Figure 9-22.—Examples of work mounted between centers.MOUNTING THE WORK.—Figure 9-22 showscorrect and incorrect ways to mount work betweencenters. In the correct example, the driving dog isattached to the work and held rigidly by the setscrew.The tail of the dog rests in the slot of the faceplate,without touching the bottom of the slot. The tail extendsbeyond the base of the slot so that the work rests firmlyon both the headstock center and the tailstock center.In the incorrect example, note that the tail of the dogrests on the bottom of the slot on the faceplate at A andpulls the work away from the center’s point, as shownat B and C. This causes the work to revolveeccentrically.In mounting work between centers for machining,be sure there is no end play between the work and thedead center. However, do not have the work held tootightly by the tailstockcenter. If you do, as the workrevolves, it will heat the center’s point, destroying bothitself and the center. To help prevent overheating,lubricate the tailstock center with grease or oil.Holding Work on a MandrelMany parts, such as bushings, gears, collars, andpulleys, require all the finished external surfaces to runtrue with their center hole, or bore.General practice is to finish the bore to a standardsize within the limit of the accuracy desired. Thus a3/4-inch standard bore would have a finished diameterof from 0.7495 to 0.7505 inch This variation is due toa tolerance of 0.0005 inch below and above the truestandard of exactly 0.750 inch. First drill the hole towithin a few thousandths of an inch of the finished size;then remove the remainder of the material with amachine reamer, following with a hand reamer if thelimits are extremely close.Then press the piece on a mandrel tightly enough sothe work will not slip while being machined Clamp adog on the mandrel, which is mounted between centers.Since the mandrel surface runs true with respect to thelathe axis, the turned surfaces of the work on the mandrelwill be true with respect to the bore of the piece.A mandrel is simply a round piece of steel ofconvenient length which has been center drilled andground true with the center holes. Commercial mandrelsare made of tool steel, hardened and ground with a slighttaper (usually 0.0005 inch per inch). This taper allowsthe standard hole in the work to vary according to theusual shop practice and still provides a drive to the workwhen the mandrel is pressed into the hole. The taper isnot great enough to distort the hole in the work Thecenter-drilled centers of the mandrel are lapped foraccuracy. The ends are turned smaller than the body ofthe mandrel and provided with flats, which give adriving surface for the lathe dog.Holding Work in ChucksThe independent chuck and universal chuck areused more often than other work-holding devices inlathe operations. The universal chuck is used for holdingrelatively true cylindrical work when the time requiredto do the job is more important than the concentricity ofthe machined surface and the holding power of thechuck When the work is irregular in shape, must beaccurately centered, or must be held securely for heavyfeeds and depth of cuts, an independent chuck is used.FOUR- JAW INDEPENDENT CHUCK.-Figure9-23 shows a rough cylindrical casting mounted in afour-jaw independent lathe chuck on the spindle of thelathe. Before truing the work, determine which part youwish to have turned true. To mount this casting in thechuck, proceed as follows:1. Adjust the chuck jaws to receive the casting. Thesame point on each jaw should touch the samering on the face of the chuck If there are no9-14
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