Modern Machine-Shop Practice Part 22

CHAPTER IV.--SCREW THREAD.

Screw threads are employed for two princ.i.p.al purposes--for holding or securing, and for transmitting motion. There are in use, in ordinary machine shop practice, four forms of screw thread. There is, first, the sharp [V]-thread shown in Fig. 246; second, the United States standard thread, the Sellers thread, or the Franklin Inst.i.tute thread, as it is sometimes called--all three designations signifying the same form of thread. This thread was originally proposed by William Sellers, and was afterward recommended by the Franklin Inst.i.tute. It was finally adopted as a standard by the United States Navy Department. This form of thread is shown in Fig. 247. The third form is the Whitworth or English standard thread, shown in Fig. 248. It is sometimes termed the round top and bottom thread. The fourth form is the square thread shown in Fig.

249, which is used for coa.r.s.e pitches, and usually for the transmission of motion.

The sharp [V]-thread, Fig. 246, has its sides at an angle of 60 one to the other, as shown; or, in other words, each side of the thread is at an angle of 60 to the axial line of the bolt. The United States Standard, Fig. 247, is formed by dividing the depth of the sharp [V]-thread into 8 equal divisions and taking off one of the divisions at the top and filling in another at the bottom, so as to leave a flat place at the top and bottom. The Whitworth thread, Fig. 248, has its sides at an angle of 55 to each other, or to the axial line of the bolt. In this the depth of the thread is divided into 6 equal parts, and the sides of the thread are joined by arcs of circles that cut off one of these parts at the top and another at the bottom of the thread. The centres from which these arcs are struck are located on the second lines of division, as denoted in the figure by the dots. Screw threads are designated by their pitch or the distance between the threads. In Fig.

250 the pitch is 1/4 inch, but it is usual to take the number of threads in an inch of length; hence the pitch in Fig. 250 would generally be termed a pitch of 4, or 4 to the inch. The number of threads per inch of length does not, however, govern the true pitch of the thread, unless it be a "single" thread.

A single thread is composed of one spiral projection, whose advance upon the bolt is equal in each revolution to the apparent pitch. In Fig. 251 is shown a double thread, which consists of two threads. In the figure, A denotes one spiral or thread, and B the other, the latter being carried as far as C only for the sake of ill.u.s.tration. The true pitch is in this case twice that of the apparent pitch, being, as is always the case, the number of revolutions the thread makes around the bolt (which gives the pitch per inch), or the distance along the bolt length that the nut or thread advances during one rotation. Threads may be made double, treble, quadruple and so on, the object being to increase the motion without the use of a coa.r.s.er pitch single thread, whose increased depth would weaken the body of the bolt.

The "ratchet" thread shown in Fig. 252 is sometimes used upon bolts for ironwork, the object being to have the sides A A of the thread at a right angle to the axis of the bolt, and therefore in the direct line of the strain. Modifications of this form of thread are used in coa.r.s.e pitches for screws that are to thread direct into woodwork.

A waved or drunken thread is one in which the path around the bolt is waved, as in Fig. 253, and not a continuous straight spiral, as it should be. All threads may be either left hand or right, according to their direction of inclination upon the bolt; thus, Fig. 254 is a cylinder having a right-hand thread at A and a left-hand one at B. When both ends of a piece have either right or left-hand threads, if the piece be rotated and the nuts be prevented from rotating, they will move in the same direction, and, if the pitches of the threads are alike, at the same rate of motion; but if one thread be a right and the other a left one, then, under the above conditions, the nuts will advance toward or recede from each other according to the direction of rotation of the male thread.

[Ill.u.s.tration: Fig. 255.]

In Fig. 255 is represented a form of thread designed to enable the nut to fit the bolt, and the thread sides to have a bearing one upon the other, notwithstanding that the diameter of the nut and bolt may differ.

The thread in the nut is what may be termed a reversed ratchet thread, and that in the bolt an undercut ratchet thread, the amount of undercut being about 2. Where this form of thread is used, the diameter of the bolt may vary as much as 1-32d of an inch in a bolt 3/4 inch in diameter, and yet the nut will screw home and be a tight fit. The difference in the thread fit that ordinarily arises from differences in the standards of measurement from wear of the threading tools, does not in this form affect the fit of the nut to the bolt. In s.c.r.e.w.i.n.g the nut on, the threads conform one to the other, giving a bearing area extending over the full sides of the thread. The undercutting on the leading face of the bolt thread gives room for the metal to conform itself to the nut thread, which it does very completely. The result is that the nut may be pa.s.sed up and down the bolt several times and still remain too tight a fit to be worked by hand. Experiment has demonstrated that it may be run up and down the bolt dozens of times without becoming as loose as an ordinary bolt and nut. On account of this capacity of the peculiar form of thread employed, to adapt itself, the threads may be made a tight fit when the threading tools are new. The extra tightness that arises from the wear of these tools is accommodated in the undercutting, which gives room for the thread to adjust itself to the opposite part or nut.

