TWIST DRILLS: TWIST DRILL PARTS, DRILL DIMENSIONS, IMPORTANT ANGLES OF A DRILL

TWIST DRILLS

Twist drill is the most widely used tool in modern drilling practice. It consists of a cylindrical body carrying two spiral flutes cut on it. Twist drills are usually made of high speed steel. 

They are made in various sizes to suit the work and are provided with either tapered shank or parallel shank. Small drills say up to 12.7 mm dia. are provided with parallel shank and larger sizes with tapered shank. 

Morse taper is commonly used for the tapered shank. Other types of shanks used on twist drills are bit shank and rachet shank, but they are not very common.

Twist drill consists of two main parts. Viz. a shank, which is gripped in the drill chuck or sleeve and the other the body, which forms the main cutting unit.

TWIST DRILLS


TWIST DRILLS


Tapered shank drills carry a tang at the end of the shank to ensure positive grip. The body consists of flutes, for removal of chips, the lips or cutting edges, the point or dead centre. The land or portion of full diameter which guides the drill in the hole and the body clearance or relief.

The drill should be so ground that the point is exactly in the centre i.e. the axis of the body and the lips are of equal angle and length. This will enable the production of a perfectly round, smooth, parallel and accurate hole of desired size and the drill will have better life. Unequal lips result in an over sized hole. 

This can be easily tested by drilling a small hole in mild steel. If the drill is correctly ground the spirals of chips that cut out will be exactly alike. If one is smaller than the other, or is broken it indicates an incorrect grinding. Alternatively, a drill grinding gauge can be used to test the same.

The advantages of using twist drills are:

1. The chips and cuttings of the metal are automatically driven out of the hole through the flutes.

2. Cutting edges are retained in good condition for a long time, thus avoiding the frequent re-grinding of the drill.

3. Heavier feeds and speeds can be used safely, resulting in a considerable saving of time.

4. For the same size and depth of the hole, they need less power in comparison to other forms of drills.

TWIST DRILL PARTS:

The twist drills are made to carry one of the following two types of spiral grooves on the body.

1. HIGH HELIX: 

They carry a helix angle of 35to 40and a heavy web. Their groove width is larger than that of the usual twist drills and therefore they enables easier and quicker disposal of chips. They are largely used for deep hole drilling especially in low tensile strength materials like Copper, Aluminum die casting alloys, Plastics, Wood etc. They are also known as Fast spiral drills.

2. LOW HELIX: 

They carry a small helix angle and are relatively more rigid. On account of their high rigidity, they are capable of taking higher torque and heavier feeds. They are widely used in general drilling work. They are also known as Slow spiral drills.

The detailed description of the different parts of a twist drill and their functions are as follows:

1. Body: It is that part of the drill which carries flutes and extends from dead centre up to the start of the neck.

2. Axis: The longitudinal centre line of the drill, along which the whole body, neck and shank of the drill are concentric, is called the axis of the drill.

3. Chisel edges or dead centre: The short edge formed at the extreme tip end of the drill, due to the inter section of the flanks, is called the chisel edge or dead centre. It coincides with axis of the drill. Some of the drills carry a screw type or spiral shaped chisel edge instead of a sharp edge type. This facilitates more accurate location of holes and lower axial thrust.

4. Shank: The portion of the drill beyond neck, which is gripped in the holding device (ex-Drill chuck, sleeve etc), is called Shank.

5. Point: The cone shaped surface at the end of the flutes, formed by grinding, and containing the dead centre, lips and flanks etc is known as point.

6. Lip or Cutting edge: It is the main cutting part of the drill and is formed by the inter section of each flank and face. There will be so many lips in a drill as the number of flutes and the faces. In a commonly used twist drill there are two lips, because it carries two flutes and two corresponding flanks. For correct drilling it is essential that both lips should be of equal length and be equally inclined with axis of the drill.

7. Body Clearance: A small reduction in the diameter of the body is provided on the drill adjacent to the land is called body clearance. It helps in reducing friction between the drill and the walls of the hole and thus helps in both metal cutting and increasing tool life.

8. Land or Margin: It is a narrow flat surface which runs all along the flutes of the drill on its leading edges. The diameter of the drill measured across its lands determines its correct size. The functions of the lands are to keep the drill aligned during the operation and produce the correct size hole.

