Centring with the turning tool is applied whenever centre drills cannot be used. Centre drills cannot be used
- for workpieces with bigger internal diameter,
- when existing centre holes are untrue or have been damaged and, consequently, must be re-centred.
The workpiece is machined on the end face and internal surface.
- The workpiece is mostly pre-machined. Therefore, the existing dimensional accuracy must be maintained.
- It is absolutely necessary to check for true running prior to centring. In case of any run-out, true running is to be ensured by suitable measures (turning out of jaws, use of steady rest as counter-support).
- The depth of straight holes is to be considered in re-centring. If necessary, the hole has to be re-drilled with a twist drill.
- Normally the top slide adjustment (Fig. 24) is utilized for centring.
Another possibility is provided by the process of positive form turning (Fig. 25) with the turning tool to be aligned prior to machining by means of a grinding gauge.
Figure 24 Centring by top slide adjustment
3 tool slide,
4 top slide,
5 direction of feed
Figure 25 Centring by form turning
2 grinding gauge,
3 turning tool,
4 direction of feed
- With the top slide adjustment, the top slide is to be swivelled by the necessary degree (mostly 30°) and the tool slide to be locked during machining.
- The clearance angle of the turning tool is to be ground so as to permit proper cutting. It is to be ensured that the side-cutting edge is clear to work.
- The feed should be operated as even and as smooth as possible so as to achieve the necessary surface finish.
- For freely rotating ends of the workpiece a steady rest can be used (see Fig. 26).
Figure 26 Use of the steady rest
- Re-centring must never be done with a multi-edged tool (centre drill, counterbore/countersink) since such tools would follow the old offset centre hole.
What is to be done if an existing centre hole is
How are workpieces re-centred?
What centring possibilities do you know?