Modern Machine-Shop Practice Part 94

[Ill.u.s.tration: Fig. 1375.]

Fig. 1375 shows a similar reading, in which the points do not coincide with the cross threads of the microscope.

[Ill.u.s.tration: Fig. 1376.]

Fig. 1376 shows the microscope adjusted for testing by turning the circle a quarter revolution.

Fig. 1377 represents one of the later forms of Ramsden's dividing engine.[21] It consists first of a three-legged table, braced so as to be exceedingly stiff. Upon this is placed a horizontal wheel with deep webs, and a flat rim. The webs stiffen the wheel as much as possible, and one of these webs, which runs round the wheel about half-way between the centre and the circ.u.mference, rests upon a series of rollers which support it, and prevent, as far as possible, the arms from being deflected by their own weight. An outer circle, which receives the graduation, is laid upon the rim of the wheel and secured in place. The edge of this circle is made concave. A very fine screw, mounted in boxes and supported independently, is then brought against this hollow edge, and, being pressed against it, the screw, when revolved, of course cuts a series of teeth in the circ.u.mference, and this tooth-cutting, facilitated by having the screw threads made with teeth, was continued until perfect [V]-shaped teeth were cut all around the edge of the wheel. This Mr. Ramsden calls ratching the wheel. The number of teeth, the circ.u.mference of the wheel, and the pitch of the screw were all carefully adjusted, so that by using 2160 teeth, six revolutions of the screw would move the wheel the s.p.a.ce of 1. When this work was finished, and the adjustment had been made as perfect as possible, a screw without teeth--that is, one in which the thread was perfect--was put in the place of that which had cut the teeth from the wheel, and the machine was perfected. The wheel A B C in the drawings is made of bell metal, and turns in a socket under the stand, which prevents the wheel from sliding from the supporting or friction rolls Z, Z. The centre R, working against the spindle M, is made so as to fit instruments of various sizes. The large wheel has a radius of 45 inches, and has 10 arms. The ring B is 24 inches in diameter by 3 inches deep. The ring C is of very fine bra.s.s, fitting exactly on the circ.u.mference of the wheel, and fastened by screws, which, after being screwed home, were well riveted. Great care was taken in making the centre on which the wheel worked exceedingly true and perfect, and in making the socket for the wheel fit as exactly as possible. The revolving mechanism is all carried on the pillar P, resting on the socket C'. We may state here that the machine, as shown in the engravings, now in the possession of the Stevens Inst.i.tute, is in some respects slightly improved on that shown in the original drawings published in "Rees' Cyclopaedia" in 1819.

After the wheel was put on its stand, and the pulleys in place, the instrument was ready for the turning mechanism. The upper part of this pillar P carries the framework in which the traversing screw revolves.

[21] From _Mechanics_.

In Fig. 1378 D is the head of this pillar, P the screw which turns the wheel. E^{1} E^{1} are the boxes, which are made conical so as to prevent any shake and to hold the screw firmly. Circles of bra.s.s, F and V, are placed on the arbor of the screw, and as their circ.u.mference is divided into 60 parts, each division consequently amounts to a motion of the wheel of 10 seconds, and 60 of them will equal 1 minute. Revolution is given to the screw by means of the treadle B' and the cord Y, which runs over the guiding screw W, Fig. 1379, and is finally attached to the box U. A spring enclosed in the box U causes it to revolve, and winds up the slack of the cord whenever the treadle is relieved. In the original drawing the head of the pillar P was carried in a parallel slip in the piece surrounding its head. The construction as shown in Fig. 1379 is somewhat different. The result attained, however, is identical, and the spindles and attachments are held so as to have no lateral motion. The wheels V and X have stops upon them, so arranged that the screw may be turned definitely to a given point and stopped. These wheels are at the opposite ends of the screw W. A detail of one of them is shown at V in Fig. 1380, where X is the ratchet-wheel. This figure also ill.u.s.trates the construction of the bearings for the screw arbor. We have not s.p.a.ce to explain the method by which the perfection of the screw was obtained, nor to discuss the means by which was obtained the success of so eliminating the errors as to make the division of the instrument more perfect than anything which had been attempted previously. Success, however, was obtained, and by means of the first or tooth-cutting screw the teeth were brought to such a considerable uniformity that, together with the fact that the screw took hold of a number of teeth at one time, most of the errors which would have been expected from this method of operation were eliminated. The method of ruling lines upon the instrument was most ingenious. The frame L L, is connected to the head D, of the pillar P in front, by the clamps I and K, and to the centre M by the block R. A frame N N stiffens the back. The blocks O, O on the frame Q' are secured to the frame L L, by set-screws C, C.

