Fig. 2062.
This device is shown in [Fig. 2062], in which a represents the grindstone spindle having journal bearing at b b, but as there are no collars on the journals, a can move endwise through b b. Fast to a are the collars c and c′ (sometimes the face of the pulley hub is made to serve instead of c′); s is a sleeve fitting easily to a, and containing a return groove, as shown; d is a fixed arm carrying a pin which projects down into the groove of s, as shown; p is the pulley driving a, and w w are suspended weights. The operation is self-acting, as follows: The shaft revolving causes the sleeve to revolve by friction, and the pin causes the sleeve to move endwise; its end face abutting against the face of the collar on one side, or the face of the pulley on the other side, as the case may be, causing the shaft to travel in that lateral direction. When the pin has arrived at the end of the groove, the stone ceases lateral motion (there being left a little play between the faces of the sleeve and of the collar and pulley face for this special purpose), while the cam travels in the opposite lateral direction, getting fairly in motion until it strikes the face, when it slowly crowds the face over. In travelling to the right it crowds against the face of the collar c′, and in traveling to the left, as shown in the figure, against the face of the collar c. The swing thus given to the stone is a slow and very regular one, the motion exciting surprise from its simplicity and effectiveness, especially when it is considered that the friction of the rotation of a shaft about 21⁄2 inches diameter in a smooth hole about 6 inches long is all that is relied upon to swing a ponderous stone.
The following are the considerations that determine in grinding tools or pieces held by the hands to the grindstone. Upon the edge of a tool that last receives the action of the stone there is formed what is termed a feather-edge, which consists of a fine web of metal that bends as the tool is ground, and does not become detached from the tool in the grinding. The amount or length of this feather-edge increases as the work is thinner, and is greater in soft than in hardened steel. It also increases as the tool or piece is pressed more firmly to the stone.
To prevent its formation on such tools as plane blades or others having thin edges, the tool is held as at g in [Fig. 2063], the top of the stone running towards the workman, and the tool is held lightly to the stone during the latter part of the grinding operation. With the tool held on the other side of the stone as at c, and pressed heavily to the stone, a feather-edge extending as long as from d to e may be formed if the tool has a moderate degree only of temper, as, say, tempered to a dark purple. The feather-edge breaks off when the tool is put to work, or when it is applied to an oil-stone, leaving a flat place instead of a sharp cutting edge. In well-hardened and massive tools, such as the majority of lathe tools, the amount of feather-edge is very small and of little moment, but in thin tapered edges, even in well-hardened tools, it is a matter of importance.
After a tool is ground it is often necessary to remove the feather-edge without having recourse to an oil stone. This may be accomplished by pressing the edge into a piece of wood lengthways with the grain of the wood, and while holding the cutting edge parallel with the line of motion, draw it towards you and along the grain of the wood, which removes the feather-edge without breaking it off low down, as would be the case if the length of the cutting edge stood at a right angle to the line of motion.
The positions in which to hold cutting tools while grinding them are as follows: The bottom faces of lathe tools and the end faces of tools such as scrapers should be ground with the tool laid upon the grindstone rest as in [Fig. 2064], the stone running in the direction of the arrow. The best position for thin work as blades is at f providing the stone runs true, for otherwise the tool edge will be liable to catch in the stone. With an untrue stone the position shown in [Fig. 2065] is the best, the blade being slowly reciprocated across the face of the stone.
If the facet requires to be ground rounding and not flat the position at c, [Fig. 2064], is the best, the work being moved to produce the roundness of surface. If the tool is to be ground hollow or somewhat to the curvature of the stone, as in [Fig. 2066], the curve being from b to c, the position at b is the best. At position d the tool cannot be held steadily; hence, that position is altogether unsuitable for tool grinding purposes.
For grinding the top faces of lathe or planer tools or other similar shaped pieces that must be held with their length at a right angle (or thereabouts) to the plane of the rotation of the stone, the tool is held in the hands, and the hands are supported by the grindstone rest as in [Fig. 2067], the fingers being so placed that should the tool catch in the stone it will slip from between the fingers and not carry them down with it upon the tool rest.
Tools to be ground to a sharp point should be ground at the back of the stone, that is to say, with the top of the stone running away from the operator, and the point should be slowly moved across the width of the stone to prevent wearing grooves in its surface.