. Cassier's magazine. ssage ofthe slide rest is obvious. Another case of support afforded tolong shafts is seen in some big bendingrolls. The usual practice has been toincrease the diameter of these rolls inproportion to length, thus making themrigid in themselves. But it has still been found difficult to maintain the samethickness in the rolled plates over thecentral areas as at the edges, since avery slight amount of spring suffices tomake y% inch of difference. The im-proved device consists in taking thespring of the rolls against a camberedgirder; the latter is provided with fric-tion roll
. Cassier's magazine. ssage ofthe slide rest is obvious. Another case of support afforded tolong shafts is seen in some big bendingrolls. The usual practice has been toincrease the diameter of these rolls inproportion to length, thus making themrigid in themselves. But it has still been found difficult to maintain the samethickness in the rolled plates over thecentral areas as at the edges, since avery slight amount of spring suffices tomake y% inch of difference. The im-proved device consists in taking thespring of the rolls against a camberedgirder; the latter is provided with fric-tion rollers which receive the pressureof the top rolls of the machine. Thelatter are then kept down to reasonabledimensions, and are, in fact, small bycomparison with those which have to bemade stiff enough in themselves. Thisis one result of the use of steel boilerplates i ^ to i y2 inches thick demandedby the present pressures. The bearing problem does not weighvery heavily on the big machine designer, BIG MACHINE TOOLS 215. because speeds are compar-atively low, although at thesame time rigidity and free-dom from chatter must beguarded against. Bearingsof gun-metal or of cast ironare suitable for nearly all bigmachine shafts and ball bearings of thesmaller lathe spindles anddrilling spindles are, there-fore, not wanted in the largershafts and spindles of the bigtools. In the vertical lathes,however, a special problemarises,—that of the bearingsfor the vertical spindle. Inthe earlier types this had aneck with the section of theSchiele curve, which is anideal form. In later ma-chines an approximation tothis is employed, flat, conedsurfaces being used, as beingmore easily machined. Alarge surface support is givenon the back of the face-platewhen doing heavy boring and turning ofsmall diameters at highspeeds the spindle is liftedclear of this face, and it runson the spindle alone. The attendant of the bigmachine is dwarfed beside results follow, one bei
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