Chapter V: Fastening Devices (1)

Bolts are usually designated for size by their diameters measured at the cylindrical stem or body, and by their lengths measured from the inner side of the head to the end of the thread, so that if a nut be used, the length of the bolt, less the thickness of the nut and washer (if the latter be used), is the thickness of work the bolt will hold. If the work is tapped, and no nut is used, the full length of the bolt stem is taken as the length of the bolt.

A _black_ bolt is one left as forged. A finished bolt has its body, and usually its head also, machine finished, but a finished bolt sometimes has a black head, the body only being turned.

A square-headed bolt usually has a square nut, but if the nut is in a situation difficult of access for the wrench, or where the head of the bolt is entirely out of sight (as secluded beneath a flange) the nut is often made hexagon. A machine-finished bolt usually has a machine-finished and hexagon nut. Square nuts are usually left black.

The heads of bolts are designated by their shapes, irrespective of whether they are left black or finished. Fig. 370 represents the various forms: _a_, square head; _b_, hexagon head; _c_, capstan head; _d_, cheese head; _e_, snap head; _f_, oval head, or button head; _g_, conical head; _h_, pan head; _i_, countersink head.

The square heads _a_ are usually left black, though in exceptional cases they are finished. Hexagon heads are left black or finished as circumstances may require; when a bolt head is to receive a wrench and is to be finished, it is usually made hexagon. Heads _c_ and _d_ are almost invariably finished when used on operative parts of machines, as are also _e_ and F. Heads _g_ are usually left black, while _h_ and _i_ are finished if used on machine work, and left black when used as rivets or on rough unfinished work.

The heads from _e_ to _i_ assume various degrees of curve or angle to suit the requirements, but when the other end of the bolt is threaded to receive a nut, some means is necessary to prevent them from rotating in their holes when the nut is screwed up, thus preventing the nut from screwing up sufficiently tight. This is accomplished in woodwork by forging either a square under the head, as in Fig. 371, or by forging under the head a tit or stop, such as shown in Figs. 372 and 373 at P. Since, however, forging such stops on the bolt would prevent the heads from being turned up in the lathe, they are for lathe-turned bolts put in after the bolts have been finished in the lathe, a hole being subsequently drilled beneath the head to receive the pin or stop, P, Fig. 372, which may be tightly driven in. A small slot is cut in the edge of the hole to receive the stop.

Bolts are designated for kinds, as in Fig. 374, in which _k_ is a machine bolt; _l_ a collar bolt, from having a collar on it; _m_ a cotter bolt, from having a cotter or key passing through it to serve in place of a nut; _n_ a carriage bolt, from having a square part under the head to sink in the wood and prevent the bolt from turning with the nut; and _o_ a countersink bolt for cases where the head of the bolt comes flush.

The simple designation "machine bolt" is understood to mean a black or unfinished bolt having a square head and nut, and threaded, when the length of the bolt will admit it, and still leave an unthreaded part under the bolt head, for a length equal to about four times the diameter of the bolt head. If the bolt is to have other than a square head it is still called a machine bolt, but the shape of the head or nut is specially designated as "hexagon head machine bolt," this naturally implying that a hexagon nut also is required.

In addition to these general names for bolts, there are others applied to special cases. Thus Fig. 375 represents a patch bolt or a bolt for fastening patches (as plate C to plate D), its peculiarity being that it has a square stem A for the wrench to screw it in by. When the piece the patch bolt screws into is thin, as in the case of patches on steam boilers, the pitch of the thread may, to avoid leakage, be finer than the usual standard.

In countersink head bolts, such as the patch bolt in Fig. 375, the head is very liable to come off unless the countersink in the work (as in C) is quite fair with the tapped hole (as in D) because the thread of the bolt is made a tight fit to the hole, and all the bending that may take place is in the neck beneath the head, where fracture usually occurs. These bolts are provided with a square head A to screw them in by, and are turned in as at B to a diameter less than that at the bottom of the thread, so that if screwed up until they twist off, they will break in the neck at B.

Instead of the hole being countersunk, however, it may be cupped or counterbored, as in Fig. 376, in which the names of the various forms of the enlargement of holes are given. The difference between a faced and a counterbored hole is that in a counterbored hole the head or collar of the pin passes within the counterbore, the use of the counterbore being in this case to cause the pin to stand firmly and straight. The difference between a dished and a cupped is merely that cupped is deeper than dished, and that between grooved and recessed is that a recess is a wide groove.

Eye bolts are those having an eye in place of a head, as in Fig. 377, being secured by a pin passing through the eye, or by a second bolt, as in the figure. When the bolt requires to pivot, that part that is within the eye may be made of larger diameter than the thread, so as to form a shoulder against which the bolt may be screwed firmly home to secure it without gripping the eye bolt.

Fig. 378 represents a foundation bolt for holding frames to the stone block of a foundation. The bolt head is coned and jagged with chisel cuts. It is let into a conical hole (widest at the bottom) in the stone block, and melted lead is poured around it to fill the hole and secure the bolt head.

