Fig. 1399Fig. 1399.
Fig. 1399.
“Certain builders of locomotives put their stub ends together with tapered bolts, but do not use tapered bolts in any other part of the structure. The Baldwin Works use tapered bolts wherever they are body bound bolts. They make a universal taper of1⁄16inch to the foot. An inch bolt 12 inches long would be 11⁄16inches diameter under the head. They make all their bolts under 9 inches long1⁄16larger under the head than the name of the bolt implies. Thus a3⁄4inch bolt would be13⁄16inch under the head, provided it was 9 inches long or under. Anything over 9 inches long is made1⁄8inch larger under the head, and still made a taper of1⁄16inch to the foot. A locomotive builder informs me that a taper of1⁄8inch to the foot is sometimes called for, and the Pennsylvania road calls for3⁄32inch to the foot. But the majority of specifications call for1⁄16inch to the foot. The advantage of1⁄16inch taper lies in the fact that a bolt headed in the ordinary manner can be made to fill the requirements, provided it is made of iron. You may decide that bolts should be tapered, for the reason that when a tapered bolt is driven into its place it can be readily knocked loose, or if that bolt, when in its place, proves to be too loose, you have merely to drive it in a little farther: these are arguments in favor of tapered bolts, showing their advantage. It is easier to repair work that has tapered bolts than work that has straight bolts. If you adopt a tapered bolt, say, with a taper of1⁄16inch to the foot, you are going to effect the making of those bolts and the boring of those holes in a commercially accurate manner, so that they can be brought into the interchangeable system. To carry this out, you require some standard to start with, and the simplest system that one can conceive is this: Let us imagine that we have a steel plug and grind it perfectly true. We have the means of determining whether that is a taper of1⁄16inch, thanks to the gentlemen who are now making these admirable gauges. We have a lathe that can turn that taper. I think if you go into the manufacture of these bolts, you will be obliged to use a lathe which will always turn a uniform taper. Having made a female gauge,Fig. 1399, 8 inches long and 11⁄16inches diameter with a taper of1⁄16inch to the foot, this is the standard of what? The area of the bolt, not of the hole it goes into. We now make a plug,Fig. 1399. Taking that tapered plug we should be able to drop it into the hole. Your taper reamer is made to fit this, but you require to know how deep the hole should be. Remember, I said this is the gauge that the bolts are made by. Now let us suppose that we have this as a standard, and to that standard these reamers are made. We decide by practice how much compression we can put upon the metal. For inch bolts, and, say, all above1⁄2inch, we might, say, allow the head to stand up1⁄8of an inch. Let us make another female gauge likeFig. 1399, but turned down1⁄8of an inch shorter. We then shall have the hole smaller than it was before. It is this degree smaller, .0065 of an inch; that is a decimal representing how much smaller that hole is when you have gone down1⁄8of an inch on a taper of1⁄16inch to the foot.
Fig. 1400Fig. 1400.
Fig. 1400.
“Having got this tapered plug, you then must have the means of making the bolts commercially accurate in the shop. For that purpose you must have some cast-iron plugs. Those are reamed with a reamer that has no guard on it, but is pushed into it until the plug—this standard plug—is flush with the end of it. If you go in a little too far it is no matter. Having produced that gauge, we gauge first the one that is used on the lathe for the workman to work by, and he will fit his bolt in until the head will be pushed up against it. If you have a bolt to make from a straight piece of iron, I should advise its being done in two lathes. Here are those beautiful gauges of the Pratt and Whitney Company, which will answer the present purpose; one of these gauges measuring what the outside of the bolt will be, the other gauge1⁄16of an inch larger will mark the part under the head. Messrs. Baldwin have a very good system of gauges. All the cast-iron plugs which they use for this purpose are square. Holes are cut in the blocks the exact size of the bolts to be turned up, as shown inFig. 1400. The object of this is that there shall be no mistake as to what the gauge is. These gauges can be readily maintained, because they have to go back into the room to the inspector. He puts this plug in. If it goes in and fits flush, it is all right. If the plug goes in too far, it is worn. He then turns a little off the end and adjusts it.
