4. Centring with the centre square, centre bell and height gauge

If any parts cannot be centred on the lathe for various reasons (hole through spindle too small, part too heavy, etc.), the centre square, centre bell or height gauge may be used depending on the specific application.

The part is machined on the end face which should be as plane and as flat as possible to avoid canting of the tools.

- Centring with the centre square

Fig. 20 shows how this is done:

The centre square is to be located at the workpiece diameter and a scribed marking of crossing lines to be applied by means of the scriber.

Figure 20 Centring with centre square

The point of intersection of the crossing lines defines the centre of the workpiece and of its diameter and is to be punch-marked as per Fig. 21 with the centre punch to be applied in the centre point.

Figure 21 Punch-marking of the central position

- Centring with the centre bell

When using the centre bell, scribing is not necessary. The centre of the workpiece is defined by the central position of the centre punch.

The centre bell is to be attached and applied as per Fig. 18 in section 3. It is held by one hand while punching is done with the other hand.

The diameter of the workpiece must be smaller than the diameter of the bell; otherwise the central position cannot be clearly defined. The centre bell must not be canted; otherwise the centre will be misplaced.

- Centring with the height gauge

The workpiece is to be inserted into a Vee. The point of the scriber (section 3, Fig. 19) is to be lined up with the highest point of the workpiece and to be lowered by d/2.

Figure 22 Workpiece in Vee

1 workpiece,
2 Vee

The scribe-marking is applied in each 180° position.

The workpiece is to be punch-marked.

Which methods can be applied if centring on the lathe is not possible?
1. _______________________________________
2. _______________________________________
3. _______________________________________