Fig. 2051.
Fig. 2052.
In [Fig. 2049] t represents a table and b a guide bar thereon. The depth of cut taken will be equal to the height the emery wheel projects above the surface a of the table, hence when the bar has been moved nearly half-way across the table its surface will be as in [Fig. 2050], the bar occupying the position shown in [Fig. 2051]. Now the part of the bar that has passed over the table will not rest upon it as is shown in [Fig. 2051]. When the bar has passed over the emery wheel more than half of the bar length, its end f, [Fig. 2052], will fall to meet the half d of the table, and end e will lift from the half c of the table, causing the bar surface to be ground rounding in its length. If, however, the cut taken be a very light one the surface may be ground practically true, because the bar will bend of its own weight and lap down to fit the table at both ends. Furthermore it will be noted that in the case of a large surface in which the wheel might sensibly wear in diameter before it had operated over the whole of the work surface, the table may be lowered or the wheel may be raised (according to the construction of the machine), to offset the wear of the wheel, or rather to take it up as it were.
Polishing Wheels.—For polishing purposes as distinguished from that of grinding, various forms of polishing wheels are employed. For the rougher class of polishing, wooden wheels covered with leather coated with fine emery that is allowed to glaze are employed. For a finer degree of polish the wheels are covered with lead to which various polishing materials are occasionally applied, while for the finest polishing rag or buff wheels are the best. Wooden polishing wheels are built up of sections of soft wood fastened together by gluing, and with wooden pegs in place of nails or screws.
Fig. 2053.
The joints of the sections or segments are broken—that is to say, suppose in [Fig. 2053] that 1, 2, 3, &c., up to 6, represent the joints of the six sections of wood forming one layer of the wheel, the next six sections would have their joints come at the dotted lines a, b, c, &c., up to f. To prevent them from warping after being made into a wheel it is advisable to cut out the sections somewhere near the size in the rough and allow them to lie a day or two before planing them up and fitting them together; the object being to allow any warping that may take place to do so before the pieces are worked up into the wheel, because if the warping takes place afterwards it will be apt to throw the wheel out of true, whereas it is necessary that these wheels be very true, not only so that they may not prove destructive to their shaft bearings, but that they may run steady, and not shake or terrible, and because the work can be made much more true and smooth with a true than with an untrue wheel. Only one layer of segments should be put on in one day, and they should be put on as quickly as possible after the glue is applied, so that the latter shall not get cold. So soon as each segment is put into its place it should be clamped firmly to its seat and driven firmly up to the joint of the next one, and when the layer is completed it should be left clamped all night to dry. In the morning one clamp should be removed, and that section fastened by boring small holes and driving therein round and slightly tapered soft-wood pegs of about 1⁄4 inch diameter. The whole of the sections being pegged the next layer of segments may be added, and so on until the required width of wheel is attained. The whole wheel should then be kept two days before it is turned, and as little as possible should be taken off in the turning process. The circumferential surface should be turned slightly rounding across its width, and as smoothly as possible. It is practicable to proceed with the construction of the wheel without waiting between the various operations so long as here advised, but the wheel will in that case be more apt to get, in time, out of true. To cover the circumference of the wheel sole leather is used, its thickness being about 1⁄4 inch; it should be put on soft and not hardened by hammering at all, and with the flesh side to the wood. The joint of the leather should not be made straight but diagonal with the wheel face, the leather at the edge of the joint being chamfered off, as shown in [Fig. 2054] at a, and the joint made diagonal, as shown in [Fig. 2055] at a.