Modern Machine-Shop Practice Part 142

FOR WOOD-WORKING TOOLS.

----------------+-------------+-----------------+---------------- Name of stone. |Kind of grit.|Texture of stone.|Color of stone.

----------------+-------------+-----------------+---------------- Wickersly. |Medium to |Very soft. |Greyish yellow.

|fine. | | | | | Liverpool or |Medium to |Soft, with sharp |Reddish.

Melling. |fine. |grit. | | | | Bay Chaleur |Medium to |Soft and sharp. |Uniform light (New Brunswick).|finest. | |blue.

| | | Huron, Michigan.|Fine. |Soft and sharp. |Uniform light | | |blue.

FOR GRINDING BROAD SURFACES, AS SAWS OR IRON PLATES.

----------------+-------------+-----------------+---------------- Name of stone. |Kind of grit.|Texture of stone.| Color of stone.

----------------+-------------+-----------------+---------------- Newcastle. |Coa.r.s.e to |The hard ones. |Yellow.

|medium. | | | | | Independence. |Coa.r.s.e. |Hard to medium. |Greyish white.

| | | Ma.s.sillon. |Coa.r.s.e. |Hard to medium. |Yellowish white.

The f.l.a.n.g.es for grindstones should be trued on both faces, and should pa.s.s easily over the grindstone shaft, and there should be between these collars and the stone an elastic disk, as of wood or felt, which will bed fully against the surface of the stone. It is preferable also if the under faces of these collars be recessed to within an inch of their perimeters so as to confine the grip to the outer edges of the faces.

The process of grinding large surfaces is entirely distinct from that of small ones, because of the difficulty in the former of getting rid of the cuttings. As an ill.u.s.tration of this point it may be remarked that a stone that has become dulled and glazed from operating upon a broad area of surface, as say a large plate, may be both cleaned of the cuttings and sharpened by grinding upon it a roller of, say, 1 or 1-1/4 inches in diameter. This roller is laid across the "horn" or rut of the stone, and pressed firmly against it, the bar being allowed to slowly rotate. What is commonly termed grinding is the cla.s.s of grinding that is followed as a trade, such as file grinding, saw grinding, plate grinding, edge tool and cutlery grinding. In all this cla.s.s of grinding the speeds of the stones is very much greater than for machine-shop tool grinding. For all the above, save cutlery grinding, the stones when new are of a diameter from 5 to 8 feet, and of a width of from 8 to 15 inches. The stones used by cutlers are about 15 inches in diameter, and from 1/2 inch to 3 inches thick. The average speed of grindstones in workshops may be given as follows:--

Circ.u.mferential speed of stone.

For grinding machinists' tools, about 900 feet per minute.

" carpenters' " 600 "

The speeds of stones for file grinding and other similar rapid grinding is thus given in the "Grinders' List."

Diameter of stone.

Revolutions ft. in. per minute.

8 0 135 7 6 144 7 0 151 6 6 166 6 0 180 5 6 196 5 0 216 4 6 240 4 0 270 3 6 308 3 0 360

These speeds are obviously obtained by reducing the diameter of the pulley on the grindstone shaft each time the stone has worn down 6 inches less in diameter, and give a uniform velocity of stone if the 8 feet stone be driven with a pulley 32 inches in diameter. Each shift (or change of pulley) giving a pulley 2 inches less in diameter.

The following table (from the _Mechanical World_) is for the diameter of stones and the number of revolutions they should run per minute (not to be exceeded), with the diameter of change or shift pulleys required, varying each shift or change 2-1/2 inches, 2-1/4 inches, or 2 inches in diameter for each reduction of 6 inches in the diameter of the stone:--

+-------------+-------------+-----------------------------+ | | | Shift of pulleys in inches. | | Diameter of | Revolutions +---------+---------+---------+ | stone. | per minute. | 2-1/2 | 2-1/4 | 2 | +-------------+-------------+---------+---------+---------+ | ft. in. | | | | | | 8 0 | 135 | 40 | 36 | 32 | | 7 6 | 144 | 37-1/2 | 33-3/4 | 30 | | 7 0 | 154 | 35 | 31-1/2 | 28 | | 6 6 | 166 | 32-1/2 | 29-1/4 | 26 | | 6 0 | 180 | 30 | 27 | 24 | | 5 6 | 196 | 27-1/2 | 24-3/4 | 22 | | 5 0 | 216 | 25 | 22-1/2 | 20 | | 4 6 | 240 | 22-1/2 | 20-1/4 | 18 | | 4 0 | 270 | 20 | 18 | 16 | | 3 6 | 308 | 17-1/2 | 15-3/4 | 14 | | 3 0 | 360 | 15 | 13-1/2 | 12 | +-------------+-------------+---------+---------+---------+ | 1 | 2 | 3 | 4 | 5 | +-------------+-------------+---------+---------+---------+

"Columns 3, 4, and 5 are given to show that if you start an 8 feet stone with, say, a countershaft pulley driving a 40 inch pulley on the grindstone spindle, and the stone makes the right number (135) of revolutions per minute, the reduction in the diameter of the pulley on the grinding-stone spindle, when the stone has been reduced 6 inches in diameter, will require to be also reduced 2-1/2 inches in diameter, or to shift from 40 inches to 37-1/2 inches, and so on similarly for columns 4 and 5. Any other suitable dimensions of pulley may be used for the stone when 8 feet in diameter, but the number of inches in each shift named, in order to be correct, will have to be proportional to the number of revolutions the stone should run, as given in column 2 of the table."

