Modern Machine-Shop Practice Part 95

[Ill.u.s.tration: Fig. 1392.]

[Ill.u.s.tration: Fig. 1393.]

[Ill.u.s.tration: _VOL. I._ =DIVIDING ENGINE AND MICROMETER.= _PLATE XV._

Fig. 1385.

Fig. 1386.

Fig. 1387.

Fig. 1388.

Fig. 1389.

Fig. 1390.

Fig. 1391.]

Fig. 1392 represents a vernier caliper, in which the measurement is read by the coincidence of ruled lines upon the following principle. The vernier is a device for subdividing the readings of any equidistant lines of division. Its principle of action may be explained as follows: Suppose in Fig. 1393 A to be a rule or scale divided into inches and tenths of an inch, and B a vernier so divided that its ten equidistant divisions are equal to nine of the divisions on A; then the distance apart of the lines of division on A will be 1/10 inch; but, as the whole ten divisions on B measure less than an inch, by 1/10 inch, then each line of division is a tenth part of the lacking tenth less than 1/10 inch apart. Thus, were we to take a s.p.a.ce equal to the 1/10 inch between 9 and 10 on A, and divide it into 10 equal parts (which would give ten parts each measuring 1/100th of an inch) and add one of said parts to each of the distances between the lines of division on B, then the whole of the lines on A would coincide with those on B. It becomes evident, then, that line 1 on B is 1/100 inch below line 1 on A, that line 2 on B is 2/100 inch below line 2 on A, line 3 on the vernier B is 3/100 inch below line 3 on the rule A, and so on, until we arrive at line 10 on the vernier, which is 10/100 or 1/10 inch below line 10 on A. Suppose, then, the rule or scale to rest vertically on a truly surfaced plate, and a piece of metal be placed beneath B, the thickness of the piece will be shown by which of the lines on B coincides with a line on A. For more minute divisions it is simply necessary to have more lines of division in a given length on A and B. Thus, if the rule be divided into inches and fiftieths, and the vernier is so divided that it has 20 equidistant lines of division to 19 lines on the rule, it will then lack one division, or 1/50 inch in 20/50 inch, each division on the vernier will then be the one-twentieth of a fiftieth too short, and as 1/20 of 1/50 is 1/1000, the instrument will read to one-thousandth of an inch.

Let it now be noted that, instead of making the lines of division closer together to obtain minute measurements, the same end may be obtained by making the vernier longer. For example, suppose it be required to measure to 1/2000 part of an inch, then, if the rule or scale be graduated to inches and fiftieths, and the vernier be graduated to have 40 equidistant lines of division, and 39 of the lines on the scale, the reading will be to the 1/2000 part of an inch. But, in any event, the whole of the readings on the vernier may be read, or will be pa.s.sed through, while it is traversing a division equal to one of the divisions on the scale or rule.

In Fig. 1392 is shown a vernier caliper, in which the vernier is attached to and carried by a slide operating against the inside edge of the instrument. The bar is marked or graduated on one side by lines showing inches and fiftieths of an inch, with a vernier graduated to have 20 equidistant lines of division in 19 of the lines of division on the bar, and therefore measuring to the 1/1000th of an inch, while the other side is marked in millimetres with a vernier reading to 1/40th millimetre, there being also 20 lines of division on the vernier to 19 on the bar.

The inside surfaces of the feet or jaws are relieved from the bar to about the middle of their lengths, so as to confine the measuring surfaces to dimensions sufficiently small to insure accurate measurement, while large enough to provide a bearing area not subject to rapid wear. If the jaw surface had contact from the point to the bar, it would be impossible to employ the instrument upon a rectangular having a burr, or slight projection, on the edge. Again, by confining the bearing area to as small limits as consistent with the requirements of durability a smaller area of the measured work is covered, and the undulations of the same may be more minutely followed.

To maintain the surface of the movable jaw parallel with that of the bar-jaw, it is necessary that the edge of the slide carrying the vernier be maintained in proper contact with the edge of the instrument, which, while adjusting the vernier, should be accomplished as follows:--

The thumb-screw most distant from the vernier should be set up tight, so that that jaw is fixed in position. The other thumb-screw should be set so as to exert, on the small spring between its end and the edge of the bar, a pressure sufficient to bend that spring to almost its full limit, but not so as to let it grip the bar. The elasticity of the spring will then hold the edge of the vernier slide sufficiently firmly to the under edge of the bar to keep the jaw-surfaces parallel; to enable the correct adjustment of the vernier, and to permit the nut-wheel to move the slide without undue wear upon its thread, or undue wear between the edge of the slide and that of the bar, both of which evils will ensue if the thumb-screw nearest the vernier is screwed firmly home before the final measuring adjustment of the vernier is accomplished.