“Now practically through machine shops we find that we have to use cast-iron gauges. We take, for instance, 2-inch shafting. Shafting can only be commercially accurate. Therefore we make cast-iron rings and if those rings will go on the shafting it is near enough accurate for merchantable purposes. But this ring will wear in a certain time. Therefore it must not be used more than a certain number of days or hours. Here you have a system that is simple in the extreme. You have all this in two gauges, one gauge being made as a mere check on that tapered plug which is the origin of all things, the origin being1⁄8or7⁄16, or1⁄4of an inch shorter if the bolt is very large. There is where you have to use your own judgment. But having adopted something practical you then can use your reamer which is necessary to produce a hole of a given size. If this reamer wears, you then turn off this wrought-iron collar far enough back to let it go in that much farther. I know of no other way by which you can accomplish this result so well as by that in use at the Baldwin Locomotive Works. I think that the system originated with Mr. Baldwin himself.
“I do not feel disposed to recommend to you any particular taper to be adopted, because it is not a question like that of screw-threads. In screw-threads we throw away the dies that are used upon bolts, which are perishable articles. The taper that has once been adopted in locomotive establishments is a perpetual thing. If the Pennsylvania railroad and all its branches have adopted3⁄32, it is folly to ask them to change it to1⁄16of an inch, because their own connections are large enough to make them independent of almost any other corporation, and the need of absolute uniformity in their work would cause them to stick to that particular thing. Any of you having five, six, seven, or two or three hundred engines, must make up your minds what you will do. When we adopt a standard for screw-threads, a screw-thread is adopted which has a manifest advantage. A bolt that has one screw thread can be used on any machine. But once having adopted a taper on a road, it is very difficult to make a change; and whether it is wisdom for this Association to say that thus-and-so shall be the standard taper, is a question I am unable to answer. Therefore I am unwilling to present any taper to you, and only present the facts, but will say that1⁄16inch is enough. The less taper you have the less material you have to cut away. But to say that1⁄16inch is preferable to1⁄32inch is folly, because no human being could tell the difference. If a bolt has 5° taper on the side, it may set in place; if it has 7°, it may jump out. That is the angle of friction for iron or other metals. Five degrees would be an absurd angle for a taper bolt. Anything, then, that will hold; that is, if you drive the bolt it will set there.
“This presentation may enable you to arrive at some conclusion. Nothing is more desirable than an interchangeable system. In making turning lathes we try to make all parts interchangeable, and we so fit the sliding spindle. Every sliding spindle in the dead head of the lathe has to be fitted into its own place. We know of no method of making all holes of exactly the same size that shall be commercially profitable. The only way we could surmount that difficulty was to put two conical sleeves in that should compress. We have so solved the problem. We now make spindles that are interchangeable, and we do not fit one part to the other. But that is not the case with bolts. You cannot put the compressing thimbles on them, therefore, you have to consider the question, How can you make holes near enough, and how can you turn the bolts near enough alike?”
Fig. 1401Fig. 1401.
Fig. 1401.
Fig. 1401represents, and the following table gives the taper adopted by the Baldwin Locomotive Works.
Bolt threads, American standard, except stay bolts and boiler studs,V-threads, 12 per inch; valves, cocks and plugs,V-threads, 14 per inch, and1⁄8inch taper per 1 inch.
Standard bolt taper1⁄16inch per foot.
Length of bolts from head to end of thread equalsa.
Diameter of bolt under the head asfollows:—
Fig. 1402Fig. 1402.
Fig. 1402.
It is obvious that a plug or collar gauge simply determines what is the largest dimension of the work, and that although it will demonstrate that a piece of work is not true or round yet it will not measure the amount of the error. The work may be oval or elliptical, or of any other form, and yet fit the gauge so far as the fit can be determined by the sense of feeling. Or suppose there is a flat place upon the work, then except in so far as the bearing marks made upon the work by moving it within the gauge may indicate, there is no means of knowing whether the work is true or not. Furthermore, in the case of lathe work held between the lathe centres it is necessary to remove the work from the lathe before the collar gauge can be applied, and to obviate these difficulties we have the caliper gauge shown inFig. 1402. The caliper end is here shown to be for3⁄4inch, and the plug end for13⁄16inch. If the two ends were for the same diameter one gauge only would be used for measuring external and internal work of the same diameter, but in this case the male cannot be tested with the female gauge; whereas if the two ends are for different diameters the end of one gauge may be tested with that of another, and their correctness tested, but the workman will require two gauges to measure an external and internal piece of the same diameter.
