When a tool has once been set by means of the template to the required angle, it can readily be reset on future occasions if the distance it projects from the clamping bracket is measured and recorded, but this particular setting only holds good, of course, for other tools having a blade of similar thickness.
Where the tip of the tool has its cutting edge formed nearly at right angles to the upper surface, as in the case of a lathe tool, it would be difficult to sharpen this edge with the tool held almost vertically, for digging-in into the surface of the stone would be liable to occur ; in this case, therefore, it is usually preferable to use a jig for guiding the stone whilst the tool is held stationary.
Fig. ii and 12 illustrate a simple jig which can be used with advantage when stoning the tips of lathe, shaper, and planer tools.
Two steel side members, which support and guide the stone, are secured to a wooden base clamped to the bench top. These side-members are formed at one end to an angle of 5 deg. and at the other to 10 deg., so that the stone is tilted this amount while it is worked to and fro against the end cutting face of the tool. The tool is held in place on the baseboard with the fingers, and is moved forwards to maintain the tip in contact with the stone as the honing operation proceeds.
If the cutting edge has to be formed at right angles to
the length of the tool, as in the case of a parting tool, the tool is held in contact with the side-member as shown ; but where the tip is formed at an angle, a fence or guide piece can, if desired, be fitted to the baseboard to set and maintain the required alignment.
To stone a tool with a clearance angle of 10 deg., instead of the 5 deg. shown in the drawing, the jig is merely turned end for end.
The side members should be made of steel strip some £ in. thick to resist wear, and, as shown in the drawing, these components are provided with extended limbs to support the stone and to prevent it from rubbing against the bench top.
Jf desired, the G-clamp can be recessed into the upper surface of the baseboard so as not to encumber the surface of the tool rest.
Grinding Rests and Jigs. These appliances are used st'-.*-.'.-.»v...
to obtain both accuracy and uniformity when grinding tools, particularly those used in production work, where consistent results would not be possible if free-hand methods of tool sharpening were adopted.
Lathe tools may, of course, be hand ground on occasion, but as a regular practice it is better to use some form of guide, which will ensure that the tool is presented to form the cutting edge correctly.
Many forms of adjustable grinding rests are fitted to grinding heads, some are intended for grinding the work on the periphery of the wheel, and others are designed for use in connection with its side surfaces.
When the periphery of the wheel is used, the rest is set so that the centre line of the tool lies a little above the wheel centre, in order to form the necessary clearance angle on the front face of the tool. Reference to Fig. 13 should make this point clear ; and it will be seen that the above-centre height of the table is adjusted by raising and lowering the rest vertically. The same result can be obtained, as shown in Fig. 14, where it Fig. 14.
will be seen the table can be tilted, and at the same time adjusted at the correct distance from the wheel, by means of a clamp-bolt sliding in a slot in the supporting arm.
It will be apparent that, as set out diagrammatically in Fig. 15, a different height adjustment is required for large arid small tools ; for, while the height of the rest at A is correct for grinding a large tool, the small tool at B has its centre line below the wheel centre line, and so would be ground with a negative front clearance that would cause it merely to rub against the work.
The pillar of the vertically adjustable rest can readily be graduated for grinding tools of various thicknesses, and also for altering the size of the clearance angle formed ; but in the case of the tilting and sliding-rest the matter is more involved, and setting the rest by means of a template against the surface of the wheel may be found a more satisfactory method.
When a grinding rest provided with a means of angular adjustment is used, such as that illustrated in Fig. 16, the angular grinding of tools is greatly facilitated by using the side faces of the wheel, and the exact height of the tool in relation to the wheel centre is then immaterial Rests of this type can be readily graduated and furnished with a scale and pointer for setting the angularity ; or, if preferred, a sheet-metal template can be used for
this purpose. In addition to the commercial types of grinding rests described, the rest illustrated in Fig. 17, which was designed to meet personal requirements, may also be found useful.
This grinding rest has the advantages that it can be adjusted for both angularity and height to enable it to be used in relation either to the side or the periphery of the grinding wheel ; and, moreover, the grinding table can, if desired, be replaced by one of a form suitable for cutter grinding or other special work. At its lower end the vertical arm carries a spindle which is clamped in a bracket attached to the bench top.
