[Fig. 1993] shows the fixture employed to grind parallel cutters, s representing a stand upon slide f (which corresponds to slide f in the general view of the machine in [Fig. 1992]) in which is fixed the arbor h. The cutter c is slid by hand along arbor h and beneath the emery wheel, the method of guiding the cutter to the wheel being shown in [Fig. 1994], which represents a front view of the machine. At e is the lug (shown also at e in the general view) which has a hole to receive a rod p, and is split through at s, so that operating binding screw l locks rod p in e. At r is a rod secured to the rod p, and g is a gauge capable of swivelling in the end of r and of being secured in its adjusted position. The end of this gauge is adjusted to touch the front face of the tooth to be ground on the cutter c, which must be held close against the end of the gauge in order to grind the cutting edge to a straight line parallel to its axis.
Fig. 1995.
A not uncommon error is to place the gauge g against the tooth in front of that which is being ground, as in [Fig. 1995], the gauge being against tooth c while tooth b is the one being ground. In this case the truth of the grinding depends upon the accuracy of the tooth spacing. Suppose, for example, that teeth b and c are too widely spaced, tooth c being too far ahead, and this error of spacing would cause tooth b to be too near the centre of the emery wheel and its cutting edge to be ground too low.
The object of feeding the cutter by hand along the arbor h is twofold: first, the amount of cut must be very light and the feed very delicate, for if the grinding proceeds too fast the cutting edge will be what is termed burned, that is to say, enough heat will be generated to soften the extreme cutting edge, which may be discovered by holding the front face of the tooth to the light, when a fine blue tint will be found along the cutting edge, showing that it has been softened in the grinding, and this will cause it to dull very rapidly.
Fig. 1996.
The second object is to insure parallelism in the cutter. Suppose, for example, that the cutter c was fast upon arbor h and was fed to the wheel by moving slide f, and if the arbor h stood at an angle, as in [Fig. 1996], to the slide upon which f moved, the cutter would be ground taper, whereas if the cutter is fed along the arbor it will be ground parallel whether the arbor is true or not with the slideway of f, the only essential being that the arbor h be parallel and straight, which is much easier to test and to maintain than it is in the slideway (d, [Fig. 1992]). Here it may be noted that oil should not be applied either to arbor h or to the cutter bore or slideway d, as lubrication only increases the wear of the parts, causing the fine emery particles that inevitably fall upon them to cut more freely.