Value of Grinding as Finishing Process. When greater accuracy than that obtainable on the milling machine or the lathe is required, recourse is had to grinding. This operation depends upon the abrasive or cutting qualities of emery, corundum, and carborundum. With work properly held to a solid grinding wheel, it is not difficult to attain great accuracy. By means of the grinding machine, parts may be economically finished, even in hardened steel that could not possibly be machined on such shop tools as the lathe, planer, or shaper. One type of machine used for this purpose is shown in Fig. 247. With such a machine, round surfaces may be ground so that the variation from the nominal diameter is less than .0001 inch.

Features of Grinding Process. The grinding machine illustrated in Fig. 247 consists of a strong base A, upon which there is mounted a headstock B and a tailstock C, similar in action to those of an ordinary lathe. Back of these is an emery wheel driven by a separate belt. The principle of operation for round surfaces, is that the part to be ground is put upon the centers, and driven exactly as in the ordinary lathe. The only additional precaution to be taken is that the driving apparatus should be secure, so that none of the parts are loose. This insures a continuous motion for the piece with no possibility of backlash. The piece runs toward the operator, and the emery wheel runs in the same direction. The two surfaces of wheel and work in contact are therefore moving in opposite directions. The headstock and tailstock are mounted upon a traveling table D, which moves back and forth in the same manner as the platen of a planer. It is made to stop automatically at each end of the stroke.

Fig. 247. Cylindrical-Grinding Machine.

When work is being done, the piece is centered, with its axis parallel to the line of travel of the table. With the piece and emery wheel in motion, the former travels to and fro in front of the wheel The wheel is then gradually moved forward until it has ground the work down to the size required.

It is not intended that large amounts of metal shall be removed by this machine. Its object is to reduce to accurate dimensions the work that has already been turned in the lathe. The proper method to pursue is to turn the piece to as nearly the required diameter as possible in the lathe, care being taken that it is left a trifle large. This may be .01 inch on each 2 inches of diameter. The surplus metal may then be removed by grinding. In the machine illustrated in Fig. 247, the transverse movement of the wheel-stand is adjusted by a hand-wheel graduated to read to .001 inch on the diameter of the work. The machine is also provided with an automatic cross-feed, which gives a range of advance of the wheel varying from .00025 inch to .004 inch at each reversal of the table. This feed, furthermore, is so arranged that it can be automatically released at any point.

Finishing to Size after Casehardening. This method of finishing is also used for pieces that have been casehardened. Case-hardening always warps the metal to which it is applied. Grinding is resorted to in order to reduce it to the proper shape. An example of this may be taken in the method used in the manufacturing of wrought-iron locomotive crank pins. The pin is forged and turned to as near the working size as possible. It is then casehardened and ground to exact alignment and dimensions.

Grinding is also used for truing work that comes from the lathe. The lathe does not turn its work round, owing to difference in the density of the metal, variation in the cutting speed, dulling of the tool, lost motion on the centers and in the spindle, and springing of the work itself due to pressure of the tool. The grinding machine remedies this to a great extent-partly because only a very slight pressure is brought against the work; partly because of the greater delicacy of adjustment of the grinding machine as compared with the lathe.

The method of grinding flat surfaces is practically similar to that used for round. The work is bolted to the table and moved to and fro beneath the emery wheel, which is given a transverse movement so as to cover the whole of the surface to be operated upon. The surface speed of the wheel may range from 4,500 to 6,000 feet per minute.

Action of Typical Grinder. Fig. 248 shows a typical surface-grinding machine built by the Brown and Sharpe Manufacturing Company, which is well adapted to the work of accurately grinding flat surfaces up to quite large dimensions. The work table travels to and fro in a manner similar to that of a planer, and carries adjustable reversing dogs which may be set to limit the extent or position of the travel. The grinding wheel mechanism is supported upon a cross-rail similar to that in a planer and capable of vertical adjustment on the arc of a circle whose center is the driving shaft supplying power to drive the grinding wheel. The wheel mechanism carrying the grinding wheel has a transverse, automatic feed the entire width of the work table.

Fig. 248. Surface-Grinding Machine.

In using this machine, the work is clamped directly to the table or held in any convenient fixture, as a milling machine or planer chuck or vise. In strapping work to the table, it must be rigidly held in place; but it is not necessary to clamp it down as tightly as on the milling machine or planer table, and great care should be used to avoid springing, warping, or other distortion, as grinding work is expected to be very true and accurate; in fact, this is its chief claim as a method of finishing surfaces.

To avoid distortion from overheating, comparatively thin wheels are generally used, particularly if the piece being ground is thin and light, as a thin casting of complicated form.

Selecting the Grinding Wheel. Grinding wheels are made with abrasives as coarse as No. 46, and as fine as No. 150. There is a great difference in the degree of hardness of a wheel due to the kind of bond, or adhesive material, with which the abrasive is mixed in forming the mass of which the wheel is composed. As to the fineness of the abrasive, that as coarse as No. 46 is suited for work on rough castings, as in the cleaning room of a foundry. For general work in shop grinding, the roughing-off will be best done with a wheel of about No. 60; and ordinary finishing, with about No. 90. For very fine finishing, the wheel may be much finer.

As to the degree of hardness of a wheel, it may be generally said that the harder the material to be ground the softer should be the wheel. There are several degrees of hardness made by the manufacturers, the simplest classification being Hard, Medium, and Soft, designated by the letters H, M, and S, respectively. All letters standing before M in the alphabet, refer to wheels harder than medium; and all letters after M refer to wheels softer than medium.

A coarse wheel grinds faster than a fine one, but leaves deep scratches in the work. A soft wheel may be made of a much finer grade than a hard one.

A soft wheel grinds faster than a hard one, but it is apt to glaze over, or fill up with particles, if used on a soft material.

Lubrication

To increase the cutting capacity of an abrasive wheel, to prevent it from glazing over, and to carry off the heat generated by the friction of the wheel on the work, a stream of water is frequently used, arrangements being made in most machines-particularly in those for grinding tools and for cylindrical grinding-