Page:EB1911 - Volume 27.djvu/50

Rh formative process. The first is the older and is still by far the most important. The second is a later ideal towards which design and practice have been extending. As yet grinding cannot compete with the work of the single-edged tools and milling cutters when large quantities of material have to be removed. lust as some

FIG. 54.-Safety Devices.

4, Grinding wheel, with

coned washer to retain

broken pieces in case

of fracture.

B, Cup wheel with encircling

ring, moved backwards

as the wheel face wears.

leading firms have been desi ning stiffer machines having fuller fabrication with a view to increase the duty of grinding wheels, the advent of the high-speed steels has given a new lease of life to the single-edged cutting tools. The rivalry now lies not with the tools of carbon temper steel, but with high speed varieties. But as a corrective

process grinding never occupied so important a position as it does to-day,

and its utility continues to extend. The commoner forms in which grinding wheels are made are shown in fig. 5 3. These are varied largely in dimensions, from tiny cylindrical rollers a fraction of an inch in diameter for hole grinding, to big wheels of 3 ft. or more

in diameter. Safety mountings, two examples of which are shown in fig. 54, embody means of retaining the broken pieces of a wheel in case it bursts. Sand-blast.-The well-known erosive action of sand when driven against rocks and stones by the Wind is utilized industrially in the sand-blast apparatus, the invention of B. C. Tilghman. The sand is przapelled by a current of steam or air, an beirzig delivered through a nozzle is directe against the surface of the work, cutting it away by the action of the enormous number of grains striking the face, each removing a very minute quantity of material. The action is very gentle, and may be modified air reservoir and the blast nozzle through which the air passes and propels the sand in the form of a jet. The pressures range from 8 lb up to about 60 lb per sq. in., depending on the class of work which is done.

The peculiar advantage of the sandblast lies in its adaptability to the working of irre ular surfaces, which could not be touched by any other class of grinding. The blast penetrates hollows and recesses, and acts over an entire surface. There are many classes of operation done with the sand-blast, including cleaning, frosting, ornamentation, engraving and sharpening. In engineers' works a large amount of cleaning is effected upon castings, forgings, sheets and other products, either preparatory to machining or to painting, enamelling, tinning, galvanizing or plating. Cycle frames are cleaned with the sand-blast after brazing. The teeth of files are sharpened by directing a stream of san and water against their backs, with the result that the burr thrown up by the chisel when cutting is obliterated, and a strong form of tooth is produced. Worn files may also be sharpened up to equal new ones by sand-blasting them. Frosting glass is another useful application of the sand-blast, and by attaching suitable patterns or designs to the surface the sand may be caused to work ornamental figurines. It is a peculiar circumstance that the sand has little effect upon soft and yie ding substances in comparison with the abrasion it produces on hard surfaces, so that the pattern will remain undamaged, while the glass or other object beneath is frosted where the sand reaches it, through the openings. Not only can designs be worked on glass, or cut in stone, but perforations may be made in glass, &c., by the continued action of the sand, without any risk of fracture occurring. Much sandblasting is performed inside closed chambers, having panes through which the workman watches the progress of the operation. But when the blast must be used in the open, protection is necessary and is afforded to the operator by a special helmet, which keeps out the flying dust and gives a supply of pure air through a tube in a similar fashion to the diver's helmet.

VII.-SAWING MACHINES

Metal-sawing machines are employed extensively in engineering works for cutting off bars, shafts, rails, girders and risers on stee castings, and for getting out curved pieces which would be dlfflcult and expensive to slot. There are three classes of these saws, circular, he first named are used for straightforward work, operating at

band and reciprocating T

by varying the class of sand and its velocity. - ~- ~ f A - right or other angles, the Other materials, such as emery, chilled iron | — second for straight cuts and globules, &c., are employed for certain classes 1 j f also for curves which canof work. In some instances the powder is. 'Ziff 1 ' not be treated with circular used dry, in others it is mixed with water, '-5-l"'-}'» . =-  i - sawS,21f1d tht? third fO1' Small being then in the condition of fluid mud. The .     pieces. The circularsawsemplant includes an air-compressing engine, an   body a stiff spindle, carrying U IE the saw disk and dig;/en by II T gearing. his spin e may

li be mounted in a sliding

bearing to carry it-past the worl§ held can a fixedbtable, or the spin e may e sta; tlona and the work be

FIG. 55.-Cold-sawing Machine. (Isaac Hill & Son, Derby.) if K if V A, Saw blade. L, Three-step cone on shaft G, belted to M, connected by bevel-gears B, Splfldle-  N and worm-gear O, to the screw K.-C, Sliding spindle carriage. P, Clutch for throwing in O to drive K. D, Driving pulleys. F  Q, Gears connecting shaft of L direct to K, also through clutch P. E, First pinion, connecting through tram or gears to wheel F, driving R, Handle for operating clutch P, which thus ives slow feed when splined shaft G.

H, Wheel driven from sliding pinion on G. J, Bevel-gears, communicating the motion to spindle B. K, Screw or feeding carriage C along. clutch is in mesh with 0, and quick return wien enga ing with-P. EE Tappet rod, having dogs struck by carriage to stop fieding. Work-table, with clamp to hold 0 jects. U, H-Girder being sawn off.