Modern Machine-Shop Practice Part 92

At the time the Government established the use of the standard system of screw threads in the navy yards, ten sets of gauges were ordered from a manufacturer. His firm procured a duplicate set of these and took them to the navy yard in Boston and found that they were practically interchangeable. He also took them to the Brooklyn Yard Navy. The following tabular statement shows the difference between them:--

--------------------------------------------------------------- | |Morse Twist| |Navy Yard |Drill and | Morse Twist Drill and Size. |Male Gauge. |Machine Co.| Machine Co.

| |Male Gauge.| Female Gauge.

-------+------------+-----------+------------------------------ 1/4 | 0.25 | 0.25 | Interchanged 5/16 | .313 | .313 | "

3/8 | .375 | .3759 | 7/16 | .437 | .437 | Interchanged 1/2 | .505 | .505 | "

9/16 | .562 | .564 (-)| "

5/8 | Damaged | .626 | "

3/4 | .7505 | .751 | "

7/8 | .876 | .8758 | "

1 | 1.00075 | 1.00075 | "

| | | { Navy Yard M. T. D. & M. Co.

1-1/8 | 1.125 (+)| 1.125 (-)| { --------- ----------------- | | | { (+) (-) 1-1/4 | 1.25 | 1.25 | Interchanged 1-3/8 | 1.375 | 1.375 | "

1-1/2 | 1.5 | 1.5 (-)| (-) 1-5/8 | 1.6245 | 1.624 | (-) 1-3/4 | 1.749 | 1.749 | Interchanged 1-7/8 | 1.8745 | 1.874 | (-) 2 | 1.999 | 1.999 | ---------------------------------------------------------------

The sign (-) means that the piece is small, but not enough to measure.

The sign (+) means that the piece is large, but not enough to measure.

The advantages to be derived from having universally accepted standard subdivisions of the yard into inches and parts of an inch are as follows:--

When a number of pieces of work of the same shape and size are to be made to fit together, then, if their exact size is not known and there is no gauge or test piece to fit them to, each piece must be fitted by trial and correction to its place, with the probability that no two pieces will be of exactly the same size. As a result, each piece in a machine would have to be fitted to its place on that particular machine, hence each machine is made individually.

Furthermore, if another lot of machines are afterwards to be made, the work involved in fitting the parts together in the first lot of machines affords no guide or aid in fitting up the second lot. But suppose the measurements of all the parts of the first lot are known to within the one ten-thousandth part of an inch, which is sufficiently accurate for practical purposes, then the parts may be made to measurement, each part being made in quant.i.ties and kept together throughout the whole process of manufacture, so that when all the parts are finished they may go to the a.s.sembling or erecting room, and one piece of each part may be taken indiscriminately from each lot, and put together to make a complete machine. By this means the manufacture of the machine may be greatly simplified and cheapened, and the fit of any part may be known from its size, while at the same time a new part may be made at any time without reference to the machine or the part to which it is to fit.

Again, work made to standard size in one shop will fit to that made to standard size in another, providing the standard gauges agree.

The Pratt and Whitney Company, of Hartford, Connecticut, in union with Professor Rogers, of Cambridge University, in Ma.s.sachusetts, determined to inspect the Imperial British yard, to obtain a copy of it, and to make a machine that would subdivide this copy into feet and inches, as well as transfer the line measurements employed in the subdivisions into end measures for use in the workshops, the degree of accuracy being greater than is necessary in making the most refined mechanism, made under the interchangeable or standard gauge system. The machine made under these auspices is the Rogers-Bond Universal Comparator; Mr. Bond having been engaged in conjunction with Professor Rogers in its construction.

The machine consists of two cylindrical guides, upon which are mounted two heads, carrying microscopes which may be reversed in the heads, so as to be used at the front of the machine for line measurements and on the back for end measurements.

Fig. 1348 is a front, and Fig. 1349 a rear view of the machine, whose details of construction are more clearly shown in the enlarged views, Fig. 1350 and 1352.

[Ill.u.s.tration: Fig. 1350.]

[Ill.u.s.tration: Fig. 1351.]

[Ill.u.s.tration: Fig. 1352.]

