 | Centring, Drilling and Counterboring / Countersinking - Course: Techniques for machining of material. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 22 p.) |
 |  | (introduction...) |
| | 1. Purpose of centring, drilling and counterboring/countersinking |
| | 2. Types and application of centring, drilling and counterboring/ countersinking tools |
| | 3. Preparation of centring, drilling and counterboring/countersinking |
| | 4. Centring with the centre square, centre bell and height gauge |
| | 5. Centring with the centre drill |
| | 6. Centring with the turning tool |
| | 7. Drilling with the drill and boring with the boring tool (turning tool) |
| | 8. Counterboring/countersinking with counterbore/countersink and turning tool |
5. Centring with the centre drill
Machining of the workpiece by means of the centre drill is done on the lathe in the centre of rotation of the workpiece.
- Centring is done in a chuck using the drill chuck and the tailstock. The cutting tool to be used (centre drill) depends on the size specified, on the diameter, shape and weight of the part to be machined.In addition to different sizes (diameter), centre holes differ in types, i. e. types A, B and C (Fig. 9 - 11 in section 2). The workpieces are machined at the end face and internal surface.
- Faulty centre holes (too big, too small, too flat, too steep, too short) are to be avoided. Safe clamping between centres or proper guidance of the tools in the centre hole would not be ensured in that case, which might result in an increased scrap rate.
- The tailstock is to be checked for central position and, if necessary, to be aligned as per Fig. 23 so as to ensure centring of the centre position of the workpiece with the centre drill chucked. For this purpose the tailstock can be adjusted to either side. Height correction is not possible.
Figure 23 Possibility of tailstock
adjustment
1 work spindle end,
2 tailstock end,
3 positive guidance of the turning tool,
4 tailstock adjustment,
5 possibility of correction
- When centring with the centre drill, high rotational speeds are used since V (cutting speed) is almost 0. The workpiece and the cutting tool must be firmly chucked to avoid accidents and tool breakage.- The work feed is manually operated and should be even and smooth.
- The application of coolant will increase the life of the cutting tools.
- The contact faces of the tailstock must be lubricated. The tailstock must be fixed, otherwise it will be displaced during the operation.
- The centring operation with the centre drill is shown in Fig. 2 (in section 1).
What types of centre holes do you know?
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What kinds of faulty centre holes may occur?
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