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?
1.
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2.
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3.
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What kinds of faulty centre holes may occur?
1.
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2.
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3.
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4.
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5.
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