This bracket is similar to that used to carry the Potts drill grinding jig, and its adoption in this instance allows either the grinding-rest or the drill grinder to be used at will, and, furthermore, the change-over can be easily and quickly made.
As will be seen in the drawing, a sole-plate A, for attachment to the bench, carries a vertical spindle B> which forms a pivot for the clamping piece C. This split clamp, into which the spindle D of the rest fits, is closed by means of a lever clamp nut E, and a locking bolt F is provided to prevent the clamping piece from turning on its pivot after the components have been correctly set.
It will be apparent that the spindle D can rotate in the component C, in order to set the rest to the wheel, and in addition, it can slide endways for adjusting the gap in
the grinding table to clear the sides of the grinding wheel. When the nut G is slackened, the pillar H of the grinding table can slide in the sleeved-clamp collar Jt to afford height adjustment, and at the same time the table can be tilted to the exact angle required for grinding the tool. If desired, the vertical limb supporting the rest can be graduated and a pointer can be attached to the clamp-collar spindle to indicate the angular setting of the grinding table.
Grinding Machines. The simplest type of grinding head, which could formerly be purchased for a few shillings, consists of an iron casting with integral bearings to cany the wheel spindle. The latter are split and closed by screws to afford some measure of adjustment to counteract wear.
Needless to say, at this low price neither the bearings nor the shaft could be precision fitted ; but if the bearings are rebored, reamed, and finally lapped, and a lapped alloy steel shaft is accurately fitted, the outcome will be a very serviceable grinding head for light work.
Moreover, if a single wheel is used and a pulley is fitted to the other extremity of the shaft, two such machines, driven by a common electric motor can be employed to mount a coarse and a fine wheel for rough and finish grinding respectively.
The change-over is made by merely changing the belt and, if desired, one machine can be made to slide to one side when not in use in order to give better access to the other.
As these simple machines do not carry grinding rests, a grinding table must be improvised and secured to the bench top in a position to serve the machine. The grinding rest illustrated in Fig. 17 will be found eminently suitable for this purpose.
The more elaborate and therefore more expensive grinding machines are robustly constructed and may be fitted with ball bearings to render them suitable for long, continuous running without attention ; also, where the belt-drive is taken from the lineshafting, fast and loose pulleys with a belt-shifting gear are fitted.
As already indicated, various types of grinding rests are fitted to these machines, and some have provision for fitting a drill grinding jig ; the spindle is usually double-ended and carries two wheels suitable for rough and finish tool grinding.
In some cases, a self-contained type of machine may be preferred which has the advantage that it can be moved from place to place as required. These machines comprise an electric motor, with an extended armature-shaft carrying a grinding wheel at either end. The motor is wound to run at a speed suitable for the wheels fitted, and is controlled by a switch mounted on the body of the machine.
Grinding rests are also fitted, and in some instances a drill-grinding jig can be supplied as an extra attachment ; in addition, some makes of machine are adapted for attaching a flexible shaft to the spindle for driving small cutting tools or abrasive wheels and pencils ; the latter will be found useful for sharpening small tools such as screw-threading dies.
As mentioned in the previous chapter, small India wheels will be found useful for grinding pointed tools and fine boring tools, where only a minimum of metal has to be removed in order to preserve the tool's original form. These wheels can be conveniently mounted on the spindle of a small electric motor, such as a fan motor, which can usually be acquired at small cost, but as fan motors are generally series-wound, their speed must be controlled by means of the series resistance switch with which they are fitted.
A simplified form of the grinding rest, illustrated in Fig. 17 should be attached to the baseboard on which the motor is mounted.
Mounting Grinding Wheels. When mounting the wheel, the lead bush fitted to its centre should be an easy sliding fit on the grinding machine spindle, and any high spots found in the bore, which prevent a free fit, should be carefully removed by scraping with the blade of a sharp knife.
The wheel itself is supported between two flanges, the inner of which should be securely fixed to the spindle and turned to run true when in place.
These flanges must be relieved, as shown in Fig. 18, and the pressure they exert on the wheel should be evenly distributed at the outer diameter by interposing washers of blotting paper, or other soft material, between them and the faces of the'wheel.
For general use, the diameter of these flanges should be equal to half the diameter of the wheel, but for light
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