7/29/2019 [Metalworking] Model Engineer - Center Finding on the Lathe

1/1

CENTRE FINDING

on the lathe

IN marking-off procedure, centresare commonly located fromcrossed lines. Then the inter-

sections a r e c a r e f u l l y centre-punched for the drilling or boringwhich follows. Wi t h relatively-large holes-whether to be finishedby drilling or boring-it is oftenadvisable to locate dividers in thecentre-punch indentations and toscribe circles corresponding to theholes, as guides. Alternatively,with a height gauge, hole positionsmay be boxed in with squares.

One relies, in drilling, on the drillsfollowing accurately into the originalindentations, using small drills first,opening out gradually, and checking

B y GE OME T ER

from the scribed circles or boxed-inpositions. Before the final drilling,of course, small errors from con-

centricity can often be corrected byudicious use of a round file.In lathe work, a similar principle

can be followed, in so far as a jobcan be eased over in the independentchuck, or adjusted on the faceplateor angle plate, if it is seen that a bores not machining accurately into

position. But it is generally preferablefor the set-up to be initially precise,as a generated bore cannot wanderoff centre-unlike one produced bydrilling; and, given the accurate set-up, one can concentrate attention onthe size and finish of the bore.

Finding a centre from a pair ofcrossed lines needs only a scribingblock, and a flat plate on which touse it if the lathe bed is one withnverted vee guides. It is not necessaryto centre punch the position of thebore, scribe a circle, or box it in.The lines alone serve for location.Each in turn is centred by testinghorizontally with the scriber point,urning the lathe, and adjusting the

work and scriber as required.After a line has been brought

horizontal, the lathe is turned through180 degrees. Then the line is above

or below the point, and so this is2 6 J A N U A R Y 19 6 1

tapped up or down half the difference,and the line is brought to it by adjust-ing the work.. After a few tests andadjustments, the line is on the latheaxis.

For work in the independent chuck,A, one line S-Tis trued first, and thenattention is given to the other U-V,followed by a further check on thefirst line. For work on the faceplate,where a straight edge is parallel toa line, the line should be trued first.Then a backing strip can be bolted tothe faceplate, to keep the work in

alignment while the other line istrued. Otherwise the first setting canbe lost in unclamping to adjust thesecond line.

To locate a centre in the middle ofparallel material, a line can be scribedfrom each edge, B. Here the requiredposition is W, while X is the distancefrom the end. Two lines Y and Z areeach scribed the same distance fromtheir adjacent edge, and each broughtto the same horizontal setting withthe material in the chuck. This canavoid a scribing error in centralising.

Using a centre punchTo find the centre of work mountedon the angle plate is another job donewith a scribed line. Any convenientmetal block can be marked for heightwith the line, and then clamped to theangle plate, C. Adjusting this on thefaceplate, the line is brought true intwo positions. A further way ofmaking the setting is to use a mandrelbetween centres with a suitablegauge, D.

Finding a centre from a tiny centrepunch indentation can only be donewith a needle, E. It is fixed in a little

ball located in a countersunk hole inspringy material, so as to be able towobble. A sleeve of cycle-valverubber will hold the needle, and thespringy material can be bolted to aholder for the tool slide. The slideis eased up, the point put in theindentation, and the work trued tosteady the needle.

A piece of thin plate, F, with itstop edge at centre height, provides aquick centre finder for scribed lines,and is a means for setting the scriberpoint, for an upward tilt of which aclip with an overlapping edge is used

on the plate. q103 M O D E L E N G I N E E R