If we cut a groove around a cylinder in the form of a helix, we shall have what is called a screw thread, the thread being formed by the material which is left between the successive turns of the helical groove. A cylinder having such a helical groove cut around it is called a screw; and a piece having a cylindrical hole in it, with a helical groove cut around the hole, is called a nut. The most common uses of the screw are to fasten pieces together, to hold them at a given distance apart, and to cause one piece to move with relation to another piece.
Fig. 61. Simple Drawing for Left and Right-Handed V Screw Threads.
The form of screw thread with which we are most familiar is what is known as the V thread, shown in its simplest form in Fig. 61. Fig. 62 shows the method of drawing the true projections of this thread. The dimensions which must be known in order to make the drawing are the outside diameter AO, the pitch AC, and the depth of the thread AK. First draw the two projections of a cylinder of a diameter equal to the outside diameter of the screw. Half of the end view is sufficient. On the line AE of this cylinder lay off AC equal to the pitch; starting at A, draw the helix ABCD, as described for Fig. 52. Inside of the cylinder AO, draw a smaller cylinder KL, the diameter of which is equal to the diameter AO minus twice the depth of the thread. Now, on this smaller cylinder, starting at point H, perpendicularly under a point on the line AC which is half way from A to C, draw the helix LHJ with the same pitch as was used for the helix ABC. Draw the lines PR, X Y, ST, etc., tangent to the two helices and the projection of the thread is completed. It is necessary to draw the invisible parts of the two helices in order to draw the lines ST, XY, etc.; but they need not be left on the finished drawing. In Fig. 62 they are shown dotted for one turn of the screw, in order to indicate the construction.
Fig. 62. Accurate Projections of the Right-Hand V Screw Thread.
Fig. 63. Accurate Construction for Double V Thread.
Fig. 64. Simple Drawing of Left- and Right-Handed Square Screw Threads.
Fig. 63 shows the method of drawing a double V thread. The process is exactly the same as for drawing a single thread. Start at point A, and draw the single thread A BCD exactly as in Fig. 62; then start at point 9, half way between A and C, and draw another single thread of the same pitch as the first one. Some thought may be necessary to decide when the lines of one thread become hidden behind the other thread.
Another very common form of screw thread is that shown in Fig. 64, and known as the square thread. The method of drawing this thread is similar to that for the V thread, with the exception of a few minor points. The construction is shown in Fig. 65. The dimensions which must be known are the outside diameter AO, the pitch AC, the depth AH, and either the width of the thread AR, or the width of the groove RC. In the figure, the width of the thread AR is taken equal to one-half of the pitch; that is, AR and RC are equal. Beginning at A, draw the helix ABC; and beginning at R, draw the helix RMN, RN of course being equal to AC. Since the part between A and R is metal, forming the thread, there will be a line from A to R and from B to M, etc. Now, starting at point H, vertically under A, and at a distance from A equal to the depth of the thread, draw the helix HJV; and from S, vertically under R, draw helix STW. Draw the lines SV, TK, etc. Here, as in the case of the V thread, the invisible lines must be drawn when making the drawing, but need not be inked.
Fig. 65. Accurate Projections for Right-Handed Square Screw Threads.
Fig. 66 shows the construction of a double square thread. An explanation is not necessary, since the difference between this and the single square thread is practically the same as between the single and double V thread.
Fig. 66. Accurate Construction for Double Square Thread.
The V and square threads are the two fundamental forms of thread in use, and all other forms are modifications of one or the other of these two. Figs. 67 to 70 show some of the more common modifications.
Figs. 67 and 68 show the two forms of the V thread which are commonly used in practice.
In Fig. 67 we have what is known as the Sellers or United States standard thread, an enlarged drawing of which is shown in Fig. 71. Referring to this figure, we see that the angle between the two sides of the thread is 60°, so that if the thread came to a point at the top and bottom, as indicated by the dotted lines, the depth of the thread D would be about 87/100 of the pitch P. The sharp corners, however, are a disadvantage, since on the outside they are likely to be bruised and to give trouble in putting on the nut, and at the bottom of the groove they tend to weaken the bolt or screw. In order to avoid these sharp corners, the threads are flattened in the United States standard thread, as shown in Fig. 71, the amount of this flattening being such that the distance C is 1/8 of the pitch, or - what amounts to the same thing - the distance A is 1/8 of D. This gives a thread whose depth E is 65/100 of the pitch. Whitworth Standard Thread. Fig. 68 illustrates what is known as the Whitworth standard thread, shown enlarged in Fig. 72. Here the angle between the sides of the thread is 55°, so that if the threads came to sharp corners, as shown by the dotted lines, the depth D would be 96/100 of the pitch. The top and bottom of the thread, instead of being flattened, are rounded off so that the distance A is 1/6 of D, or the depth E is 64/100 of the pitch.
Fig. 67. U. S. Standard Thread.
Fig. 68. Whitworth Standard Thread.
Fig. 69. Lag Screw V Thread.
Fig. 70. Variation of Square Thread.
Fig. 69 shows the V thread as used on lag screws and other wood screws. Here the groove is much larger than the thread, because the wood into which it is to screw is weaker than the iron of which the screw is made.
Fig. 70 shows a slightly modified form of square thread, the only difference between this and the square thread previously described being that the sides of the groove taper slightly.