When the measurement is completed the second thumb-screw must be set home and the reading examined again, for correctness, to ascertain if tightening the screw has altered it, as it would be apt to do if the thumb-screw was adjusted too loose.

The jaws are tempered to resist wear, and are ground to a true plane surface, standing at a right angle to the body of the bar. The method of setting the instrument to a standard size is as follows:--

The zero line marked 0 on the vernier coincides with the line 0 on the bar when the jaws are close together; hence, when the 0 line on the vernier coincides with the inch line on the bar, the instrument is set to an inch between the jaws. When the line next to the 0 line on the vernier coincides with the line to the left of the inch line on the bar, the instrument is set to 1-1/1000 inches. If the vernier slide then be moved so that the second line on the vernier coincides with the second line, on the left of the inch on the bar, the instrument is set to 1-2/1000 inches, and so on, the measurement of inches and fiftieths of an inch being obtained by the coincidence of the zero line on the vernier with the necessary line on the bar, and the measurements of one-thousands being taken as described.

But if it is required to measure, or find the diameter of an existing piece of work, the method of measuring is as follows:--

The thumb-screws must be so adjusted as to allow the slide to move easily or freely upon the work without there being any play or looseness between the slide and the bar. The slide should be moved up so as to very nearly touch the work when the latter is placed between the jaws.

The thumb-screw farthest from the vernier should then be screwed home, and the other thumb-screw operated to further depress the spring without causing it to lock upon the bar. The nut-wheel is then operated so that the jaws, placed squarely across the work, shall just have perceptible contact with it. (If the jaws were set to grip the work tight they would spring from the pressure, and impair the accuracy of the measurements.) The thumb-screw over the vernier may then be screwed home, and the adjustment of the instrument to the work again tried. If a correction should be found necessary, it is better to ease the pressure of the thumb-screw over the vernier before making such correction, tightening it again afterwards. The reading of the measurement is taken as follows:--

If the 0 line on the vernier coincides with a line on the bar, the measurement will, of course, be shown by the distance of that line from the 0 line on the bar, the measurement being in fiftieths of inches, or inches and fiftieths (as the case may be), but if the 0 line on the vernier does not coincide with any line of division on the bar, then the measurement in inches and fiftieths will be from the next line (on the bar) to the right of the vernier, while the thousandths of an inch may be read by the line on the vernier which coincides with a line on the bar.

Suppose, for example, that the zero line of the vernier stands somewhere between the 1 inch and the 1-1/50 inch line of division on the bar, then the measurement must be more than an inch, but less than 1-1/50 inches.

If the tenth or middle line on the vernier is the one that coincides with a line on the bar, the reading is 1-10/1000 inches. If the line marked 5 on the vernier is the one that coincides with a line on the bar, the measurement is an inch and 5/1000, and so on.

For measuring the diameters of bores or holes, the external edges of the jaws are employed; the width of the jaw at the ends being reduced in diameter to enable the jaw ends to enter a small hole. These edges are formed to a circle, having a radius smaller than the smallest diameter of hole they will enter when the jaws are closed, which insures that the point of contact shall be in the middle of the thickness of each jaw. In this case the outside diameter of the jaws must be deducted from the measurement taken by the vernier, or if it be required to set the instrument to a standard diameter, the zero line on the vernier must be set to a distance on the bar less than that of the measurement required to an amount equal to the diameter of the jaw edges when the jaws are closed. This diameter is, as far as possible, made to correspond to the lines of division on the bar. Thus in the instrument shown in Fig. 1392, these lines of division are 1/50 inch; hence the diameter across the closed bars should, to suit the reading (for internal measurements) on the bar, be measurable also in fiftieths of an inch; but the other side of the bar is divided into millimetres, hence to suit internal measurements (in millimetres or fractions thereof) the width of the jaws, when closed, should be measurable in millimetres; hence, it becomes apparent that the diameter of the jaws used for internal measurements can be made to suit the readings on one side only of the bar, unless the divisions on one side are divisible into those on the other side of the bar. When the diameter of the jaws is measurable in terms of the lines of division on the bar, the instrument may be set to a given diameter by placing the zero of the vernier as much towards the zero on the bar as the width of the jaws when closed. Thus, suppose that width (or diameter, as it may be termed) be 10/50 of an inch, and it be required to set the instrument for an inch interval or bore measurement, then the zero on the vernier must be placed to coincide with the line on the bar which denotes 40/50 of an inch, the lacking 10/50 inch being accounted for in the diameter or width of the two jaws.

