“A wise man changes his mind often; a fool never.”—Carlyle.

If every turner and machinist was allowed to design the tools he uses such a multiplicity of fancy and useless shapes would result, that inefficiency, with all its attendant evils, would be inevitable.

The Production Engineer has long since realised this truth, hence, in every modern machine-shop the adoption of what is termed “Standard Tools” is universal.

What are “Standard Tools?”

A standard tool does not necessarily imply a definitely fixed and unalterable shape, but a range of tools fashioned to meet the full requirements of the work involved.

A first year apprentice knows that it would be mid-summer madness to attempt to use, on mild steel, a tool designed for brass. He knows, or should know, that to get the best results the metal to be machined and the nature of the operation, are the factors that determine the variations that comprise a set of standard tools.

In average workshop practice, two, three and sometimes four, variations are included in a make-up. This make-up is of a standard profile, but the rake of the top face, the radius of the cutting point, the incline of the cutting edge to the tool shank and the clearance of the cutting sides, are varied to suit the machine, the material to be machined plus the depth of cut and the desired feed. None of these variables need affect the decision to lay down certain standards, as they only influence and relate to a flexibility that is selected to meet the everyday workshop requirements.

Useful Definitions of Tool Parts.

The Base, is the side of the shank which rests against the tool support that takes the pressure of the cut.

The Tap Face, is that part of the tool which takes the friction of the chip as it is cut from the material.

The Rake, is the angle at which the top face is presented to the material. It is usually a combination of side and front cutting angle (the knife and parting tools being the exceptions, although one has front and the other side clearance).

The Profile, is the plan when looking down at the top face. It is determined by the combination of the side and front angles of the cutting edges.

The Clearance, is that part of the tool nose that is ground away to prevent rubbing on the material whilst cutting. If measured from the nose it is called “front clearance,” but if from the side, “side clearance.”

The Radius, is the amount of curve or corner between the front and side angles round the cutting edge or point.

High Speed v. Carbon Steel.

In the heavy machine shops (on tough and high tensile metals) high speed steel is supreme, and no workshop manager would tolerate the use of carbon steel. Where soft metals are machined, carbon steel not only holds its own but is essential to output, lower steel costs, and high-class finish. This is due to the fact that no high speed steel reaches the same test of hardness that can be obtained and maintained with carbon steel (subject to the material being kept at a low temperature).

In large brass machine shops, where articles require a high finish prior to lacquering, or bronzing and polishing on the buff, all such work is machined very, smoothly, wrapt in tissue paper, and sent direct to the lacquering shop. The tools used on this class of highly finished work are called “Planning Tools,” and are made exclusively of carbon steel.

These same tools, after being ground to the required shape, are finished off on a smooth oilstone, and when correctly finished they produce a polished mirror-like surface on the work that page 35 only an expert can detect from the machine mop and buff finish.

Forging and Grinding of Tools.

It is sometimes claimed that tools should be forged, and that the cutting properties are enhanced thereby. This was possibly true half a century ago, but it is most certainly not true to-day.

Modern tool steel, if subjected to uncertain temperatures and hammering at heats under or above certain limits, loses its most essential properties. Strict obedience to the manufacturer's instructions is the only sure means of obtaining the best results. Forging in some instances cannot be avoided; but the modern method of grinding, which covers a very large range of tools, is the desirable method. Experience has conclusively proved that intelligent grinding has no deleterious effect on high-speed steel.

Don'ts When Grinding.

Don't try to save time by using an untrue wheel.

Don't try to cut with a loaded wheel—a loaded wheel does not cut, but burns off the steel by friction, and surface cracks result.

Don't try to economise with the water service when grinding a hardened tool. It is often better to use no cooling agent than to use it too sparingly.

Don't take too heavy a cut; a grinding wheel is essentially a cutting tool, and has working limitations.

Don't use a dirty cooling agent; such glazes and loads the wheel.

Don't run a wheel above 6,000 feet per minute—glazing will result.

Don't run a wheel below 4,500 feet per minute—the wear on the wheel will be out of all proportion to the metal removed.

Don't force a glazed or loaded wheel—it will break. (To be continued.)