Mechanical Drawing Self-Taught Part 8

_EXAMPLES IN BOLTS, NUTS, AND POLYGONS._

[Ill.u.s.tration: Fig. 149.]

[Ill.u.s.tration: Fig. 150.]

[Ill.u.s.tration: Fig. 151.]

Let it be required to draw a machine screw, and it is not necessary, and therefore not usual in small screws to draw the full outline of the thread, but to represent it by thick and thin lines running diagonally across the bolt, as in Figure 149, the thick ones representing the bottom, and the thin ones the top of the thread. The pencil lines would be drawn in the order shown in Figure 150. Line 1 is the centre line, and line 2 a line to represent the lower side of the head; from the intersection of these two lines as a centre (as at A) short arcs 3 and 6, showing the diameter of the thread, are marked, and the arcs 5 and 6, representing the depth of the thread, are marked. The arc 7, representing the head, is then marked. The vertical lines 8, 9, 10, and 11 are then marked, and the outline of the screw is complete. The thick lines representing the bottom of the thread are next marked in, as in Figure 151, extending from line 9 to line 10. Midway between these lines fine ones are made for the tops of the thread. All the lines being pencilled in, they may be inked in with the drawing instruments, taking care that they do not overrun one another. When the pencil lines are rubbed out, the sketch will appear as in Figure 149.

[Ill.u.s.tration: Fig. 152.]

For a bolt with a hexagon head the lines would be drawn in the order shown in Figure 152. At a right-angle to centre line 1, line two is drawn. The pencil-compa.s.ses are then set to half the diameter of the bolt, and from point A arcs 3 and 6 are pencilled, thus showing the width of the front flat of the head, as well as the diameter of the stem. From the point where these arcs meet line 2, and with the same radius, arcs 5 and 6 are marked, showing the widths of the other two flats of the head. The thickness of the head and the length of the bolt head may then be marked either by placing a rule on line 1 and marking the short lines (such as line 7) a cross line 1, or the pencil-compa.s.ses may be set to the rule and the lengths marked from point A. In the United States standard for bolt heads and nuts the thickness of the head is made equal to the diameter of the bolt. With the compa.s.ses set for the arcs 3 and 4, we may in two steps, from A along the centre line, mark off the thickness of the head without using the rule. But as the rule has to be applied along line 1 to mark line 7 for the length of the bolt, it is just as easy to mark the head thickness at the same time.

The line 8 showing the length of the thread may be marked at the same time as the other lengths are marked, and the outlines 9, 10, 11, 12, 13 may be drawn in the order named. We have now to mark the arcs at the top of the flats of the head to show the chamfer, and to explain how these arcs are obtained we have in Figure 153 an enlarged view of the head. It is evident that the smallest diameter of the chamfer is represented by the circle A, and therefore the length of the line B must equal A. It is also evident that the outer edge of the chamfer will meet the corners at an equal depth (from the face of the nut), as represented by the line C C, and it is obvious that the curves that represent the outline of the chamfer on each side of the head or nut will approach the face of the head or nut at an equal distance, as denoted by the line D D. It follows that the curve must in each case be such as will, at each of its ends, meet the line C, and at its centre meet the line D D, the centres of the respective curves being marked in the figure by X.

[Ill.u.s.tration: Fig. 153.]

It is sufficiently accurate, therefore, for all practical purposes to set the pencil on the centre-line at the point A in Figure 152 and mark the curve 14, and to then set the compa.s.ses by trial to mark the other two curves of the chamfer, so that they shall be an equal distance with arc 14 from line 9, and join lines 10 and 13 at the same distance from line 9 that 14 joins lines 3 and 4, so that as in Figure 153 all three of the arcs would touch a line as C, and another line as D.

[Ill.u.s.tration: Fig. 154.]

The United States standard sizes for forged or unfinished bolts and nuts are given in the following table, Figure 154 showing the dimensions referred to in the table.

UNITED STATES STANDARD DIMENSIONS OF BOLTS AND NUTS.

KEY: A: Nominal. D.

B: Effective.[*]

C: Standard Number of threads per inch.

