Side Play of Calipers in Boring Holes Larger than a Piece of Known Diameter The following is an approximate rule for obtaining the variation in the size of a hole corresponding to a given amount of side play in the calipers. The rule has the merit of extreme simplicity and can be applied equally well to all diameters except the very smallest. In most cases the calculation is so simple that it can be done mentally without having recourse to pencil or paper. The Calculation Let A in Fig. 11 = side play of calipers or end measuring rod in sixteenths of an inch. B = dimensions to which calipers are set, or length of measuring rod in inches. C = difference between diameter of hole and length of B in thousandths of an inch. A2 2B' Example: A standard end measuring rod, 5 inches long, has inch of side play in a hole. case A C = = 6 and B 6 X 6 36 II II = What is the size of the hole? In this 5. Apply the above formula: 3.27 thousandths of an inch, or 0.00327 inch. The diameter of the hole, therefore, is 5+ 0.00327 or 5.50327. The method will be found to be correct within a limit of about 0.0002 inch if the amount of side play is not more than one eighth of the diameter of the hole for holes up to 6 inches diameter; within 0.0005 inch for holes from 6 inches up to 12 inches; and within 0.001 for holes from 12 inches up to 24 inches. Allowing for Running and Driving Fits This rule has been found to be useful for boring holes of large diameters in which allowances have to be made for running or driving fits, as only a single measuring rod for each nominal size is required. The rods should be of standard length, or a known amount less than standard, the allowances being obtained by varying the amount of side play when boring. The rule is also capable of determining limits, as the maximum and minimum amount of side play allowable can be specified. The measuring rods should be tapered at each end and the points slightly rounded. For accurate work, the body of the rod should be encased in some non-conducting material to nullify the effect of the heat of the hand. In comparing this method with that described on page 233, it should be remembered that the conditions are reversed - that is to say, the first method is for setting calipers to a given dimension smaller than a hole of known diameter, whereas the method now described is for boring a hole a given amount larger than a gage of known length. In measuring the side play it is sufficient to take it to the nearest sixteenth of an inch, and if anything like accuracy is required it should be measured not guessed at. DIMENSIONS OF KEYS AND KEY-SEATS THE following rules and table on page 237, as prepared by Baker Bros., Toledo, Ohio, give dimension of keys and key-seats. The width of the key should equal one fourth the diameter of the shaft. The thickness of the key should equal one sixth the diameter of the shaft. The depth in the hub for a straight key-seat should be one half the thickness of the key. The depth in the hub at the large end, for a taper key-seat, should be three fifths the thickness of the key. The taper for all key-seats should be inch in 1 foot of length. The depth to be cut in the hub for taper key-seats, at the large end, is greater than those cut straight, for the reason that unless this is done the depth in the hub at the small end will not be sufficient, especially in long key-seats. The depths of key-seats in the table are given in thousandths of an inch and measured from the edge of the key-seat, and not from the center. In this manner the exact depth of key-seat can be measured at any time after it is cut. For extra long key-seats the depth cut in the hub may be slightly increased, but for the average work the table will be found correct. DIMENSIONS OF STRAIGHT KEYS ANOTHER System of keys used by a good many manufacturers is given in the table following, the sizes of shafts ranging by sixteenths from inch to 4 inches and by eighths from 4 to 6 inches. The keys are square until the 13 inch shaft is reached, when the thickness of the key becomes less than the width. With the 4 size the thickness of the key becomes inch less than the width and this difference is constant up to the 5 shaft when the width exceeds the thickness by inch, this difference in the two dimensions continuing throughout the remainder of the table. SQUARE FEATHER KEYS AND STRAIGHT KEY SIZES THE tables on page 239 give the sizes of square feather keys and regular straight keys in accordance with the practice of Jones & Laughlin, Pittsburg. For taper keys, this concern and many others use a -inch per foot taper. |