Another method of securing a foundation bolt head within a stone block is shown in Fig. 379; a similar coned hole is cut in the block, and besides the bolt head B a block W is inserted, the faces of the block and bolt being taper to fit to a taper key K, so that driving K locks both the bolt and the block in the stone. When the bolt can pass entirely through the foundation (as when the latter is brickwork) it is formed as in Fig. 380, in which B is a bolt threaded to receive a nut at the top. At the bottom it has a keyway for a key K, which abuts against the plate P. To prevent the key from slackening and coming out, it has a recess as shown in the figure at the sectional view of the bolt on the right of the illustration, the recess fitting down into the end of the keyway as shown.

Another method is to give the bolt head the form at B in Fig. 381, and to cast a plate with a rectangular slot through, and with two lugs A C. The plate is bricked in and a hole large enough to pass the bolt head through is left in the brickwork. The bolt head is passed down through the brickwork in the position shown at the top, and when it has passed through the slot in the plate it is given a quarter turn, and then occupies the position shown in the lower view, the lugs A C preventing it from turning when the nut is screwed home. The objection to this is that the hole through the brickwork must be large enough to admit the bolt head. Obviously the bolt may have a solid square head, and a square shoulder fitting into a square hole in the plate, the whole being bricked in.

Figs. 382 and 383 represent two forms of hook bolt for use in cases where it is not desired to have bolt holes through both pieces of the work. In Fig. 382 the head projects under the work and for some distance beneath and beyond the washer, as is denoted by the dotted line, hence it would suspend piece A from B or piece B from A. But in Fig. 383 the nut pressure is not beneath the part where the hook D grips the work, hence the nut would exert a pressure to pull piece B in the direction of the arrow; hence if B were a fixed piece the bolt would suspend A from it, but it could not suspend B from A.

In woodwork the pressure of the nut is apt to compress the wood, causing the bolt head and nut to sink into the wood, and to obviate this, anchor plates are used to increase the area receiving the pressure; thus in Fig. 384 a plate is tapped to serve instead of a nut, and a similar plate may of course be placed under the bolt head.

The Franklin Institute or United States Standard for the dimensions of bolt heads and nuts is as follows. In Fig. 385, D represents the diameter of the bolt, J represents the short diameter or width across flats of the bolt head or of the nut, being equal to one and a half times the diameter of the bolt, plus 1/16 inch for finished heads or nuts, and plus 1/8 inch for rough or unfinished heads or nuts. K represents the depth or thickness of the head or nut, which in finished heads or nuts equals the diameter of the bolt minus 1/16 inch, and in rough heads equals one half the distance between the parallel sides of the head, or in other words one half the width across the flats of the head.

H represents the thickness or depth of the nut, which for finished nuts is made equal to the diameter of the bolt less 1/16 inch, and therefore the same thickness as the finished bolt head, while for rough or unfinished nuts it is made equal to the diameter of the bolt or the same as the rough bolt head. I represents the long diameter or diameter across corners, which, however, is a dimension not used to work to, and is inserted in the following tables merely for reference:--

TABLE OF THE FRANKLIN INSTITUTE STANDARD DIMENSIONS FOR THE HEADS OF BOLTS AND FOR THEIR NUTS, WHEN BOTH HEADS AND NUTS ARE OF HEXAGON FORM, AND ARE POLISHED OR FINISHED.

+------------+-------------+-----------+---------------+-----------+
| Diameter | Diameter at | Number of | Diameter | Thickness |
| at top | bottom of | Threads | across Flats, | or |
| of Thread. | Thread. | per inch. | or short | Depth. |
| | | | diameter. | |
+------------+-------------+-----------+---------------+-----------+
| 1/4 | .185 | 20 | 7/16 | 3/16 |
| 5/16 | .240 | 18 | 17/32 | 1/4 |
| 3/8 | .294 | 16 | 5/8 | 5/16 |
| 7/16 | .345 | 14 | 23/32 | 3/8 |
| 1/2 | .400 | 13 | 13/16 | 7/16 |
| 9/16 | .454 | 12 | 29/32 | 1/2 |
| 5/8 | .507 | 11 | 1 | 9/16 |
| 3/4 | .620 | 10 | 1-3/16 | 11/16 |
| 7/8 | .731 | 9 | 1-3/8 | 13/16 |
| 1 | .837 | 8 | 1-9/16 | 15/16 |
| 1-1/8 | .940 | 7 | 1-3/4 | 1-1/16 |
| 1-1/4 | 1.065 | 7 | 1-15/16 | 1-3/16 |
| 1-3/8 | 1.160 | 6 | 2-1/8 | 1-5/16 |
| 1-1/2 | 1.284 | 6 | 2-5/16 | 1-7/16 |
| 1-5/8 | 1.389 | 5-1/2 | 2-1/2 | 1-9/16 |
| 1-3/4 | 1.491 | 5 | 2-11/16 | 1-11/16 |
| 1-7/8 | 1.616 | 5 | 2-7/8 | 1-13/16 |
| 2 | 1.712 | 4-1/2 | 3-1/16 | 1-15/16 |
| 2-1/4 | 1.962 | 4-1/2 | 3-7/16 | 2-3/16 |
| 2-1/2 | 2.176 | 4 | 3-13/16 | 2-7/16 |
| 2-3/4 | 2.426 | 4 | 4-3/16 | 2-11/16 |
| 3 | 2.629 | 3-1/2 | 4-9/16 | 2-15/16 |
| 3-1/4 | 2.879 | 3-1/2 | 4-15/16 | 3-3/16 |
| 3-1/2 | 3.100 | 3-1/4 | 5-5/16 | 3-7/16 |
| 3-3/4 | 3.377 | 3 | 5-11/16 | 3-13/16 |
| 4 | 3.567 | 3 | 6-1/16 | 3-15/16 |
| 4-1/4 | 3.798 | 2-7/8 | 6-7/16 | 4-3/16 |
| 4-1/2 | 4.028 | 2-7/8 | 6-13/16 | 4-7/16 |
| 4-3/4 | 4.256 | 2-5/8 | 7-3/16 | 4-11/16 |
| 5 | 4.480 | 2-1/2 | 7-9/16 | 4-15/16 |
| 5-1/4 | 4.730 | 2-1/2 | 7-15/16 | 5-3/16 |
| 5-1/2 | 4.953 | 2-3/8 | 8-5/16 | 5-7/16 |
| 5-3/4 | 5.203 | 2-3/8 | 8-11/16 | 5-11/16 |
| 6 | 5.423 | 2-1/4 | 9-1/16 | 5-15/16 |
+------------+-------------+-----------+---------------+-----------+

