Drilling, Tapping and Reaming

Drilling — Tools, Geometry and Process

Drilling is the operation of producing a cylindrical hole in a workpiece using a rotating cutting tool called a twist drill. A standard twist drill has two cutting lips, a chisel edge, flutes, and a shank. The point angle (included angle) is 118° for general-purpose work on steel. The helix angle is 20°–30°. The lip clearance angle is 8°–12°. Drills are held in a drill chuck (straight shank) or directly in the spindle (taper shank, Morse taper). Feed is the downward movement per revolution, measured in mm/rev. Cutting fluid (coolant) reduces heat and improves finish.

Tapping — Internal Thread Cutting

Tapping is the process of cutting internal threads in a pre-drilled hole using a hardened tool called a tap. A standard tap set contains three taps: (1) Taper tap — has 8–10 chamfered threads, used first to start the thread; (2) Second (Plug) tap — has 3–5 chamfered threads, used after taper tap; (3) Bottoming tap — has 1–2 chamfered threads, used to cut threads to the bottom of a blind hole. Tap drill size = Major diameter − Pitch. For M12×1.75: drill = 12 − 1.75 = 10.25 mm. A tap wrench is used to rotate the tap. Turn ½ turn forward, ¼ turn back to break chips.

Reaming — Hole Finishing Operation

Reaming is a finishing operation that enlarges and accurately sizes a drilled hole to a precise dimension with a smooth surface finish. The tool used is called a reamer. Types: (1) Hand reamer — straight shank with a square end, used manually; (2) Machine reamer — used in drill press or lathe; (3) Adjustable reamer — can be set to different sizes; (4) Taper reamer — for producing tapered holes. Reaming allowance left after drilling is 0.1–0.3 mm for holes up to 12 mm and 0.3–0.5 mm for larger holes. Reaming speed is kept at half the drilling speed to get a good finish. Cutting oil is essential during reaming.

Cutting Speeds, Feeds and Cutting Fluids

Cutting speed (V) = π × D × N / 1000 m/min, where D = drill diameter in mm, N = RPM. For mild steel, cutting speed is 20–30 m/min; for cast iron 15–25 m/min; for aluminium 60–90 m/min. Feed for drilling is 0.1–0.3 mm/rev for small drills. Cutting fluids serve to cool the tool and workpiece, lubricate, and flush away chips. For steel: soluble oil or cutting oil is used. For cast iron: dry cutting or air blast (cast iron is self-lubricating due to graphite). For aluminium: kerosene or soluble oil. Correct speed and feed prevent drill breakage and overheating.

01Drilling Operation

• Twist drill geometry — point angle 118°, helix angle 20°–30°, lip clearance 8°–12°
• Types of drills — straight shank (chuck held) and taper shank (Morse taper, direct spindle)
• Drilling machines — sensitive drill press, pillar drill, radial drill
• Cutting speed formula — V = πDN/1000; feed in mm/rev

02Tapping — Internal Thread Cutting

• Three-tap set — Taper (1st), Plug/Second (2nd), Bottoming (3rd)
• Tap drill size calculation — Major diameter minus Pitch
• Tap wrench usage — ½ turn forward, ¼ turn back to break chips
• Blind hole vs through hole tapping — bottoming tap essential for blind holes

03Reaming Operation

• Types of reamers — hand reamer, machine reamer, adjustable reamer, taper reamer
• Reaming allowance — 0.1–0.3 mm for holes up to 12 mm; 0.3–0.5 mm for larger holes
• Reaming speed — always half of the corresponding drilling speed
• Surface finish improvement — reaming achieves IT7–IT8 tolerance and Ra 1.6 µm finish

04Cutting Fluids and Their Selection

• Purpose — cooling, lubrication, chip flushing, improved tool life
• Steel and wrought iron — soluble oil (suds) or neat cutting oil
• Cast iron — dry cutting or compressed air (graphite acts as lubricant)
• Aluminium and non-ferrous metals — kerosene or light soluble oil

05Drill and Tap Failures, Precautions and Safety

• Common drill failures — overheating, chipping of cutting edge, drill breakage due to excessive feed
• Tap breakage causes — insufficient tap drill size, no back-turn to break chips, misalignment
• Precautions — clamp workpiece firmly, use correct drill size, apply cutting fluid, check drill sharpness
• Safety — wear safety goggles, never hold workpiece by hand while drilling, remove chips with brush not hand