Fig. 1350 is a top view, and Fig. 1352 a front view, the upper part of the machine being lifted up for clearness of ill.u.s.tration. X, X, are the cylindrical guides, upon which are the carriages I, K, for the microscopes. The construction of these carriages is more fully seen in Fig. 1351, which represents carriage K. It is provided with a hand-wheel R, operating a pinion in a rack (shown at T in the plan view figure of the machine) and affording means to traverse the carriage along the cylindrical guides. The microscope may be adjusted virtually by the screw M^{4}. The base upon which the microscope stands is adjustable upon a plate N, by means of the two slots and binding screws shown, and the plate N fits in a slideway running across the carriage. U is one of the stops used in making end measurements, the other being fixed upon the frame of the machine at V in the plan view, Fig. 1350. The micrometric arrangement for the microscope is shown more clearly in Fig.

1353. The screw B holds the box in position, the edge of the circular base on which it sits being graduated, so that the position of M may be easily read. In the frame M is a piece of gla.s.s having ruled upon it the crossed lines, or in place of this a frame may be used, having in it crossed spider web lines. These lines are so arranged as to be exactly in focus of the upper gla.s.s of the microscope, this adjustment being made by means of the screw S. The lines upon the bar are in the focus of the lower gla.s.s; hence, both sets of lines can be seen simultaneously, and by suitable adjustment of the microscope can be brought to coincide.

[Ill.u.s.tration: Fig. 1353.]

Beneath the cylindrical guides, and supported by the rack T that runs between and beneath them, are the levers P, in Fig. 1352, upon which weights may be placed to take up the flexure or sag of the cylindrical guides.

In Fig. 1352, H, H, are heads that may be fixed to the cylindrical guides at any required point, and contain metallic stops, against which corresponding stops on the microscope carriages may abut, to limit and determine the amount to which these carriages may be moved along the cylindrical guides.

The pressure of contact between the carriage and the fixed stops is found to be sufficiently uniform or constant if the carriage is brought up to the stops (by means of the hand-wheel R, Fig. 1351) several times, and a microscope reading taken for each time of contact. But this pressure of contact may be made uniform or constant for all readings by means of an electric current applied to the carriage through the metallic stops on heads H, H, and those on the carriage.

We have now to describe the devices for supporting the work and adjusting it beneath the microscopes.

[Ill.u.s.tration: Fig. 1354.]

Referring, then, to Fig. 1352, E is a bed or frame that may be raised or lowered by means of the hand-wheel C, so as to bring the plate S (on which rests the bar whose line measure is to be compared) within range of the microscopes. The upper face of E is provided with raised [V]

slideways, which are more clearly seen in the end view of this part of the machine shown in Fig. 1354. Upon these raised [V]s are the devices for adjusting the height of the eccentric rollers S^{3}, upon which the bars to be tested are laid, S^{2} representing one of these bars. To adjust the bars in focus under the microscope, these eccentric rollers are revolved by means of levers S^{4}. At S^{5} is a device for giving to the table a slight degree of longitudinal movement in the base plate that rests upon the raised [V]s; on the upper face of E and at S^{6} is a mechanism for adjusting the height of that end of the plate S. The base plate may be moved along the raised [V]s of E by the hand-wheel D.

To test whether the cylindrical guides are deflected by their own weight or are level, a trough of mercury may be set upon the eccentric rollers S^{3}, Fig. 1352, and the fine particles of dust on its surface may be brought into focus in the microscope, whose carriage may then be traversed to various positions along the cylindrical guides, and if these dust particles remain in focus it is proof that the guides are level with the mercury surface.

[Ill.u.s.tration: Fig. 1355.]

The methods of using the machine are as follows: The standard bar has marked upon its upper face (which is made as true as possible and highly polished) a line B (Fig. 1355), which is called the horizontal line, and is necessary in order to set the bar parallel to the cylindrical guides of the machine. The lines A, A, are those defining the measurement as a yard, a foot, or whatever the case may be, and these are called the vertical lines or lines of measurement. Now, suppose we require to test a bar with the standard and the lines on its face are marked to correspond to those on the standard.

The first operation will be to set the standard bar on the eccentric rollers S^{3} in Fig. 1352, and it and the microscopes are so adjusted that the spider web lines in the microscope exactly intersect the lines A and B on the standard, when the microscope carriage abuts against the heads H, Fig. 1352. The standard bar is then replaced by the bar to be tested, which is adjusted without altering the microscope adjustment or the heads H, and if the spider web lines in the microscope exactly coincide with and intersect the lines A and B, the copy corresponds to the standard. But if they do not coincide, then the amount of error may be found by the micrometer wheel G, Fig. 1353.

[Ill.u.s.tration: Fig. 1356.]