------------------------+----------------------------+--------------------- BOLT. BOLT HEAD AND NUT. ------------------------+-----------------+----------+----------+---------- Diameter. Long diameter, Short -------+------ I, or diameter diameter across corners. of --------+-------- hexagon A B C Hexa- Square. and gon. square, Depth of Depth of or width nut, bolt across J. H. head, K.

-------+------+---------+--------+--------+----------+----------+---------- 1/4 .185 20 9/16 23/32 1/2 1/4 1/4 5/16 .240 18 11/16 27/32 19/32 5/16 19/64 3/8 .294 16 25/32 31/32 11/16 3/8 11/32 7/16 .345 14 29/32 1-3/32 25/32 7/16 25/64 1/2 .400 13 1 1-1/4 7/8 1/2 7/16 9/16 .454 12 1-1/8 1-3/8 31/32 9/16 31/64 5/8 .507 11 1-7/32 1-1/2 1-1/16 5/8 17/32 3/4 .620 10 1-7/16 1-3/4 1-1/4 3/4 5/8 7/8 .731 9 1-21/32 2-1/32 1-7/16 7/8 23/32 1 .837 8 1-7/8 2-5/16 1-5/8 1 13/16 1-1/8 .940 7 2-3/32 2-9/16 1-13/16 1-1/8 29/32 1-1/4 1.065 7 2-5/16 2-27/32 2 1-1/4 1 1-3/8 1.160 6 2-17/32 3-3/32 2-3/16 1-3/8 1-3/32 1-1/2 1.284 6 2-3/4 3-11/32 2-3/8 1-1/2 1-3/16 1-5/8 1.389 5-1/2 2-31/32 3-5/8 2-9/16 1-5/8 1-9/32 1-3/4 1.491 5 3-3/16 3-7/8 2-3/4 1-3/4 1-3/8 1-7/8 1.616 5 3-13/32 4-5/32 2-15/16 1-7/8 1-15/32 2 1.712 4-1/2 3-19/32 4-13/32 3-1/8 2 1-9/16 2-1/4 1.962 4-1/2 4-1/32 4-15/16 3-1/2 2-1/4 1-3/4 2-1/2 2.176 4 4-15/32 5-15/32 3-7/8 2-1/2 1-15/16 2-3/4 2.426 4 4-29/32 6 4-1/4 2-3/4 2-1/8 3 2.629 3-1/2 5-11/32 6-17/32 4-5/8 3 2-5/16 3-1/4 2.879 3-1/2 5-25/32 7-1/16 5 3-1/4 2-1/2 3-1/2 3.100 3-1/4 6-7/32 7-19/32 5-3/8 3-1/2 2-11/16 3-3/4 3.317 3 6-5/8 8-1/8 5-3/4 3-3/4 2-7/8 ... 3.567 3 7-1/16 8-21/32 6-1/8 4 3-1/16 4-1/4 3.798 2-7/8 7-1/2 9-3/16 6-1/2 4-1/4 3-1/4 4-1/2 4.028 2-3/4 7-15/16 9-23/32 6-7/8 4-1/2 3-7/16 4-3/4 4.256 2-5/8 8-3/8 10-1/4 7-1/4 4-3/4 3-5/8 5 4.480 2-1/2 8-13/16 10-25/32 7-5/8 5 3-13/16 5-1/4 4.730 2-1/2 9-1/4 11-5/16 8 5-1/4 4 5-1/2 4.953 2-3/8 9-11/16 11-27/32 8-3/8 5-1/2 4-3/16 5-3/4 5.203 2-3/8 10-3/32 12-3/8 8-3/4 5-3/4 4-3/8 6 5.423 2-1/4 10-17/32 12-29/32 9-1/8 6 4-9/16 -------+------+---------+--------+--------+----------+----------+---------- * Diameter at the root of the thread.

The basis of the Franklin Inst.i.tute or United States standard for the heads of bolts and for nuts is as follows:

The short diameter or width across the flats is equal to one and one-half times the diameter plus 1/8 inch for rough or unfinished bolts and nuts, and one and one-half times the bolt diameter plus, 1/16 inch for finished heads and nuts. The thickness is, for rough heads and nuts, equal to the diameter of the bolt, and for finished heads and nuts 1/16 inch less.

