Drills and Drilling Processes

A twist drill is a rotary cutting tool with two helical flutes and two cutting lips, used to produce round holes.

  shank      body (flutes)         point
 |<---->|<------------------->|<-------->
 ====== |  ///////////////  |  /\
 tang   |  helical flutes   | /  \  lips
                              \/  chisel edge

Main parts

• Shank: straight (small drills, held in a chuck) or Morse taper (large drills, with a tang that drives and helps ejection).
• Body: carries two helical flutes that form the cutting edges, carry chips out and let coolant in.
• Land: raised strip following the flute; the margin on it guides the drill and keeps diameter.
• Web: central core thickness between flutes; it increases toward the shank for strength.
• Point: cone formed by two lips and the chisel edge.

Geometry angles

• The two lips must be equal in length and angle; otherwise the hole will be oversize and the drill may break.
• The chisel edge does not cut but pushes metal sideways, producing high thrust; web thinning reduces it.

Given: D = 20 mm, t = 50 mm, v = 25 m/min, f = 0.25 mm/rev, point angle 118°, overtravel = 2 mm.

(a) Spindle speed

$$N = \frac{1000\,v}{\pi D} = \frac{1000 \times 25}{\pi \times 20} = 397.9\ \text{rpm}$$

(b) Drill approach (point allowance)

The drill must travel until its full diameter enters the plate.

$$A = \frac{D}{2}\cot\left(\frac{118^\circ}{2}\right) = \frac{20}{2 \times \tan 59^\circ} = \frac{10}{1.664} = 6.01\ \text{mm}$$

(c) Feed rate

$$f_m = f \times N = 0.25 \times 397.9 = 99.5\ \text{mm/min}$$

(d) Time

Total tool travel:

$$L = t + A + \text{overtravel} = 50 + 6.01 + 2 = 58.01\ \text{mm}$$ $$T = \frac{L}{f_m} = \frac{58.01}{99.5} = 0.583\ \text{min} \approx 35\ \text{s}$$

(e) Metal removal rate

$$\text{MRR} = \frac{\pi}{4}D^2 \times f_m = \frac{\pi}{4}(20)^2 \times 99.5 = 31{,}250\ \text{mm}^3/\text{min}$$

Answer: N = 398 rpm, approach = 6.01 mm, feed rate = 99.5 mm/min, time = 0.583 min (35 s), MRR = 31.25 cm³/min.

All three are finishing operations that bring a hole or surface to close tolerance and fine finish, but they differ in tool and action.

Counterboring

Enlarging the end of a hole to a larger diameter with a flat bottom so that a bolt head, nut or cap screw sits flush or below the surface. A pilot on the counterbore guides it in the original hole.

Countersinking

Making a conical enlargement at the mouth of a hole (usually 60°, 82° or 90°) so a flat-head screw or rivet lies flush. It is also used to chamfer and deburr holes and to make the centre-hole for lathe centres.