Precision locating and dividing methods

by Anonymous

Adjustable Jig for Accurate Hole Spacing

An adjustable jig for accurately spacing small holes is shown in [Fig. 21].

Fig. 20. Special Disk-jig for Precision Drilling

This form is especially adapted for locating a number of equally spaced holes between two previously drilled or bored holes, and the accuracy of the method lies in the fact that a slight error in the original spacing of the guide bushing is multiplied, and, therefore, easily detected. There are two of these guide bushings A and B which are carried by independent slides. These slides can be shifted along a dovetail groove after loosening the screws of clamp-gib C. To illustrate the method of using this jig, suppose five equally-spaced holes are to be located between two holes that are 12 inches apart. As the center-to-center distance between adjacent holes is 2 inches, slides A and B would be set so that the dimension x equals 2 inches plus the radii of the bushings. A straightedge is then clamped to the work in such position that a close-fitting plug can be inserted through the end holes which were previously drilled or bored. Then with a plug inserted through, say, bushing B and one of the end holes, the first hole is drilled and reamed through bushing A; the jig is then shifted to the left until the plug in B enters the hole just made. The second hole is then drilled and reamed through bushing A and this drilling and shifting of the jig is continued until the last hole is finished. The distance between the last hole and the original end hole at the left is next tested by attempting to insert close-fitting plugs through both bushings. Evidently, if there were any inaccuracy in the spacing of the bushings, this would be multiplied as many times as the jig was shifted, the error being accumulative. To illustrate how the error accumulates, suppose that the bushings were 0.001 inch too far apart; then the distance to the first hole would be 2.001 inch, to the second hole, 4.002 inch, and finally the distance from the first to the sixth hole would be 10.005 inches; consequently, the distance between the sixth and seventh holes would equal 12-10.005 = 1.995 inch, or 0.005 inch less than the required spacing, assuming, for the sake of illustration, that the first and last holes were exactly 12 inches apart. In case of an error of 0.005 inch, the bushings would be set closer together an amount equal to one-fifth of this error, as near as could be determined with a micrometer, and all of the holes would then be re-reamed.

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].”

Spacing by Correcting the
Accumulated Error

Another indexing method of spacing holes equi-distant, is illustrated by the diagram at D, [Fig. 22]. An accurately fitting plug is inserted in the central hole of the plate in which holes are required. Two arms h are closely fitted to this plug but are free to rotate and are provided with a fine-pitch screw and nut at the outer ends for adjusting the distance between the arms. Each arm contains an accurately made, hardened steel bushing k located at the same radial distance from the center of the plate. These bushings are used as a guide for the drill and reamer when machining the holes in the plate.

To determine the center-to-center distance between the bushings, divide 360 by twice the number of holes required; find the sine corresponding to the angle thus obtained, and multiply it by the diameter of the circle upon which the holes are located. For example, if there were to be eleven holes on a circle 8 inches in diameter, the distance between the centers of the bushings would equal

360	= 16.36 degrees.
2 × 11

The sine of 16.36 degrees is 0.2815, and 0.2815 × 8 = 2.252 inches. The arms are adjusted to locate the centers of the bushings this distance apart, by placing closely fitting plugs in the bushings and measuring from one plug to another with a micrometer or vernier caliper. Of course, when taking this measurement, allowance is made for the diameter of the plugs.

After the arms are set, a hole is drilled and reamed; an accurately fitting plug is then inserted through the bushing and hole to secure the arms when drilling and reaming the adjacent hole. The radial arms are then indexed one hole so that the plug can be inserted through one of the arms and the last hole reamed. The third hole is then drilled and reamed, and this operation is repeated for all of the holes. Evidently, if the center-to-center distance between the bushings is not exactly right, the error will be indicated by the position of the arms relative to the last hole and the first one reamed; moreover, this error will be multiplied as many times as there are holes. For instance, if the arms were too far apart, the difference between the center-to-center distance of the last pair of holes and the center-to-center distance of the bushings in the arms, would equal, in this particular case, eight times the error, and the arms should be re-adjusted accordingly. Larger bushings would then be inserted in the arms and the holes re-reamed, this operation being repeated until the holes were all equi-distant.

As will be seen, the value of this method lies in the fact that it shows the accumulated error. Thus, if the arms were 0.0005 inch too far apart, the difference between the first and last hole would equal 8 × 0.0005 = 0.004 inch. This same principle of dividing can be applied in various ways. For instance, the radial arms if slightly modified, could be used for drilling equally-spaced holes in the periphery or disk of a plate, or, if a suitable marking device were attached, a device of this kind could be used for accurately dividing circular parts.