In a second form of self-locking thread, the thread on the bolt is made of the usual [V]-shape United States standard. The thread in the nut, however, is formed as ill.u.s.trated in Fig. 256, which is a section of a 3/4-inch bolt, greatly enlarged for the sake of clearness of ill.u.s.tration. The leading threads are of the same angle as the thread on the bolt, but their diameters are 3/4 and 1-16th inch, which allows the nut to pa.s.s easily upon the bolt. The angle of the next thread following is 56, the succeeding one 52, and so on, each thread having 4 less angle than the one preceding, while the pitch remains the same throughout. As a result, the rear threads are deeper than the leading ones. As the nut is screwed home, the bolt thread is forced out or up, and fills the rear threads to a degree depending upon the diameter of the bolt thread. For example, if the bolt is 3/4 inch, its leading or end thread will simply change its angle from that of 60 to that of 44, or if the bolt thread is 3/4 and 1-64th inch in diameter, its leading thread will change from an angle of 60 to one of 44. It will almost completely fill the loose thread in the nut. The areas of s.p.a.ces between the nut threads are very nearly equal, although slightly greater at the back end of the nut, so that if the front end will enter at all, the nut will screw home, while the thread fit will be tight, even under a considerable variation in the bolt itself. From this description, it is evident that the employment of nuts threaded in this manner is only necessary in order to give to ordinary bolts all the advantages of tightness due to this form of thread.

The term "diameter" of a thread is understood to mean its diameter at the top of the thread and measured at a right angle to the axis of the bolt. When the diameter of the bottom or root of the thread is referred to it is usually specified as diameter at the bottom or at the root of the thread.

The depth of a thread is the vertical height of the thread upon the bolt, measured at a right angle to the bolt axis and not along the side of the thread.

A true thread is one that winds around the bolt in a continuous and even spiral and is not waved or drunken as is the thread in Fig. 253. An outside or male thread is one upon an external surface as upon a bolt; an internal or female thread is one produced in a bore or hole as in a nut.

[Ill.u.s.tration: Fig. 256.]

The Whitworth or English standard thread, shown in Fig. 248, is that employed in Great Britain and her colonies, and to a small extent in the United States. The [V]-thread fig. 246 is that in most common use in the United States, but it is being displaced by the United States standard thread. The reasons for the adoption of the latter by the Franklin Inst.i.tute are set forth in the report of a committee appointed by that Inst.i.tute to consider the matter. From that report the following extracts are made.

"That in the course of their investigations they have become more deeply impressed with the necessity of some acknowledged standard, the varieties of threads in use being much greater than they had supposed possible; in fact, the difficulty of obtaining the exact pitch of a thread not a multiple or sub-multiple of the inch measure is sometimes a matter of extreme embarra.s.sment.

"Such a state of things must evidently be prejudicial to the best interests of the whole country; a great and unnecessary waste is its certain consequence, for not only must the various parts of new machinery be adjusted to each other, in place of being interchangeable, but no adequate provision can be made for repairs, and a costly variety of s.c.r.e.w.i.n.g apparatus becomes a necessity. It may reasonably be hoped that should a uniformity of practice result from the efforts and investigations now undertaken, the advantages flowing from it will be so manifest, as to induce reform in other particulars of scarcely less importance.

"Your committee have held numerous meetings for the purpose of considering the various conditions required in any system which they could recommend for adoption. Strength, durability, with reference to wear from constant use, and ease of construction, would seem to be the princ.i.p.al requisites in any general system; for although in many cases, as, for instance, when a square thread is used, the strength of the thread and bolt are both sacrificed for the sake of securing some other advantage, yet all such have been considered as special cases, not affecting the general inquiry. With this in view, your committee decided that threads having their sides at an angle to each other must necessarily more nearly fulfil the first condition than any other form; but what this angle should be must be governed by a variety of considerations, for it is clear that if the two sides start from the same point at the top, the greater the angle contained between them, the greater will be the strength of the bolt; on the other hand, the greater this angle, supposing the apex of the thread to be over the centre of its base, the greater will be the tendency to burst the nut, and the greater the friction between the nut and the bolt, so that if carried to excess the bolt would be broken by torsional strain rather than by a strain in the direction of its length. If, however, we should make one side of the thread perpendicular to the axis of the bolt, and the other at an angle to the first, we should obtain the greatest amount of strength, together with the least frictional resistance; but we should have a thread only suitable for supporting strains in one direction, and constant care would be requisite to cut the thread in the nut in the proper direction to correspond with the bolt; we have consequently cla.s.sed this form as exceptional, and decided that the two sides should be at an angle to each other and form equal angles with the base.

