It must be borne in mind that in the final analysis, all of the power used in cutting plastics or metal is ultimately transformed into heat. The cutting of the chip by the shearing action of the tool-bit, the small particles of material sliding over each other, the back of the chip sliding over the top of the tool, and the compression of the material at the point of the tool, all generate heat which must be dissipated. This is problem enough when working metals, but cast resin plastics are infinitely finegrained—no grain at all in fact—and one of the best heat-insulators known, which means that they cannot get rid of the heat themselves in anything like the degree that metals can. That means that the entire burden of dissipating the heat generated while cutting devolves upon the tool-bit, and therein lies the reason why plastics will dull a tool edge quicker than steel, why stellite or tungsten carbide points work better (because they can be operated at red heat without affecting their temper) and why hard bronze bits, a relatively soft material, stand up better than tool steel (because they are better heat conductors). Cutting-speeds range from 200 to 500 surface feet per minute.
The same comments made in the section on woodworking lathes regarding chucking, a loose tail-center and mandrels apply when working plastics on a metal-working lathe.
Drilling, routing, sawing, surfacing and milling are among the various operations which can be performed on plastics in addition to turning and boring, in the metal-working lathe, using regular metal-working equipment and accessories. One of the photographs illustrates the milling operation and drilling. Regular milling cutters, driven at as high a speed as possible, and with as slight a rake as possible, are used, and comments about altering drill-bits have already been made in a previous section.
Sanding, buffing and polishing should all be done in the lathe if possible before removing the work, providing the lathe is so set up that fairly high speeds are possible for the latter operation, as this can be performed much more quickly in the lathe than on a buffing-wheel, and more uniformly.
Faceting A very frequent commercial operation in working plastics, and one which the craftsman would do well to try, is faceting. Commercial shops use a sort of automatic dividing-head and tool-post-grinder with a fine-grain disc for this work, but the amateur can do just as good, although slower work, either freehand or with one of the gem-cutting machines recently placed on the market for amateur lapidary work. The material is so soft and polishes so quickly that a 32-facet stone can be cut and polished in a few minutes, since each facet requires only a touch to the wheel and it is cut. If properly polished, in such a way that the square corners are not rounded off, a colorful piece of material has been selected, it would be difficult to tell the "gem" from a real stone except on very close examination. These gem-cutting machines, incidentally, have several features interesting to plastics workers, including the "irrigation system" which eliminates the dust in grinding, sanding and carving operations. They are equipped with various types of grinding-wheels and polishing laps, are easily handled and quite safe, quiet, and also might be rigged up to do considerable carving by means of home-made cutters described in the next chapter. The author is shown working with one of the more popular makes in Plate Y.
How To Make A High-Speed Abrasive Cut-off Wheel This type of saw, as previously stated, is the most popular cut-off device in commercial plastic-working shops, because of its speed, the clean cut which requires practically no finishing, and the fact that it cuts without heating or scoring the material. The high cost of such equipment, however, prevents its use in small shops, amateur or professional, and the strength, weight and precision required in its construction, together with the water-stream and so on, have daunted the average craftsman from trying to construct one.
However, I am presenting herewith a design which I believe is safe, sensible, and above all within the range of a good craftsman, simplified so as to require no special machine-work or expensive parts in its construction, and yet which will serve the purpose for which it is intended as well as equipment selling for two or three hundred dollars. No doubt many variations in the design will be made by those having certain parts or equipment on hand which can be incorporated in the design, and others may want to substitute more expensive parts than those indicated. All of this can be done, within reason, provided only that the few cardinal principles of design enumerated below are scrupulously adhered to.
Not only is the water-cooled abrasive cut-off wheel suitable for plastics, but it is widely used for cutting practically all types of material from fibre to high-speed steel, in a fraction of the time required by any other type of cut-off device. The wheel will slice thru a bar of brass almost as fast as it will through cast resin, and other materials at proportionate speeds. It is no toy, and not worth while making up for a once-a-month job, but for any shop having sufficient cut-off work of any kind to do (except wood of course) it will be found to be an extremely useful device.
Abrasive cut-off wheels (the cutting disc itself) come in a great variety of grit, hardness, binding and thickness, each company having its own nomenclature and designations. The safest way to choose the proper wheel for the work in hand is to ask the advise of the particular maker whose wheels you are buying.
Such a wheel running dry on a bench-saw or similar arbor must be very coarse and open, as otherwise it will burn, but a wheel running under a water-stream as this one does can be much finer-grained without any danger of heating, leaving a much finer finish on the work. In selecting a wheel, a compromise is made between speed and finish, depending on the type of material to be cut, and the balance of cost between a slower speed and the finishing cost.