The following points highlight the fifteen main parts of twist drill. The parts are: 1. Body 2. Shank 3. Dead Centre 4. Point 5. Cutting Edge 6. Lips 7. Lip Clearance 8. Margin 9. Body Clearance 10. Webs 11. Rake Angle of a Drill 12. Helix Angle 13. Point Angle 14. Chisel-Edge Angle.
Part # 1. Body:
It is the part of the drill that is fluted and relieved.
Part # 2. Shank:
It is the part that fits into the holding device.
Part # 3. Dead Centre:
It is the sharp edge at the extreme tip end of the drill, formed by the intersection of the cone- shaped surfaces of the point. It should always be in the exact centre of the axis of the drill.
Part # 4. Point:
It is the entire cone-shaped surface of the cutting end of the drill.
Part # 5. Cutting Edge:
It is the part of the point which actually cuts away the material when drilling a hole. It is ordinarily as sharp as the edge of a knife. There is a cutting edge for each flute of the drill.
Part # 6. Lips:
There are the main cutting edges of the drill and are formed by the intersection of the flank and flute surfaces. For efficient cutting, the lips should be straight, equal in length and symmetrical with the axis of the drill.
Part # 7. Lip Clearance:
It is the surface of the point that is ground away or relieved just back of the cutting edge of the thrill.
Part # 8. Margin:
It is the narrow surface (between A and B in Fig. 18.16) along the groove that determines the size of the drill and keeps the drill aligned. It’s surface is part of a cylinder which is interrupted by the flutes and what is known as body clearance. The diameter of the margin at the shank end of the drill is 0.01 to 0.05 mm smaller than the diameter at the point. This allows the drill to revolve without binding when drilling deep holes.
Part # 9. Body Clearance:
The portion of the drill from B to C in Fig. 18.16 is smaller in diameter than the margin between A and B. This reduction in size, called body clearance, reduces the friction between the drill and walls of the hole being drilled, while the margin ensures the hole being of accurate size.
Part # 10. Webs:
It is the metal column in the drill which separates the flutes. It runs through the entire length of the drill between the flutes and the supporting section of the drill. It is in fact the ‘back bone’ of the drill. It gradually increases in thickness towards the shank. This thickness of the web gives additional rigidity to the drill.
Part # 11. Rake Angle of a Drill:
It is the angle of the flute in relation to the work. For ordinary drilling, the rake angle established by the manufacturer of the drill is correct and should remain untouched. If this angle was 90° or more, it would not give a good cutting edge. If the angle is ground too small, it makes the cutting edge so thin that it breaks down under the strain of the work.
The rake angle also partially governs the tightness with which the chips curl and hence the amount of space which the chips occupy. Other conditions being the same, a very large rake angle makes a tightly rolled chip, while a rather small rake angle makes a chip tend to curl into a more loosely rolled helix.
Part # 12. Helix Angle:
It determines the rake angle of the cutting edge of the drill. As it decreases, rake angle also decreases and makes the cutting edge stronger. Usual helix angles for normal materials are 16°, 18°, 20°, 25°, 30° for diameter ranges 0 – 0.6 mm, 0.6 to 1, 1 to 3.2, 3.2 to 5, 5 to 10 and above 10 mm respectively. For harder materials helix angles are smaller of the order of 10° to 13° and for softer materials helix angles of the order of 35° to 45° are used.
The twist drills are made in three tool types, viz. normal with helix angles from 16° for 0.6 mm hole to 30° for 10 mm hole; hard type with helix angles from 10° for 1 to 3 mm diameter holes; to 13° for 10 mm diameter holes; and soft type with helix angles from 35° for 1 to 3 mm hole to 40° for holes of 10 mm diameter.
Part # 13. Point Angle:
It is usually taken as 118° as it gives satisfactory results for a wide variety of materials. Lesser point angle increases width of cut and is used for brittle materials. Point angle of 80° is used for moulded, laminated plastics, hard rubber and marble.
Higher point angle reduces width of cut and produces thicker chips for same feed rate and is used for hard and tough materials. 140° point angle is used for celluloid, copper, aluminium alloys, stainless steel and austenitic steels.
Part # 14. Chisel-Edge Angle:
It is the angle between the chisel edge and the cutting lip, as viewed from the end of the drill. Larger it is, larger will be clearance on the cutting lip near the chisel edge. It varies from 130° to 145°. Large values are used for small diameter drills.