Drills and drilling

Drilling is one of the cutting operations producing cylindrical holes of different diameter in solid material by means of rotating tools called "drills" (Fig. 22).

The most common type of drill in use¹ is the twist drill made of a tempered steel round bar stock. Twist drills are made with two, three, or four spiral grooves or flutes milled from the solid. These grooves or flutes winding around the body of The drill serve for forming the cutting edges of the drill, as well as for removing the chips formed in drilling from the hole.

Straight-Shank Drill

Tapered-Shank Drill

Fig. 22. Drills.

The twist drill comprises three principal parts: body, shank and point. The twist drill has two cutting edges known as the "lips". These cutting edges, or lips, are connected e by a third edge, called a "web". It is this part that gives rigidity and strength to the drill. When in use the first two cutting edges remove the material

from the work, while the third one penetrates into the material by rubbing rather than cutting.

In order to drill holes in a metal the cutting edges of a drill should be correctly ground to a certain angle. When a drill is ground correctly, its cutting edges, or lips, should have equal length whereupon each of them should make the same angle with the centre line. Otherwise the drill will make holes the diameter of which is larger than that of the drill. This may disable the drill and cause an undesirable waste of material. Drills have shanks of various types, the most commonly used being those having straight and tapered shanks. The shank of the drill serves for clamping the drill either in the chuck spindle or socket of a drilling machine. The above part of the drill may be either of a cylindrical shape, like in² straight-shank drills, or of tapered shape in tapered-shank drills.

The third part of the drill is called a "drill point". It is always ground to a cutting angle varying with the kind of material to be drilled. For hard materials this cutting angle equals 140° and for soft materials it equals 90°. The cutting edges of flat drills used for drilling holes in steel or in cast iron are ground to an angle of 100° to 120°. All the drills get worn while drilling and they should be re-ground from time to time.

1. The most common type of drill in use -самым обычным используемым типом сверла...

2. like in - как в, подобно

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