Page 112 - Turner 1st sem Practicals

Skill sequence

Centre drilling on lathe

Objective : This shall help you to
• centre drill a work held in a chuck.

Round workpieces can be quickly and accurately centre- Lock the tailstock in this position.
drilled without the necessity of centre punch marks.
Start the machine and slowly feed the centre drill into the
The procedure to centre drill a work held in a chuck is given work by rotating the tailstock hand wheel.
below in sequence. (Fig 1)
Withdraw the drill frequently from the workpiece to clean
the chips and to apply the cutting fluid.
Continue drilling until about three fourths of the tapered
portion of the centre drill has entered the work. (Fig 2)

Ensure that uniform continuous pressure is
applied during feeding and no extra force is
given.

Hold the work in a four jaw chuck about 50 mm outside and
true.
Finish face the work with a facing tool.

Ensure no ‘pip’ is left out in the centre and the
face is at right angles to the axis.

Mount the drill chuck in the tailstock spindle.

Remove dirt on the taper shank of the chuck After drilling to the correct depth, withdraw the tailstock
and the tailstock spindle taper bore. spindle.
Note
Mount a suitable centre drill securely in the drill chuck.
When the diameter of the work is more than 150 mm with
Set the spindle speed about 1000 r.p.m.
the same amount outside the chuck, and when irregular
Slide the tailstock over the bed until the centre drill is close work is held in the chuck, running the machine at 1000
to the work face. r.p.m. for centre drilling will cause undue load to the
spindle. Avoid this method of centre drilling.

Common errors in centre drilling

Condition of How to avoid and
centre hole Errors correct the errors

No clearance for point Drill pilot hole.
of centre.
Countersink pilot hole at 60°.
Centre hole incomplete.

Insufficient bearing Drill centre hole
surface for lathe centre. with a centre drill.

Producltion & Manufacturing : Turner - Exercise 1.3.33 99