When it is essential that the various working points of a tool, part of a machine, or apparatus shall be in exact relation to one or more given points, a receiving gage is used. This gage, as the name implies, is made to receive, or take in, the work; that is, the piece of work is placed in the gage, and the location of the different points is determined by the eye.
Fig. 439 shows a gun hammer, while Fig. 440 represents a receiving gage for accurately gaging the points C, D, E, F, G, and H, in relation to the fulcrum screw hole A and the face B. These points must also be in exact relation to each other - hence the necessity for a gage of this character. When making the gage, it is customary in most shops to gage only those parts that must be located accurately with relation to some other point or points.
In the case of the gun hammer under consideration, the fulcrum screw hole A must be the main working point, because when in use the gun hammer is pivoted at this point, and, consequently, every point must be in exact relation to this hole; the point of next importance is the face B which strikes the firing pin. In order that the face of the hammer may be the proper distance from the firing pin when half-cocked or full-cocked, it is necessary that the half-cock notch D, and the full-cock notch E be correctly located with regard to the face of the hammer. They must also be in exact location as regards the fulcrum screw hole A. If the main spring 13 to exert the proper amount of force on the hammer, it is necessary that the spring seat G be accurately located. As the portions marked C and H are intended just to fill the opening in the gun frame when the hammer is in any position, it is necessary that they be located the proper distance from the center of the fulcrum screw hole A; hence the need of a gage that will determine the exact location of all points as related to A and B and to each other. As the portions marked J, J, K, L, M, and N must be in precise location to the other points or to each other, they are gaged with a separate tool because each additional gaging point complicates matters.
When gages of this character are being made, a piece of machine steel is usually taken for the base; this is planed to size and ground or filed for finish; a hole is drilled and reamed to receive a pin the size of the fulcrum screw hole. This pin is made of a piece of drill rod a few thousandths of an inch larger than the desired pin. The piece of drill rod should be long enough to be held in the chuck of the grinding machine, and should be cut of the proper length, as shown in Fig. 441. The short end should be hardened and the temper drawn to a straw color, after which the pin may be placed in the chuck and ground to the desired size. It may then be broken off and the end ground; this can be done by holding the pin in the chuck, leaving the broken end out in order that it may be ground square; the pin should then be forced to place in the hole in the base.