Fig. 1403Fig. 1403.
Fig. 1403.
For small lathe work of odd size as when it is required to turn work to fit holes reamed by a worn reamer that is below the standard size, a gauge such as inFig. 1403, is sometimes used, the mouthaserving as a caliper and the holebas a collar gauge for the same diameter of work. It is obvious that such a gauge may be applied to the work while it is running in the lathe, and that when the size atawears too large the jaw may be closed to correct it; a plan that is also pursued to rectify the caliper gauge shown inFig. 1402.
Fig. 1404Fig. 1404.
Fig. 1404.
Fig. 1405Fig. 1405.
Fig. 1405.
On large work, as, say, of six inches in diameter, a gauge, such as inFig. 1404, is used, being short so that it may be light enoughto be conveniently handled; or sometimes a piece such as inFig. 1405is used as a gauge, the ends being fitted to the curvature of the bore to be tested. Gauges of these two kinds, however, are generally used more in the sense of being templates rather than measuring tools, since they determine whether a bore is of the required size rather than determine what that size is.
Fig. 1406Fig. 1406.
Fig. 1406.
Fig. 1407Fig. 1407.
Fig. 1407.
For gauging work of very large diameter, as, say, several feet, to minute fractions of an inch, as is necessary, for example, for a shrinkage fit on a locomotive tire, the following method is employed. InFig. 1406letarepresent a ring, say, 5 feet bore, and requiring its bore to be gauged to within, say,1⁄100inch. Thenrrepresents a rod made, say,1⁄2inch shorter than the required diameter of bore, andw,Fig. 1407, represents a wedge whose upper surfacecdis curved, its lower surface being a true plane. The thickness at the endcis made, say,51⁄100inch, while that atdis48⁄100inch; or in other words, there is3⁄100of an inch taper in the length of the wedge. Suppose then that the rodris placed in the bore ofaas in figure, and that the wedge just has contact with the work bore and with the end of the rod when it has entered as far aseinFig. 1407, and that pointeis one-third of the length of the wedge, then the bore ofawill measure the length of the rodrplus49⁄100of an inch. But if the wedge passed in to linef, the latter being two-thirds the length of the wedge fromd, then the bore would be50⁄100larger than the length of the rodr. It is obvious that with this method the work may be measured very minutely, and the amount of error, if there be any, may be measured.
The rod must be applied to the work in the same position in which its measurement was made, otherwise its deflection may vitiate the measurement. Thus, if the rod measures 4 feet 111⁄2inches when standing vertical, it must be applied to the work standing vertical; but if it was measured lying horizontal, it must be applied to the work lying horizontal, as there will be a difference in its length when measured in the two positions, which occurs on account of variations in its deflection from its own weight.
Fig. 1408Fig. 1408.
Fig. 1408.
Fig. 1409Fig. 1409.
Fig. 1409.
For simply measuring a piece of work to fit it to another irrespective of its exact size as expressed in inches and parts of an inch the common calipers are used.Fig. 1408represents a pair of spring calipers, the bow acting as a spring to keep the two legs apart, and the screw and nut being used to close them against the spring pressure. The slightness of the legs enables these calipers to be forced or to spring over the work, and thus indicate by the amount of pressure it requires to pass them over the work how much it is above size, and therefore how much it requires to be reduced. But, on the other hand, this slightness renders it somewhat difficult to measure with great correctness. A better form of outside calipers is shown inFig. 1409, in which in addition to the stiffness of the pivoted joint a bow spring acts to close the caliper legs, which are operated, to open or close them, by operating the hand screw shown, the nuts in which the screw operates being pivoted to the caliper legs. The advantage of this form is that the calipers may be set very readily, while there is no danger of the set or adjustment of the calipers altering from any slight blow or jar received in laying them down upon the bench.
