For this operation a wet-grinder is to be recommended. The tension of hardened high-speed steel is very great, and so is the heat generated by grinding at a dry-wheel-especially if much pressure is used The result of this is very local and uneven heating and expansion of the steel. This is a frequent though often unsuspected cause of fine cracks. If, therefore, a dry grinding-wheel is employed, extremely light pressure should be used, and the point of contact changed constantly. A wet grindstone helps to keep down the heat, but even with a wet grinding-wheel hardened high-speed steel should be ground with as little pressure as possible.

Speaking of their Stag Air-hardening Steel, Messrs. Edgar Allen say, " In the absence of definite details, such as shape and size of tool, make of machine used, experience of worker, etc., it is impossible to lay down hard and fast rules for running this steel. The table of speeds and feeds given below is for rough general guidance only, and is not to be taken as definite recommendations."

Speed a Material. Minute. Feed. Cut, Cast-iron ram 33ft. 1/12in. 5/8in Rough - hammered steel

On mild-steel connecting rods........108 ft. 1/16in.1/4 1/4in

Steel shafting 160 1/12 ,, 1/4 ,, On mild-steel bars 33/4in.

diam. .... 306 ,, 1/60 „ Special Alloy Steels.-In buying tool-steel the kind of work to be done largely decides the quality that shall be chosen. It mav be laid down as an axiom that for every kind of tool and work there is an economical steel.

What exactly is meant by an " economical " steel ? An economical steel is the cheapest that will do the work efficiently. For example, high-speed steel with a high tungsten-content is the economical steel for turning worn locomotive-tyres hardened on the tread by skidding and the friction of brakes. It is the economical steel in this instance because no other tool steel will either keep its edge or do the work so well. You could buy carbon tool-steel or special alloy tool-steel for much less money, but if the only result was tools that broke down and work that made no progress, neither of these cheaper steels could be called economical.

On the other hand, high-speed steel would not be an economical steel for a carpenter's drill. The reason is that it is expensive, and does the work little if any better than an ordinary carbon tool-steel. The amount of heat generated in drilling wood is not sufficient to destroy the temper of the carbon-steel drill, thus spoiling it, nor is the character of the work so difficult as to throw a heavier strain than it can bear on a tool made from carbon tool-steel. There is, therefore, no reason why the costly high-speed steel should be used for this purpose, and it is. indeed, the least economical.

When selecting a suitable and, to the best of their belief, economical quality of steel for their work, many users imagine that the choice lies between the high-speed and the carbon tool steels, the one class varying chiefly in tungsten-content, and the other in carbon-content. In reality, neither highspeed nor carbon-tool steels may be economical in their particular instance, and only a special alloy tool-steel could really be so. Being unaware, however, of the existence of this intermediate class of steels, these users sometimes order consistently a steel that is not economical. A special alloy tool-steel contains, in varying percentages and either singly or in combination, some specific element, such as tungsten, vanadium, chromium, or molybdenum. The introduction of these elements raises certain properties of carbon steel to a strength not previously possessed. For instance, the addition of tungsten and chromium increases the hardness, toughness and red-hardness of the steel, while vanadium acts as a general stimulant to heighten the effect of these alloying elements. In speaking of the treatment of a special alloy-steel a definite make and brand must be taken, and the three examples taken are manufactured bv Messrs. Edgar Allen & Co. Ltd

Imperial Steel (Label : Black On Gold)

This is a water-hardening alloy tool-steel made by the crucible process. It is less costly than a high-speed steel, but its manufacture is as carefully supervised. It contains tungsten, though not in large enough proportions to rank it as a high-speed steel.

Its chief properties are permanent hardness, fineness of grain, and ability to keep a cutting-edge. The hardness is given to it by the tungsten, and care in selection of raw materials and in manufacture produces the fine grain.

Fine grain is important in tool-steel. The edge of any cutting-tool, magnified, appears irregular. The extent of this irregularity depends upon the fineness or coarseness of the crystals or " grain." The finer the texture of the steel, the smoother and less irregular will he the edge. The smoother the edge the longer it will last, because there are fewer irregularities to turn it up and blunt it. The grain of this tool-steel is delicate and silky in appearance and in its hardened condition this steel will, in consequence, take and hold an extremely fine cutting edge. It is suitable for the best finishing work, where the class of job is not too severe for machining chilled iron, and is specially useful for brass-working.

Treatment

Imperial tool-steel is water hardening and is supplied in the annealed condition, but for those who wish to take special precautions and prefer to oil-harden a steel of this high quality, an oil-hardening Imperial tool-steel can be supplied on request.

Correct working-instructions are as follows :

Heat the steel (away from the blast) to a bright red, then forge and set aside to cool. Don't hammer it when it has cooled below a dull red. Re-heat (only the cutting-edge) to a dull red heat (760° C. to 790° C.) (1385-1435° F.) and cool down in lukewarm water. It is not necessary to temper lathe- or planing-tools, but drills should be tempered to a dark straw colour (230° C).

Precautions against too rapid or irregular heating should be taken, as this is certain to cause fracture.

For Re-Annealing When Desired

Anneal at 730° C. Heat up to and thoroughly soak at this temperature and allow to cool.

Red Label Steel (Label : White On Red)

The chief properties of Red Label tool-steel are the density of its structure and its keen hardening. Further, owing to the care with which it is made, and the uniformly good quality of the raw materials put into it it has a fine grain.

The meaning and importance of fine grain in steel are well known, and previously mentioned. The keen hardening ensures durability in the tools, and Red Label tool-steel will take and hold a razor-sharp edge.

The tungsten in this steel gives it a higher tensile strength than ordinary carbon tool-steel, enabling certain tools made from it to bear up under heavy stresses. In addition, it enables the tools to stand a much higher friction-heat, a point of great importance in some tools and classes of work.

When To Use Red Label Tool-Steel

The properties outlined above make this steel eminently suitable for twist-drills, taps, milling-cutters, key-seating cutters, end-mills for brass-work, reamers, and other tools of a similar function. In many instances it might be more profitable to buy a costlier steel. In some cases a lower-priced steel might do the work equally well.

Treatment Of Red Label Tool-Steel

Heat carefully and thoroughly to darkish cherry-red heat, say 760° to 780° Cent. (1400°-1436° Fahr.) and harden in clear water at about 15° Cent. (60° Fahr.). It should be remembered that small sections of steel will harden at a lower temperature than more bulky pieces. Small sections may be hardened at an even lower heat than stated, and the lowest heat at which a steel will harden is always the best. Afterwards ternper to a straw colour for such things as taps, etc.

For Reannealing When Desired

Anneal at 730° C. Heat up to and thoroughly soak at this temperature and allow to cool.

The experience of the user, with the above general indications as to temperature and treatment, will ensure uniform and satisfactory results, but as pyrometers and furnaces vary considerably, a trial should be made with a small sample.

K.9. Oil-hardening Steel (Label : white on black).-The effective performance of a tool often depends upon its shape remaining unvaried by hardening. To meet this demand for a steel that would not shrink, warp, crack, or become distorted by treatment, and that would be comparatively inexpensive, K.9 steel was perfected.

A water - hardened tool often varies slightly in shape through treatment, though the amount of variation is not constant, and is not always important. K.9 steel is designed to harden in oil, and in consequence the risk of contraction or expansion is minimised.

Its Properties

K.9 steel possesses a very fine grain. The treatment given to it from ingot to bar is responsible for this, and also for its toughness ami resistance to wear, two of the properties that make it economical to use. It has perfect uniformity, and is very easy to heat-treat. Its handling presents no difficulties.