[Ill.u.s.tration: Fig. 155.]

[Ill.u.s.tration: Fig. 156.]

The hexagonal or hexagon (as they are termed in the shop) heads of bolts may be presented in two ways, as is shown in Figures 155 and 156.

The latter is preferable, inasmuch as it shows the width across the flats, which is the dimension that is worked to, because it is where the wrench fits, and therefore of most importance; whereas the latter gives the length of a flat, which is not worked to, except incidentally, as it were. There is the objection to the view of the head, given in Figure 156, however, that unless it is accompanied by an end view it somewhat resembles a similar view of a square head for a bolt. It may be distinguished therefrom, however, in the following points:

If the amount of chamfer is such as to leave the chamfer circle (as circle A, in Figure 153) of smaller diameter than the width across the flats of the bolt-head, the outline of the sides of the head will pa.s.s above the arcs at the top of the flats, and there will be two small flat places, as A and B, in Figure 156 (representing the angle of the chamfer), which will not meet the arcs at the top of the flats, but will join the sides above those arcs, as in the figure; which is also the case in a similar view of a square-headed bolt. It may be distinguished therefrom, however, in the following points:

If the amount of chamfer is such as to leave the chamfer circle (A, Figure 153) of smaller diameter than the width across the flats of the bolt-head, the outline of the sides will pa.s.s above the arc on the flats, as is shown in Figure 157, in which the chamfer A meets the side of the head at B, and does not, therefore, meet the arc C. The length of side lying between B and D in the side view corresponds with the part lying between E and F in the end view.

[Ill.u.s.tration: Fig. 157.]

If we compare this head with similar views of a square head G, both being of equal widths, and having their chamfer circles at an equal distance from the sides of the flats, and at the same angle, we perceive at once that the amount of chamfer necessary to give the same distance between the chamfer circle and the side of the bolt (that is, the distance from J to K, being equal to that from L to M), the length of the chamfer N for the square head so greatly exceeds the length A for the hexagon head that the eye detects the difference at once, and is instinctively informed that G must be square, independently of the fact that in the case of the square head, N meets the arc O, while in the hexagon head, A, which corresponds to N, does not meet the arc C, which corresponds to O.

When, however, the chamfer is drawn, but just sufficient to meet the flats, as in the case of the hexagon H, and the square I, in Figure 157, the chamfer line pa.s.ses from the chamfer circle to the side of the head, and the distinction is greater, as will be seen by comparing head H with head I, both being of equal width, having the same angle of chamfer, and an amount just sufficient to meet the sides of the flats. Here it will be seen that in the hexagon H, each side of the head, as P, meets the chamfer circle A. Whereas, in the square head these two lines are joined by the chamfer line Q, the figures being quite dissimilar.

[Ill.u.s.tration: Fig. 158.]

It is obvious that whatever the degree or angle of the chamfer may be, the diameter of the chamfer circle will be the same in any view in which the head may be presented. Thus, in Figure 158, the line G in the side view is in length equal to the diameter of circle G, in the end view, and so long as the angle of the chamfer is forty-five degrees, as in all the views. .h.i.therto given, the width of the chamfer will be equal at corresponding points in the different views; thus in the figure the widths A and B in the two views are equal.

[Ill.u.s.tration: Fig. 159.]

If the other view showing a corner of the head in front of the head be given, the same fact holds good, as is shown in Figure 159. That the two outside flats should appear in the drawing to be half the width of the middle flat is also shown in Figure 158, where D and E are each half the width of C. Let us now suppose, that the chamfer be given some other angle than that of 45 degrees, and we shall find that the effect is to alter the curves of the chamfer arcs on the flats, as is shown in Figure 160, where these arcs E, C, D are shown less curved, because the chamfer B has more angle to the flats. As a result, the width or distance between the arcs and line G is different in the two views. On this account it is better to draw the chamfer at 45 degrees, as correct results may be obtained with the least trouble.

