Precision locating and dividing methods

by Anonymous

Originating a Precision Dividing Wheel

There are various methods employed for making accurate indexing wheels for a definite number of divisions. One of these methods, suitable particularly for small numbers of divisions, employs a split wheel with a series of taper holes reamed through the two divisions. By shifting the two divisions from point to point (as explained in connection with sketch B, [Fig. 22]) and reaming and re-reaming the taper holes at each shifting, they may finally be brought very accurately into position. Another method that has been employed consists in clamping about the rim of the dividing wheel a number of precisely similar blocks, fitting close to each other and to the wheel itself. These blocks are then used for locating the wheel in each of its several positions in actual work. A third and simpler method (a modification of the one last described) consists in grinding a series of disks and clamping them around a rim of such diameter that the disks all touch each other and the rim simultaneously, as explained in connection with sketch C, [Fig. 22]. The wheel described in the following, which is illustrated in [Fig. 23], was made in this way.

Fig. 23. Precision Dividing Wheel

Disks A are clamped against an accurately ground face of the wheel B and are supposed to just touch each other all around, and to be each of them in contact with the ground cylindrical surface at x. They are held in proper position by bolts C and nuts D. The bolts fit loosely in the holes of the disks or bushings A so that the latter are free to be located as may be desired with reference to the bolts.

One or two improvements in the construction of this type of dividing wheel may be noted before proceeding to a description of the way in which it is made. For one thing, instead of having an indexing bolt enter the V-space between two adjoining disks, a smaller diameter y is ground on each of them, over which locking finger or pawl passes, holding the wheel firmly from movement in either direction. This construction has the advantage of a probable lessening of error by locating on each bushing instead of between two bushings; moreover, it gives a better holding surface and better holding angles than would be the case if this smaller diameter were not provided.

A second improvement lies in the method of clamping the bushings A in place. Instead of providing each bolt with a separate washer, a ring F is used. This ring fits closely on a seat turned to receive it on the dividing wheel B. When one bushing A has been clamped in place, the disk is locked from movement so that there is no possibility, in clamping the remaining bushings, of having their location disturbed in the slightest degree by the turning of the nuts in fastening them in place.

The bushings A, of which there were in this case 24, were all ground exactly to the required diameters on their locating and locking surfaces. The important things in this operation are, first, that the large or locating diameter of the bushing should be exactly to size; and second, that this surface should be in exact alignment with the diameter in which the locking is done; and, finally, that the face of the bushing should be squared with the cylindrical surfaces. A refined exactness for the diameter of the locking surfaces is not so important, as the form of locking device provided allows slight variations at this point without impairment of accuracy. This dimension was kept within very close limits, however. The truth of the two cylindrical surfaces and the face of the bushing was assured by finishing all these surfaces in one operation on the grinding machine.

The sizing of the outer diameter of the bushing, which was 1.158 inch, must be done so accurately that it was not thought wise to trust to the ordinary micrometer caliper. An indexing device was therefore made having a calipering lever with a long end, in the ratio of 10 to 1, which actuated the plunger of a dial test indicator of the well-known type made by the Waltham Watch Tool Co. The thousandth graduations on the dial of this indicator would then read in ten-thousandths, permitting readings to be taken to one-half or one-quarter of this amount. The final measurements with this device were all taken after dipping the bushings in water of a certain temperature, long enough to give assurance that this temperature was universal in all the parts measured. It will be understood, of course, in this connection, that getting the diameter of these bushings absolutely to 1.158 inch was not so important as getting them all exactly alike, whether slightly over or slightly under this dimension; hence, the precaution taken in measurement.

Wheel B was next ground down nearly to size, great care being taken that it should run exactly concentric with the axis. As soon as the diameter of the surface x was brought nearly to the required dimension as obtained by calculation, the disks were tried in place. The first one was put in position with its loose hole central on the bolt and clamped in place under ring F. The next bushing was then pressed up against it and against the surface x of the wheel and clamped in place. The third one was similarly clamped in contact with its neighboring bushing and the wheel, and so on, until the whole circle was completed. It was then found that the last disk would not fill the remaining space. This required the grinding off of some stock from surface x, and a repetition of the fitting of the bushings A until they exactly filled the space provided for them.