Fig. 1410Fig. 1410.
Fig. 1410.
Fig. 1410gives views of a common pair of outside calipers such as the workman usually makes for himself. When this form is made with a sufficiently large joint, and with the legs broad and stiff as in the figure, they will serve for very fine and accurate adjustments.
Fig. 1411Fig. 1411.
Fig. 1411.
Fig. 1412Fig. 1412.
Fig. 1412.
Fig. 1413Fig. 1413.
Fig. 1413.
Fig. 1411represents a pair of inside calipers for measuring the diameters of holes or bores. The points of these calipers should be at an angle as shown in theFig. 1412, which will enable the points to enter a long distance in a small hole, as is denoted by the dotted lines in the figure. This will also enablethe extreme points to reach the end of a recess, as inFig. 1413, which the rounded end calipers, such as in this figure, will not do.
Fig. 1414Fig. 1414.
Fig. 1414.
Fig. 1414represents a pair of inside calipers with an adjustment screw having a right-hand screw ataand a left-hand one atb, threaded into two nuts pivoted into the arms, so that by operating the screw the legs are opened or closed, and are locked in position, so that they cannot move from an accidental blow. But as the threads are apt to wear loose, it is preferable to provide a set screw to one of the nuts so as to take up the wear and produce sufficient friction to prevent looseness of the legs.
Fig. 1415Fig. 1415.
Fig. 1415.
Fig. 1416Fig. 1416.
Fig. 1416.
Calipers are sometimes made double, that is to say, the inside and the outside calipers are provided in the one tool, as inFig. 1415, which represents a pair of combined inside and outside calipers having a set screw atcto secure the legs together after the adjustment is made. The object of this form is to have the measuring points equidistant from the centre of the pivotainFig. 1416, so that when the outside legs are set to the diameter of the work as atb, the inside ones will be set to measure a hole or bore of the same diameter as atc.
This, however, is not a desirable form for several reasons, among which are thefollowing:—
In the first place outside calipers are much more used than inside ones, hence the wear on the points are greatest. Again, the pivot is apt to wear, destroying the equality of length of the points from the centre of the pivot; and in the third place the shape of the points of calipers as usually made vitiates the correctness of the measurements.
Fig. 1417Fig. 1417.
Fig. 1417.
Fig. 1417, for example, represents the ordinary form, the points being rounded; hence, when the legs are closed the point of contact between the inside and outside calipers will be ata, while when they are opened out to their fullest the points of contact will be atb. This may, however, be remedied to a great extent by bevelling off the ends from the outside as shown inFig. 1416.
Fig. 1418Fig. 1418.
Fig. 1418.
The end faces of outside calipers should be curved in their widths, as inFig. 1418, so that contact shall occur at the middle, and it will then be known just where to apply the points of the inside calipers when testing them with the outside ones.
Fig. 1419Fig. 1419.
Fig. 1419.
Inside and outside calipers are capable of adjustment for very fine measurements; indeed, from some tests made by the Pratt and Whitney Company among their workmen it was found that the average good workman could take a measurement with them to within the twenty-five thousandth part of an inch. But the workman of the general machine shop who has no experience in measuring by thousandths has no idea of the accuracy with which he sets two calipers in his ordinary practice. The great difference that the one-thousandth of an inch makes in the fit of two pieces may be shown as inFig. 1419, which represents a collar gauge of5⁄8inch in diameter, and a plug1⁄1000inch less in diameter, and it was found that with the plug inserted1⁄8inch in the collar it could be moved fromatob, a distance of about5⁄16inch, which an ordinary workman would at once recognise as a very loose fit.
Fig. 1420Fig. 1420.
Fig. 1420.
Fig. 1421Fig. 1421.
Fig. 1421.
Fig. 1422Fig. 1422.
Fig. 1422.