[Ill.u.s.tration: Fig. 2059.]

[Ill.u.s.tration: Fig. 2060.]

In all grinding operations it is necessary that the stone should run true. This is sometimes accomplished by so mounting the stones in their frames that their perimeters touch at the back of each stone, one stone running slightly faster than the other. Or sometimes the work is placed between the two stones, as in Fig. 2059, which represents a plan frequently used to grind circular saws; _c_ in the figure represents the grinding-stone and _a_ the saw. Long saws are mounted vertically as in Fig. 2060, _a_ representing a frame to which the upper end of the saw is attached and driven by a disk crank and connecting rod as shown, the two stones _c_ _e_ may, in this case, be of equal diameter.

[Ill.u.s.tration: Fig. 2061.]

Fig. 2061 represents a grindstone truing device (for tool-grinding stones) in which a series of serrated disks are employed in place on a threaded roll. The disks are fed to the stone by the hand wheel and screw, and are traversed back and forth across the stone face by means of the lever handle shown.

The fast running grindstones used for heavy and coa.r.s.e grinding are trued by a process known as hacking. The high spots of the stone are marked by holding a piece of coal to the stone while it revolves slowly, and a tool similar to an adze is used to cut or chop indentations in the stone. The highest spots will be most plainly marked by the coal, and the hacking is s.p.a.ced closer together in these places, the hacking marks crossing each other and varying in depth to suit, obviously being deepest where the marks are blackest. The hacking also sharpens the stone. To prevent the stone from wearing uneven across its face the file grinder mounts the stone in a very ingenious manner, causing it to traverse automatically, back and forth, while rotating.

[Ill.u.s.tration: 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 ab.u.t.ting 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 2-1/2 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 ma.s.sive 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 sc.r.a.pers 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.

To produce a finer edge than is possible with the grindstone, the oil-stone is brought into requisition, the shape of the oil-stone being varied to suit the shape of the tool. Three kinds of oilstone are in general use, Turkey stone, Arkansas stone, and Was.h.i.ta stone, the latter being softer and of inferior quality to the two former. The best quality of Arkansas stone is of a milky white color, of very fine and even grain, and very hard, being impervious to a file; but there are softer grades. An oil-stone should be of even grain throughout, so that it may wear even throughout, and produce a smooth and unscored edge. Arkansas stone is rarely obtainable in lengths above 6 inches, on account of the presence of fine seams of hard quartz, which wears less than the stone, and forms a projection that scores the cutting edge of the tool, and the same applies to the Turkey stones.

For tools fully hardened and not tempered the hardest oilstones are best; but for tools that are tempered, as tools for woodwork, a softer grade of stone is preferable, since it will cut the most free.

When an oil-stone has worn out of shape it may be dressed on a grindstone; but if a flat surface is required it is best to true it by a piece of coa.r.s.e sand-paper laid upon a flat true surface.

The action of an oil-stone is to smooth the surfaces; but while doing this the oil-stone itself forms what is termed a wire-edge, which resembles a feather-edge, except that it is smoother and more continuous. It is caused by the weak edge of the blade giving way under the pressure with which it is held to the stone. To reduce the wire-edge as much as possible the tool is pressed very lightly to the oil-stone during the latter part of the stoning, and is frequently turned over. If the motion of the tool upon the oil-stone is parallel with the line of cutting edge, the wire-edge will be greater than if the line of motion were at a right angle to it.

Again, the strokes performed while the cutting edge is advancing upon the oil-stone produce less wire-edge than the return strokes, hence the finishing process consists of a few light strokes upon one and then upon the other facet repeated several times. Now let it be observed that, the wire-edge will never be turned toward the facet last oil-stoned, and cannot be obviated by the most delicate use of the stone; but after the stoning proper is finished, the operator will lay one facet quite level with the face of the stone, and then give to the blade, under a very light pressure, forward diagonal motion, and then perform the same operation with the other facet upon the stone, the last facet operated upon being usually the straight and not the bevelled one. To still further reduce the wire-edge for very fine work, the operator sometimes uses a piece of leather belt, either glued to a piece of wood, as upon the lid of the oil-stone box, or some attach it at each end to projecting pieces of wood, while yet others lap the tool upon the palm of the hand. In giving an edge to a razor, the process may be carried forward in the usual way by means of straps, the first strokes being long ones made under a slight pressure, the strokes getting shorter and the pressure lighter as the process proceeds, until at last the motion and contact are scarcely perceptible.

[Ill.u.s.tration: Fig. 2068.]

When, as in the case of plane blades and carpenters' chisels, the area of face is large, it is advantageous to grind the face somewhat concave, as in Fig. 2068, so that the heel and the point only of the tool has contact with the oil-stone, thus reducing the area to be stoned and steadying the tool, because, the area being small, the heel as well as the edge may be allowed to rest upon the oil-stone without unduly prolonging the stoning operation.

[Ill.u.s.tration: _VOL. II._ =GRINDSTONE GRINDING.= _PLATE V._

Fig. 2063.

Fig. 2064.

Fig. 2065.

Fig. 2066.

Fig. 2067.]

[Ill.u.s.tration: _VOL. II._ =FULL AUTOMATIC GEAR CUTTER.= _PLATE VI._