Basic Cutting Process DRILLING Definition Drilling is the

Basic Cutting Process DRILLING

Definition • Drilling is the production of cylindrical holes of specific diameters in workpieces using drill machine/tools. The drill bit has wedge-shape edges and operates by a chip removal process.

Types of drill bit • • • Flat drill Twist drill Countersink drill Counterbore drill Center drill

Flat drill • The earliest form of drill was the flat drill and it was from all other types were developed. • The flat drill was very easy and inexpensive to produce, but it had the disadvantages that it was difficult to hold on course, its chip removal was poor and its operating efficiency was very low.

Twist drill • The twist drill is made from High Speed Steel, tempered to give maximum hardness throughout the parallel cutting portion. It comprises a cutting point at the tip of a cylindrical shaft with helical flutes. Flutes are incorporated to carry away the chips of metal and the outside surface is relieved to produce a cutting edge along the leading side of each flute. Twist drill-type N 24 • For normal materials like mild steel.

Twist drill – cont. Twist drill-type H 12 • For hard and brittle materials, high-strength steels, moulted plastics and stone. Twist drill-type W 36 • For soft materials, aluminium, copper and zinc alloys.

Twist drill – cont. • The twist drill are always made with a right hand helix. This will forces the chips up the flutes and away from the cutting edges. • While drilling, the drill is held by the shank. The shank may be parallel or tapered. • The tapered shank drills have a taper called the morse taper. • These drills are provided with a tongue (tang) at the end of the tapered shank. • The shank fits into the slot in a drill sleeve or direct in the machine spindle. • It drives the drill and keeps the shank from slipping especially on large drills.

Twist drill – cont. 3 jaws chuck

3 jaws chuck • A Chuck has movable jaws, for gripping drill bit tightly. Installing Drill Bits in the spindle • Using the chuck key, open the chuck just far enough so that the drill bit can be inserted centrally into the chuck jaws. • Carefully tighten the chuck so that it grips the drill bit on the blank portion of the drill shank. • Do not tighten the jaws on the drill flutes. Remove the chuck key. • Rotate the chuck by hand to confirm that the drill bit is inserted properly and not wobbling.

Angles of Twist Drills 1. Cutting angle or angle of lip. 2. Lip clearance angle. 3. Rake angle

Cutting angle • The two lips must be the same length and angle. The cutting angle for ordinary work is 59° and varies with the metal to metal. • Note: – If the cutting angle is more, the drill will not cut the metal easily and the will not hold its position centrally because of being too flat. – If the angle is less, more power will be needed to turn the drill and it will cut as slower rate due to longer cutting edges. – If the angles on the cutting edges are different, then one cutting edge will wear quickly and the hole will be larger than the drill. This results the wobbling of spindle and the drill wears out quickly.

Lip clearance angle • It is the angle formed by grinding away the heel behind the cutting edge. Usually 12 – 15 o • This angle will increase for drilling soft materials under heavy feeds.

Rake angle • It is the angle between the flute and the work. It is usually 70 – 75 o. • This helps to secure the lip over the correct space to curl and chips. • If the angle is more, then there will be no edge for cutting and if it is less, the cutting edge is too thin and may break under the strain of the work.

Countersink drill • A countersink drill has cone shaped cutting edges and is used to cut a chamfer round the edge of a hole so that the conical heads of countersink screws can be accommodated.

Countersink drill – cont. • Countersinks are normally made with included cutting angles of 60°, 75° or 90°

Counterbore drill • A counterbore has parallel cutting edges and is used to open out holes so that screw heads can be accommodated flush with the workpiece surface.

Center drill • Center drill bits are used in metalworking to provide a starting hole for a larger-sized drill bit or to make a conical indentation in the end of a workpiece in which to mount a lathe center. • They are designed to drill a small diameter pilot hole followed by a 60° countersink that provides a bearing surface for the 60° point of the lathe center.

Factors affecting the drilling operation • The three main factors governing drill performances are: Cutting speed, feed rate and cooling method. • Followings are the most common used cutting speed and feed rate. Cutting Speed Material Cutting Speed Mild steel 6 - 9 m/min Stainless Steel 4 - 9 m/min Aluminium 30 - 36 m/min Feed Rate 5. 5 mm diameter twist drill 0. 08 - 0. 15 mm/rev 30 mm diameter twist drill 0. 04 - 0. 55 mm/rev

Cutting speed (CS) • This cutting speed is the peripheral speed of the drill and it is normally stated in metres per minute. The cutting speed is governed largely by the machineability of material from which the workpiece is made, and by the drill diameter.

Spindle speed (RPM) • To determine the correct number of RPM of a drilling machine spindle for a given size drill, the following should be known. 1. The type of material to be drilled. 2. The recommended cutting speed of the material. 3. The type of material from which the drill is made. • The speed for the drill in revolutions per minute can be found using the following formula: RPM = CS 1000 / d Where • RPM = Drill speed (rev/min. i. e. machine spindle speed) • CS = Cutting speed (metres / min) • d = Drill diameter (mm)

Example: Calculate the RPM for drilling mild steel plate using 18 mm diameter High Speed Steel drill at a cutting speed of 30 meters per minute. RPM = CS 1000 / d = 30 x 1000 / 3. 14 x 18 = 30. 000 / 56. 52 = 530 RPM

Feet Rate & Cutting fluid • The rate of movement of a drill into the materials is called the 'feed rate'. As with the cutting speed, the feed rate is governed by the machineability of the material and the drill diameter. Feed rate is normally stated in millimeters per revolution (mm/rev). • Although it is possible to carry out some drilling operations dry, the use of an appropriate cutting fluid will give a better surface finish, allow a higher cutting speed and extend the tool life.

Safety and Care on Drilling • Twist drill must be clamped in the drill chuck tightly • The workpiece to be drilled must be firmly secured by vice, or clamps. • Drill guard (figure 32) must be closed before switch on the machine. • Use the correct drilling speed and apply suitable drilling force It is advisable to release the drill occasionally, lift the drill, and clear the hole of cutting. • Apply cutting fluid in the cutting except for drilling Cast iron. • Take care, when the drill is nearly penetrated through the workpiece. • The "screw in" action can lift up the workpiece.

Safety and Care on Drilling