If the joints of outside calipers are well made the calipers may upon small work be closed upon the work as inFig. 1420, and the adjustment may be made without requiring to tap or lightly knock the caliper legs against the work as is usually done to set them. But to test the adjustment very finely the work should be held up to the light, as inFig. 1421, the lower leg of the calipers rested against the little finger so as to steady it and prevent it from moving while the top leg is moved over the work, and at the same time moving it sideways to find when it is helddirectly across the work. For testing the inside and outside calipers together they should for small diameters be held as inFig. 1422, the middle finger serving to steady one inside and one outside leg, while one leg only of either calipers is grasped in the fingers.
Fig. 1423Fig. 1423.
Fig. 1423.
For larger dimensions, as six or eight inches, it is better, however, to hold the calipers as inFig. 1423, the forefinger of the left hand serving to rest one leg of each pair on the contact being thus tested between the legs that are nearest to the operator.
The adjustment of caliper legs should be such that contact between the caliper points and the work is scarcely, if at all, perceptible. If with the closest of observation contact is plainly perceptible, the outside calipers will be set smaller than the work, while in the case of inside calipers, they would be set larger; and for this reason it follows that if a bore is to be measured to have a plug fitted to it, the inside calipers should have barely perceptible contact with the work bore, and the outside calipers should have the same degree of contact, or, if anything, a very minute degree of increased contact. On the other hand, if a bore is to be fitted to a cylindrical rod the outside calipers should be set to have the slightest possible contact with the rod, and the inside ones set to have as nearly as possible the same degree of contact with the outside ones, or, if anything, slightly less contact. For if in any case the calipers have forcible contact with the work the caliper legs will spring open and will therefore be improperly set.
Calipers should be set both to the gauge and to the work in the same relative position. Let it be required, for example, to set a pair of inside calipers to a bore, and a pair of outside calipers to the inside ones, and to then apply the latter to thework. If the legs of the inside calipers stand vertical to the bore for setting they should stand vertical while the outside calipers are set to them, and if the outside calipers are held horizontally while set to the inside ones they should be applied horizontally to the work, so as to eliminate any error due to the caliper legs deflecting from their own weight.
To adjust calipers so finely that a piece of work may be turned by caliper measurement to just fit a hole; a working or a driving fit without trying the pieces together, is a refinement of measurement requiring considerable experience and skill, because, as will be readily understood from the remarks made when referring to gauge measurements, there are certain minute allowances to be made in the set of the calipers to obtain the desired degree of fit.
In using inside calipers upon flat surfaces it will be found that they can be adjusted finer by trusting to the ear than the eye. Suppose, for example, we are measuring between the jaws of a pillow-block. We hold one point of the calipers stationary, as before, and adjust the other point, so that, by moving it very rapidly, we can just detect a scraping sound, giving evidence of contact between the calipers and the work. If, then, we move the calipers slowly, we shall be unable, with the closest scrutiny, to detect any contact between the two.
Calipers possess one great advantage over more rigid and solid gauges, in that the calipers may be forced over the work when the degree of force necessary to pass them on indicates how much the work is too large, and therefore how much it requires reducing. Thus, suppose a cylindrical piece of work requires to be turned to fit a hole, and the inside calipers are set to the bore of the latter, then the outside calipers may be set to the inside ones and applied to the work, and when the work is reduced to within, say,1⁄100inch the calipers will spring open if pressed firmly to the work, and disclose to the workman that the work is reduced to nearly the required size. So accustomed do workmen become in estimating from this pressure of contact how nearly the work is reduced to the required diameter, that they are enabled to estimate, by forcing the calipers over the work, the depth of the cut required to be taken off the work, with great exactitude, whereas with solid gauges, or even caliper gauges of solid proportions, this cannot be done, because they will not spring open.
The amount to which a pair of calipers will spring open without altering their set depends upon the shape: thus, with a given joint they will do so to a greater extent in proportion as the legs are slight, whereas with a given strength of leg they will do so more as the diameter of the joint is large and the fit of the joint is a tight one. But if the joint is so weak as to move too easily, or the legs are so weak as to spring too easily, the calipers will be apt in one case to shift when applied to the work, and in the other to spring so easily that it will be difficult to tell by contact when the points just touch the work and yet are not sprung by the degree of contact. For these reasons the points of calipers should be made larger in diameter than they are usually made: thus, for a pair of calipers of the shape shown inFig. 1410, the joint should be about 11⁄4inches diameter to every 6 inches of length of leg. The joint should be sufficiently tight that the legs can just be moved when the two legs are taken in one hand and compressed under heavy hand pressure.
