Singe the earliest times, the abrasive properties of natural stones and rocks have been utilised for sharpening metal tools and weapons. These natural deposits vary from the hard, close - grained quartz rocks to the softer and more friable sand stones.
Anyone who has tried to sharpen a pocket knife on a doorstep will have discovered that, when the stone is used dry, it will soon become filled with metal particles, and in that state it will have but little abrasive action. To obviate this, water or oil is applied to the stone to enable the metal dust to be carried away and prevent its becoming adherent to the surface of the stone.
Whether water or oil should be used will depend on the character of the stone employed. In general, the thinner the oil or other liquid used the faster the stone will cut, for the presence of the oil has a cushioning effect on the cutting properties of the stone, in addition to preventing the adherence of metallic dust.
The Arkansas Stone. This is, perhaps, the best known form of oilstone and the most generally used. It is found as a natural deposit in the American State of Arkansas, and the best samples consist of nearly pure quartz almost white in colour.
There is no apparent grain in the stone and its extreme hardness prevents the surface from being worn unevenly, and scored or grooved when tools with narrow cutting edges are sharpened on it. The outstanding characteristic of this stone is the facility with which it imparts an extremely accurate and highly finished edge to all classes of cutting tools.
As the Arkansas stone is expepsive, especially in the best grades, it should be well cared for and protected from damage by means of a wooden casing fitted with a lid, as illustrated in Fig. i. To prevent the stone from sliding on the bench when in use, points may be fitted to the sole of the casing, or a cross-piece can be attached to engage the edge of the bench or the lathe bed.
The Washita Stone. Although this oilstone is much cheaper than the foregoing variety, its qualities are greatly inferior. It is comparatively coarse grained, and its softer texture does not well resist wear or scoring when it is used to sharpen pointed or narrow-edged tools.
foft the: jc po'N ts at tALM LNU allow bed to crip bench firmly
fqtt the tupncft f i ll c t at each end to fit transversely on lathe bed
Turkey Stone. The texture of this form of oilstone varies greatly ; the hardest stones are capable of producing a very fine cutting edge, whilst the softer qualities cut more rapidly but are too readily grooved to be suitable for sharpening tools such as gravers and small chisels.
Oilstone Slips. In addition to the ordinary bench forms or blocks, oilstones of ail types, but Arkansas stones in particular, are made as slips for holding in the hand and applying to the tool. These slips are made in a great variety of shapes for use when sharpening tools of special form, such as gouges and reamers, but the flat rectangular patterns are suitable for touching-up the cutting edges of lathe and shaper tools while clamped in the toolpost of the machine. Hard Arkansas slips are particularly suited for this purpose, for not only do they produce a very fine cutting edge, but they are not so liable to be scored, as are the softer stones, when applied to narrow cutting edges. The various forms of slips are illustrated later in Fig. 2, in connection with hones composed of artificial abrasive material.
Razor Hones. These stones are usually of soft texture, and free-cutting is promoted by using water, instead of oil, as a lubricant. The best known stones come from Germany and Belgium, but the British varieties are now largely used in this country.
Stones of this type are employed almost exclusively for honing razors, where the blade is laid flat to the surface of the stone during the sharpening operation, but if a tool, such as a chisel, is applied to the stone at the usual angle for sharpening, the cutting edge will tend to dig in and plough up the surface.
Grindstones. These circular stones, formed from natural sandstone deposits, were formerly used exclusively in the cutlery industry and may still be found in carpenters' shops.
In order to prevent heating of the work and to promote free-cutting, the stone is kept flooded with water by means of a drip can or by dipping into a self-contained water tank.
As these stones are of soft texture and have a definite grain, they tend to wear irregularly and may require truing from time to time by means of a piece of hoop-iron or with a tool specially designed for the purpose.
Artificial Abrasives. It will be evident that the abrasive stones, found as natural deposits in many parts of the world, have certain disadvantages ; the finished product has to be fashioned from the quarried stone either by grinding or cutting, which is a laborious and expensive process : the material is seldom homogeneous, and may be interspersed with veins of impurities : the nature of the stone may vary in different parts of the same quarry, thus rendering uniformity of output hardly possible.
For these reasons, a natural abrasive material was sought which could be embedded in a bonding substance to afford all the advantages of the natural stone, and, at the same time, ensure uniformity and allow of precise composition capable of being varied to meet special requirements.
As has been stated, the Arkansas stone is composed of nearly pure quartz and is the hardest of the natural hones, but this material is relatively much softer than the diamond which is the hardest substance known.
A search for a hard natural abrasive resulted in the finding of deposits of both emery and corundum, which are aluminium oxides associated with iron-oxide and other impurities. These substances were then bonded with ceramic or pottery materials to form hones and grinding wheels, but uniformity of performance was lacking owing to the variation of the natural abrasives even when quarried from the same mine.
However, research led to the discovery of the artificial production of the natural materials in their pure form.
The formation of Silicon carbide in the laboratory, by the fusion of sand and coke in the electric furnace, was later followed by the production of aluminium oxide abrasives from the aluminium ore bauxite.
Grinding wheels and hones made from either the silicon or the aluminium artificial compounds are superior to those where the natural materials are used, both as regards the uniformity and uie quality of the work produced.