Note that square heads are supposed to be always unfinished, hence there is no standard for their sizes if finished.

The Franklin Institute standard dimensions for hexagon and square bolt heads and nuts when the same are left unfinished or rough, as forged, are as follows:--

+----------+-------------+-------------+---------------+-----------+
| | Diameter | Diameter | Short | Thickness |
| Bolt | across | across | diameter, | or |
| Diameter | corners, or | corners or | or diameter | depth for |
| in | long | long | across flats | square or |
| Inches. | diameter of | diameter of | for square or | hexagon |
| | hexagon | square | hexagon heads | heads. |
| | heads. | heads. | and nuts. | |
+----------+-------------+-------------+---------------+-----------+
| | Inch. | Inch. | Inch. | Inch. |
| 1/4 | 37/64 | 7/10 | 1/2 | 1/4 |
| 5/16 | 11/16 | 10/12 | 19/32 | 19/64 |
| 3/8 | 51/64 | 63/64 | 11/16 | 11/32 |
| 7/16 | 9/10 | 1-7/64 | 25/32 | 25/64 |
| 1/2 | 1 | 1-15/64 | 7/8 | 7/16 |
| 9/16 | 1-1/8 | 1-23/64 | 31/32 | 31/64 |
| 5/8 | 1-7/32 | 1-1/2 | 1-1/16 | 17/32 |
| 3/4 | 1-7/16 | 1-49/64 | 1-1/4 | 5/8 |
| 7/8 | 1-21/32 | 2-1/32 | 1-7/16 | 23/32 |
| 1 | 1-7/8 | 2-19/64 | 1-5/8 | 13/16 |
| 1-1/8 | 2-2/32 | 2-9/16 | 1-13/16 | 29/32 |
| 1-1/4 | 2-5/16 | 2-53/64 | 2 | 1 |
| 1-3/8 | 2-17/32 | 3-3/32 | 2-3/16 | 1-3/32 |
| 1-1/2 | 2-3/4 | 3-23/64 | 2-3/8 | 1-3/16 |
| 1-5/8 | 2-31/32 | 3-5/8 | 2-9/16 | 1-9/32 |
| 1-3/4 | 3-3/16 | 3-57/64 | 2-3/4 | 1-3/8 |
| 1-7/8 | 3-13/32 | 4-5/32 | 2-15/16 | 1-15/32 |
| 2 | 3-5/8 | 4-27/64 | 3-1/8 | 1-9/16 |
| 2-1/4 | 4-1/16 | 4-61/64 | 3-1/2 | 1-3/4 |
| 2-1/2 | 4-1/2 | 5-31/64 | 3-7/8 | 1-15/16 |
| 2-3/4 | 4-29/32 | 6 | 4-1/4 | 2-1/8 |
| 3 | 5-3/8 | 6-17/32 | 4-5/8 | 2-5/16 |
| 3-1/4 | 5-13/16 | 7-1/16 | 5 | 2-1/2 |
| 3-1/2 | 6-7/64 | 7-39/64 | 5-3/8 | 2-11/16 |
| 3-3/4 | 6-21/32 | 8-1/8 | 5-3/4 | 2-7/8 |
| 4 | 7-3/32 | 8-41/64 | 6-1/8 | 3-1/16 |
| 4-1/4 | 7-9/16 | 9-3/16 | 6-1/2 | 3-1/4 |
| 4-1/2 | 7-31/32 | 9-3/4 | 6-7/8 | 3-7/16 |
| 4-3/4 | 8-13/32 | 10-1/4 | 7-1/4 | 3-5/8 |
| 5 | 8-27/32 | 10-49/64 | 7-5/8 | 3-13/16 |
| 5-1/4 | 9-9/32 | 11-23/64 | 8 | 4 |
| 5-1/2 | 9-23/32 | 11-7/8 | 8-3/8 | 4-3/16 |
| 5-3/4 | 10-5/32 | 12-3/8 | 8-3/4 | 4-3/8 |
| 6 | 10-19/32 | 12-15/16 | 9-1/8 | 4-9/16 |
+----------+-------------+-------------+---------------+-----------+

The depth or thickness of both the hexagon and square nuts when left rough or unfinished is, according to the above standard, equal to the diameter of the bolt.