Fig. 1424Fig. 1424.
Fig. 1424.
For measuring the distance of a slot or keyway from a surface, the form of calipers shown inFig. 1424is employed; the straight leg has its surface a true plane, and is held flat against the surfacebof the slot or keyway, and the outside or curved leg is set to meet the distance of the work surface measuring the distancec. These are termed keyway calipers.
There are in general machine work four kinds of fit, as follow: The working or sliding fit; the driving fit; the hydraulic press fit; and the shrinkage fit. In the first of these a proper fit is obtained when the surfaces are in full contact, and the enveloped piece will move without undue friction or lost motion when the surfaces are oiled. In the second, third, and fourth, the enveloped piece is made larger than the enveloping piece, so that when the two pieces are put together they will be firmly locked.
It is obvious that in a working or sliding fit the enveloped piece must be smaller than that enveloping it, or one piece could not pass within the other. But the amount of difference, although too small to be of practical importance in pieces of an inch or two in diameter and but few inches in length, is appreciable in large work, as, say, of two or more feet in diameter. A journal, for example, of1⁄10inch diameter, running in a bearing having a bore of1⁄1000inch larger diameter, and being two diameters in length, would be instantly recognised as a bad fit; but a journal 6 inches in diameter and two diameters or 12 inches long would be a fair fit in a bearing having a bore of 61⁄1000inches. In the one case the play would be equal to one one-hundredth of the shaft’s diameter, while in the other case the play would equal but one six-thousandth part of the shaft’s diameter. In small work the limit of wear is so small, and the length of the pieces so short, that the1⁄1000of an inch assumes an importance that does not exist in larger work. Thus, in watch work, an error of1⁄1000inch in diameter may render the piece useless; in sewing machine work it may be the limit to which the tools are allowed to wear; while in a steamship or locomotive engine it may be of no practical importance whatever.
A journal1⁄10inch in diameter would require to run, under ordinary conditions, several years to become1⁄1000inch loose in its bearing. Some of this looseness, and probably nearly one half of it, will occur from wear of the bearing bore; hence, if a new shaft of the original standard diameter be supplied the looseness will be reduced by one-half. But a 6-inch journal and bearing would probably wear nearly1⁄1000inch loose in wearing down to a bearing which may take but a week or two, and forthese reasons among others, standard gauges and measuring tools are less applicable to large than to small work.
The great majority of fits made under the standard gauge system consist of cylindrical pieces fitting into holes or bores. Suppose then that we have a plug and a collar gauge each of an inch diameter, and a reamer to fit the collar gauge, and we commence to ream holes and to turn plugs to fit the collar gauge, then as our work proceeds we shall find that as the reamer wears, the holes it makes will get smaller, and that as the collar gauge wears, its bore gets larger, and it is obvious that the work will not go together. The wear of the gauge obviously proceeds slowly, but the wear of the reamer begins from the very first hole that it reams, although it may perform considerable duty before its wear sensibly affects the size of the hole. Theoretically, however, its size decreases from the moment it commences to perform cutting duty until it has worn out, and the point at which the wearing-out process may have proceeded to its greatest permissible limit is determined by its reduction of size rather than by the loss of its sharpness or cutting capacity. Obviously then either the reamer must be so made that its size may be constantly adjusted to take up the wear, as in the adjustable reamer, or else if solid reamers are used there must be a certain limit fixed upon as the utmost permissible amount of wear, and the reamer must be made above the standard size to an amount equal to the amount of this limit, so that when the reamer has worn down it will still bore a hole large enough to admit the plug gauge. To maintain the standard there should be in this case two sets of gauges, one representing the correct standard and the other the size to which the reamer is to be made when new or restored to its proper size.
