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11/09/2007 - Holes machining

Holes machining is one of the most important operations in the industrial field, for its huge diffusion in mechanical components production processes. There is a bunch of hole kinds that can be realized (cylindrical, conical, tapered, etc.) by means of a sequence of operations that are a function of desired geometry and quality. Principal operations on holes are drilling, widening, reaming and tapping. In this section, only drilling will be discussed, being surely the most diffused operation.

Drilling


Drilling is surely the more common operations on holes. This process is characterized by a poor quality and by low level dimensional and geometrical tolerances.
The used tool is the helicoidal point, which geometry is depicted in Figure 1.
The most used helix inclination angle is 118°, that can work on a wide range of materials. For harder materials angles until 130°-150° can be used while narrow angles, 60°-90° are used for “soft” materials. Points are classified as a function of these angles (and of the workable materials) as: N for construction steels, cast iron and non ferrous alloys of medium hardness, D for hard and tenacious materials, T for soft and malleable materials.
Helicoidal points are both keft and right handed, and the last ones are more diffused. They are composed by three different sections: tang (that can be straight or tapered), point (that cut the material) and the central body, as depicted in Figure 2.
The intersection of helicoidal surface with the extreme surface of the point generates the two cutters (AB and CE), connected by a segment (AC) that doesn’t participate to the cut but is used to harden material. The two helicoidal guide edges (BB’ and EE’) have instead the function of guiding the point inside the hole.
The function of the helicoidal grooves is to evacuate metallic chips that forms during the operation and, if used, to bring lubrication fluid to the cutting zone.
Resistance section of the point is constituted by the central core, that usually have a diameter of 10-20% of the external point diameter.
This kind of points are the most widely adopted, but has the disadvantage that they need a preliminary hole for centering (made with a special point, Figure 3). This kind of points has in fact some difficulties in initial material penetration. To resolve this problem without the need for pre-drilling, special points are used (Figure 4), but they usually have an higher cost.
There are several material used for the construction of drilling points. In particular, the cheapest one are made of carbon steel, but present a limit in the cutting speed. Higher cutting speeds can be obtained using tungsten, cobalt and titan points, used for particular applications.

Figure 4: Autocentering point.

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