The following are the sizes of finished bolts and nuts according to the present Whitworth Standard. The exact sizes are given in decimals, and the nearest approximate sizes in sixty-fourths of an inch:--

+-------------+------------------------+----------------------+
| Diameter of | Width of nuts across | Height of bolt |
| bolts. | flats. | heads. |
+-------------+----------+-------------+--------+-------------+
| 1/8 | .338 | 21/64 _f_ | .1093 | 7/64 |
| 3/16 | .448 | 29/64 _b_ | .1640 | 5/32 |
| 1/4 | .525 | 33/64 _f_ | .2187 | 7/32 |
| 5/16 | .6014 | 19/32 _f_ | .2734 | 17/64 |
| 3/8 | .7094 | 45/64 _f_ | .3281 | 21/64 |
| 7/16 | .8204 | 53/64 _b_ | .3828 | 3/8 _f_ |
| 1/2 | .9191 | 29/32 _b_ | .4375 | 7/16 |
| 9/16 | 1.011 | 1-1/64 _b_ | .4921 | 31/64 _f_ |
| 5/8 | 1.101 | 1-3/32 _f_ | .5468 | 35/64 |
| 11/16 | 1.2011 | 1-13/64 _b_ | .6015 | 19/32 _f_ |
| 3/4 | 1.3012 | 1-19/64 _f_ | .6562 | 21/32 |
| 13/16 | 1.39 | 1-25/64 _b_ | .7109 | 45/64 _f_ |
| 7/8 | 1.4788 | 1-31/64 _b_ | .7656 | 49/64 |
| 15/16 | 1.5745 | 1-37/64 _b_ | .8203 | 13/16 _f_ |
| 1 | 1.6701 | 1-43/64 _b_ | .875 | 7/8 |
| 1-1/8 | 1.8605 | 1-55/64 _f_ | .9843 | 63/64 |
| 1-1/4 | 2.0483 | 2-3/64 _f_ | 1.0937 | 1-3/32 |
| 1-3/8 | 2.2146 | 2-7/32 _b_ | 1.2031 | 1-13/64 |
| 1-1/2 | 2.4134 | 2-13/32 _f_ | 1.3125 | 1-5/16 |
| 1-5/8 | 2.5763 | 2-37/64 _b_ | 1.4128 | 1-27/64 |
| 1-3/4 | 2.7578 | 2-3/4 _f_ | 1.5312 | 1-17/32 |
| 1-7/8 | 3.0183 | 3-1/16 _f_ | 1.6406 | 1-41/64 |
| 2 | 3.1491 | 3-5/32 _b_ | 1.75 | 1-3/4 |
| 2-1/8 | 3.337 | 3-11/32 _b_ | 1.8523 | 1-55/64 |
| 2-1/4 | 3.546 | 3-35/64 _b_ | 1.9687 | 1-31/32 |
| 2-3/8 | 3.75 | 3-3/4 | 2.0781 | 2-5/64 |
| 2-1/2 | 3.894 | 3-57/64 _f_ | 2.1875 | 2-3/16 |
| 2-5/8 | 4.049 | 4-3/64 _f_ | 2.2968 | 2-19/64 |
| 2-3/4 | 4.181 | 4-3/16 _b_ | 2.4062 | 2-13/32 |
| 2-7/8 | 4.3456 | 4-11/32 _f_ | 2.5156 | 2-33/64 |
| 3 | 4.531 | 4-17/32 _b_ | 2.625 | 2-5/8 |
+-------------+----------+-------------+--------+-------------+

The thickness of the nuts is in every case the same as the diameter of the bolts: _f_ = full, _b_ = bare.

When bolts screw directly into the work instead of passing through it and receiving a nut, they come under the head of either tap bolts, set screws, cap screws, or machine screws. A tap bolt is one in which the full length of the stem or body is threaded, and differs from a set screw, which is similarly threaded, in the respect that in a set screw the head is square and its diameter is the same as the square bar of steel or iron (as the case may be) from which the screw was made, while in the tap bolt the head is larger in diameter than the bar it was made from. Furthermore a tap bolt may have a hexagon head, which is usually left unfinished unless ordered to be finished, as is also the case with set screws.

Cap screws are made with heads either hexagon, square, or round, and also with a square head and round collar, as in Fig. 386, the square heads being of larger diameter than the iron from which they were made. When the heads of cap screws are finished they are designated as "milled heads."

A machine screw is a small screw, such as in Fig. 387, the diameter of the body being made to the Birmingham wire gauge, the heads being formed by upsetting the wire of which they are made. They have saw slots S for a screw driver, the threads having special pitches, which are given hereafter. The forms of the heads are as in Fig. 387, A being termed a Fillister, B a countersink, and C a round head. The difference between a Fillister head of a machine screw and the same form of head in a cap screw is that the former is upset cold, and the latter is either forged or cut out of the solid metal.

When the end of a screw abuts against the work to secure it, it is termed a set screw. The ordinary form of set screw is shown in Fig. 389, the head being square and either black or polished as may be required. The ends of the set screws of commerce, that is to say, that are kept on sale, are usually either pointed as at A, Fig. 388, slightly bevelled as at B, or cupped as at D. If left flat or only slightly bevelled as at B, they are liable, if of steel and not hardened, or if of iron and case-hardened only, to bulge out as at C. This prevents them from slacking back easily or prevents removal if necessary, and even though of hardened steel they do not grip very firmly. On this account their points are sometimes made conical, as at A. This form, however, possesses a disadvantage when applied to a piece of work that requires accurate adjustment for position, inasmuch as it makes a conical indentation in the work, and unless the point be moved sufficiently to clear this indentation the point will fall back into it; hence the conical point is not desirable when the piece may require temporary fixture to find the adjustment before being finally screwed home. For these reasons the best form of set screw end is shown at D, the outside of the end being chamfered off and the inside being cupped, as denoted by the dotted lines. This form cuts a ring in the work, but will hold sufficiently for purposes of adjustment without being screwed home firmly.

