A snap gage may be held in a vise on the universal grinder when the openings are ground to size, provided it is held in such a way that it cannot spring. If sprung in any man-ner while being held, it would assume its normal shape when taken from the vise, and consequently the measuring surfaces would not be parallel. As this would destroy its accuracy, it is highly important that the measuring surfaces of the openings be parallel.
A snap gage may be clamped to an angle iron held in the vise while grinding, Fig. 428, or it may be clamped to a piece of machine steel or cast iron centered, Fig. 429. This holder should be placed between the centers of the grinding machine.
If the opening whose gaging surfaces are to be ground is of sufficient width, an emery wheel of the form shown in Fig. 430 may be used; or a wheel may be recessed on its sides as shown in Fig. 431. If the wheel is of the form shown in Fig. 430, it will be necessary to remove it after grinding one wall of the opening and to reverse it to grind the other. If, however, the opening is too narrow to allow this type of wheel, a very thin wheel may be made to answer the purpose, but it will be necessary to swivel the head of the grinder a little, in order that the wheel may touch the surface to be ground only at the corner of the wheel. An engine lathe or a bench lathe can be substituted if a grinding machine is not available. If the lathe is provided with a grinding attachment, the holder to which the gage is attached maybe placed between the centers of the lathe, and the grinding attachment used in the ordinary manner. If the lathe is not provided with a grinding attachment, the emery wheel may be mounted on an arbor between the centers of the lathe. The arbor may be driven from any accessible pulley, either on some overhead countershaft or else on some machine whose driving pulley is in line with a small pulley on the arbor. If this method is used, it will be necessary to have hardened centers in both head and tail spindles of the lathe.
Fig. 427. Friction Block.
Fig. 428. Snap Gage Clamped in Vise for Grinding.
Fig. 429. Snap Gage Clomped to Centered Piece.
Figs. 430 and 431. Typical Wheels.
A thin wheel used in grinding the surfaces of a narrow opening necessitates that the tail center of the lathe be set over each way to give the desired amount of clearance to the side of the emery wheel. The holder mentioned may be fastened to the tool rest, or the gage may be fastened to the rest, Fig. 432. At the right is shown a side view of one of the straps used to hold the work to the rest while grinding; the center 13 represented as being cut away in order that it may bear at its ends, thus removing any chance of its tipping the work that is being ground.
Fig. 432. Method of Fastening Gage for Grinding with Thin Wheels.