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.