In some cases the end of the set screw is tapped through the enveloping piece (as a hub) and its end projects into a plain hole in the internal piece of the work, and in this case the end of the thread is turned off for a distance of two or three threads, as at A in Fig. 390. Similarly, when the head of the screw is to act or bear upon the work, the thread may be turned off as at B in the figure.

When a bolt has no head, but is intended to screw into the work at one end, and receive a nut at the other, it is termed a stud or standing bolt. The simplest form of standing bolt is that in which it is parallel from end to end with a thread at each end, and an unthreaded part in the middle, but since standing bolts or studs require to remain fixed in the work, it is necessary to screw them tightly into their places, and therefore firmly home. This induces the difficulty that some studs may screw a trifle farther into the work than others, so that some of the stud ends may project farther through the nuts than others, giving an appearance that the studs have been made of different lengths. The causes of this may be slight variations in the tapping of the holes and the threading of the studs. If those that appear longest are taken out and reduced to the lengths of the others, it will be found sometimes that the stud on the second insertion will pass farther into the work than at the first, and the stud will project less through the nut than the others. To avoid this those protruding most may be worked backward and forward with the wrench and thus induced to screw home to the required distance, but it is better to provide to the stud a shoulder against which it may screw firmly home; thus in Fig. 391 is a stud, whose end A is to screw into the work, part B is to enter the hole in the work (the thread in the hole being cut away at the mouth to receive B). In this case the shoulder between B and C screwing firmly against the face of the work, all the studs being made of equal length from this shoulder to end E, then the thickness of the flange or work secured by the nut being equal, the nuts will pass an equal distance on end D, and E will project equally through all the nuts. The length of the plain part C is always made slightly less than the thickness of the flange or foot of the work to be bolted up, so that the nut shall not meet C before gripping the flange surface.

There are, however, other considerations in determining the shape and size of the parts A and C of studs.

Thus, suppose a stud to have been in place some time, the nut on end E being screwed firmly home on the work, and perhaps somewhat corroded on E. Then the wrench pressure applied to the nut will be in a direction to unscrew the stud out of the work, and if there be less friction between A and the thread in the work than there is between D and the thread in the nut, the stud and not the nut will unscrew. It is for this purpose that the end A requires firmly screwing into the work. But in the case of much corrosion this is not always sufficient, and the thread A is therefore sometimes made of a larger diameter than the thread at D. In this case the question at once arises, What shall be the diameter of the plain part C?

If it be left slightly larger than D, but the depth of the thread less than A, then it may be held sufficiently firmly by the fit of the threads (without the aid of screwing against a shoulder) to prevent unscrewing when releasing the nut, and may be screwed within the work until its end projects the required distance; thus all the studs may project an equal distance, but there will be the disadvantage that when the studs require removing and are corroded the plain part is apt to twist off, leaving the end A plugging the hole. The plain part C may be left of same diameter as A, both being larger than D; but in this case the difficulty of having all the studs project equally when screwed home, as previously mentioned, is induced; hence C may be larger than A, and a shoulder left at B, as in the figure; this would afford excellent facility for unscrewing the stud to remove it, as well as insuring equal projection of E. The best method of all is, so far as quality goes, to make the plain part C square, as in Fig. 392, which is an English practice, the square affording a shoulder to screw up against and secure an equal projection while serving to receive a wrench to put in or remove the stud. In this case the holes in the flange or piece bolted up being squared, the stud cannot in any case unscrew with the nut. The objection to this squared stud is that the studs cannot be made from round bar iron, and are therefore not so easily made, and that the squaring of the holes in the flange or part of the work supported by the stud is again extra work, and for these reasons studs with square instead of cylindrical mid-sections have not found favor in the United States.

An excellent method of preventing the stud from unscrewing with the nut is to make the end A longer than the nut end, as in Fig. 393, so that its threads will have more friction; and this has the further advantage that in cast iron it serves also to make the strength of the thread equal to that of the stud. As the faces of the nuts are apt when screwed home to score or mark the face of the work, it adds to the neatness of the appearance to use a washer W beneath the nut, which distributes the pressure over a greater area of work surface.

In some practice the ends A of studs are threaded taper, which insures that they shall fit tight and enables their more easy extraction.

An excellent tool for inserting studs of this kind to the proper distance is shown in Fig. 394. It consists of a square body _a_ threaded to receive the stud whose end is shown at _c_. The upper end is threaded to receive an adjusting screw _b_, which is screwed in so that its end _d_ meets the end _c_ of the stud. It is obvious that _b_ may be so adjusted that when _a_ is operated by a wrench applied to its body until its end face meets the work and the stud is inserted to the proper depth, all subsequent studs may be put into the same depth.