[Ill.u.s.tration: Fig. 1377.]

Fig. 1381 shows a side view of the frame Q', which it is seen carries a [V]-shaped piece Q, which in turn carries another [V]-shaped piece S, Fig. 1378. The piece Q is supported on pointed screws _d_, _d_, and the piece S is supported on two similar screws _f_, _f_. The point of this piece S carries the cutting tool E, Fig. 1378. Of course S can move only in a radial line from the centre M towards the circ.u.mference. If the s.e.xtant, octant, or other instrument be fastened to the large wheel A, with its centre at M, and the large wheel be rotated by the screw, all lines drawn upon it by E will be radial, and the distances apart will be governed by the number of turns made by the screw. This improvement, we think, was originated by Mr. Ramsden, and was a very great advance over the old method of the straight-edge, and has been used in some of the Government comparators and dividing engines. The following is Mr.

Ramsden's own description of the graduation of the machine, and of his method of operating it. It shows the extreme care which he took in correcting the mechanical errors in the construction:--

"From a very exact centre a circle was described on the ring C, about 4/10 inch within where the bottom of the teeth would come. This circle was divided with the greatest exactness I was capable of, first into five parts, and each of these into three. These parts were then bisected four times; that is to say, supposing the whole circ.u.mference of the wheel to contain 2160 teeth, this being divided into five parts, and these again divided into three parts, each third part would contain 144, and this s.p.a.ce, bisected four times, would give 72, 36, 18, 9; therefore, each of the last divisions would contain 9 teeth. But, as I was apprehensive some error might arise from quinquesection and trisection, in order to examine the accuracy of the divisions, I described another circle on the ring C, Fig. 1378, 1/10 inch within the first, and divided it by continual bisection, as 2160, 1080, 540, 270, 135, 67-1/2, 33-3/4, and, as the fixed wire (to be described presently) crossed both the circles, I could examine their agreement at every 135 revolutions (after ratching could examine it at every 33-3/4); but not finding any sensible difference between the two sets of divisions, I, for ratching, made choice of the former, and, as the coincidence of the fixed wire with an intersection could be more exactly determined with a dot or division, I therefore made use of intersections on both sides, before described.

"The arms of the frame L, Fig. 1381, were connected by a thin piece of bra.s.s, 3/4 inch broad, having a hole in the middle 4/10 inch in diameter; across this hole a silver wire was fixed, exactly in a line to the centre of the wheel; the coincidence of this wire with the intersections was examined by a lens of 1/10 inch focus, fixed in a tube which was attached to one of the arms L. Now (a handle or winch being fixed on the end of the screw) the division marked 10 on the circle F was set to its index, and, by means of a clamp and adjusting-screw for that purpose, the intersection marked I on the circle C' was set exactly to coincide with the fixed wire. The screw was then carefully pressed against the circ.u.mference of the wheel by turning the finger-screw _h_; then, removing the clamp, I turned the screw by its handle nine revolutions, till the intersection marked 240 came nearly to the wire.

Then, turning the finger-screw _h_, I released the screw from the wheel, and turned the wheel back till the intersection marked 2 exactly coincided with the wire, and by means of the clamp before mentioned, the division 10 on the circle being set to its index, the screw was pressed against the edges of the wheel by the finger-screw _h_, the clamps were removed, and the screw turned nine revolutions, till the intersection marked I nearly coincided with the fixed wire; the screw was released from the wheel by turning finger-screw _h_ as before, the wheel was turned back till intersection marked 3 coincided with the fixed wire; the division 10 in the circle being set to its index, the screw was pressed against the wheel as before, and the screw turned nine revolutions, till intersection 2 was nearly coincident with the fixed wire, and the screw released, and I proceeded in this manner till the teeth were marked round the whole circ.u.mference of the wheel. This was repeated three times round to make the impressions deeper. I then ratched the wheel round continuously in the same direction, without ever disengaging the screw, and, in ratching the wheel about 300 times round, the teeth were finished.