"The general form of the thread having been determined upon the above considerations, the angle which the sides should bear to each other has been fixed at 60, not only because this seems to fulfil the conditions of least frictional resistance combined with the greatest strength, but because it is an angle more readily obtained than any other, and it is also in more general use. As this form is in common use almost to the exclusion of any other, your committee have carefully weighed its advantages and disadvantages before deciding to recommend any modification of it. It cannot be doubted that the sharp thread offers us the simplest form, and that its general adoption would require no special tools for its construction, but its liability to accident, always great, becomes a serious matter upon large bolts, whilst the small amount of strength at the sharp top is a strong inducement to sacrifice some of it for the sake of better protection to the remainder; when this conclusion is reached, it is at once evident a corresponding s.p.a.ce may be filled up in the bottom of the thread, and thus give an increased strength to the bolt, which may compensate for the reduction in strength and wearing surface upon the thread. It is also clear that such a modification, by avoiding the fine points and angles in the tools of construction, will increase their durability; all of which being admitted, the question comes up, what form shall be given to the top and bottom of the thread? for it is evident one should be the converse of the other. It being admitted that the sharp thread can be made interchangeable more readily than any other, it is clear that this advantage would not be impaired if we should stop cutting out the s.p.a.ce before we had made the thread full or sharp; but to give the same shape at the bottom of the threads would require that a similar quant.i.ty should be taken off the point of the cutting tool, thus necessitating the use of some instrument capable of measuring the required amount, but when this is done the thread having a flat top and bottom can be quite as readily formed as if it was sharp. A very slight examination sufficed to satisfy us that in point of construction the rounded top and bottom presents much greater difficulties--in fact, all taps and screws that are chased or cut in a lathe require to be finished or rounded by a second process. As the radius of the curve to form this must vary for every thread, it will be impossible to make one gauge to answer for all sizes, and very difficult, in fact impossible, without special tools, to shape it correctly for one.

"Your committee are of opinion that the introduction of a uniform system would be greatly facilitated by the adoption of such a form of thread as would enable any intelligent mechanic to construct it without any special tools, or if any are necessary, that they shall be as few and as simple as possible, so that although the round top and bottom presents some advantages when it is perfectly made, as increased strength to the thread and the best form to the cutting tools, yet we have considered that these are more than compensated by ease of construction, the certainty of fit, and increased wearing surface offered by the flat top and bottom, and therefore recommend its adoption. The amount of flat to be taken off should be as small as possible, and only sufficient to protect the thread; for this purpose one-eighth of the pitch would seem to be ample, and this will leave three-fourths of the pitch for bearing surface. The considerations governing the pitch are so various that their discussion has consumed much time.

"As in every instance the threads now in use are stronger than their bolts, it became a question whether a finer scale would not be an advantage. It is possible that if the use of the screw thread was confined to wrought iron or bra.s.s, such a conclusion might have been reached, but as cast iron enters so largely into all engineering work, it was believed finer threads than those in general use might not be found an improvement; particularly when it was considered that so far as the vertical height of thread and strength of bolt are concerned, the adoption of a flat top and bottom thread was equivalent to decreasing the pitch of a sharp thread 25 per cent., or what is the same thing, increasing the number of threads per inch 33 per cent. If finer threads were adopted they would require also greater exact.i.tude than at present exists in the machinery of construction, to avoid the liability of overriding, and the wearing surface would be diminished; moreover, we are of opinion that the average practice of the mechanical world would probably be found better adapted to the general want than any proportions founded upon theory alone."

[Ill.u.s.tration: Fig. 257.]

[Ill.u.s.tration: Fig. 258.]

The princ.i.p.al requirements for a screw thread are as follows: 1. That it shall possess a strength that, in the length or depth of a nut, shall be equal to the strength of the weakest part of the bolt, which is at the bottom of the bolt thread. 2. That the tools required to produce it shall be easily made, and shall not alter their form by reason of wear.

