3. Tools for metal drilling

There are various kinds of drills:

Drills type “hard”

They are used for materials harder than steel, for instance chromium-nickel alloys, cast steel and alloyed steels of a tensile strength of more than 500 MPa

Figure 3 - Drill type “hard”

Drills type “normal”

Application with general-purpose constructional steel, low-alloy steels, cast iron

Figure 4 - Drill type “normal”

Drills type “soft”

Used for materials softer than steel such as long-chip aluminium alloys, copper, zinc, plastics, a number of compression moulding materials

Figure 5 - Drill type “soft”

Drills with cemented carbide tips

Used with very hard and resistant materials as well as natural materials (such as stone, concrete, ceramics, glass)

Figure 6 - Drill with cemented carbide tips

3.1. Construction and drill point of a drill type “normal”

This type of drill is the most common tool for drilling of steel:

Figure 7 - Construction of the drill type “normal”

1	chisel edge	a	- lip relief angle (4° - 6°)
2	principal cutting edges	b	- lig angle
3	flanks	g	- rake angle (25°)
4	land	d	- point angle (116° - 118°)
5	helical flute	y	- complementary angle of the chisel edge angle (55°)
6	shank

The chisel edge is an edge at the point of the drill. It presses and squeezes the material in the bore hole and, in doing so, consumes approximately one third of the thrust.

Therefore, larger bore holes are predrilled with smaller drills (according to the length of the chisel edge of the big drill) or the drill is equipped with a laterally sharpened chisel edge. The principal cutting edges do the actual chipping work; they must always be sharp and provided with a flank (behind the cutting edges), so that these cutting edges have the effect of a wedge.

The land is in contact with the bore hole wall; due to its narrowness, there is only a little friction during the boring process and a good guidance of the drill (no walking off centre) is achieved.

The helical flute transports the chips out of the bore hole and, at the same time, enables the lubricating and cooling agent to reach the point of the drill.

Figure 8 - Comparison of the rake angles

1	Drill type “hard”,
2	Drill type “normal”,
3	Drill type “soft”

The three types of drills are distinguished by the rake angle, which determines the slope of the helical flute (type “hard” -15°; type “soft” - 35°).

The shank - with drills of diameters up to 10 mm - is normally cylindrical, with diameters of more than 10 mm it is conical, because with these drills, the very strong power transmission is effected through the favourable reception of the taper sleeve in the machine.

3.2. Special drill point

In addition to the type of drill, the drill point can be different with hard or soft materials in order to achieve a good chip removal:

Figure 9 - Special drill points

With hard materials

- Point angle 80° - 90° (1) or double-cone drill point (2)

With soft materials

- Point angle 130° - 140° (3) or flat drill point with centre point (4)

For drilling of thin sheet metal, too, the drill with “flat drill point and centre point” is used in order to prevent the bore hole from tearing out. This drill may also be used for cylindrical counterboring instead of a counterboring tool with pilot.

Figure 10 - Use of the drill with “flat drill point and centre point” for thin sheet metal

What kinds of drills are there?
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When do you use the “normal” type of drill?
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What main parts does the drill consist of?
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What is the disadvantage of the chisel edge and how can it be compensated for?
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Which kinds of drill points are recommendable for drilling hard materials?
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Which kinds of drill points are recommendable for drilling soft materials?
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What fields of application are there for the use of the drill with “flat drill point and centre point”?
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