[Ill.u.s.tration: Fig. 1378.]

"Now, it is evident that if the circ.u.mference of the wheel was even one tooth, or ten minutes, greater than the screw would require, this error would, in the first instance, be reduced by 1/240 part of a revolution, or two seconds and a half, and these errors or inequalities of the teeth were equally distributed round the wheel at the distance of nine teeth from each other. Now, as the screw in ratching had continual hold of several teeth at the same time and thus constantly changing, the above-mentioned irregularities soon corrected themselves, and the teeth were reduced to a perfect equality. The piece of bra.s.s which carried the wire was now taken away, and the cutting-screw was also removed, and a plain one put in its place. At one end of the screw arbor, or mandrel was a small bra.s.s circle F, having its edge divided into 60 parts, numbered at every sixth division, as before mentioned. On the other end of the screw is a ratchet-wheel V (X, Fig. 1380) having 60 teeth, covered by the hollow circle (V, Fig. 1380), which carries two clicks that catch upon opposite sides of the ratchet-wheel. When the screw is to be moved forward, the cylinder W turns on a strong steel arbor E", which pa.s.ses through the piece X'; this piece, for greater firmness, is attached to the screw-frame by the braces _w_. A spiral groove or thread is cut upon the outside of the cylinder W, which serves both for holding the string and also giving motion to the lever I on its centre, by means of a steel tooth _v_, that works between the threads of the spiral. To the lever is attached a strong steel pin _m_, on which a bra.s.s socket turns; this socket pa.s.ses through a slit in the piece _u_, and may be tightened in any part of the slit by the finger-nut _y_. This piece serves to regulate the number of revolutions of the screw for each tread of the treadle B'."

[Ill.u.s.tration: Fig. 1379.]

[Ill.u.s.tration: Fig. 1380.]

[Ill.u.s.tration: Fig. 1381.]

[Ill.u.s.tration: Fig. 1382.]

Figs. 1382, 1383, and 1384 represent a method adopted to divide a circle by the Pratt and Whitney Company. The principle of the device is to enable the wheel to be marked, to be moved through a part of a revolution equal to the length of a division, and to test the accuracy of the divisions by the coincidence of the line first marked with that marked last when the wheel has been moved as many times as it is to contain divisions. By this means any error in the division multiplies, so that the last division marked will exhibit it multiplied by as many times as there are divisions in the whole wheel. The accuracy of this method, so long as variations of temperature are avoided, both in the marking and the drilling of the wheel, appears to be beyond question. In the figures, W represents a segment of the wheel to be divided, and C what may be termed a dividing chuck. The wheel is mounted on an arbor in a gear-cutting machine. On the hub of the wheel (which has been turned up for the purpose) there is fitted, to a close working fit, a bore at the end of an arm, the other end of the arm being denoted by A in the figures. The dividing chuck is fitted to the slide S of the gear-cutting machine, and is of the following construction.

Between two lugs, B and B', it receives the end of arm A. These lugs are provided with set-screws, the distance between the ends of which regulate the amount of movement of the end of arm A. Upon A is the slide D, carrying the piece E, in which is the marking tool F, the latter being lifted by a spring G, and, therefore, having no contact with the wheel surface until the spring is depressed. H is an opening through the arm A to permit the marking tool F to meet the wheel face, as shown in Fig. 1384, which is an end view of the slide showing the arm A in section. The face of the wheel rests upon the chuck on each side of the arm at the points I, J, and may be clamped thereto by the clamps K. The arm may be clamped to the wheel by the clamp shown dotted in at L, the bolt pa.s.sing up and through the screw handle M. N is simply a lever with which to move the arm A, or arm A and the wheel. Suppose all the parts to be in the position shown in the cuts, the clamps being all tightened up, the slide D may be moved forward towards K, while the spring is depressed, and F will mark a line upon the wheel. The handle M may then be released and arm A moved until it touches the set-screw in B', when M may be tightened and another line marked. Clamps K are then tightened, and the wheel, with the arm A fast to it, moved back to the position shown in the cut, when the clamps may be tightened again and another line marked, the process being continued all round the wheel. To detect and enable the correction of any discoverable error in a division, there is provided the plate P, having upon it three lines of division (which have been marked simultaneously with three of the lines marked on the wheel). This plate is supported by an arm or bracket Q, on the rear edge of which are three notches R to hold a microscope, by means of which the lines on P may be compared with those on the wheel face, so that if any discrepancy should appear it may be determined which line is in error.

