Fig. 359Fig. 359.
Fig. 359.
The single wrench has its hole at one end, as shown inFig. 359atd, and is employed for tapping holes in locations where the double wrench could not be got in.
Fig. 360Fig. 360.
Fig. 360.
In some cases double tap wrenches are made with two or three sizes of square holes to serve as many different sizes of taps, but this is objectionable, because unless the handles of the wrench extend equally on each side of the tap, the overhanging weight on one side of the tap exerts an influence to pull the tap over to one side and tap the hole out of straight. For taps that have square heads the wrench should be a close but an easy fit to the tap head, otherwise the square corners of the tap become rounded. For the smaller sizes of taps, adjustable wrenches, such as shown inFig. 360, are sometimes employed. These contain two dies; the upper one, which meets the threaded end ofc, being a sliding fit, and the joint faces being formed as shown ata,b. By rotating the handlecits end leaves the upper die, which may be opened out, leaving the square hole between the dies large enough to admit the squared tap end. After the wrench is placed on the tap,cis rotated so as to close the dies upon the tap.
When the location of the tapping hole leaves room for the wrench to rotate a full circle,cis screwed up so that the dies firmly grip the tap head, which preserves the tap head; but when the wrench can only be rotated a part of a revolution,cis adjusted to leave the dies an easy fit to the tap head, so as to enable the wrench to be removed from the tap head with facility and again placed upon the tap head.cis operated by a round lever or pin introduced in a hole in the collar, or the collar may be squared to receive a wrench.
Fig. 361Fig. 361.
Fig. 361.
Fig. 362Fig. 362.
Fig. 362.
To insure that a tap shall tap a hole straight, the machinist, in the case of hand tapping, applies a square to the work and the tap, as shown inFig. 361, in whichwrepresents a piece of work,ta tap, ands stwo squares. If the tap is a taper one the square is sighted with the shank of the tap, as shown in position 1, but if the thread of the tap is parallel, the square may be applied to the thread of the tap, as in position 2. If the tap leans over to one side, as inFig. 362, it is brought upright by exerting a pressure on the tap wrench handleb(on the high side) in the direction of the arrowa, while the wrench is rotated; but if the tap leans much to one side it is necessary to rotate the tap back and forth, exerting the pressure on the forward stroke only.
It is necessary to correct the errors before the tap has entered the hole deeply, because the deeper the tap has entered the greater the difficulty in making the correction. If the pressure on the tap wrench be made excessive, it is very liable to cause the tap to break, especially in the case of small taps, that is to say, those of5⁄8inch or less in diameter. The square should be applied as soon as the tap has entered the hole sufficiently to operate steadily, and should be applied several times during the tapping operation.
Fig. 363Fig. 363.
Fig. 363.
When the tap does not pass through the hole it may be employed with a guide which will keep it true, as shown inFig. 363, in whichwis a piece of work,tthe tap, andsa guide, the latter being bolted or clamped to the work atb. In this case the shank of the tap is made fully as large in diameter as the thread. In cases where a number of equidistant holes require tapping, as in the case of cylinder ends, this device saves a great deal of time and insures that the tapping be performed true, the hole to receive the boltband that to receive the tap being distant apart to the same amount as are the holes in the work.
Fig. 364Fig. 364.
Fig. 364.
Fig. 365Fig. 365.
Fig. 365.
In shops where small work is made to standard gauge, and on the interchangeable system, devices are employed, by means ofwhich a piece that has been threaded will screw firmly home to its place, and come to some definite position, as in the following examples. InFig. 364let it be required that the studashall screw in the slides; the armato stand vertical when collarbis firmly home, and a device such as inFig. 365may be employed.pis a plate on which is fixed a chuckcto receive the slides. In platepis a groovegto hold the headhat a right angle to the slideway inc, there being a projection beneathhand beneathcto fit intog. The taptis threaded throughh, but not fluted at the part that winds throughhwhen the tapping is being done, so as not to cause the thread inhto wear.hacts as a guide to the tap and causes it to start the thread at the same point in the bore of each pieces, and the stem will be so threaded that the screw starts at the same point in the circumference of each piece.
Fig. 366Fig. 366.
Fig. 366.
Fig. 367Fig. 367.
Fig. 367.
Fig. 368Fig. 368.
Fig. 368.
Fig. 369Fig. 369.
Fig. 369.
A second example of uniform tapping is shown inFigs. 366,367, and368. The piece,Fig. 366, is to have its boreatapped in line with the slotc, and the thread is to start at a certain point in its bore. InFig. 367this piece is shown chucked on a plated.fis a chuck having a lugefitting into the slot (c,Fig. 366) of the work. This adjusts the work in one direction. The facedof the plate adjusts the vertical height of the work, and the alignment of the hole to the axis of the tap is secured in the construction of the chuck, as is shown inFig. 369. A lugkis at a right angle to the facebof the chuck and stands in a line with luge, as denoted by the dotted lineg g, and as lugkfits into the slotg,Fig. 367, the work will adjust itself true when bolted to the plate.
