This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
A drawing being inked in, the next things are tints, dimensions, and centre lines. The centre line should be in red ink, and pass through all points of the drawing that have an axial centre, or where the work is similar and balanced on each side of the line. This rule is a little obscure, but will be best understood if studied in connection with the drawing.
Dimension lines should be in blue, but may be in red. Where to put them is a great point in drawing. To know where dimensions are required involves a knowledge acquired by practice. The lines should be fine and clear, leaving a space in their centre for figures when there is room. The distribution of centre lines and dimensions over a drawing must be carefully studied, for the double purpose of giving it a good appearance and to avoid confusion. Figures should be made like printed numerals; they are much better understood by the workman, look more artistic, and when once learned require but little if any more time than written figures. If the scale employed is feet and inches, dimensions to 3 ft. should be in inches, and above this in feet and inches; this corresponds to shop custom, and is more comprehensible to the workman, however wrong it may be according to other standards.
In shading drawings, be careful not to use too deep tints, and to put the shades in the right place. Many will contend, and not without good reasons, that working drawings require no shading; yet it will do no harm to learn how and where they can he shaded : it is hotter to omit the shading from choice than from necessity. Sections must, of course, he shaded - with lines is the old custom, yet it is certainly a tedious and useless one; sections with light ink shading of different colours, to indicate the kind of material, are easier to make, and look much better. By the judicious arrangement of a drawing, a large share of it may be in sections, which in almost every case are the best views to work by. The proper colouring of sections gives a good appearance to a drawing, and makes it "stand out from the paper." In shading sections leave a margin of white between the tints and the lines on the upper and left-hand sides of the section: this breaks the connection or sameness, and the effect is striking; it separates the parts, and adds greatly to the clearness and general appearance of a drawing.
Cylindrical parts in the plane of sections, such as shafts and holts, should be drawn full, and have a "round shade," which relieves the flat appearance - a point to be avoided as much as possible in sectional views.