The labor involved in the operation of marking a large wheel is very great. Suppose, for example, that a wheel has 200 lines of division, and that after going round the wheel as described it is found that the last division is 100th inch out; then in each division the error is the two-hundredth part of this 100th inch, and that is all the alteration that must be made in the distance between set-screws B and B'.

[Ill.u.s.tration: Fig. 1383.]

[Ill.u.s.tration: Fig. 1384.]

Figs. 1385 and 1386 represent a method of originating an index wheel, adopted by R. Hoe and Co., of New York City.

In this method the plan was adopted of fitting round a wheel 180 tapering blocks, which should form a complete and perfect circle. These blocks were to serve the same purpose as is ordinarily accomplished by holes perforated on the face of an index wheel. In their construction, means of correcting any errors that might be found, without the necessity of throwing away any portion of the work done, would also be provided. Further, this means would provide for taking up wear, should any occur in the course of time, and thus restore the original truth of the wheel.

Fig. 1385 of the engravings shows the originating wheel mounted upon a machine or cutting engine. Upon the opposite end of the shaft is the worm-wheel in the process of cutting. After the master worm-wheel has been thus prepared by means of the originating wheel, it is used upon the front end of the shaft, in the position now occupied by the originating wheel, and operated by a worm in the usual manner.

Subdivisions are made by change wheels. The construction of the originating wheel will be understood by the smaller engravings.

Fig. 1386 is an enlarged section of a segment of the wheel, while Fig.

1387 is an edge view of this segment. Fig. 1388 is a view of one of the blocks employed in the construction of the wheel, drawn to full size.

In the rim of the originating wheel there was turned a shoulder, C, Fig.

1387, 5 feet in diameter. Upon this shoulder there were clamped 180 blocks, of the character shown in Fig. 1386, as indicated by the section, Fig. 1387. These blocks were secured to the face of the wheel D by screws E, and were held down to the shoulder by the screw and clamp G F, shown in Fig. 1387. (They are omitted in Fig. 1385 for clearness of ill.u.s.tration.) In the preparation of these blocks each was fitted to a template T, in Fig. 1388, and was provided with a recess B, to save trouble in fitting and to insure each block seating firmly on the shoulder C. The shoulder, after successive trials, was finally reduced to such a diameter that the last block exactly filled the s.p.a.ce left for it when it was fully seated on the shoulder C. The wheel thus prepared was mounted on a Whitworth cutting engine, as shown in Fig. 1385. The general process of using this wheel is as follows: The blocks forming the periphery of the originating wheel are used in place of the holes ordinarily seen in the index plates. One of them is removed to receive a tongue, shown in the centre of Fig. 1385, which, exactly filling the opening or notch thus made, holds the wheel firmly in place. After a tooth has been cut in the master worm-wheel, shown at the back of Fig.

1385, the block in the edge of the originating wheel corresponding to the next tooth to be cut is removed. The tongue is withdrawn from the first notch, the wheel is revolved, and the tongue is inserted in the second position. The block first removed is then replaced, and the cutting proceeds as before. This operation is repeated until all the teeth in the master wheel have been cut. The s.p.a.ce being a taper, the tongue holds the originating wheel more firmly than is possible by means of cylindrical pins fitting into holes. The number of blocks in the originating wheel being 180, the teeth cut in the master wheel may be 180 or some exact divisor of this number.

The advantages of this method of origination are quite evident. Since 180 blocks were made to fill the circle, the edges of each had 2 taper.

This taper enabled the blocks to be fitted perfectly to the template, because any error in fit would be remedied by letting the block farther down into the template. Hence, it was possible to correct any error that was discovered without throwing the block away. Further, as the blocks themselves are removed to form a recess for locking the originating wheel in position while cutting the worm-wheel, the truth of the work is not subject to the errors that creep in when holes or notches require to be pierced in the originating wheel. Such errors arise from the heating due to the drilling or cutting, from the wear of the tools or from their guides, from soft or hard spots in the metal and other similar causes.

