Drawing temper of T slot cutters.

When drawing the temper of a tool of this description it is necessary, in order that the necked portion be as strong as possible (especially if it is slender), to draw it to a purple or even a blue color, while the cutting teeth need drawing to a straw color.

It is surprising to one not thoroughly posted in the effects of different degrees of heat on steel to find how hard a cutter of this kind may be left if it was properly heated when hardened. This is best seen by comparing with one that was heated a trifle too hot, yet not to a degree that is generally considered harmful to the steel. In the case of the cutter properly heated—that is, to the refining heat-it may be left when tempering at a faint straw color, while if given a trifle more heat, it is necessary to draw it to a full straw, a difference of 30° of heat, and a vast difference in the amount of work it will do between grindings. In order to successfully draw the temper, the necked portion may be placed in the flame of a gas jet, a Bunsen burner, the flame of a spirit lamp; or, if none of these are available, and it is necessary to use a blacksmith's forge for all work of this description, a piece of sheet iron having a hole in it may be placed over the fire. A jet of flame will come through the hole, which may be made to strike the necked portion. In this way the desired temper may be obtained.

Hollow Mills.

When articles having a hole running part way through them, as, for instance, the hollow mill shown in Fig. 90, are to be hardened, it is advisable to dip

Hardening nollow mills.

them in the bath, with the opening uppermost, as represented in Fig. 91. If the mill were dipped with the opening down, it would be almost impossible to get water to enter the hole for any considerable distance,

Figure 90. A hollow mill.

As

as the steam generated would blow the water out. a consequence, the walls of the hole would not harden, and the steam would in all probability cause the steel to crack.

Then again, best results will follow if the frail end is not chilled until after the heavier, solid portions have contracted somewhat. If the lighter portions are chilled and contracted before the heavier ones, the tendency is for the heavier parts, which are stronger than the lighter, to pull them into conformity with themselves, and as the steel is hard and rigid, it must crack. While this principle is explained elsewhere in this work, it seems wise to show the adaptability of this peculiarity of steel to pieces of this description.

When making articles having holes, as shown, if the piece is to be hardened, the liability of cracking will be lessened if the stock at the end of hole is left, If, however, the piece is made

as shown in Fig. 92.

Tepid water for hardening hollow mills.

with a sharp corner, as shown in Fig. 93, it is advisable to fill in this sharp corner with fire clay, or graphite, in order that there may be no pronounced difference in the contraction of the two portions.

When hard

ening pieces of this character, it is, generally speaking, good practice to use a bath of tepid water or brine.

When it is considered desirable to harden a piece a certain distance, and no farther, and the facilities for heating do not allow of heating exactly the right distance, it is necessary to dip in the bath with the teeth down.

In order to over

come the tendency of the steam to blow the water from the hole, a small vent hoie is drilled through the wall of the piece, as shown in Fig. 94. If this hole is large enough to allow the steam to escape, good results will follow if a bath is used having a jet of water coming up from the bottom, as, by this means, water is

Various types of hollow mills.

forced into the hole. However, the operator should bear in mind that it is never good practice to have

the hardening stop at a shoulder, either inside or outside of a piece of steel. Where possible, stop the

Figure 93. Hollow mill with

sharp corners.

hardening somewhat short of the shoulder, but if this does not meet the requirements, harden a trifle

Figure 94.

Hollow mill with hole

to allow escape

of steam.

beyond the shoulder. This may seem like a little thing to bother about, but it generally means the difference between a good job and a poor one, and it's

Hardening thin articles.

one of the little points that count in making a successful hardener.

Thin Articles.

Thin articles, as screw slotting saws, metal slitting saws, etc., may be hardened between two plates whose faces are covered or rubbed with oil. If reasonable care is exercised in the operation, they will be very straight.

It is essential, in order to get good results, to heat the pieces on a flat plate. They should be heated no hotter than is necessary to accomplish the desired result. When at the proper heat, the saw may be taken by a pair of tongs, of the form shown in Fig. 95, and placed on a plate whose face is covered with lard, sperm or raw linseed oil. The advantage derived from using tongs of this description is, the saw is held by the portion near the hole, rather than by the teeth, as would be the case if a pair of the ordinary style were used. In that case, the teeth grasped by the tongs would not be of the same temperature as the balance of the saw; and, as a consequence, the hardening would not be uniform. Another plate, whose face has been treated in a similar manner, may be placed on top of the saw and held there until the saw is cold. It is necessary to

place the top plate in position as quickly as possible, after the saw has been placed on the lower plate.

If the saw should become chilled before the upper