These are used when it is necessary to change the size of counterbore and pilot frequently. A shank or bar is made to accommodate different sizes of cutters, and sleeves serve as pilots. In Fig. 161, A is the cutter, and B the pilot which is tapped in the end to receive a screw to hold the sleeves, and C is the shank which is held in a chuck or collet when the counterbore is in use.
After taking a roughing chip off the bar, the end B is run in the steady rest and the hole for the screw F is drilled and tapped. The outside end is countersunk to a 60-degree angle to run on a center. When machining the holder, the portions B, C, and D should be left about .010 inch larger than finish size, to allow for grinding; if more convenient, however, they may be left a few thousandths of an inch above size, and filed to finish dimensions.
The body, or cutter, A, should have a hole 1/14 inch smaller than finish size drilled through it; the outside surface should be turned off, and the piece annealed. If a grinder having an internal grinding attachment is at hand, the hole in the cutter should be left .005 inch small for grinding. If the worker does not have the tool, the hole may be reamed to finish size. The outside diameter should be left about .010 inch large; the ends should be faced to length, and the teeth cut. If four teeth are to be cut, the work may be done with the side milling cutter, shown in Fig. 156. The counterbore should be held in a chuck on the spiral head spindle, which should be tipped to produce a strong tooth, as shown in Fig. 161. Before hardening, the hole should be drilled and tapped for the screw H, which holds the counterbore to the bar.
Fig. 181. Combination Counterbore.
To harden, the counterbore should be given an even, low, red heat, and plunged into water or brine in such a manner that the bath will come in contact with the teeth. If the teeth are stubbed and strong, the temper need not be drawn more than to a light straw color.
The screw H should be made of tool steel and have a projection 1/8 inch long on one end, turned to the bottom of the thread. This is to enter a hole drilled in the bar or holder and keep the counterbore from turning. The end of the screw should be about .005 inch smaller than the hole. The screw should be hardened and drawn to a blue color. The sleeve intended to go on the pilot E should be made of tool steel, hardened, and ground to size inside and out. The screw F may be made of machine steel, casehardened to the proper depth, by heating it to a red and sprinkling with powdered cyanide of potassium, then reheating and plunging it into water.