Drilling, Tapping and Threading Operations
The exam covers twist drill geometry, spindle speed calculation, tap drill size and thread terminology.
Twist drill geometry
A twist drill has a shank (straight or taper), a fluted body and a point. The standard point angle is 118 degrees for general steel, 135 degrees for hard materials and about 90 degrees for plastics and soft materials. Lip clearance is 8-12 degrees. The chisel edge at the centre does not cut but extrudes metal, which is why centre punching and pilot drilling matter.
Speed and feed
Spindle speed N = 1000V divided by pi times D, with V in metres per minute and D in millimetres. For high speed steel in mild steel, V is about 25-30 m/min; cast iron 20-25; aluminium 60-100. Use coolant and withdraw the drill periodically to clear chips.
Related operations
Reaming removes 0.05-0.1 mm for accuracy and finish; counterboring gives a flat seat for a socket head screw; countersinking forms the cone for a countersunk screw; spot facing cleans a seating surface; boring enlarges and trues a hole.
Tapping and threading
For ISO metric threads, tap drill size = major diameter - pitch. M8 x 1.25 gives 6.75 mm (use the standard 6.8 mm); M10 x 1.5 gives 8.5 mm. Taps come as taper, plug and bottoming. Turn forward, back off a quarter turn to break chips, and use lubricant. Dies in a die stock cut external threads. Know pitch (axial distance between crests), lead (advance per revolution, equal to pitch for single start), major, minor and pitch diameters, and the 60 degree metric flank angle.
Worked example
A 10 mm high speed steel drill in mild steel at V = 25 m/min: N = 1000 x 25 / (3.142 x 10) = 796 rpm, so set about 800 rpm. For M12 x 1.75 the tap drill is 12 - 1.75 = 10.25 mm, standard 10.2 mm.
Traps that cost marks
- Using major diameter minus twice the pitch for tap drill size.
- Running a tap at drilling speed instead of slowly by hand.
- Not backing off, which breaks taps.
- Confusing pitch with lead in multi-start threads.