The Norton Grinding Wheel Co., who were pioneers in the field of artificially-produced abrasives, have adopted the name Crystolon to indicate the silicon carbide compound, and have designated the factory-produced aluminium oxide, Alundum.
Artificial Abrasive Stones. Carborundum stones for bench or hand use are made by many of the leading manufacturers of artificial abrasive materials. The hones manufactured by the Norton Grinding Wheel Co., are named India stones, and these are made in the ordinary bench size of 8 in. long, 2 in. wide, and 1 in. thick, and also in the form of slips or hand stones.
All these varieties can be obtained in either fine, medium, or coarse grit-size suitable for fine or rough sharpening respectively.
India stones are very resistant to wear, and their surface hardness protects them against scoring when tools with a narrow or pointed cutting edge are sharpened.
In addition to the size mentioned, the bench stones are made in a variety of sizes and also in the form of combination stones, composed of two portions of different grit-size cemented together. Some of the more generally used forms of slips manufactured are illustrated in Fig. 2 ; and the special use of these will be described in the appropriate place later in this book.
In accordance with the manufacturer's recommendations, adoption of the following measures are essential for the preservation of the cutting efficiency of these stones ; the initial sharpness is preserved by keeping the stone clean and moist, that is to say by wiping after use and applying a little thin oil to prevent hardening of the surface, also when not in use the stone should be kept in its covered box to exclude dust and air. Secondly, the flat surface of the stone should be preserved by careful use, and to prevent uneven wear the whole surface should be employed and not merely the centre portion. As a further precaution, the side-face of the stone should be used if there is any danger of the surface being damaged when sharpening a pointed tool. Glazing and loading should be prevented by regular oiling and cleaning, for the surface oil floats off the metallic dust and prevents its entry into the substance of the stone. Should the stone become glazed or gummed with dried oil, its cutting properties can usually be restored by cleaning the surface with petrol or ammonia ; but if this does not suffice, the stone should be rubbed with a piece of fine sandpaper fixed to a smooth, flat board.
Grinding Wheels. Although grinding wheels are nc?w made by a large number of manufacturers in various parts of the world, fortunately, the terms adopted to designate the character and properties of the wheels are common to all the well-known makes.
As has already been stated, the two artificial abrasives most commonly used are silicon carbide and aluminium oxide, which are manufactured by the Norton Co., as Crystolon and AlunduiH respectively ; the former is used chiefly for grinding cast-iron and non-ferrous metals, and the latter for tool sharpening and surface grinding.
Grain Size. It is important that the abrasive grains should be of uniform size, otherwise the small grains will not do their share of the work and any large grains will tend to produce scratch marks and thus spoil the surface finish.
The segregation of the grains according to size is carried out by an accurately controlled sieving operation. The Alundum grain sizes range from 4 to 900 ; that is to say these figures represent the number of meshes per linear inch in the screen through which the abrasive material has passed.
For rough-grinding small tools a grain-size of from 24 to 40 is generally used, whilst for finishing the work a 50 to 80 grit will be found to give good results.
Grade. The abrasive grains are embedded in a matrix or bonding substance of which there are two forms in general use, the vitrified and the silicate. The former is more generally suitable for the large variety of work undertaken in the small workshop.
The bonding material is graded not according to its hardness, but by the degree of tenacity with which it holds the abrasive grains.
When a grinding wheel is in operation the abrasive grains gradually become blunted, and if the wheel \s to continue to cut efficiently and without unduly heating the work, it is important that these blunted grai;^ should be shed so that fresh, sharp grains can take their place.
The grade, or tenacity of holding of the bonding material must, therefore, be such that the sharp, free-cutting grains are securely held, whilst the blunted grains are removed by the pressure of the work.
The grade of the wheel is denoted by a letter ; H signifies hard, M medium, and S soft ; likewise, the intermediate grades are represented by the intervening letters of the alphabet.
For general workshop use and rough grinding, a wheel with a vitrified bond of from H to K grade should be used, whilst for fine grinding a softer grade of from J to M will be found suitable.
When sharpening tools such as scribers and very small boring tools, the ordinary wheel used for finish-grinding will probably be found to remove the metal too quickly for the operation to be readily controlled. In this case, the small India wheels made by the Norton Go. will be found to give excellent results, but thin oil should be applied to the wheel-surface to prevent its becoming loaded with metal dust and thus causing heating of the work.
A special green grit type of wheel is necessary for sharpening Tungsten carbide-tipped tools, and, to obtain a really fine cutting edge, this must be followed by lapping on a wheel impregnated with diamond dust.
Care of Grinding Wheels. If a wheel is to continue to give good service it must be run at the correct speed and be properly cared for. The question of mounting and truing the wheel will be dealt with later when grinding appliances are considered. Normally the wheel should be set to run at some 5,000 ft. per min., measured at the periphery ; or if the revolutions per min. are required, this is ascertained by dividing the number 20,000 by the diameter of the wheel expressed in inches.
The table on page 9 gives the exact calculated speeds.
The two common adverse conditions that may arise to interfere with the effective working of the wheel are glazing and loading.
Wheel Diam in inches revolutions per minute for surface speeds of
15.279 7*639 5,093 3,820 3,056
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