The limit allowed for reamer wear varies in practice from1⁄1000to1⁄10000of an inch, according to the requirements of the work. As regards the wear of the standard gauges used by the workmen they are obviously subject to appreciable wear, and must be returned at intervals to the tool room to be corrected from gauges used for no other purpose.
To test if a hole is within the determined limit of size a limit gauge may be used. Suppose, for example, that the limit is1⁄1000of an inch, then a plug gauge may be made that is1⁄1000of an inch taper, and if the large end of this plug will enter the hole, the latter is too large, while if the small end will not enter, the hole is too small.
When only a single set of plug and collar gauges are at hand the plug or the collar gauge may be kept to maintain the standard, the other being used to work to, both for inside and outside work. Suppose, for example, that a plug and collar gauge are used for a certain piece of work and that both are new, then the reamer may be made from either of them, because their sizes agree, but after they have become worn either one or the other must be accepted as the standard of size to make the reamer to. If it be the collar gauge, then the plug gauge is virtually discarded as a standard, except in that if the plug gauge be not used at all it may be kept as a standard of the size to which the collar gauge must be restored when it has worn sufficiently to render restoration to size necessary. If this system be adopted the size of the reamer will be constantly varying to suit the wear of the collar gauge, and the difficulty is encountered that the standard lathe arbors or mandrels will not fit the holes produced, and it follows that if standard mandrels are to be used the reamers must when worn be restored to a standard size irrespective of the wear of the gauges, and that the standard mandrels must be made to have as much taper in their lengths as the limit of wear that is allowed to the reamers. Suppose, for example, that it is determined to permit the reamer to wear the1⁄2000of an inch before restoring it to size, then in an inch mandrel the smallest end may be made an inch in diameter and the largest 11⁄2000inch in diameter, so that however much the reamer may be worn within the limit allowed for wear the hole it produces will fit at some part in the length of the standard mandrel. But as the reamer wears smaller its size must be made as much above its designated standard size as the limit allowed for wear; hence, when new or when restored to size, the reamer would measure 11⁄2000inches, and the hole it produced would fit the large end of the mandrel. But as the reamer wore, the hole would be reamed smaller and would not pass so far along the mandrel, until finally the limit of reamer wear being reached the work would fit the small end of the mandrel. The small end of the mandrel is thus the standard of its size, and the wear of the collar gauge is in the same direction as that of the reamer. Thus, so long as the collar gauge has not worn more than the1⁄2000of an inch it will, if placed upon the mandrel, fit it at some part of its length.
Now suppose that the plug gauge be accepted as the standard to which the reamer is to be made, and that to allow for reamer wear the reamer is made, say,1⁄2000inch larger than the plug gauge, the work being made to the collar gauge. Then with a new reamer and new or unworn gauges the hole will be reamed above the standard size to the1⁄2000inch allowed for reamer wear. As the reamer wears, the hole it produces will become smaller, and as the collar gauge wears, the work turned to it will be larger, and the effect will be that, to whatever extent the collar gauge wears, it will reduce the permissible amount of reamer wear, so that when the collar gauge had worn the1⁄2000inch the work would not go together unless the reamer was entirely new or unworn.
In a driving fit one piece is driven within the other by means of hammer blows, and it follows that one piece must be of larger diameter than the other, the amount of the difference depending largely upon the diameter and length of the work.
It is obvious, however, that the difference may be so great that with sufficiently forcible blows the enveloping piece may be burst open. When a number of pieces are to be made a driving fit, the two pieces may be made to fit correctly by trial and correction, and from these pieces gauges may be made so that subsequent pieces may be made correct by these gauges, thus avoiding the necessity to try them together.
In fitting the first two pieces by fit and trial, or rather by trial and correction, the workman is guided as to the correctness of the fit by the sound of the hammer blows, the rebound of the hammer, and the distance the piece moves at each blow. Thus the less the movement the more solid the blow sounds, and the greater the rebound of the hammer the tighter the fit, and from these elements the experienced workman is enabled to know how tightly the pieces may be driven together without danger of bursting the outer one.
