E L E I E Whena component is required to be machined on a lathe, the following principal points must be settled: (r) the manner in which the work should be mounted; (2) the tool set-up to be employed; and (3) the speeds and feeds to be employed. but a setting below centre may cause slender work to deflect and spring on the top of the tool and out of the centres. For all taper turning and screw cutting operations, it is essential that the cutting edge of the tool be mounted exactly on the centre line of the work, oti!erwise discrepancies in taper and thread form will result. A simple way to obtain correct tool height is by setting to one of the lathe centres. Turning between Centres.
This method of turning necessitates centre holes being drilled in both ends of the work. The operation should be performed by a special drill giving a countersink of 60 deg. angle to suit the lathe centres, the centre drill bei..'1g held in a drill chuck mOUJ."1ted i.11 the tailstock spindle with the work held in a chuck. For parting-off operations, use a tool with a rigid shank and with the overhang from the toolpost kept to a minimum. The cutting edge must be set on the work centre and square to the lathe axis. Do not attempt parting-off unless the work is well supported ar. .. ensure that overhang from the chuck is not excessive.
It is usual practice to carry out a facing operation on the end of the work piece before centre-drilling and itis essential that work overhang from chuck jaws, for both facing and drilling operations, should be a minimum, to ensure concentricity. Centre drills are delicate tools and easily broken, largely owing to lack of sensitivity in feeding the drill, and to work speed being too low. To prevent breakage use a high speed of work revolution and a very fine feed. ( Chuck Work. When a chuck is not in use it should not be left so that cast iron dust or other cuttings may enter the bore or parts of the mechanism.
The bore may be protected by plugging with a doth, nevertheless before mounting the chuck on the lathe spindle, dean the bore of the chuck and the spindle nose and lubricate with Molybdenum Disulphide a tube of which, is supplied with the machine. Wit.1J, the work mounted between centres, and fitted with a driving dog and with t.1Je tailstock centre we111ubricated, it is important that the work should be free enough to turn by hand but without any end movement. Owing to heat generated by the cutting action, the work expands during machining, and if screwed up tightly before cutting commences, the result is that the centre end may become overheated causing damage ~o both centre and work.
When mounting or removing a chuck, protect the slides of the bed with a piece of wood upon which to rest the chuck before lifting it onto t..'1e spindle nose. When tightening the chuck jaws, never try to obtain increased gripping power by lengthening the arm of the box key. When using carbide turning tools, the work speed may be so high that damage to a dead centre may take place however much care is used. It is better then, to use a revolving centre which rotates with the work, and is not therefore affected by high work speeds.
For second operation work, or for work which may be'difficult to hold by standard jaws, the use of soft jaws shaped as required will often prove a time saving factor and ensure greater accuracy. Tool Settings. Correct tool setting 1S Important, for accurate grinding of tool angles is of no avail if the tool is improperly set in relation to the work. While normally the tool point should be on the centre line of the work, a setting slightly above centre is permissible, When setting work in an independent four-jaw chuck, make use t 15 ;l~