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		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

1. Purpose of centring, drilling and counterboring/countersinking

Centring, drilling and counterboring/countersinking are metal-cutting operations with single-, double- and multi-edged cutting tools which are constantly in action. They are used to produce internal surfaces of rotationally symmetrical parts (rotating around their axis of rotation).

Figure 1 Inside machining of parts by centring, drilling, counter-boring/countersinking

1 centre hole,
2 drilled hole,
3 bored/counterbored hole

Centring is a cutting operation with primarily double-edged tool (centre drill) but also with single-edged tool (turning tool). It is used prior to other cutting operations, such as turning between centres, taper turning, drilling and grinding. The work-piece is fixed in a certain position (mostly central position) to the axis of rotation.

Figure 2 Centring on the lathe

1 drill chuck,
2 sleeve,
3 tailstock,
4 workpiece

The main types of centre holes are centre holes types A and B. Type C (protected cylindrical centre holes) are rarely used. Drilling is like centring a cutting operation with single- or double-edged tool.

Drilled holes are required, for example, to locate screws, rivets, bolts, pins. shafts and other cylindrical parts.

Figure 3 Drilling on the lathe

1 chuck,
2 workpiece,
3 twist drill

Counterboring/countersinking is a cutting operation with a multi-edged tool for the purpose of enlarging, boring, spot facing, deburring and chamfering drilled holes. Counterboring/counter-sinking is required, for instance, for inserting bolt, rivet and screw heads into a part.

Figure 4 Boring/counterboring with the turning tool

Figure 5 Counterboring with the piloted counterbore

Counterboring/countersinking is also used to produce plane seating surfaces for screw heads and other machine elements on castings with uneven surface.

Figure 6 Counterboring with the counterbore

What is the basic purpose of centring, drilling and counter-boring/sinking tools?
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