But when the width of the jaws when closed is not measurable in terms of the lines of division on the bar, the measurement shown by the vernier will, of course, be too small by the amount of the widths of the two jaws, and the measurement shown by the vernier must be reduced to the terms of measurement of the width of the jaws, or what is the same thing, the measurement of the diameter of the jaws must be reduced to the terms of measurement on the bar, in order to subtract one from the other, or add the two together, as the case may require.

For example: Suppose the diameter of the jaws to measure, when they are close together, 250/1000 of an inch, and that the bar be divided into inches and fiftieths. Now set the zero of the vernier opposite to the line denoting 49/50 inch on the bar. What, then, is the measurement between the outside edges of the jaws? In this case we require to add the 250/1000 to the 49/50 in order to read the measurement in terms of fiftieths and thousandths of an inch, or we may read the measurement to one hundredths of an inch, thus: 49/50 equal 98/100, and 250/1000 equal 25/100, and 98/1000 added to 25/100 are 123/100, or an inch and 23/100.

To read in 1/1000ths of an inch, we have that 49/50 of an inch are equal to 980/1000, because each 1/50 inch contains 20/1000 inch, and this added to 250/1000 makes 1230/1000, that is 1-230/1000 inches.

The accuracy of the instrument may be maintained, notwithstanding any wear which may in the course of time take place on the inside faces of the jaws, by adjusting the zero line on the vernier to exactly coincide with the zero line on the bar, but the fineness of the lines renders this a difficult matter with the naked eye, hence it is desirable to read the instrument with the aid of a magnifying gla.s.s. If the outer edges of the jaws should wear, it is simply necessary to alter the allowance made for their widths.

[Ill.u.s.tration: Fig. 1394.]

Fig. 1394 represents standard plug and collar gauges. These tools are made to represent exact standard measurements, and obviously do no more than to disclose whether the piece measured is exactly to size or not.

If the work is not to size they will not determine how much the error or difference is, hence they are gauges rather than measuring tools. It is obvious, however, that if the work is sufficiently near to size, the plug or male gauge may be forced in, or the collar or female gauge may be forced on, and in this case the tightness of the fit would indicate that the work was very near to standard size. But the use of such gauges in this way would rapidly wear them out, causing the plug gauge and also the collar to get smaller than its designated size, hence such gauges are intended to fit the work without friction, and at the same time without any play or looseness whatever. Probably the most accurate degree of fit would be indicated when the plug gauge would fit into the collar sufficiently to just hold its own weight when brought to rest while within the collar, and then slowly fall through if put in motion within the collar. It is obvious that both the plug and the collar cannot theoretically be of the same size or one would not pa.s.s within the other, but the difference that is sufficient to enable this to be done is so minute that it is practically too small to measure and of no importance.

[Ill.u.s.tration: Fig. 1395.]

When these gauges are used by the workmen, to fit the work to their wear is sufficient to render it necessary to have some other standard gauge to which they can be from time to time referred to test their accuracy, and for this purpose a standard such as in Fig. 1395 may be employed. It consists of a number of steel disks mounted on an arbor and carefully ground after hardening each to its standard size.

But a set of plug and collar gauges provide within themselves to a certain extent the means of testing them. Thus we may take a collar or female gauge of a certain size and place therein two or three plug gauges whose added diameters equal that of the female or collar gauge.

[Ill.u.s.tration: Fig. 1396.]

In Fig. 1396, for example, the size of the female gauge A being 1-1/2 inches, that of the male B may be one inch, and that of C 1/2 an inch, and the two together should just fit the female. On the other hand, were we to use instead of B and C two males, 7/8 and 5/8 inches respectively, they should fit the female; or a 1/2 inch, a 5/8 inch and a 3/8 inch male gauge together should fit the female. By a series of tests of this description, the accuracy of the whole set may be tested; and by judicious combinations, a defect in the size of any gauge in the set may be detected.

The wear of these gauges is the most at their ends, and the fit may be tested by placing the plug within the collar, as in Fig. 1397, and testing the same with the plug inserted various distances within the collar, exerting a slight pressure first in the direction of A and then of B, the amount of motion thus induced in the plug denoting the closeness of the fit.

