Fig. 21. Adjustable Jig for Accurate Hole Spacing
Methods of Accurately Dividing
a Circle
Sometimes it is necessary to machine a number of holes in a plate so that all the holes are on a circle or equi-distant from a central point, and also the same distance apart, within very small limits. A simple method of spacing holes equally is illustrated at A, [Fig. 22]. A number of buttons equal to the number of holes required are ground and lapped to exactly the same diameter, preferably by mounting them all on an arbor and finishing them at the same time. The ends should also be made square with the cylindrical surface of the button. When these buttons are finished, the diameter is carefully measured and this dimension is subtracted from the diameter of the circle on which the holes are to be located, in order to obtain the diameter d (see illustration). A narrow shoulder is then turned on the plate to be bored, the diameter being made exactly equal to dimension d. By placing the buttons in contact with this shoulder, they are accurately located radially and can then be set equi-distant from each other by the use of a micrometer. In this particular case, it would be advisable to begin by setting the four buttons which are 90 degrees apart and then the remaining four. The buttons are next used for setting the work preparatory to boring. (See “[Button Method of Accurately Locating Work].”)
Fig. 22. Four Methods of Accurately Dividing a Circle
Correcting Spacing Errors
by Split Ring Method
Another method of securing equal spacing for holes in indexing wheels, etc., is illustrated at B, [Fig. 22]. This method, however, is not to be recommended if the diameter of the circle on which the holes are to be located, must be very accurate. The disk or ring in which the holes are required, is formed of two sections e and f, instead of being one solid piece. The centers for the holes are first laid out as accurately as possible on ring e. Parts e and f are then clamped together and the holes are drilled through these two sections. Obviously, when the holes are laid out and drilled in this way, there will be some error in the spacing, and, consequently, all of the holes would not match, except when plate e is in the position it occupied when being drilled. Whatever errors may exist in the spacing can be eliminated, however, by successively shifting plate e to different positions and re-reaming the holes for each position. A taper reamer is used and two pins should be provided having the same taper as the reamer. Ring e is first located so that a hole is aligned quite accurately with one in the lower plate. The ring is then clamped and the hole is partly reamed, the reamer being inserted far enough to finish the hole in plate e and also cut clear around in the upper part of plate f. One of the taper pins is then driven into this hole and then a hole on the opposite side is partly reamed, after which the other pin is inserted. The remaining holes are now reamed in the same way, and the reamer should be fed in to the same depth in each case. If a pair of holes is considerably out of alignment, it may be necessary to run the reamer in to a greater depth than was required for the first pair reamed, and in such a case all the holes should be re-reamed to secure a uniform size.
The next step in this operation is to remove the taper pins and clamps or turn index plate e one hole and again clamp it in position. The reaming process just described is then repeated; the holes on opposite sides of the plate are re-reamed somewhat deeper, the taper pins are inserted, and then all of the remaining holes are re-reamed to secure perfect alignment for the new position of the plate. By repeating this process of shifting plate e and re-reaming the holes, whatever error that may have existed originally in the spacing of the holes, will practically be eliminated. It would be very difficult, however, to have these holes located with any great degree of accuracy, on a circle of given diameter.
Circular Spacing by Contact
of Uniform Disks
When an accurate indexing or dividing wheel is required on a machine, the method of securing accurate divisions of the circle illustrated at C, [Fig. 22], is sometimes employed. There is a series of circular disks or bushings equal in number to the divisions required, and these disks are all in contact with each other and with a circular boss or shoulder on the plate to which they are attached. The space between adjacent disks is used to accurately locate the dividing wheel, engagement being made with a suitable latch or indexing device. When making a dividing wheel of this kind, all of the disks are ground and lapped to the same diameter and then the diameter of the central boss or plate is gradually reduced until all of the disks are in contact with each other and with the boss. For an example of the practical application of this method see “[Originating a Precision Dividing Wheel].”