15 Main Parts Of Twist-Drill

The next points highlight the fifteen predominant parts of twist drill. The elements are: 1. Body 2. Shank 3. Dead Centre 4. Point 5. Leading 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's the half that fits into the holding system.

Part # 3. Dead Centre:

It's the sharp edge on the excessive tip end of the drill, formed by the intersection of the cone- shaped surfaces of the point. It should at all times be in the precise centre of the axis of the drill.

Part # 4. Point:

It is the complete cone-formed floor of the reducing end of the drill.

Part # 5. Leading edge:

It is the a part of the purpose which actually cuts away the fabric when drilling a hole. It is ordinarily as sharp as the sting of a knife. There's a leading edge for each flute of the drill.

Part # 6. Lips:

There are the principle chopping edges of the drill and are formed by the intersection of the flank and flute surfaces. For efficient reducing, the lips should be straight, equal in length and symmetrical with the axis of the drill.

Part # 7. Lip Clearance:

It's the surface of the point that's floor away or relieved just back of the leading edge of the fun.

Part # 8. Margin:

It's the narrow surface (between A and B in Fig. 18.16) along the groove that determines the scale of the drill and retains the drill aligned. It’s floor is a part of a cylinder which is interrupted by the flutes and what is named body clearance. The diameter of the margin on the shank finish 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:

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The portion of the drill from B to C in Fig. 18.Sixteen is smaller in diameter than the margin between A and B. This discount in size, referred to as physique clearance, reduces the friction between the drill and partitions of the opening being drilled, while the margin ensures the opening being of accurate measurement.

Part # 10. Webs:

It is the steel column within the drill which separates the flutes. It runs by the whole 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 progressively increases in thickness towards the shank. This thickness of the online provides additional rigidity to the drill.

Part # 11. Rake Angle of a Drill:

It's the angle of the flute in relation to the work. For extraordinary drilling, the rake angle established by the manufacturer of the drill is right and will remain untouched. If this angle was 90° or extra, it would not give a great innovative. If the angle is ground too small, it makes the innovative so thin that it breaks down below the strain of the work.


The rake angle additionally partially governs the tightness with which the chips curl and hence the amount of space which the chips occupy. Other conditions being the identical, a really giant rake angle makes a tightly rolled chip, while a relatively small rake angle makes a chip are inclined to curl into a extra loosely rolled helix.

Part # 12. Helix Angle:

It determines the rake angle of the innovative of the drill. Because it decreases, rake angle also decreases and makes the leading edge stronger. Usual helix angles for normal materials are 16°, 18°, 20°, 25°, 30° for diameter ranges zero - 0.6 mm, 0.6 to 1, 1 to 3.2, 3.2 to 5, 5 to 10 and above 10 mm respectively. For more durable supplies 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. regular with helix angles from 16° for 0.6 mm hole to 30° for 10 mm gap; exhausting type with helix angles from 10° for 1 to three mm diameter holes; to 13° for 10 mm diameter holes; and smooth sort 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 often taken as 118° as it provides passable outcomes for a wide number of supplies. Lesser level angle increases width of lower and is used for brittle supplies. Point angle of 80° is used for moulded, laminated plastics, laborious rubber and marble.


Higher level angle reduces width of reduce and produces thicker chips for same feed rate and is used for arduous and tough materials. 140° level 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 reducing lip, as considered from the end of the drill. Larger it is, larger shall be clearance on the cutting lip near the chisel edge. It varies from 130° to 145°. Large values are used for small diameter drills.