When the work pivots upon a stem, as in Fig. 395, the bolt is termed a standing pin, and as in such cases the stem requires to stand firm and true it is usual to provide the pin with a collar, as shown in the figure, and to secure the pivoted piece in place with a washer and a taper pin because nuts are liable to loosen back of themselves. Furthermore, a pin and washer admit of more speedy disconnection than a nut does, and also give a more delicate adjustment for end fit.

In drilling the tapping holes for standing bolts, it is the practice with some to drill the holes in cast iron of such a size that the tap will cut three-quarters only of a full thread, the claim being that it is as strong as a full thread. The difference in strength between a three-quarter and a full thread in cast iron is no doubt practically very small indeed, while the process of tapping is very much easier for the three-quarter full thread, because the tap may, in that case, be wound continuously forward without backing it at every quarter or half revolution, as would otherwise be necessary, in order to give the oil access to the cutting edges of the tap--and oil should always be used in the process of tapping (even though on cast iron it causes the cuttings to clog in the flutes of the tap, necessitating in many cases that the tap be once or twice during the operation taken out, and the cuttings removed) because the oil preserves the cutting edges of the tap teeth from undue abrasion, and, therefore, from unnecessarily rapid dulling. With a tap having ordinarily wide and deep flutes, and used upon a hole but little deeper than the diameter of the tap, the cuttings due to making a three-quarter full thread will not more than fill the flutes of the tap by the time its duty is performed. We have also to consider that with a three-quarter full thread it is much easier to extract the standing bolt when it is necessary to do so, so that all things considered it is permissible to have such a thread, providing the tapping hole does not pass through into a cylinder or chamber requiring to be kept steam-tight, for in that case the bolt would be almost sure to leak. As a preventive against such leakage, the threads are sometimes cut upon the standing bolts without having a terminal groove, and are then screwed in as far as they will go; the termination of the thread upon the standing bolt at the standing or short end being relied upon to jam into and close up the thread in the hole. A great objection to this, however, is the fact that the bolts are liable to screw into the holes to unequal depths, so that the outer ends will not project an equal distance through the nuts, and this has a bad appearance upon fine work. It is better, then, in such a case, to tap the holes a full thread, the extra trouble involved in the tapping being to some extent compensated for in the fact that a smaller hole, which can be more quickly drilled, is required for the full than for the three-quarter thread.

The depth of the tapping hole should be made if possible equal to one and a half times the diameter of the tap, so that in case the hole bottoms and the tap cannot pass through, the taper, and what is called in England the second, and in the United States the plug tap, will finish the thread deep enough without employing a third tap, for the labor employed in drilling the hole deeper is less than that necessary to the employment of a third tap. If the hole passes through the work, its depth need not, except for cast-iron holes, be greater than 1/8 inch more than the diameter of the bolt thread, which amount of excess is desirable so that in case the nut corrodes, the nut being as thick as the diameter of the tap, and therefore an inch less than the depth of the hole at the standing end, will be more likely to leave the stud standing than to carry it with it when being unscrewed.

When it is desirable to provide that bolts may be quickly removed, the flanges may be furnished with slots, as in Fig. 396, so that the bolts may be passed in from the outside, and in this case it is simply necessary to slacken back the nut only. It is preferable, however, in this case to have the bolt square under the head, as in Fig. 397, so as to prevent the bolt from turning when screwing up or unscrewing the nut. The bolt is squared at A, which fits easily into the flange. The flanges, however, should in this case be of ample depth or thickness to prevent their breakage, twice the depth of the nut being a common proportion.

In cases where it is inconvenient for the bolt head to pass through the work a [T] groove is employed, as in Fig. 398. In this case the bolt head may fit easily at A B to the sides A B of the groove, so that while the bolt head will slide freely along the groove, the head, being square, cannot turn in the slot when the nut is screwed home. This, however, is more efficiently attained when there is a square part beneath the bolt head, as in Fig. 399, the square A of the bolt fitting easily to the slot B of the groove.

When it is undesirable that the slots run out to the edge of the work they may terminate in a recess, as at A in Fig. 400, which affords ingress of the bolt head to the slot; or the bolt head may be formed as in Fig. 401, the width A B of the bolt head passing easily through the top A B of the slot, and the bolt head after its insertion being turned in the direction of the arrow, which it is enabled to do by reason of the rounded corners C D. In this case, also, there may be a square under the head to prevent the bolt head from locking in the slot, but the corners of the square must also be rounded as in Fig. 402.

The underneath or gripping surface of a bolt head should be hollow, as at A in Fig. 403, rather than rounding as at B, because, if rounding, the bolt will rotate with the nut when the latter grips the work surface. It should also be true with the axial line of the bolt so as to bear fairly upon the work without bending. The same remarks apply to the bedding surface of the nut, because to whatever amount the face is out of true it will bend the threaded end of the bolt, and this may be sufficient to cause the bolt to break.

In Fig. 404, for example, is shown a bolt and nut, neither of which bed fair, being open at A and B respectively, and it is obvious that the strain will tend to bend or break the bolt across the respective dotted lines C, D. In the case of the nut there is sufficient elasticity in the thread to allow of the nut forcing itself to a bed on the work, the bolt bending; but in the case of the bolt head the bending is very apt to break off the bolt short in the neck under the head. In a tap bolt where the wrench is applied to the bolt head, the rotation, under severe strain, of the head will usually cause it to break off in all cases where the bolt is rigidly held, so that it cannot cant over and allow the head to bed fair.

