waste of time, and as a consequence the following approximate methods are adopted in drawing offices:

BUTTRESS THREAD.

V Threads (Fig. 110, a, b, c).-(a) For sizes diameter the screw is drawn in by hand as shown; with larger sizes the proper number of threads per inch may be set out, and a 55° set square used. After a little practice it is possible to show threads in this way with good accuracy and considerable ease.

(b) Same as (a), but adding the cross lines, those joining the bottoms of the threads being darker than those joining the tops.

(c) Parallel lines, representing the diameter, are drawn for the whole length of the bolt, and cross lines, at the proper angle and distance apart (the pitch), are then drawn as shown, the darker alternate lines not going right across and representing the bottoms of the threads. This is perhaps the neatest method, and will be generally adopted in the examples of this book.

To obtain the correct angle of slope of the lines, it is best to set off a dis

Fig. 109.

up to 1" or 14"

(a)

(8)

(c)

(d)

Fig. 110.

tance equal to half the pitch as shown at ab (Fig. 110, c).

Then the line joining the point s to b represents one thread, the other lines being drawn parallel.

(26) Square Threads.-These are usually represented as shown

Ρ 2

in Fig. 110, d. Start by setting off a distance of to obtain the correct inclination of the lines as in Fig. 110, c, and then draw

Ρ

parallel lines a distance apart of 2་

(27) Right- and Left-Handed Screws.-Screw threads may be either right-handed or left-handed. The threads in Fig. 110 are all right-handed, and it will be seen that if a nut be turned round on the thread in the same direction as the hands of a watch it will move along the thread away from the starting point. A left-handed thread is exactly the reverse of this, and the lines representing the thread would, therefore, slope from left to right, and not as with right-handed threads from right to left.

(28) Gas Threads.-There are many parts, such as pipes and their connections for gas, water, and steam, which cannot be screwed with such deep threads as the Whitworth standard. The thread adopted is of V form but of small pitch, the number of threads per inch of length being as follows:

Diameter of pipe,

4

No. of threads per inch, X X X X X

14 14 11

N.B. The diameter of iron pipes is measured internally and of brass pipes externally.

EXAMPLES.

(1) Draw correctly in sectional outline and fully dimension four or five threads of (a) Whitworth V thread, (b) square thread, (c) buttress thread.

(2) Draw a screw thread upon a cylinder 2" diameter for a length of 3" by drawing office method, (a) Whitworth V thread, (b) square thread.

(3) A hollow cylinder 3" internal diameter, 5′′ external diameter, 4" long, is screwed internally. Draw a longitudinal section showing the thread (a) Whitworth V thread, (b) square thread.

SECTION XVI.

NUTS.

(29) Hexagonal Nuts (Fig. 111).-Thickness

size across flats = 11" d+1".

A hexagonal nut only differs from a plain hollow hexagonal prism in having the corners of one base bevelled or "chamfered" off to give greater finish. Hence the drawing of a nut in different positions is almost identical with the projection of a hexagonal prism, and should present no difficulty after working the problems of pp. 108, 110, 118.

Fig. 111.

The "chamfer" of the corners is usually done at 45° to the axis of the nut, and proceeds until a complete circle is formed on the base. Many engineers adopt the practice of chamfering nuts on both faces, or if they use nuts only chamfered on one face, they fit them with the chamfer next the work in order to prevent the sharp corners of the nuts cutting the metal against which they bear.

B

(30) Drawing of Nuts.-The number of nuts (or bolt heads) required to be shown on machine drawings is generally very great, and it is necessary to adopt a quick and ready method of drawing them. It is frequently necessary to draw either or both of the views A and B (Fig. 112), without on the same drawing requiring to show the other view C, therefore it is necessary

Fig. 112.

to have a method which will allow of this being done without requiring the separate drawing of the view C.

The following are the usual methods adopted :

(a) For showing a nut across the corners (3 faces) as at A, Fig. 112, make the distance across the corners equal to twice the diameter, and the width of the middle face equal to the diameter.

(b) For showing a nut across the flats (2 faces) Fig. 112, B, adopt the rule for sizes already given (§ 29).

But when the view C, Fig. 112, must be shown upon a drawing, it is better to draw it before the views A and B, making the circle, round which the hexagon is drawn, equal in diameter to 11⁄2d +, and then projecting either the view A or B from it. When this is done the side of A across the corner will not be exactly 2 d, especially for nuts above 1" diameter.

One very important rule, which should be always adhered to, is that when only one of the two views, A or B, is required, the first one should be adopted—that is, a nut should always be shown across the corners rather than across the flats. The reason for

this is that one seldom wants to know the greatest clearance between parts of machines, but the least clearance. If a nut is drawn across the flats it does not show the least, but the greatest, and is thus misleading.

Students will find this apparently small point is really one of great importance, especially when designing such parts as flanges, covers, &c., the size of which depends, to a large extent, upon the size of the bolt and nut used.

The sizes of ordinary bolts and nuts need not be marked upon drawings, except the diameter and length. It is a waste of time to dimension the nut or head, or to write on the pitch of the thread except in unusual cases, as bolts and nuts are all made to standard sizes, and the workman only requires to know the diameter and length in order to obtain them from the stores.

The rule as to making the size of a nut across the corners equal to twice the diameter is intended to hold rather for drawing a nut than as a safe rule for determining sizes when designing. In such cases it is better to draw the hexagon, knowing the size across the flats equals 1 d + ", and measure the size across the corners from it, or to consult Table III. (p. 165).

=

Fig. 113 shows the drawing office method of showing the chamfer using the rule that, size across corners 2 d. A semicircle is drawn of radius = diameter of bolt, which gives both the height and the limit of the width. The completion of the view is then easily and quickly accomplished. To completely draw a nut it is necessary to draw lines at 45,° as shown in Fig. 112, A, but in most draw

Fig. 113.

ings this is omitted.

(31) Lock Nuts.—In pieces subject to rapid movement, which are held together by bolts and nuts, there is a considerable

tendency for the nut to work loose owing to constant vibration, this is prevented by using two nuts, one screwed tightly down upon the other, the top one being termed a lock nut. As the duty of this second nut is only to jamb the first and not to take much or any of the stress, it may be much thinner than the usual standard. As a general rule, the thin nut is made equal to half the bolt diameter.

In ordinary practice the nuts are arranged as shown in Fig. 114, the thin nut being the lock-nut. Many authorities, however, say the thin nut should be inside, an arrangement which is not practically as convenient, owing to the fact that ordinary spanners are frequently too thick to admit of fitting on the thin nut when it is so placed. Many engineers use an ordinary nut for a lock-nut, thus having both nuts the same thickness.

Lock-nuts are chamfered on both faces.

Fig. 114.

(32) Methods of Locking Nuts. There are many other ways of locking nuts. The most common, perhaps, is to drive a

taper or split-pin through the bolt just above the nut (Fig. 115a), but this scarcely locks the nut, although it prevents it screwing off. A better arrangement is to drive a strong taper-pin with a split end through the nut and bolt when screwed home, the end being then opened out (Fig. 1156).

Still more certain guards are obtained by using one of the three methods, abc (Fig. 116). In the first (a), a set-screw jambs against a part of the nut, turned cylindrical, to form a collar; in the second (b), a pin is screwed into the piece against which the nut bears, close up to one face of the nut, diameter of pin not less than 1⁄2′′; and in the third (c), a guard plate is fitted accurately

Fig. 116a.