Referring to Fig. 62, the die _A_ has a surface _B_ which fits the face of the teeth of the gear _C_. This surface is perforated by a large number of holes which permit the quenching oil to circulate freely. The die _A_ is set in the upper end of the plunger _A_ of the tempering machine, shown in Fig. 63, a few inches above the surface of the quenching oil in the tank _N_. Inside the die _A_ are the centering jaws _D_, Fig. 62, which are an easy fit for the bore of the gear _C_. The inner surface of the centering jaws is in the shape of a female cone. The upper die is shown at _E_. In the center (separate from it, but a snug sliding fit in it) is the expander _G_, which, during quenching, enters the taper in the centering jaws _D_, expanding them against the bore of the gear _C_. The faces _F_ of the upper die _E_ fit two angles at the back of the gear and are grooved for the passage of the quenching oil. The upper die _E_ is secured to the die carrier _B_, shown in Fig. 9, and inside the die is the expander _G_, which is backed up by compression springs. [Illustration: FIG. 62.--Dies for Gleason gear-hardening machine.] HARDENING OPERATION.--Hardening a gear is accomplished as follows: The gear is taken from the furnace by the furnaceman and placed in the lower die, surrounding the centering jaws, as shown at _H_ in Fig. 62 and _C_ in Fig. 63. Air is then turned into the cylinder _D_, and the piston rod _E_, the die carrier _B_, the top die _F_ and the expander _G_ descend. The pilot _H_ enters a hole in the center of the lower die, and the expander _G_ enters the centering jaws _I_, causing them to expand and center the gear _C_ in the lower die. On further advance of the piston rod _E_, the expander _G_ is forced upward against the pressure of the springs _J_ and the upper die _F_ comes in contact with the upper surface of the gear. Further downward movement of the dies, which now clamp the work securely, overcomes the resistance of the pressure weight _K_ (which normally keeps up the plunger _A_), and the gear is submerged in the oil. The quenching oil is circulated through a cooling system outside the building and enters the tempering machine through the inlet pipe _L_. When the machine is in the position shown, the oil passes out through the ports _M_ in the lower plunger to the outer reservoir _N_, passing to the cooling system by way of the overflow _O_. When the lower plunger _A_ is forced downward, the ports _M_ are automatically closed and the cool quenching oil from the inlet pipe _L_, having no other means of escape, passes through the holes in the lower die and the grooves in the upper, circulating in contact with the surfaces of the gear and passes to the overflow. When the air pressure is released, the counterweights return the parts to the positions shown in Fig. 63, and the operator removes the gear. [Illustration: FIG. 63.--Gleason tempering machine.]