In trying the fit of the plug by pa.s.sing it well into or through the collar, the axis of the plug should be held true with that of the collar, and the plug while being pressed forward should be slightly rotated, which will cause the plug to enter more true and therefore more easily. The plug should be kept in motion and not allowed to come to rest while in the collar, because in that case the globules of the oil with which the surfaces are lubricated maintain a circular form and induce rolling friction so long as the plug is kept in motion, but flatten out, leaving sliding friction, so soon as the plug is at rest, the result being that the plug will become too tight in the collar to permit of its being removed by hand.

[Ill.u.s.tration: Fig. 1397.]

The surfaces of both the plug and the collar should be very carefully cleaned and oiled before being tried together, it being found that a film of oil will be interposed between the surfaces, notwithstanding the utmost accuracy of fit of the two, and this film of oil prevents undue abrasion or wear of the surfaces.

When great refinement of gauge diameter is necessary, it is obvious that all the gauges in a set should be adjusted to diameter while under an equal temperature, because a plug measuring an inch in diameter when at a temperature of, say, 60 will be of more than an inch diameter when under a temperature of, say, 90.

It follows also that to carry this refinement still farther, the work to be measured if of the same material as the standard gauge should be of the same temperature as the gauge, when it will fit the gauge if applied under varying temperatures; but if a piece of work composed, say, of copper, be made to true gauge diameter when both it and the gauge are at a temperature of, say, 60, it will not be to gauge diameter, and will not fit the gauge, if both be raised to 90 of temperature, because copper expands more than steel.

To carry the refinement to its extreme limit then, the gauge should be of the same metal as the work it is applied to whenever the two fitting parts of the work are of the same material. But suppose a steel pin is to be fitted as accurately as possible to a bra.s.s bush, how is it to be done to secure as accurate a fit as possible under varying temperatures?

The two must be fitted at some equal temperature; if this be the lowest they will be subject to, the fit will vary by getting looser, if the highest, by getting tighter; in either case all the variation will be in one direction. If the medium temperature be selected, the fit will get tighter or looser as the temperature falls or rises. Now in workshop practice, where fit is the object sought and not a theoretical standard of size, the range of variation due to temperature and, generally, that due to a difference between the metals, is too minute to be of practical importance. To the latter, however, attention must, in the case of work of large diameter, be paid: thus, a bra.s.s piston a free fit at a temperature of 100 to a 12-inch cast-iron cylinder, will seize fast when both are at a temperature of, say, 250. In such cases an allowance is made in conformity with the co-efficients of expansion.

In the case of the gauges, all that is practicable for ordinary work-shop variation of temperature is to make them of one kind and quality of material--as hard as possible and of standard diameter, when at about the mean temperature at which they will be when in use. In this case the limit of error, so far as variation from temperature is concerned, will be simply that due to the varying co-efficients of expansion of the metals of which the work is composed.

[Ill.u.s.tration: Fig. 1398.]

To provide a standard of lineal measurement which shall not vary under changes of temperature it has been proposed to construct a gauge such as shown in Fig. 1398, in which A and B are bars of different metals whose lengths are in the inverse ratio of their co-efficients of expansion. It is evident that the difference of their lengths will be a constant quant.i.ty, and that if the two bars be fastened together at one end, the distance from the free end of B to the free end of A will not vary with ordinary differences in temperature.

Plug and collar gauges may be used for taper as well as for parallel fits, the taper fit possessing the advantage that the bolt or pin may be let farther into its hole to take up the wear. In a report to the Master Mechanics a.s.sociation upon the subject of the propriety of recommending a standard taper for bolts for locomotive work, Mr. Coleman Sellers says:--

"As the commission given to me calls for a decision as to the taper of bolts used in locomotive work, it presupposes that taper bolts are a necessity. In our own practice we divide bolts into several cla.s.ses, and our rule is that in every case where a through bolt can be used it must be used. If we cannot use a through bolt we use a stud, and where a stud cannot be used we put in a tap bolt, and the reason why a tap bolt comes last is because it is part and parcel of the machine itself. There are also black bolts and body bound bolts, the former being put into holes 1/16 inch larger than the bolt. It is possible in fastening a machine or locomotive together to use black bolts and body bound bolts. With body bound bolts it is customary for machine builders to use a straight reamer to true the hole, then turn the bolt and fit it into its place.

It is held by many locomotive builders that the use of straight bolts is objectionable, on the score that if they are driven in tight there is much difficulty in getting them out, and where they are got out two or three times they become loose, and there is no means of making them tighter.

"There is no difficulty in making two bolts of commercially the same size. But there is a vast difference between absolute accuracy and commercial accuracy. Absolute accuracy is a thing that is not obtainable. What we have to strive for, then, is commercial accuracy.