If mechanical drawing is to be of any practical use to a person, he must be able to thoroughly understand the form and arrangement of the various parts of a machine from an inspection of the drawings of the machine without reference to the machine itself. He ought also to be able to make drawings of a machine or the parts of a machine from the machine itself. As mechanical drawing is simply the application of the principles of geometry to the representation of machines, a person who wishes to become thoroughly conversant with mechanical drawing and machine design should commence by studying the geometrical problems given in this work. The student in following up the problems given, should not content himself by merely copying the drawings, but should do each example over and over, until he is thoroughly familiar with the principles involved in their construction, and also understands why each line is drawn.

In working over these examples several times the student is not only committing them to memory, but is at the same time becoming proficient in the handling of the various drawing tools.

While many draftsmen are familiar with all of the problems given in this section of the work, it is not to be expected that all draftsmen or students are thoroughly conversant with all of them, and it is intended that this section of the work shall be used not only as reference data but for practical examples of elementary mechanical drawing. If the different problems given in this section are drawn with great accuracy, the technical skill acquired in drawing and proper handling of the different instruments will be found to be of great value. It will not be necessary to ink in these simple geometrical problems, as it is better to acquire precisian or accuracy in pencil work before going further. These problems are believed to be an essential part of a work on mechanical drawing. To understand geometry certain qualities of mind are absolutely necessary, and many persons find it impossible to grasp even the simple problems of this study. The draftsman or student who is without practical knowledge of geometry is very poorly equipped for his duties.


MACHINE DRAWING

The draftsman should not as a role be content with simply reproducing the views shown in the different examples given, to the dimensions marked on them, but should lay out other views and cross sections. The great importance of the value of being able to make intelligible free hand sketches of machine details cannot be overestimated, the draftsman should practice this art, not only from the illustrations given herewith, but from actual machine details. Fully dimensioned free hand sketches of actual machines or their details, form excellent examples for drawing practice. All such sketches should be made in a book kept for the purpose, always putting in the dimensions where possible. The description of the various applications of the mechanical powers herein before given is more for reference than for the purpose of teaching these principles. As machine drawing is simply the application of the principles of geometry to the representation of machines, if the draftsman or student is not already familiar with the study of geometry, he should make himself acquainted with the problems given in this work, before going further.


U. S. Standard Hexagonal Bolt-head and Nut

Two types of head and nut are illustrated, the rounded or spherical, and the chamfered or conical, as shown in Fig. 156. Three dimensions are fixed by this standard: First, the distance across the flats or short diameter, commonly indicated by H, and equal to one and one-half times the diameter of the bolt plus one-eighth of an inch, second, the thickness of the head, which is equal to one half its short diameter, third, the thickness of the nut, which is equal to the diameter of the bolt.