In this test the carriage is moved up against the stops H several times, and several readings or tests are made, so as to see that the force of the contact of the carriage against the stops H is uniform at each test, and if any variation is found, the average of a number of readings is taken. It is found, however, that with practice the carriage may be moved against the head H by means of the hand-wheel with such an equal degree of force that an error of not more than one fifty-thousandth of an inch is induced. It is found, however, that if too much time is occupied in this test, the heat of the operator's body will affect the temperature of the bars, and therefore expand them and vitiate the comparison. But in this connection it may be noted that if a bar is at a temperature of 40, and is placed in an ice bath, it does not show any contraction in less than one minute, and that when it does so, the contraction is irregular, taking place in sudden movements or impulses.

Professor Rogers' methods of testing end measures are as follows: To compare a line with an end measure, a standard bar is set upon the machine, its horizontal and vertical lines being adjusted true to the cylindrical guides by the means already described, and the microscope carriage is so adjusted that the spider web lines of the microscope coincide with the horizontal and vertical lines marked on the standard, while at the same time the stop (U, Fig. 1350) on the carriage K has contact with the fixed stop (V, Fig. 1350.) Carriage K is then moved along the cylindrical guides so as to admit the bar (whose end measure is to be compared with the lines on the standard) between the two stops, and if, with the bar touched by both stops U and V, the microscope spider lines intersect the vertical and horizontal line on the standard bar, then the end measure corresponds to the line measure; whereas, if such is not the case, the amount of error may be found by noting how much movement of the micrometer wheel of the microscope is required to cause the lines to intersect.

It is obvious that in this test, if the cylindrical guides had a horizontal curvature, the test would not be perfect.

THE HORIZONTAL CURVATURE.--The copy or bar to be tested may be set between the stops, and the standard bar may be placed on one side of it, as in Fig. 1356, and the test be made as already described. It is then set the same distance from the bar to be tested, but on the other side of it, as in figure, and again adjusted for position and tested, and if the readings on the standard bar are the same in both tests, it is proof that the measurements are correct.

Suppose, for example, that the cylindrical guides were curved as in Fig.

1356, it is evident that the vertical lines would appear closer together on the standard bar when in the first position than when in the second position.

In the Rogers machine the amount of error due to curvature in the cylindrical guides in this direction is found to be about 1/5000 part of an inch in 39 inches, corresponding to a radius of curvature of five miles.

[Ill.u.s.tration: Fig. 1357.]

[Ill.u.s.tration: Fig. 1358.]

Another method of testing an end with a line measure is as follows: The bar to be measured is shaped as in Fig. 1357, the end measurement being taken at A, and the projection B at each end serving to preserve the end surfaces A from damage. The standard bar is then set upon the machine and its horizontal and vertical lines adjusted in position as before described. In connection with this adjustment, however, the bar to be tested is set as in Fig. 1358; C being a block of metal (having marked centrally upon it horizontal and vertical lines), placed between the bar and the fixed stop U, its vertical line being in line with the vertical line on the standard. This adjustment being made, the block C is removed and placed at the other end of the bar, as shown in Fig. 1359, when, if the end measure on the bar corresponds with the line measure on the standard, the vertical line at the other end of the standard will correspond with the vertical line on block C.

[Ill.u.s.tration: Fig. 1359.]

To prove that the vertical line is exactly equidistant from each end of the block C, all that is necessary is to place it between the bar and the fixed stop U, Fig. 1350, adjust the microscope to it and then turn it end for end, and if its vertical line is still in line with the spider web of the microscope it is proof that it is central on the block, while if it is not central the necessary correction may be made.

It is obvious that it is no matter what the length of C may be so long as its vertical line is central in its length.

In this process the coincidence of the vertical lines on the standard and on the piece C are employed to test the end measure on the bar with the line measure on the standard.

[Ill.u.s.tration: Fig. 1360.--General View.]

[Ill.u.s.tration: Fig. 1361.--Plan.]

Figs. 1360 and 1361 represent the Whitworth Millionth Measuring Machine, in which the measurement is taken by the readings of an index wheel, and the contact is determined from the sense of touch and the force of gravity.

It is obvious that in measuring very minute fractions of an inch one of the main difficulties that arise is that the pressure of contact between the measuring machine and the surfaces measured must be maintained constant in degree, because any difference in this pressure vitiates the accuracy of the measurement. This pressure should also be as small as is consistent with the a.s.surance that contact actually exists, otherwise the parts will spring, and this would again impair the accuracy of the measurement.