If no chamfer at all is to be given, a hexagon head may still be distinguished from a square one, providing that the view giving three sides of the head, as in Figure 158, is shown, because the two sides D and E being half the width of the middle one C, imparts the information that it is a hexagon head. If, however, the view showing but two of the sides and a corner in front is given, and no chamfer is used, it could not be known whether the head was to be hexagon or square, unless an end view be given, as in Figure 161.

[Ill.u.s.tration: Fig. 160.]

If the view showing a full side of the head of a square-headed bolt is given, then either an end view must be given, as in Figure 162, or else a single view with a cross on its head, as in Figure 163, may be given.

It is the better plan, both in square and hexagon heads, to give the view in which the full face of a flat is presented, that is, as in Figures 155 and 163; because, in the case of the square, the length of a side and the width across the head are both given in that view; whereas if two sides are shown, as in Figure 161, the width across flats is not given, and this is the dimension that is wanted to work to, and not the width across corners. In the case of a hexagon the middle of the three flats is equal in width to the diameter of the bolt, and the other two are one-half its width; all three, therefore, being marked with the same set of compa.s.ses as gives the diameter of the body of the bolt, were as shown in Figure 152. For the width across flats there is an accepted standard; hence there is no need to mark it upon the drawing, unless in cases where the standard is to be departed from, in which event an end view may be added, or the view showing two sides may be given.

[Ill.u.s.tration: Fig 161.]

[Ill.u.s.tration: Fig. 162.]

[Ill.u.s.tration: Fig. 163.]

[Ill.u.s.tration: Fig. 164.]

To draw a square-headed bolt, the pencil lines are marked in the order shown by figures in Figure 164. The inking in is done in the order of the letters _a_, _b_, _c_, etc. It will be observed that pencil lines 2, 9, and 10 are not drawn to cross, but only to meet the lines at their ends, a point that, as before stated, should always be carefully attended to.

[Ill.u.s.tration: Fig. 165]

To draw the end view of a hexagon head, first draw a circle of the diameter across the flats, and then rest the triangle of 60 degrees on the blade _s_ of the square, as at T 1, in Figure 165, and mark the lines _a_ and _b_. Reverse the triangle, as at T 2, and draw lines _c_ and _d_. Then place the triangle as in Figure 166, and draw the lines _e_ and _f_.

[Ill.u.s.tration: Fig. 166.]

If the other view of the head is to be drawn, then first draw the lines _a_ and _b_ in Figure 167 with the square, then with the 60 degree triangle, placed on the square S, as at T 1, draw the lines _c_, _d_, and turning the square over, as at T 2, mark lines _e_ and _f_.

[Ill.u.s.tration: Fig. 167.]

If the diameter across corners of a square head is given, and it be required to draw the head, the process is as follows: For a view showing one corner in front, as in Figure 168, a circle of the given diameter across corners is pencilled, and the horizontal centre-line _a_ is marked, and the triangle of 45 degrees is rested against the square blade S, as in position T 1, and lines _b_ and _c_ marked, _b_ being marked first; and the triangle is then slid along the square blade to position T 1, when line _c_ is marked, these two lines just meeting the horizontal line _a_, where it meets the circle. The triangle is then moved to the left, and line _d_, joining the ends of _b_ and _c_, is marked, and by moving it still farther to the left to position T 2, line _e_ is marked. Lines _b_, _c_, _d_, and _e_ are, of course, the only ones inked in.

[Ill.u.s.tration: Fig. 168.]

[Ill.u.s.tration: Fig. 169.]

If the flats are to lie in the other direction, the pencilling will be done as in Figure 169. The circle is marked as before, and with the triangle placed as shown at T 1, line _a_, pa.s.sing through the centre of the circle, is drawn. By moving the triangle to the right its edge B will be brought into position to mark line _b_, also pa.s.sing through the centre of the circle. All that remains is to join the ends of these two lines, using the square blade for lines _c_, _d_, and the triangle for _e_ and _f_, its position on the square blade being denoted at T 3; lines _c_, _d_, _e_, _f_, are the ones inked in.