Modern Machine-Shop Practice Part 30

TABLE OF THE FRANKLIN INSt.i.tUTE 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 Inst.i.tute 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 pa.s.sing 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."

[Ill.u.s.tration: Fig. 386.]

[Ill.u.s.tration: Fig. 387.]

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 pa.s.s 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 s.c.r.e.w.i.n.g 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 f.l.a.n.g.e or work secured by the nut being equal, the nuts will pa.s.s 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 f.l.a.n.g.e or foot of the work to be bolted up, so that the nut shall not meet C before gripping the f.l.a.n.g.e 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 s.c.r.e.w.i.n.g 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?

[Ill.u.s.tration: Fig. 388.]

[Ill.u.s.tration: Fig. 389.]

[Ill.u.s.tration: Fig. 390.]

[Ill.u.s.tration: Fig. 391.]

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 s.c.r.e.w.i.n.g against a shoulder) to prevent uns.c.r.e.w.i.n.g 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 uns.c.r.e.w.i.n.g 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 f.l.a.n.g.e 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 f.l.a.n.g.e 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.

[Ill.u.s.tration: Fig. 392.]

[Ill.u.s.tration: Fig. 393.]

An excellent method of preventing the stud from uns.c.r.e.w.i.n.g 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.

[Ill.u.s.tration: Fig. 394.]

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.

[Ill.u.s.tration: Fig. 395.]

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 pa.s.s 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 pa.s.s 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 pa.s.ses 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.

[Ill.u.s.tration: Fig. 396.]

When it is desirable to provide that bolts may be quickly removed, the f.l.a.n.g.es may be furnished with slots, as in Fig. 396, so that the bolts may be pa.s.sed 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 s.c.r.e.w.i.n.g up or uns.c.r.e.w.i.n.g the nut.

The bolt is squared at A, which fits easily into the f.l.a.n.g.e. The f.l.a.n.g.es, 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.

[Ill.u.s.tration: Fig. 397.]

[Ill.u.s.tration: Fig. 398.]

In cases where it is inconvenient for the bolt head to pa.s.s 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.

[Ill.u.s.tration: Fig. 399.]

[Ill.u.s.tration: Fig. 400.]

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 pa.s.sing 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.

[Ill.u.s.tration: Fig. 401.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig. 403.]

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.