TESTINGLATHE ACCURACY

THB most important test of alathe lies, no doubt, in theaccuracy of the work and

the manner in which it is produced-a skilled operator being able toovercome considerable basic in-accuracy. Even if the work is ofa relatively simple character, amplescope for error exists-providedthere is sufficient variation to testall aspects of alignment.

This is to say, a lathe may beaccurate for one type of work but notfor another, and experience of itcan indicate where to expect errors,though the reason why may notalways be immediately apparent. How-ever, there are various simple testswhich are largely a substitute for“ work experience ” and which canbe useful for discovering errors, fortruing the machine (if possible), or onoccasion for setting up.

FIXED CENTRE /

13 DECEMBER 1956 831

A simple but important test, A, isthe meeting of headstock and tail-stock centres. When’ the fixed centrehas noticeably dropped, wear of theunderside of the tailstock and possiblyof the bed itself is indicated. Theeffect on between-centre turning maybe small or non-existent, but carewill be necessary when using centredrills or boring cutters from thetailstock-a degree of “lift ” thenbeing necessary for the tools to centre.

The same effect also obtains whensupporting chucked work from thetailstock. This test should be madewith the barrel both close in and wellextended. A sideways error of thesame kind can often be corrected byadjusting the tailstock.

A more severe test of the same sort,B, can be made with an indicator,which can be a dial type or one of thesmall inexpensive varieties. Theindicator is mounted in a chuck or on

a driving plate with its plunger bearingon the fixed centre. Then the machinespindle is turned, when a steadyreading shows perfect alignment, usinga small mirror to see the instrumentupside down and from behind.

Where there are variations in read-ings, as is virtually always the case, itcan be seen in which direction (verticallyor sideways) they occur, and the tail-stock adjusted to correct sidewayserror.

A live centre test for running truth,B, can be made of the one normallyin the spindle and of others of thesame taper with the indicator mountedon the slide-rest. This can sometimesreveal that the centres themselveshave not been ground true; and in useit may be advisable to keep them to Jone position-spindle or tailstock-marking for fitting in a certain manner.

GENERAL ALIGNMENTA test for general alignment of

headstock and tailstock for between-centre turning can be made employinga mandrel, C. Any suitable piece ofrod can be used, carefully centred,reduced in its length, and with theends turned the same size.

A tool mounted on the slide-restcan be brought close to one diameter,leaving a small gap, tchecked on the otherpiece of white paper on the bedproviding a light background againstwhich to see the gaps. Finally, thetool may be set to touch the diameterslightly when traversing the saddle.This aids reasonable setting of alathe before work begins, and as analternative to a tool an indicatorcan be used.

A faceplate may be checked with arule, D, and when mounted on thespindle and rotated is tested for facewobble. If chuck work is machinedtrue, a test can be made on the face-plate of cross-slide alignment-in twostages, E and F, using a tool or anindicator.

Testing along the near side alongline X-Xl, no error may be shown ifthe faceplate was machined on thelathe, for alignment corresponds tothe cross-slide. Testing on the farside, however, on line X2-X3, anyerror is doubled and can be easilyseen.

Topslide setting can be tested asG, an indicator on a mandrel and arounded rod on the slide-rest. Withthe slide out of alignment, movementis along such as X4-X5, and checkingwith saddle traverse, variations areshown, whereas with a true settingreadings will be all the same. rzl

MODEL ENGINBBR

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