Centre of Lathe: Meaning and Methods | Machine Tools | Engineering

In this article we will discuss about:- 1. Meaning of Centering 2. Methods for Locating Centres 3. Turning Work Supported between Centres 4. Rough Turning between Centres.

Meaning of Centering:

Work to be turned in the lathe may be either held in the centres; or fastened in a chuck, or clamped to the faceplate. The work which is to be faced or turned true with a finished hole is held either no a mandrel between centres, or on a special mandrel the shank of which fits the spindle. In the case of work to be held between the live and dead centres of the lathe, first sixty degree counter-sink holes (which fit the sixty degree lathe centres) are drilled and reamed in both ends of the work.

The work is fitted in the centres and is usually driven from the face-plate by means of a dog which is securely clamped to it on the live-centre end. The work thus turns with the live centre which acts both as a support and a bearing.

In attaching a lathe dog to the work, it should be ensured that the set screw is securely fastened. If it is required to put a finished end in a dog, it should be protected by a piece of copper or soft brass between the work and the hardened end of the set screw.

It is very important that the work to be turned is carefully centered. If this is not done then a big chip will be removed from one side of the diameter and a small chip from the opposite. This unevenness of cut takes more time and may cause inaccuracy. In some cases where the un-machined work is not having much stock to be removed then it may be difficult to obtain the exact size even.

Methods for Locating Centres:

The centres may be located by any one of the following methods:

(i) Using Divider:

In this method the end of the work is chalked and the divider set to approximately one half the diameter of the piece and four lines  are scribed. The center of the square formed by these lines will be the centre of the work.

(ii) Caliper-Method:

In this method the hermaphrodite caliper is set to about the radius of the piece. One leg is placed on the circumference at the extreme end of the piece and with the other leg having pointed scriber, four arcs (from 4 positions 90° apart on the circumference) near the centre of the piece are drawn. The four arcs will form an approximate square, the centre of which is the centre required.

(iii) Centre-Head Method:

In this method both limbs of the centre-head of a combination square are held against the surface of the work and with a scriber two lines are scribed at about right angles to each other. The intersection of these lines is the required centre.

After the centre is located it may be marked with a centre punch having a point ground to about 90°, keeping it vertically at the centre point and tapping with a hammer to a sufficient depth so that it may be possible to revolve the workpiece on the centres of lathe freely and thus checking the accuracy of centre-punch marks with a chalk-mark.

After that the centre holes may be made by holding the work in the chuck of the lathe and the counter sunk drill in the tailstock. Centre should not be made too big; it should be just large enough to withstand the resistance of the cut.

A centre drill is used for drilling the work at each end to produce a conical shape for supporting work for turning between centres. Centre holes should be drilled very carefully. For precision work, the centres should be machined ground, or lapped out with a conical lap and fine grinding paste.

The cylindrical portion at the bottom of the conical hole ensures that the lathe centre engages only the cone shaped part, and clears at the point. A bar when supported between ends prepared like this, cannot move in any way, except to rotate about its own axis. The conical surfaces act as datum faces from which all diameters are produced.

The chief advantage of working between centres is that the work can be removed from the machine and replaced, and will still revolve about the same axis. The work may be transferred from one machine to other, and same datum faces are used again and again. The work supported between centres is driven by a carrier dog fastened at one end, the finger of dog being driven by a clutch-plate mounted on the spindle nose.

Turning Work Supported between Centres:

For this purpose centre holes on end faces of workpiece are drilled using a combination drill and countersink tool. The angle of tool is identical with the centre point. The straight drilled hole not only provides clearance for the centre point but also serves as a reservoir for the lubricant.

The centre hole should be drilled deep enough to provide adequate support, otherwise the centre point will burn off. On the other hand if the hole is drilled too deeply, centre would ride on lip of holes, and groove will wear on dead centre.

For accurate turning between centres, the live centre should run true and in precise alignment, otherwise a tapered piece will be produced. The live centre can be tried by taking a light truing cut or by a tool post grinder (if it is hardened). The trueness of centres may be checked visually by bringing the centre points together and by checking the witness lines on the back of the tailstock for alignment.

For close tolerance work the precise method of checking alignment should be used. A steel test bar is mounted between the centres and a dial indicator mounted in the tool post and at right angles to the work. The indicator is moved along the test bar and if no change in dial indicator reading is noted then the centres are aligned.

For mounting the work between centres, use of dog is made. The dog is clamped to one end of the workpiece. A little lubricant (white lead or graphite and oil) is applied in the centre holes of workpiece. The work is mounted in the centres and tail centre adjusted until the work is snug. If workpiece is too loose, it will chatter.

If it is adjusted too tightly, then scoring or burning of centre will take place. It is very important that dog tail should not bind on the face plate slot because in that case the work will be pulled off centre and out of round (non-concentric) hole will be produced. The situation can be remedied by using different face plate so that centre hole seats properly on the headstock centre.

Rough Turning between Centres:

Rough turning operation is used where excessive stock is to be removed and surface finish not critical. For such operation, deep cuts with coarse feed are used. The compound rest is set at 30 degrees from a right angle to the work. Such an arrangement permits the tool to cut as close to the left end of the work as possible.

However, it should always be checked and ensured before starting the lathe that the compound rest will not be fed beyond the distance without striking the dog or chuck. Usual a left-hand tool holder is used and the tool post is positioned as far to the left in the compound rest T-slot as possible. Excessive over change of tool should be avoided as it results into chatter and caused a rough machined surface.

For rough turning the cutting edge of the tool is located about 1.0 mm above work centre, for each 16 mm of diameter. It must also be ensured that tool holder is positioned correctly. An incorrectly positioned tool will tend to turn in the tool post due to heavy side pressure developed during the machining operation, and will force the cutting tool deeper into the work.

On the other hand, a correctly positioned tool holder will cause the cutting tool to pivot away from the work. Due to quick heating of centre in rough machining, it should be lubricated from time to time. It is advisable to use heavy duty ball bearing centre for rough turning.

The rough as well as finish machining operation should be carried out toward the headstock and avoided toward the tailstock since the former is more solidly mounted.