3. That these tools shall (in the case of lathe work) be easily sharpened, and set to correct position in the lathe. 4. That a minimum of measuring and gauging shall be required to test the diameter and form of the thread. 5. That the angles of the sides shall be as acute as is consistent with the required strength. 6. That it shall not be unduly liable to become loose in cases where the nut may require to be fastened and loosened occasionally.

Referring to the first, by the term "the strength of a screw thread," is not meant the strength of one thread, but of so many threads as are contained in the nut. This obviously depends upon the depth or thickness of the nut-piece. The standard thickness of nut, both in the United States and Whitworth systems, as well as in general practice, or where the common [V]-thread is used, is made equal to the diameter of the top of the thread. Therefore, by the term "strength of thread" is meant the combined strength of as many threads as are contained in a nut of the above named depth. It is obvious, then, when it is advantageous to increase the strength of a thread, that it may be done by increasing the depth of the nut, or in other words, by increasing the number of threads used in computing its strength. This is undesirable by reason of increasing the cost and labor of producing the nuts, especially as the threading tools used for nuts are the weakest, and are especially liable to breakage, even with the present depth of nuts.

It has been found from experiments that have been made that our present threads are stronger than their bolts, which is desirable, inasmuch as it gives a margin for wear on the sides of the threads. But for threads whose nuts are to remain permanently fastened and are not subject to wear, it is questionable whether it were not better for the bolts to be stronger than the threads. Suppose, for instance, that a thread strips, and the bolt will remain in place because the nut will not come off the bolt readily. Hence the pieces held by the bolt become loosened, but not disconnected. If, on the other hand, the bolt breaks, it is very liable to fall out, leaving the piece or pieces, as the case may be, to fall apart, or at least become disconnected, so far as the bolt is concerned.

But since threads are used under conditions where the threads are liable to wear, and since it is undesirable to have more than one standard thread, it is better to have the threads, when new, stronger than the bolts.

[Ill.u.s.tration: Fig. 259.]

Referring to the second requirement, screw threads or the tools that produce them are originated in the lathe, and the difficulty with making a round top and bottom thread lies in shaping the corner to cut the top of the thread. This is shown in Fig. 257, where a Whitworth thread and a single-toothed thread-cutting tool are represented. The rounded point A of the tool will not be difficult to produce, but the hollow at B would require special tools to cut it. This is, in fact, the plan pursued under the Whitworth system, in which a hob or chaser-cutting tool is used to produce all the thread-cutting tools. A chaser is simply a toothed tool such as is shown in Fig. 258. Now, it would manifestly be impracticable to produce a chaser having all the curves, A and B, at the top and at the bottom of the teeth alike, by the grinding operations usually employed in the workshop, and hence the employment of the hob.

Fig. 259 represents a hob, which is a threaded piece of steel with a number of grooves such as shown at A, A, A, which divide the thread into teeth, the edges of which will cut a chaser, of a form corresponding to that of the thread upon the hob. The chaser is employed to produce taps and secondary hobs to be used for cutting the threads in dies, &c., so that the original hob is the source from which all the thread-cutting tools are derived.

[Ill.u.s.tration: Fig. 260.]

For the United States standard or the common [V]-thread, however, no standard hob is necessary, because a single-pointed tool can be ground with the ordinary grinding appliances of the workshop. Thus, for the United States standard, a flat-pointed tool, Fig. 260, and for the common [V]-thread, a sharp-pointed tool, Fig. 260, may be used. So far as the correctness of angle of pitch and of thread depth are concerned, the United States standard and the common [V]-thread can both be produced, under skilful operation, more correctly than is possible with the Whitworth thread, for the following reasons:--

To enable a hob to cut, it must be hardened, and in the hardening process the pitch of the thread alters, becoming, as a general rule (although not always) finer. This alteration of pitch is not only irregular in different threads, but also in different parts of the same thread. Now, whatever error the hob thread receives from hardening it transfers to the chaser it cuts. But the chaser also alters its form in hardening, the pitch, as a general rule, becoming coa.r.s.er. It may happen that the error induced in the hob hardening is corrected by that induced by hardening the chaser, but such is not necessarily the case.

[Ill.u.s.tration: Fig. 261.]

The single-pointed tool for the United States standard or for the common [V]-thread is accurately ground to form after the hardening, and hence need contain no error. On the other hand, however, the rounded top and bottom thread preserves its form and diameter upon the thread-cutting tools better than is the case with threads having sharp corners, for the reason that a rounded point will not wear away so quickly as a sharp point. To fully perceive the importance of this, it is necessary to consider the action of a tool in cutting a thread. In Fig. 261 there is shown a chaser, A, applied to a partly-formed thread, and it will be observed that the projecting ends or points of the teeth are in continuous action, cutting a groove deeper and deeper until a full thread is developed, at which time the bottoms of the chaser teeth will meet the perimeter of the work, but will perform no cutting duty upon it. As a result, the chaser points wear off, which they will do more quickly if they are pointed, and less quickly if they are rounded. This causes the thread cut to be of increased and improper diameter at the root.

