A bolt is a cylindrical bar upset at one end to form a head and having a screw thread cut at the other end.

A nut is a hollow piece of metal in which a screw thread has been cut.

A right-hand bolt has its thread so cut that its nut goes on, or advances along the axis of the bolt, when turned in the same direction as the hands of a watch. A left-hand bolt has its thread so cut that its nut must be turned opposite to the hands of a watch in order to have it go on.

Fig. 21. Conventional Drawing for Machine Bolt and Nut.

## Hexagonal Bolt Head And Nut

Figs. 21 and 22 show conventional drawings of a 7/8 - inch machine bolt having a hexagonal head and nut. The head is simply a hexagonal prism which has been chamfered, i.e., the corners rounded off so that the top view shows a circle inscribed in a hexagon, see Fig. 23. This top view has been omitted in Figs. 21 and 22, the conventional drawing being considered sufficient to show that the head is hexagonal, yet some draftsmen prefer to specify whether the head and nut are hexagonal or square,thus, "SQ.HD.", "HEX. N." In this case the view across the long diameter is given in preference to the view across the flats, so that in close quarters the clearance of the corners may be readily seen. The shank of the bolt is represented as explained above for conventional threads. The point is chamfered a little in the figure so that it appears as the frustum of a cone.

Fig. 22. Conventional Drawing for Machine Bolt and Nut.

Fig. 23. Conventional Drawing for Machine Bolt Head.

Another style of point is shown in Fig. 24, where the end is rounded off with a radius equal to about 1½ times the diameter. The lines which represent the thread should not cross the line drawn square across the bolt where the chamfer or rounding of the point begins. Note that in Fig. 21 the threads have been shown dotted through the nut, while in Fig. 22 the simpler and more common method is followed of omitting the dotted threads and showing the long diameter of the nut.

Fig. 24. Conventional Drawing for Machine Bolt.

The dimensions given are all that are necessary for the workman to make a standard bolt and nut. Some draftsmen prefer to show the length of bolt and thread from the base of the frustum or spherical end, as in Fig. 22, but this does not at once give the total length under the head, which is usually the important figure. In case dimensions for the head are needed the thickness and the distance across flats should be given, as obtained from the table of proportions on page 25. If standard threads are not used, then the number of threads per inch must be given.

Fig. 25. Machine Bolt with U. S. Standard Thread.

A method for making a conventional drawing of a hexagonal bolt head or nut is shown in Fig. 24. From A as a center describe an are with a radius equal to the diameter of the bolt, making it intersect the perpendicular to the center line through A at points B and C. Continue the sides of the bolt until they intersect the arc at E and F, and draw lines through B and C parallel to sides of bolt. Draw a tangent to the arc parallel to BC for the top of the head. Find by trial the radius G and draw the arcs for the sides of the head. It will be noted that the long diameter of the hexagon by this method is twice the diameter of bolt, which is practically true to the standard table for bolts under 1-inch diameter, and sufficiently exact for the larger sizes in common use.

Fig. 25 is a drawing to scale of a 2½ - inch rough bolt, having a hexagonal head and United States standard thread. Dimensions for the height and width of the head have been taken from table of bolt heads on page 25. The width of the head, 3 7/8", is the diameter of the chamfer circle and is the first portion of the plan view to be drawn. Then the hexagon is circumscribed about the chamfer circle. Project the width of the faces and BD for the flat portion of the top. Assuming the chamfer to be conical and at 45° with the axis, draw lines BC and DF. The curves of intersection are approximated by arcs of circles springing from F and C and drawn tangent to BD.

The enlarged thread section below shows that the thread is flattened at top and bottom by cutting off ⅛ of the depth F of the V thread. Note that the pitch of the thread is laid off on a line located outside of the true diameter of the bolt by an amount equal to the portion cut off the ordinary sharp V thread. The end is rounded with a radius equal to the diameter, or preferably 1½ times the diameter of the bolt.

Two views, plan and elevation, of two faces of a hexagonal bolt head are given in Fig. 23. The chamfer circle is tangent to the sides of the hexagon, which means that the tool, in making the chamfer, cuts off the corners of the top as at ABC. The true curves DAB and BCE are lines of intersection of a cone or sphere with a hexagonal prism and may be easily obtained by the principles of projection. A simpler and much more convenient method is to approximate these curves with arcs of circles, using the height of the head as a radius, as shown.

In case it is desired to show more chamfer as in Fig. 26, the top of the head may be cut off at FK and GH at an angle of 30° or 45°, and the diameter of the chamfer circle is projected to the plan view as shown. The width of the hexagon is the same as before and is readily projected from the plan view. The curves of intersection DAB and BCE are drawn as arcs of circles, but instead of being tangent to the top they are tangent to the line KH, which indicates the place where the chamfer cuts the flat sides of the hexagon.

Fig. 26. Hexagonal Bolt Head with Chamfer.