Fig. 24. Precision Dividing Wheel and its Indexing Mechanism

This operation required, of course, considerably more skill than a simple description of the job would indicate. One of the points that had to be carefully looked out for was the cleaning of all the surfaces in contact. A bit of dust or lint on one of the surfaces would throw the fitting entirely out. The temperature of the parts was another important consideration. As an evidence of the accuracy with which the work was done, it might be mentioned that it was found impossible to do this fitting on a bench on the southern or sunny side of the shop, the variations of temperature between morning and noon, and between bright sunshine and passing clouds, being such that the disks would not fit uniformly. The variation from these minute temperature changes resulted from the different coefficients of expansion of the iron wheel and the steel bushings. The obvious thing to do would be to build a room for this work kept at a constant temperature and preferably that of the body, so that the heat of the body would make no difference in the results. It was found sufficient in this case, however, to do the work on the northern side of the shop where the temperature was more nearly constant, not being affected by variations in sunshine.

The dividing wheel, the construction of which has just been described, was made by the Fellows Gear Shaper Co. It is used for indexing the Fellows gear cutters in the machine in which the teeth are ground. The indexing mechanism of this machine is shown in [Fig. 24]. It is operated by a handle or lever pinned to rock-shaft H, to which is keyed arm J. Pivoted to J is a pawl K engaging the teeth of ratchet L, which revolves loosely on shaft H. This ratchet L controls the movement of locking finger E. The parts are shown in their normal or locked position in the engraving.

As the handle on shaft H is pulled in the direction indicated by the arrow, arm J is raised, carrying the ratchet wheel around to the right. This allows flat spring M to drop off of the ratchet tooth, permitting helical spring O to raise latch E and thus leave the wheel free. The continued movement of the hand-lever and of rock-shaft H, by means of gear N, intermediate pinion P and gear Q, causes the indexing pawl R, which is pivoted to gear Q and acts on the head of one of the bolts C ([see Fig. 23]), to index the wheel one step. Just before reaching its new location the new tooth of ratchet wheel L coming up, bears down on the top of spring M, pressing latch E into place against the tension of coil spring O. By this means the wheel is locked in position.

When the operator pushes the handle on shaft H back again to its position of rest, the pawl R is retracted into position to act on the next bolt head for the next indexing. Star-wheel L remains stationary on this backward movement, being prevented from revolving by the notch on the top of the tooth into which spring M fits. Pawl K on its return engages with the next tooth of this wheel, ready for the next indexing operation.

A slight rotary adjustment of dividing wheel B, independent of this indexing mechanism, is required for the feeding of the machine. This is accomplished by the end movement of latch E, which is pivoted in slide S. This slide is pressed to the right by spring plunger T, and is adjusted positively in the other direction by feed-screw U, which is finely graduated to permit accurate adjustment. The accuracy in indexing obtained by the use of a wheel thus made was required to bring the finished cutters within the very narrow limits allowed for them in the final inspection.

CHAPTER III
LOCATING WORK FOR BORING
ON MILLING MACHINE

It is often desirable to perform boring operations on the milling machine, particularly in connection with jig work. Large jigs, which because of their size or shape could not be conveniently handled in the lathe, and also a variety of smaller work, can often be bored to advantage on the milling machine. When such a machine is in good condition, the necessary adjustments of the work in both vertical and horizontal planes, can be made with considerable accuracy by the direct use of the graduated feed-screw dials. It is good practice, however, when making adjustments in this way, to check the accuracy of the setting by measuring the center distances between the holes directly. For the purpose of obtaining fine adjustments when boring on the milling machine, the Brown & Sharpe Mfg. Co. makes special scales and verniers that are attached to milling machines, so that the table may be set by direct measurement. By attaching a scale and vernier to the table and saddle, respectively, and a second scale to the column with a vernier on the knee, both longitudinal and vertical measurements can be made quickly and accurately, and the chance of error resulting from inaccuracy of the screw, or from lost motion between the screw and nut, is eliminated.

Checking Location of Holes by
Micrometer-and-plug Method

One method of checking the accuracy of the location of holes bored in the milling machine, is to insert closely fitting ground plugs into the bored holes and then determine the center-to-center distance by taking a direct measurement across the plugs with a micrometer or vernier caliper. For example, if holes were to be bored in a jig-plate, as shown in [Fig. 1], assuming that hole A were finished first, the platen would then be moved two inches, as shown by the feed dial; hole B would then be bored slightly under size. Plugs should then be accurately fitted to these holes, having projecting ends, preferably of the same size. By measuring from one of these plugs to the other with a vernier or micrometer caliper, the center distance between them can be accurately determined, allowance being made, of course, for the radii of each plug. If this distance is incorrect, the work can be adjusted before finishing B to size, by using the feed-screw dial. After hole B is finished, the knee could be dropped 1.5 inch, as shown by the vertical feed dial, and hole C bored slightly under size; then by the use of plugs, as before, the location of this hole could be tested by measuring center distances between C-B and C-A.

