Material Removal Processes: “Abrasive and Non-Traditional”

A grinding wheel is a bonded mass of abrasive grains. Each grain acts as a tiny cutting tool, and the wheel renews itself as the dull grains break out and fall away.

Standard marking, example A 60 K 5 V

  A     60     K      5      V
  |     |      |      |      |
abrasive grit  grade structure bond

Selection guide

• Steel and soft metals use aluminium oxide; cast iron, carbide and non-ferrous use silicon carbide.
• Hard materials need a soft grade so that dull grains drop out; soft materials need hard grades.
• Fine grit for good finish; coarse grit for fast removal.
• Wide contact area or soft material needs open structure.

Glazing and loading

• Glazing: the abrasive grains become blunt and the wheel surface turns smooth and shiny because the grade is too hard. Cutting efficiency falls, heat and burning appear.
• Loading: the spaces between the grains fill with chips of the work material (soft ductile metals such as aluminium or copper). The wheel cuts poorly.

Truing and dressing

• Truing restores the wheel shape and makes it run concentric, using a single-point diamond or a diamond roll.
• Dressing restores sharpness and exposes new grains by removing the glazed or loaded layer, using a dressing stick, star dresser (Huntington) or diamond. Often both are done together.

(a) EDM

In EDM, the work and the shaped tool (electrode) are both immersed in a dielectric fluid (kerosene, transformer oil or deionised water) and kept a small gap apart (0.01-0.5 mm). A pulsed DC supply makes a spark discharge across the gap; each spark generates temperature of 8000-12000 degree C in a tiny spot, melting and vaporising a small crater in the work. The dielectric flushes out the debris and re-insulates the gap. A servo system keeps the gap constant as the tool advances, and the tool shape is reproduced in the work.

   +---- pulse generator ----+
   |                         |
 tool (+/-)   spark gap    work (-/+)
   |__ dielectric fluid + flushing __|
          servo feed (down)

Advantages: machines any electrically conductive material regardless of hardness (hardened steel, carbide); complex cavities, dies and small deep holes; no cutting force, no burr; good accuracy.

Limitations: only conductive materials; slow metal removal; tool wear and recast (heat-affected) layer; high power; sharp inside corners not possible.

(b) ECM

In ECM, the work is the anode and the tool is the cathode. A low-voltage (10-20 V), high current (several thousand A/cm$^2$) DC flows through a fast electrolyte (NaCl or NaNO$_3$ solution) in a gap of about 0.1-0.5 mm. The work metal dissolves according to Faraday's law, and the electrolyte carries the hydroxide sludge away. The tool is not worn.

By Faraday's laws, mass removed per second:

$$\dot{m} = \frac{\eta\,I\,A_w}{z\,F}$$

Volumetric rate:

$$\dot{V} = \frac{\eta\,I\,A_w}{z\,F\,\rho}$$

with $A_w = 55.85$ g/mol, $z = 2$, $F = 96\,485$ C/mol, $\rho = 7.86$ g/cm$^3$, $\eta = 0.9$, $I = 1000$ A.

$$\dot{V} = \frac{0.9\times1000\times55.85}{2\times96\,485\times7.86} = 0.0331\ \text{cm}^3/\text{s} = 33.1\ \text{mm}^3/\text{s}$$

This equals $33.1\times60 = 1988\ \text{mm}^3/\text{min}$ (about 2.0 cm$^3$/min).

Tool feed rate for the cavity cross-section of 400 mm$^2$:

$$v = \frac{\dot{V}}{A} = \frac{33.1}{400} = 0.0828\ \text{mm/s} = 4.97\ \text{mm/min}$$

Answer: volumetric removal rate $= 33.1$ mm$^3$/s (1.99 cm$^3$/min); feed rate $= 4.97$ mm/min.