A plain tap bolt should be turned up along its body, because if out of true the hole it passes through must be made large enough to suit the eccentricity of the bolt, or else a portion of the wrench pressure will be expended in rotating the bolt in the hole instead of being expended solely in screwing the bolt farther into the work.

It is obvious therefore, that if a tap bolt be left black the hole it passes through must be sufficiently large to make full allowance for the want of truth in the bolt. For the same reasons the holes for tapped bolts require to be tapped very true.

Black studs possess an advantage (over tap bolts) in this respect, inasmuch as that if the holes are not tapped quite straight the error may be to some extent remedied by screwing them fully home and then bending them by hammer blows.

Nuts are varied in form to suit the nature of the work. For ordinary work, as upon bolts, their shape is usually made to conform to the shape of the bolt head, but when the nut is exposed to view and the bolt head hidden, the bolt end and the nut are (for finished work) finished while the bolt heads are left black.

The most common form of hexagon nut is shown in Fig. 405, the upper edge being chamfered off at an angle of about 40°. In some cases the lower edge is cut away at the corners, as in Fig. 405 at A, the object being to prevent the corners of the nut from leaving a circle of bearing marks upon the work, but this gives an appearance at the corners that the nut does not bed fair. Another shape used by some for the end faces of deep nuts, that is to say, those whose depth exceeds the diameter of the bolt, is shown in Fig. 406. Nuts of extra depth are used when, from the nut being often tightened and released, the thread wear is increased, and the extra thread length is to diminish the wear.

To avoid the difficulty of having some of the bolt ends project farther through some nuts than others on a given piece of work, as is liable to occur where the flanges to be bolted together are not turned on all four radial faces, the form of nut shown in Fig. 407 is sometimes employed, the thread in the nut extending beyond the bolt end.

As an example of the application of this nut, suppose a cylinder cover to be held by bolts, then the cylinder flange not being turned on its back face is usually of unequal thickness; hence to have the bolt ends project equally through the nuts, each bolt would require to be made of a length to suit a particular hole, and this would demand that each hole and bolt be marked so that they may be replaced when taken out, without trying them in their places. Another application of this nut is to make a joint where the threads may be apt to leak. In this case the mouth of the hole is recessed and coned at the edge; the nut is chamfered off with a similar cone, and a washer W, Fig. 408, is placed beneath the nut to compress and conform to the coned recess; thus with the aid of a cement of some kind, as red or white lead (usually red lead), a tight joint may be made independent of the fit of the threads.

When the hole through which the bolt passes is considerably larger in diameter than the bolt, the flange nut shown in Fig. 409 is employed, the flange covering the hole. A detached washer may be used for the same purpose, providing that its hole fit the bolt and it be of a sufficient thickness to withstand the pressure and not bend or sink into the hole.

Circular nuts are employed where, on account of their rotating at high speed, it is necessary that they be balanced as nearly as possible so as not to generate unbalanced centrifugal force. Fig. 410 represents a nut of this kind: two diametrically opposite flat sides, as A, affording a hold for the wrench. Other forms of circular nuts are shown in Figs. 411 and 412. These are employed where the nuts are not subject to great strain, and where lightness is an object.

That in Fig. 411 is pierced around its circumference with cylindrical holes, as A, B, C, to receive a round lever or rod or a wrench, such as shown in Fig. 459.

That shown in Fig. 412 has slots instead of holes in its circumference, and the form of its wrench is shown in Fig. 461.

When nuts are employed upon bolts in which the strain of the duty is longitudinal to the bolt, and especially if the direction of motion is periodically reversed, and also when a bolt is subject to shocks or vibrations, a single nut is liable to become loose upon the bolt, and a second nut, termed a check nut, jamb nut, or safety nut, becomes necessary, because it is found that if two nuts be employed, as in Fig. 413, and the second nut be screwed firmly home against the first, they are much less liable to come loose on the bolt.

Considerable difference of practice exists in relation to the thickness of the two nuts when a check nut is employed. The first or ordinary nut is screwed home, and the second or check nut is then screwed home. If the second nut is screwed home as firmly as the first, it is obvious that the strain will fall mainly on the second. If it be screwed home more firmly than the first, the latter may be theoretically considered to be relieved entirely of the strain, while if it be screwed less firmly home, the first will be relieved to a proportionate degree of the strain. It is usual to screw the second home with the same force as applied to the first, and it would, therefore, appear that the first nut, being relieved of strain, need not be so thick as the first, but it is to be considered that, practically, the first nut will always have some contact with the bolt threads, because from the imperfections in the threads of ordinary bolts the area and the force of contact is not usually the same nor in the same direction in both nuts, unless both nuts were tapped with the same tap and at about the same time.

When, for example, a tap is put into the tapping machine, it is at its normal temperature, and of a diameter due to that temperature, but as its work proceeds its temperature increases, notwithstanding that it may be freely supplied with oil, because the oil cannot, over the limited area of the tap, carry off all the heat generated by the cutting of a tap rotated at the speeds usually employed in practice. As a result of this increase of temperature, we have a corresponding increase in the diameter of the tap, and a variation in the diameter of the threads in the nuts. The variation in the nuts, however, is less than that in the tap diameter, because as the heated tap passes through the nut it imparts some of its heat to the nut, causing it also to expand, and hence to contract in cooling after it has been tapped, and, therefore, when cold, to be of a diameter nearer to that of the tap.

