6. Centring, drilling and counterboring or countersinking

Centring is made by means of the drill chuck and the centre drill in the marked centre of the workpiece. It is often substituted by centre punching. The choice of the centre drill depends on the kind of the necessary centre bore.

If subsequent operations (drilling, etc.) are to be carried out, use a centre drill according to Fig. 25.

Figure 25. Centre drill (A-centring 60°)

Figure 26. Centre drill (B-centring 60°, 120°)

Feeding is made by hand. Uniform guiding is necessary to prevent tool breakage.

Drilling can be made by means of the drill chuck or taper sleeves by using a twist drill. Preparation by centring or centre punching is necessary. Twist drills are available in different designs with parallel shank (standard type up to a diameter of 10 mm) and taper shank (tool taper size MK 1-6 for drills with a diameter of more than 10 mm). For especially hard materials carbide-tipped drills are applied.

Almost every drill makes larger bores than is specified by its diameter. It is advisable to make a test bore.

If chips collect in the spiral flutes of the drill, heavy friction occurs on the bore wall. This may lead to drill breakage.

For this reason the drill must often be withdrawn from the bore hole and the chips must be removed. On principle, the drill cutting edge must be cooled, not the workpiece! For an economic machining it is also possible to use combination tools (stepped drill, multi-cut drill).

Figure 27. Stepped drill

Figure 28. Multi-cut drill

Counterboring is made by means of counterbores with cylindrical shank by using the drill chuck. If tools have a taper shank, taper sleeves are used. Combination tools are possible here as well (piloted counterbore, step counterbore).

Figure 29. Piloted counterbore

Figure 30. Step counterbore

The spiral-flute counterbore (see Fig. 11) is often applied. Being a three-lipped tool, it is used for expanding holes (preparation) for reaming. Thanks to its three-lip shape it has good guiding properties and corrects centre-distance displacements in bores. As for counterboring and countersinking always multiple-cutting-edge tools are applied, chatter marks (surface defects of the machined surfaces) may occur at high cutting values. In this case the cutting values are to be reduced.

A fitting bore of Ø 32 E8 is to be machined into a workpiece by means of the technique of reaming. Determine the limits.
____________________________________________________
____________________________________________________

As far as the machine reamers are concerned which are used as tools, we distinguish between non-adjustable (see Fig. 12), adjustable, shell, helically fluted and taper reamers. Non-adjustable machine reamers are most frequently used. Helically fluted machine reamers are used for workpieces with interrupted cut (grooved workpieces).

Figure 31. Reamer with helical flutes

In addition to this, hand reamers are used for assembly purposes. It is to be considered that the final size of a reamed bore does not only depend on the correct diameter of the reamer. The kind of material to be reamed, the axial alignment between workpiece and tool and the kind of coolant and lubricant used also influence the dimensional stability. One and the same reamer, for example, can produce quite different diameters with tough steel and with brittle cast iron. Therefore, a test bore must always be made. If there is a deviation from the dimensional stability, the tool must be changed, as there is no reamer which completely equals another one in its dimensions.

What machining allowance is to be chosen for a Ø 16 F7 bore?
____________________________________________________
____________________________________________________

What commonly used possibilities of combination of counterboring or countersinking tools exist?
____________________________________________________
____________________________________________________