Flats on cylindrical or shaped surfaces

It is not always obvious that surfaces are flat when they are on otherwise curved, cylindrical or spherical surfaces. In this case, flat surfaces such as squares, tapered squares and other flat surfaces may be indicated by thin 'St Andrew' cross type diagonal lines. An example of this is shown in the entirely fictitious gear shaft in Figure 3.17. The extreme right-hand end of the shaft has a reduced diameter and approximately half of this cylindrical length has been flat milled to produce a square cross-section. The fact that the crosssectional shape of this region is square and not cylindrical is seen in the end view as a square and in the right-hand side elevation by the crosses. 

Engineering Drawing for Manufacture
by Brian Griffiths
Publisher: Elsevier Science & Technology Books