[Ill.u.s.tration: Fig. 262.]

The same defect occurs on the tools for cutting internal threads, or threads in holes or bores. In Fig. 262, for example, is shown a tool cutting an internal thread, which tool may be taken to represent one tooth of a tap. Here again the projecting point of the tool is in continuous cutting action, while this, being a single-toothed tool, has no bottom corners to suffer from wear. As a result of the wear upon the tools for cutting internal threads, the thread grooves, when cut to their full widths, will be too shallow in depth, or, more correctly speaking, the full diameter of the thread will be too small to an amount corresponding to twice the amount of wear that the tool point has suffered. In single-pointed tools, such as are used upon lathe work, this has but little significance, because it is the work of but a minute or two to grind up the tool to a full point again, but in taps and solid dies, or in chasers in heads (as in some bolt-cutting machines) it is highly important, because it impairs the fit of the threads, and it is difficult to bring the tools to shape after they are once worn.

[Ill.u.s.tration: Fig. 263.]

The internal threads for the nuts of bolts are produced by a tap formed as at T in Fig. 263. It consists of a piece of steel having an external thread and longitudinal flutes or grooves which cut the thread into teeth. The end of the thread is tapered off as shown, to enable the end of the tap to enter the hole, and as it is rotated and the nut N held stationary, the teeth cut grooves as the tap winds through, thus forming the thread.

[Ill.u.s.tration: Fig. 264.]

The threads upon bolts are usually produced either by a head containing chasers or by a solid die such as shown at A in Fig. 264, B representing a bolt being threaded. The bore of A is threaded and fluted to provide cutting teeth, and the threads are chamfered off at the mouth to a.s.sist the cutting by spreading it over several teeth, which enables the bolt to enter the die more easily.

We may now consider the effect of continued use and its consequent wear upon the threads or teeth of a tap and die or chaser.

The wear of the corners at the tops of the thread (as at A B in Fig.

265) of a tap is greater than the wear at the bottom corners at E F, because the tops perform more cutting duty.

[Ill.u.s.tration: Fig. 265.]

First, the top has a larger circle of rotation than has the bottom, and, therefore, its cutting speed is greater, to an amount equal to the difference between the circ.u.mferences of the thread at the top and at the bottom. Secondly, the tops of the teeth of tap perform nearly all the cutting duty, because the thread in the nut is formed by the tops and sides of the tap, which on entering cut a groove which they gradually deepen, until a full thread is formed, while the bottoms of the teeth (supposing the tapping hole to be of proper diameter and not too small) simply meet the bore of the tapping hole as the thread is finished. If, as in the case of hot punched nuts, the nut bore contains scale, this scale is about removed by the time the bottoms of the top teeth come into action, hence the teeth bottoms are less affected by the hardness of the scale.

In the case of the teeth on dies and chasers, the wear at the corners C D, in Fig. 266, is the greatest. Now, the tops of the teeth on the tap (A B, in Fig. 265) cut the bottom or full diameter of the thread in the nut, while the tops of the teeth (C D, in Fig. 266) in the die cut the bottom of the thread on the bolt; hence the rounded corners cut on the work by the tops of the teeth in the one case, meet the more square corners left by the tops of the teeth in the other, and providing that under these circ.u.mstances the thread in the nut were of equal diameter to that on the bolt the latter would not enter the former.

If the bolt were made of a diameter to enable the nut to wind a close fit upon the bolt, the corners only of the threads would fit, as shown in Fig. 267, which represents at N a thread in a portion of a nut and at S a portion of a thread upon a tap or bolt, the two threads being magnified and shown slightly apart for clearness of ill.u.s.tration. The corners A, B of the nut are then cut by the corners A B of the tap in Fig. 265, and the corners C, C, D correspond to those cut by the corners C, D of the die teeth in Fig. 266; corners E, F, Fig. 267, are cut by corners C, D, in Fig. 266, and corners G, H are cut by corners G, H in Fig. 266, and it is obvious that the roundness of the corners A, B, C, and D in Fig. 267 will not permit the tops of the thread on the bolt to meet the bottoms of the thread in the nut, but that the threads will bear at the corners only.

[Ill.u.s.tration: Fig. 266.]