An example of work requiring the micrometer-and-plug test, is shown set up in the milling machine in [Fig. 25]. The large circular plate shown has a central hole and it was necessary to bore the outer holes in correct relation with the center hole within a limit of 0.0005 inch. The center hole was first bored and reamed to size; then an accurately fitting plug was inserted and the distances to all the other holes were checked by measuring from this plug. This method of testing with the plugs is intended to prevent errors which might occur because of wear in the feed-screws or nuts, that would cause the graduated dials to give an incorrect reading. On some jig work, sufficient accuracy could be obtained by using the feed-screw dials alone, that is, without testing with the plugs, in which case the accuracy would naturally depend largely on the condition of the machine.

Fig. 25. Example of Precision Boring on Milling Machine

A method that is a modification of the one in which plugs are used to test the center distance is as follows: All the holes are first drilled with suitable allowance for boring, the location being obtained directly by the feed-screw dials. A special boring-tool, the end of which is ground true with the shank, is then inserted in the spindle and the first hole, as at A in [Fig. 1] is finished, after which the platen is adjusted for hole B by using the dial as before. A close-fitting plug is then inserted in hole A and the accuracy of the setting is obtained by measuring the distance between this plug and the end of the boring-tool, which is a combination tool and test plug. In a similar manner, the tool is moved from one position to another, and, as all the holes have been previously drilled, all are bored without removing the tool from the spindle.

Another modification of the micrometer-and-plug method is illustrated in Figs. [26] and [27]. It is assumed that the plate to be bored is finished on the edges, and that it is fastened to an angle-plate, which is secured to the table of the milling machine and set square with the spindle. A piece of cold-rolled steel or brass is first fastened in the chuck (which is mounted on the spindle) and turned off to any diameter. This diameter should preferably be an even number of thousandths, to make the calculations which are to follow easier. The turning can be done either by holding the tool in the milling machine vise, or by securing it to the table with clamps. In either case, the tool should be located near the end of the table, so as to be out of the way when not in use.

Fig. 26. Obtaining Vertical Adjustment by Means of
Depth Gage and turned Plug in Chuck

After the piece in the chuck is trued, the table and knee are adjusted until the center of the spindle is in alignment with the center of the first hole to be machined. This setting of the jig-plate is effected by measuring with a micrometer depth gage from the top and sides of the work, to the turned plug, as illustrated in [Fig. 26]. When taking these measurements, the radius of the plug in the chuck is, of course, deducted. When the plate is set the plug is removed from the chuck and the first hole drilled and bored or reamed to its proper size. We shall assume that the holes are to be located as shown by the detail view, [Fig. 26], and that hole A is the first one bored. The plug is then again inserted in the chuck and trued with the tool, after which it is set opposite the place where the second hole B is to be bored; this is done by inserting an accurately fitting plug in hole A and measuring from this plug to the turned piece in the chuck, with an outside micrometer as indicated in [Fig. 27]. Allowance is, of course, again made for the radii of the two plugs. The horizontal measurement can be taken from the side of the work with a depth gage as before. The plug is then removed and the hole drilled and bored to the proper size. The plug is again inserted in the chuck and turned true; the table is then moved vertically to a position midway between A and B, and then horizontally to the proper position for hole C, as indicated by the depth gage from the side of the work. The location can be verified by measuring the center distances x with the micrometer. In a similar manner holes D, E, F and G are accurately located.

Fig. 27. Adjusting for Center-to-center Distance
by use of Plugs and Micrometer

If the proper allowances are made for the variation in the size of the plug, which, of course, is made smaller each time it is trued, and if no mistakes are made in the calculations, this method is very accurate. Care should be taken to have the gibs on all sides fairly tight at the beginning, and these should not be tightened after each consecutive alignment, as this generally throws the work out a few thousandths. If the reductions in the size of the plug, each time it is turned, are confusing, new plugs can be used each time a test is made, or the end of the original plug can be cut off so that it can be turned to the same diameter for every test. If the center distances x are not given, it is, of course, far more convenient to make all the geometric calculations before starting to work.