9. Lip clearance: That part of the conical surface of point, which is ground to provide relief near the cutting edge, is called lip clearance.

10. Face: The curved surface of the flute near the lip is called face. The chips cut from the material, slide up wards along this surface.

11. Flutes: The helical grooves in the body of the drill are known as flutes. Commonly used drills carry two flutes, while special drills carry four. These flutes make the chips curl and provide passage for their exit. Cutting edges are formed on the point due to machining of these flutes and the cutting fluid reaches the cutting area through these flutes only.

12. Flank: It is the curved surface, on either side of the dead centre, which is confined between the cutting edge on its one side and the face of the other flutes on the other side.

13. Web: The central metal column of the drill body that separates the flutes from one another is known as web. Its thickness gradually increases from the tip side towards the shank side, where it is maximum. This will provide strength and rigidity to the drill.

14. Chisel edge corner: The point of inter section of the chisel edge and the lip is known as chisel edge corner.

15. Outer Corner: The extreme of the dead centre, where the face and flank intersect to form a corner is called outer corner.

16. Neck: The smaller diameter cylindrical portion, which separates the body and shank of a drill, is called neck. All necessary particulars of the drill are engraved on this portion.

17. Tang: The flat portion of rectangular cross-section provided at the end of the tapered shank is known as tang. This fits in to a matching slot in the holding device, such as socket, sleeve or spindle to provide a positive drive. Also for driving the drill out of the sleeve or spindle, the drift is applied over this part of the drill.

18. Heel: An edge is formed where the body clearance and the flute of the drill intersect. This edge is known as Heel.

DRILL DIMENSIONS:

The important dimensions of drill are:

1. Diameter: The linear measurement perpendicular to the axis and across the lands of a drill, at the outer corners.

2. Length of body: It is the length measured along the axis between the dead centre and start

of neck of the drill.

3. Flute length: The length measured parallel to the axis between the dead centre and the point of termination of the flute near the neck.

4. Lip length: The distance between the chisel edge corner and the outer corner is called lip length.

5. Overall length: The distance measured along the axis between the dead centre and the extreme end of the tang is known as overall length. In the case of parallel shank drills, which carry no tang, it is the distance between dead centre and extreme end of shank measured along the axis of drill.

6. Depth of body clearance: The body diameter is slightly reduced either side and is measured along the radius of the drill, perpendicular to axis.

7. Core taper: It is the measure of the increase in web thickness or core thickness, starting from the minimum at the point end and to the maximum at the shank end of the drill.

8. Lead of helix: It is the axial distance measured between two corresponding points on a flute in its one complete turn.

IMPORTANT ANGLES OF A DRILL

Different angles are provided on a drill for efficient metal cutting. The main angles are:

1. Rake angle: 

It is also known as helix angle. It is the angle formed between a plane containing drill axis and the leading edge of the land. It can be positive, negative and zero value. For right hand flutes it is positive, for left hand flute it is negative and for parallel flutes it is zero. For most drills the rake angle varies from 0 to 480.

However 160  to 320 is common for normal materials. Higher values are suitable for soft material and lower value for hard materials. Larger the value of this angle, lesser will be the torque required and viceversa.

2. Point angle: 

It is also known as cutting angle. The most commonly used value for large variety of materials is 1180. However it varies from 80to 140smaller cutting angle for brittle materials and larger for harder and tougher materials. It is the angle included between the two opposite lips of a drill measured in a plane containing the axis of the drill and both the lips.

3. Lip Clearance angle: 

The angle formed between the flank and a plane normal to the axis, measured at the periphery of the drill. Its value varies from 8to 15for most of the drills but 12are most common. It enables easy entry of the drill.

4. Chisel edge angle: 

When the drill is viewed from its end, there appears to be an obtuse angle formed between the lip and chisel edge. This is called chisel edge angle. It determines the clearance on the cutting lip near the chisel edge. The greater this angle larger will be the clearance. Normally this angle varies between 1200 to 1350

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TWIST DRILLS


STRAIGHT FLUTE DRILLS

Twist drill is not suitable for drilling holes in softer materials like brass as it will pull through the back of the metal. A straight fluted drill is proper tool for such work. 

The only disadvantage is chips clog in the flutes then in spiral flutes. Mainly used for drilling holes in sheet metal.

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