What the actual difference in diameter between two pieces may require to be to make a driving fit is governed, as already said, to a great extent by the dimensions of the pieces, and also by the nature of the material and the amount of area in contact. Suppose, for example, that the plug is 6 inches long, and the amount of pressure required to force it within the collar will increase with the distance to which it is enveloped by the collar. Or suppose one plug to be 3 inches and another to be 6 inches in circumference, and each to have entered its collar to the depth of an inch, while the two inside or enveloped pieces are larger than the outside pieces by the same amount, the outside pieces being of equal strength in proportion to their plugs, so that all other elements are equal, and then it is self-evident that the largest plug will require twice as much power as the small one will to force it in another inch into the collar, because the area of contact is twice as great. It is usual, therefore, under definite conditions to find by experiment what allowance to make to obtain a driving or a forcing fit. Thus, Mr. Coleman Sellers, at a meeting of the Car Builders Association, referring to the proper amount of difference to be allowed between the diameters of car axles and wheel bores in order to obtain a proper forcing or hydraulic fit, said, “Several years ago some experiments were made to determine the difference which should be made between the size of the hole and that of the axle. The conclusion reached was that if the axle of standard size was turned 0.007 inch larger than the wheel was bored it would require a pressure of about 30 tons to press the axle into the wheel.” The wheel seat on the axle here referred to was 47⁄8inches in diameter and 7 inches long. It is to be remarked, however, that the wheel bore being of cast iron and the axle of wrought iron the friction between the surfaces was not the same as it would be were thetwo composed of the same metal. This brings us to a consideration of what difference in the forcing fit there will be in the case of different metals, the allowance for forcing being the same and the work being of the same dimensions.
Suppose, for example, that a wrought-iron plug of an inch in diameter is so fitted to a bore that when inserted therein to a distance of, say, 2 inches, it requires a pressure of 3 lbs. to cause it to enter farther, then how much pressure would it take if the bore was of cast iron, of yellow brass, or of steel, instead of wrought iron. This brings us to another consideration, inasmuch as the elasticity and the strength of the enveloping piece has great influence in determining how much to allow for a driving, forcing, or a shrinkage fit.
Obviously the allowance can be more if the enveloping piece be of wrought iron, copper, or brass, than for cast iron or steel, because of the greater elasticity of the former. Leaving the elasticity out of the question, it would appear a natural assumption that the pieces, being of the same dimensions, the amount of force necessary to force one piece within the other would increase in proportion as the equivalents of friction of the different metals increased.
This has an important bearing in practice, because the fit of pieces not made to standard gauge diameter is governed to a great extent by the pressure or power required to move the pieces. Thus, let a steel crosshead pin be required to be as tight a fit into the crosshead as is compatible with its extraction by hand, and its diameter in proportion to that of the bore into which it fits will not be the same if that bore be of wrought iron, as it would be were the bore of steel, because the coefficient of friction for cast steel on cast iron is not the same as that for steel on wrought iron. In other words, the lower the coefficient of friction on the two surfaces the less the power required to force one into the other, the gauge diameters being equal. In this connection it may be remarked that the amount of area in contact is of primary importance, because in ordinary practice the surfaces of work left as finished by the steel cutting tools are not sufficiently true and smooth to give a bearing over the full area of the surfaces.
This occurs for the following reasons. First, work to be bored must be held (by bolts, plates, chuck-jaws, or similar appliances) with sufficient force to withstand the pressure of the cut taken by the cutting tool, and this pressure exerts more or less influence to spring or deflect the work from its normal shape, so that a hole bored true while clamped will not be so true when released from the pressure of the holding clamps.
To obviate this as far as possible, expert workmen screw up the holding devices as tight as may be necessary for the heavy roughing cuts, and then slack them off before taking the finishing cuts.
Secondly, under ordinary conditions of workshop practice, the steel cutting tools do not leave a surface that is a true plane in the direction of the length of the work, but leave a spiral projection of more or less prominence and of greater or less height, according to the width of that part of the cutting edge which lies parallel to the line of motion of the tool feed, taken in proportion to the rate of feed per revolution of the work.