To avoid any error from the heating due to the cut on the worm-wheel, in producing master wheels, Messrs. Hoe and Co. allowed the wheel to cool after each cut. The teeth were cut in the following order: The first three were cut at equidistant points in the circ.u.mference of the wheel.

The next three also were at equidistant points, and midway between those first cut. This plan was continued until all the teeth were cut, thus making the expansion of the wheel from the heat as nearly equal as possible in all directions.

There is one feature in this plan that is of value. It is that a certain number of blocks, for example six, may be taken out at two or three different parts of the originating wheel and interchanged, thus affording a means of testing that does not exist in any other method of dividing.

The tools applied by the workmen to measure or to test work may be divided into cla.s.ses.

1st. Those used to determine the actual size or dimension of the work, which may be properly termed measuring tools.

2nd. Those used as standards of a certain size, which may be termed gauges.

3rd. Those used to compare one dimension with another, as in the common calipers.

4th. Those used to transfer measurements or distances defined by lines.

5th. Those used to test the accuracy of plane or flat surfaces, or to test the alignment of one surface to another.

Referring to the first, their distinctive feature is that they give the actual dimensions of the piece, whether it be of the required dimension or not.

The second determine whether the piece tested is of correct size or not, but do not show what the amount of error is, if there be any.

The third show whatever error there may be, but do not define its amount; and the same is true of the fifth and sixth.

Fig. 1389 represents a micrometer caliper for taking minute end measurements. This instrument is capable of being set to a standard measurement or of giving the actual size of a piece, and is therefore strictly speaking a combined measuring tool and a gauge. The [U]-shaped body of the instrument is provided with a hub _a_, which is threaded to receive a screw C, the latter being in one piece with the stem D, which envelops for a certain distance the hub _a_. The thread of C has a pitch of 40 per inch; hence one revolution of D causes the screw to move endways 1/40 of an inch.

The vertical lines of division shown on the hub _a_ are also 1/40 of an inch apart, hence the bevelled edge of the sleeve advances one of the divisions on _a_ at each rotation.

This bevelled edge is divided into 25 equal divisions round its circ.u.mference, as denoted by the lines marked 5, 10, &c. If, then, D be rotated to an amount equal to one of its points of division, the screw will advance 1/25 of 1/40 of an inch. In the cut, for example, the line 5 on the sleeve coincides with the zero line which runs parallel to the axial line of the hub. Now suppose sleeve D to be rotated so that the next line of division on the bevelled edge of D comes opposite to the zero line, then 1/25 part of a revolution of D will have been made, and as a full revolution of D would advance the screw 1/40 of an inch, then 1/25 of a revolution will advance it 1/25 of 1/40 inch, which is 1/1000 inch.

The zero line being divided by lines of equal division into 40ths of an inch, then, as shown in the cut, the instrument is set to measure 3/40ths and 5/25ths of a fortieth.

It is to be observed that to obtain correct measurements the work must be held true with the face of the foot B, and the contact between the end of screw _c_ and the work must be just barely perceptible, otherwise the pressure of the screw will cause the [U]-piece to bend and vitiate the accuracy of the measurement. Furthermore, if the screw be rotated under pressure upon the work, its end will wear and in time impair the accuracy of the instrument. To take up any wear that may occur, the foot-piece B is screwed through the hub, holding it so that it may be screwed through the hub to the amount of the wear.

To avoid wear as much as possible, the screws of instruments of this kind are sometimes hardened, and to correct the error of pitch induced in the hardening, each screw is carefully tested to find in what direction the pitch of the hardened thread has varied, and provision is made for the correction as follows:--

The zero line on the hub _a_ stands, if the thread is true to pitch, parallel to the axis of the screw C, but if the pitch of the thread has become coa.r.s.er from hardening, this zero line is marked at an angle, as shown in Fig. 1390, in which A A represents the axial line of the screw and B the zero line.

If the screw pitch becomes finer from hardening, the zero line is made at an angle in the opposite direction, as shown in Fig. 1391, the amount of the angle being that necessary to correct the error in the screw pitch. The philosophy of this is, that if the pitch has become coa.r.s.er a less amount of movement of the screw is necessary, while if it has become finer an increased movement is necessary. It is obvious, also, that if the pitch of the thread should become coa.r.s.er at one end and finer at the other the zero line may be curved to suit.