The effect of not paying attention.

harden in a satisfactory manner. This difficulty is more pronounced in the case of a piece glazed from the action of grinding wheels. While a glazed surface might not be considered objectionable, if it was to be ground away after hardening, yet it is not always considered advisable to grind the cutting faces of reamer and milling machine cutter teeth. There is no good

Figure 156. The spring of a mandrel.

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excuse for using dull tools when machining steel. Not only does it lead to trouble when the pieces are hardened, but it is a means of wearing the tools out much faster than if they were kept sharp. Neither can as much nor as good work be done with dull tools.

It is often the case that a careless workman will mill the flutes in a long reamer, tap, or similar tool, without supporting the work properly. In this way the tool is sprung, first one way, then the other. This not only results in a crooked tool, but there is no knowing where it may go when hardened. Many times hardened pieces are sprung by heating when grinding. This is especially true with pieces that may have sprung when hardened. Take, for instance, a long mandrel which may have gone in the direction shown in Fig. 156. Now, if this mandrel were placed in a grinder and ground in a manner that caused it to become heated on the side that is already curved out, as shown in cut, it would spring still more.

Many times thin, flat pieces are sprung from the

What caused the cracks.

expansion of one side when ground in a surface grinder. The side which comes in contact with the wheel becomes heated, while the opposite side, from contact with a mass of cold iron-the table-remains cool. The side which heats must expand, with the result that the piece is curved in the direction of the heated side.

When flat pieces which are hardened are ground with a glazed wheel or cne too fine for the purpose, they are very liable to crack, commencing at the edge or end where the wheel leaves the work. Fig. 157 represents a rectangular gauge which cracked as a result

of grinding. The fault was laid at the hardener's door, and he, poor fellow, was doing his best to harden the gauges in a satisfactory manner, so he said the steel was no good. An investigation showed the cracks to be from the end where the wheel left the gauge when grinding in the surface grinder. The vise which held the piece was turned one-quarter way around, and it was found that the cracks were from one. side, instead of the end of the piece. An examination of the wheel revealed the fact that it was too fine for the purpose, and that it was badly glazed. A coarse wheel, free from

Cracking from the wrong use of water.

glaze, was substituted and the gauges were found to be sound after grinding.

Not only may hardened steel be sprung and cracked from heat generated when grinding, but it may also be cracked if water is run on it, unless due care is observed. If the operation is hurried to the extent that it becomes heated, even when the water is running on it, the water cools the piece, which is instantly heated again and then cooled. This sudden expansion and contraction causes the steel

to become

cracked in innumerable.

places, these cracks running in all directions. This trouble may occur when grinding pieces of almost any shape. The cracks may occur on the surface of a cylin

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Fig. 158. Disc cracked from being
ground too rapidly.

drical piece, on the flats of a square, or on the face of an article being ground. Fig. 158 represents a disc whose face was cracked, as represented, when ground, with a stream of water running on the work. The fault did not lay in using water, but in forcing the grinding faster than the wheel could properly cut the metal.

These few facts are pointed out, because it often happens that when these troubles arise, the party doing the hardening is blamed, and unless he is sufficiently

A proper emery wheel for cutter teeth.

versed in the action of emery wheels on surfaces of steel, he naturally thinks the fault is either in the steel or in his method of treating it.

Very often milling machine cutter teeth are softened when ground, the hardener being blamed as a consequence. It is not good practice to use a very fine wheel when grinding tools of this description, neither should too hard a wheel be used. Ordinarly an emery wheel made of 60 to 90 emery will be found about right, and be sure the face of the wheel is not glazed. Should it become glazed, use a piece of emery wheel somewhat coarser than the one in use to remove the glaze. This also makes the face of the wheel open, and lessens the liability of heating.

Many times the writer has seen workmen using a tool ground in a manner that made it impossible for it to cut. It was forced into the stock, and broke it off. The tool could not stand this treatment, and gave right out, the workman in the meantime saying things about the hardener. When the tool was properly ground, it worked all right.

Some mechanics do not seem to realize that there is a proper speed to run stock or cutting tools, in order to get desired results. As a consequence, they either run them much too fast, with the result the tools can not stand up, or they are afraid they will exceed the proper speed, and, as a consequence, do not produce anywhere near the amount of work they might.

When cutting a key way or spline in a tool that is to be hardened, the tool maker should avoid sharp corners, as they are an invitation for a crack when the steel is rapidly cooled in the bath. While an article having sharp corners is not as liable to crack when

How tools are weakened by grinding.

hardened by the process termed Pack Hardening as when treated in the ordinary manner, it is not advisable to in any way weaken a tool, or give it an invitation to crack. Consequently, avoid sharp corners as far as possible, or cuts or deep scratches that tend to weaken the article.

A milling machine cutter, made with light, weak teeth, can not be made to stand up when in use; the teeth being slender and weak, break like pipe-stems. Cutters with teeth of this description require greater care when hardening, to avoid overheating. Being slender, they spring and break. Do not blame the · hardener if they fail to give satisfaction when in use.

Another source of trouble is fine teeth in milling cutters, reamers, and similar tools. The teeth, being fine, fill with chips, and in the case of milling machine cutters, the oil not being able to get to the teeth, can not conduct away the heat generated, which has the effect of drawing the temper to a degree that makes it impracticable to use them.

A short time ago the writer's attention was called to a side tool for use in an engine lathe. The tool was made from a well-known brand of steel, which is generally considered one of the best steels on the market. It was claimed that the tool could not be made to keep an edge on a mild grade of machine steel running at a periphery speed of 30 feet per minute, taking a fair cut.

An examination of the tool revealed the fact that it was ground in such a manner that the cutting edge had no backing. It might possibly have stood up if the material being machined had been wood instead of steel. Because the tool would not stand, the hardener was considered as being to blame. When