The most commonly used form of flute is that cut with a convex milling cutter for milling half-circles, Fig. 101. The advantages claimed for this form are (1) that the flutes are deep enough to provide for the chips, and yet leave the lands as strong as need be; and (2) that the form of the. back of the land is such that the chips cannot be wedged between the land and the work when the motion of the tap is reversed. The form of groove made with this cutter is shown in Fig. 102. In order to support the tap when starting to cut, and prevent cutting the hole large at the outer end, hand taps have their lands left wider. A, Fig. 102, than the lands on machine taps. If the forms of cutter illustrated in Fig. 101 or Fig. 103 are used, the width of lands as shown at A may be one-fourth the diameter of the tap. Fig. 104 shows a special form of cutter. It does not make so deep a groove, Fig. 105, in proportion to the width, as a tap and reamer cutter.
After cutting the grooves, the lands should be backed off to give the tap cutting edges; this is usually done with a file. Commence at the heel of the land A, Fig. 106; file the top of the land and gradually approach the cutting edge, making sure that no stock is removed at that portion - simply bring it to a sharp edge. Enough should be filed off the heel A to make it cut readily, yet not enough to cause it to chatter. The size and number of threads per inch should be stamped on the shank of the tap. If it has a thread differing from the one in general use in the shop, that should also be stamped on the shank, as "U. S. S." if it is a United States Standard thread.
Fig. 102. Suction of Milling Cutter.
Below are given the numbers of the cutters for different diameters of taps when the form shown in Fig. 104 is used:
No. 1 cutter cuts taps up.......... to 1-inch diameter.
No. 2 cutter cuts tape from 5/12-inch to 1/4-inch diameter No. 3 cutter cuts taps from 6/32 inch to 1-inch diameter No. 4 cutter cuts taps from 1/16-inch to 5/8-inch diameter No. 5 cutter cuts taps from 11/14-inch to 1/4-inch diameter No. 6 cutter cuts taps from 15/16-inch to 1 1/4-inch diameter No. 7 cutter cuts taps from 1 5/16-inch to 1 5/8-inch diameter No. 8 cutter cuts taps from 1 11/16-inch. to 2 -inch diameter.
Fig. 104. Special Milling Cutler for Grooving Tape Courtesy of Beckor Milling Machine Company, Hyde Park, Massachusetts.
Fig. 105. Groove Mads by Becker Special Cutler.
If but a few taps are to be hardened at a time, it is customary to heat them in a gas jet or an open fire of charcoal or hard coal. It is advisable, however, to heat them gradually in a tube. They should be plunged one at a time into the bath a little above the threads, and worked up and down and around in the bath to prevent soft spots. Excellent results follow the use of the bath shown in Fig. 79. Taps of 1-inch diameter and smaller should be left in the bath until cold; larger ones may be removed from the bath as soon as the singing noise ceases, immediately plunged into oil, and left until cold. For taps of less than 1/4-inch diameter, the citric acid bath will be found satisfactory; for larger taps, strong brine is advisable.
Fig. 106. Tap Showing Method or Filing Away Tooth at the Point.
Caurtesy of Wiley, and Russell Manufacturng Company, Greenfield, Massachusetts.
To have the tap retain as Dearly as possible its size and correctness of pitch, use the pack-hardening process. Run taps 1/8 inch in diameter and smaller for 1/2 hour after they are red hot; taps 1/5 to 1/4 inch in diameter, 1 hour; taps 1/4 to 1/2 inch in diameter, 1 1/4 hours; taps of a diameter larger than 1 inch, 2 hours. Harden in a bath of raw linseed oil.
It is advisable to grind the flutes of the taps with an emery wheel of the proper shape in order to brighten the surface so that the color will be readily seen when drawing the temper. Grinding also sharpens the cutting edges, and breaks the burrs that have been thrown between the teeth when cutting the flutes. The temper should be drawn to a full straw color. Much more satisfactory results may be obtained by heating the taps in a kettle of oil, drawing the temper to a point from 460° F. to 500° F., according to the size of the tap and the nature of the stock to be cut.