Furthermore, as the tap becomes heated it expands in length, and its pitch increases, hence here is another influence tending to cause the pitches of the nut threads to vary, because although the temperature of the tap when in constant use reaches a limit beyond which, so long as its speed of rotation is constant, it never proceeds; yet, when the tap is taken from the machine to remove the tapped nuts which have collected on its shank, and it is cooled in the oil to prevent it from becoming heated any more than necessary, the pitch as well as the diameter of the tap is reduced nearer to its normal standard.

So far, then, as theoretical correctness, either of pitch or diameter in nut threads, is concerned, it could only be attained (supposing that the errors induced by hardening the tap could be eliminated) by employing the taps at a speed of rotation sufficiently slow to give the oil time to carry off all the heat generated by the cutting process. But this would require a speed so comparatively slow as not to be commercially practicable, unless followed by all manufacturers. Practically, however, it may be considered that if two nuts be tapped by a tap that has become warmed by use, they will be of the same diameter and pitch, and should, therefore, have an equal area and nature of contact with the bolt thread, supposing that the bolt thread itself is of equal and uniform pitch. But the dies which cut the thread upon the bolt also become heated and expanded in pitch. But if the temperature of the dies be the same as that of the tap, the pitches on both the bolt and in the nut will correspond, though neither may be theoretically true to the designated standard.

In some machines for nut tapping the tap is submerged in oil, and thus the error due to variations of temperature is practically eliminated, though even in this case the temperature of the oil will gradually increase, but not sufficiently to be of practical moment.

Let it now be noted that from the hardening process the taps shrink in length and become of finer pitch, while the dies expand and become of coarser pitch, and that this alone precludes the possibility of having the nut threads fit perfectly to those on the bolt. It becomes apparent, then, that only by cutting the threads in the lathe, and with a single-toothed lathe tool that can be ground to correct angle after hardening, can a bolt and nut be theoretically or accurately threaded. Under skilful operation, however, both in the manufacture of the screw-cutting tools and in their operation, a degree of accuracy can be obtained in tapped nuts and die-threaded bolts that is sufficient with a single nut for ordinary uses, but in situations in which the direction of pressure on the nut is periodically reversed, or in which it is subject to shocks or vibrations, the check nut becomes necessary, as before stated.

An excellent method of preventing a nut from slackening back of itself is shown in the safety nut in Fig. 414; it consists of a second nut having a finer thread than the first one, so that the motion of the first would in unscrewing exceed that of the second, hence the locking is effectually secured.

Work may be very securely fastened together by the employment of what are called differential screws, the principle of whose action may be explained with reference to Fig. 415, which is extracted from "Mechanics." It represents a piston head and piston rod secured together by means of a differential screw nut. The nut contains an internal thread to screw on the rod, and an external one to screw into the piston head, but the internal thread and that on the rod differ from the external one, and that in the head by a certain amount, as say one tenth of the pitch. The nut itself is furnished with a hexagonal head, and when screwed into place draws the two parts together with the same power as a screw having a pitch equal to the difference between the two pitches.

When putting the parts together the nut is first screwed upon the rod B. The outside threads are then entered into the thread in the piston C, and by means of a suitable wrench the nut is screwed into the proper depth. As shown in the engraving, the nut goes on to the rod a couple of threads before it is entered in the piston. The tightening then takes place precisely as though the nut had a solid bearing on the piston and a fine thread on the rod, the pitch of which is equal to the difference between the pitches of the two threads. Fig. 416 shows its application to the securing of a pump plunger upon the end of a piston-rod. In this case, as the rod does not pass through the nut, the latter is provided with a cap, which covers the end of the rod entirely.

The principle of the differential screw may be employed to effect very fine adjustments in place of using a very fine thread, which would soon wear out or wear loose. Thus in Fig. 417 is shown the differential foot screws employed to level astronomical instruments. C D is a foot of the instrument to be levelled. It is threaded to receive screw A, which is in turn threaded to receive the screw B, whose foot rests in the recess or cup in E F. Suppose the pitch of screw A is 30 per inch, and that of B is 40, and we have as follows. If A and B are turned together the foot C D is moved the amount due to the pitch of A. If B is turned within a the foot is moved the amount due to the pitch of B. If A is turned the friction of the foot of B will hold B stationary, and the motion of C D will equal the difference between the pitches of the threads of A and B. Thus one revolution of A forward causes it to descend through C D 1/30 inch (its pitch), tending to raise C D 1/30 inch. But while doing this it has screwed down upon the thread of B 1/40 inch (the pitch of B) and this tends to lower C D, hence C D is moved 1/120 inch, because 1/30-1/40 = 1/120.

To cause a single nut to lock itself and dispense with the second or jamb nut, various expedients have been employed. Thus in Fig. 418 is shown a nut split on one side; after being threaded the split is closed by hammer blows, appearing as shown in the detached nut. Upon screwing the nut upon the bolt the latter forces the split nut open again by thread pressure, and this pressure locks the nut. Now there will be considerable elasticity in the nut, so that if the thread compresses on its bearing area, this elasticity will take up the wear or compression and still cause the threads to bind. Sometimes a set screw is added to the split, as in Fig. 419, in which case the split need not be closed with the hammer.