Fig. 368shows a method of tapping or hobbing four chasers (as for a bolt cutter), so that if the chasers are marked 1, 2, 3 and 4, as shown, any chaser of No. 1 will work with the others, although not tapped at the same operation.cis a chuck with four dies (a,b,c,d) placed between the chasers. By tightening the set-screwss, the dies and chasers are locked ready for the tapping.nis a hub to receive a guide-pinp, which is passed through to hold the chasers true while being set in the chuck, and it is withdrawn before the tapping commences;d e fare simply to take hold of when inserting and removing the dies. It is obvious that a chuck such as this used upon a plate, as inFig. 365, with the hob guided in the headhthere shown, would tap each successive set of chasers alike as a set, and individually alike, provided, of course, that the hob guide or headhis at each setting placed the same distance from the face of the chuck, a condition that applies to all this class of work. In the case of work like chasers, where the tap or hob does not have much bearing to guide it in the work, a three-flute hob should be used for four chasers, or a four-flute hob for three chasers, which is necessary so that the hob may work steadily and tap all to the same diameter.
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.
Ablackbolt 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.
Fig. 370Fig. 370.
Fig. 370.
The heads of bolts are designated by their shapes, irrespective of whether they are left black or finished.Fig. 370represents 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 headsaare 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. Headscanddare almost invariably finished when used on operative parts of machines, as are alsoeandf. Headsgare usually left black, whilehandiare finished if used on machine work, and left black when used as rivets or on rough unfinished work.
Fig. 371Fig. 371.
Fig. 371.
Fig. 372Fig. 372.
Fig. 372.
Fig. 373Fig. 373.
Fig. 373.
The heads frometoiassume 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 inFig. 371, or by forging under the head a tit or stop, such as shown inFigs. 372and373atp. 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.
Fig. 374Fig. 374.
Fig. 374.
Bolts are designated for kinds, as inFig. 374, in whichkis a machine bolt;la collar bolt, from having a collar on it;ma cotter bolt, from having a cotter or key passing through it to serve in place of a nut;na carriage bolt, from having a square part under the head to sink in the wood and prevent the bolt from turning with the nut; andoa 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.
Fig. 375Fig. 375.
Fig. 375.
In addition to these general names for bolts, there are others applied to special cases. ThusFig. 375represents a patch bolt or a bolt for fastening patches (as platecto plated), its peculiarity being that it has a square stemafor 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 inFig. 375, the head is very liable to come off unless the countersink in the work (as inc) is quite fair with the tapped hole (as ind) 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 headato screw them in by, and are turned in as atbto 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 atb.
Fig. 376Fig. 376.
Fig. 376.
Instead of the hole being countersunk, however, it may be cupped or counterbored, as inFig. 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.
Fig. 377Fig. 377.
Fig. 377.
Eye bolts are those having an eye in place of a head, as inFig. 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, thatpart 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. 378Fig. 378.
Fig. 378.
Fig. 378represents 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.
Fig. 379Fig. 379.
Fig. 379.
Fig. 380Fig. 380.
Fig. 380.
Another method of securing a foundation bolt head within a stone block is shown inFig. 379; a similar coned hole is cut in the block, and besides the bolt headba blockwis inserted, the faces of the block and bolt being taper to fit to a taper keyk, so that drivingklocks 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 inFig. 380, in whichbis a bolt threaded to receive a nut at the top. At the bottom it has a keyway for a keyk, which abuts against the platep. 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.
Fig. 381Fig. 381.
Fig. 381.
Another method is to give the bolt head the form atbinFig. 381, and to cast a plate with a rectangular slot through, and with two lugsa 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 lugsa cpreventing 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.
Fig. 382Fig. 382.
Fig. 382.
Fig. 383Fig. 383.
Fig. 383.
Figs. 382and383represent two forms of hook bolt for use in cases where it is not desired to have bolt holes through both pieces of the work. InFig. 382the 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 pieceafrombor piecebfroma. But inFig. 383the nut pressure is not beneath the part where the hookdgrips the work, hence the nut would exert a pressure to pull piecebin the direction of the arrow; hence ifbwere a fixed piece the bolt would suspendafrom it, but it could not suspendbfroma.
Fig. 384Fig. 384.
Fig. 384.
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 inFig. 384a plate is tapped to serve instead of a nut, and a similar plate may of course be placed under the bolt head.
Fig. 385Fig. 385.
Fig. 385.
The Franklin Institute or United States Standard for the dimensions of bolt heads and nuts is as follows. InFig. 385,drepresents the diameter of the bolt,jrepresents 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, plus1⁄16inch for finished heads or nuts, and plus1⁄8inch for rough or unfinished heads or nuts.krepresents the depth or thickness of the head or nut, which in finished heads or nuts equals the diameter of the bolt minus1⁄16inch, 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.
hrepresents the thickness or depth of the nut, which for finished nuts is made equal to the diameter of the bolt less1⁄16inch, and thereforethe 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.irepresents 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 forreference:—
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 asfollows:—
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 aninch:—
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.