. Each face is shown at its true shape and true size, rather than appearing shortened by perspective. Therefore, the manufacturer can determine the form of the part from the drawing. [3]
2. The views are arranged using a fixed projection convention. This tells the reader which face each view represents without needing extra explanation. [1]
3. Dimensions must be written on the drawing rather than measured from it. A printed or copied drawing may be enlarged, reduced or distorted, so a ruler measurement may not equal the intended size. Also, a measured line cannot state the required accuracy. [2]
4. Tolerances state the permitted variation from a stated size. No manufacturing process produces every feature at one exact value. Without a tolerance, a manufacturer may either make the part unnecessarily accurately, increasing cost, or allow too much variation so parts do not fit or assemble. [2]

Examination point: dimensions give the target size; tolerances give the allowed limits around that size. Neither should be inferred by measuring a drawing. [8]