Milling uses a rotating multitooth cutter to produce many configurations; each tooth makes a chip per revolution.

Milling Types

Slab (peripheral) milling

Cutter axis parallel to the surface; teeth on the circumference. Helical teeth preferred (lower load, smoother, less chatter).

• Conventional (up) milling: most common; chip starts thin; scale doesn’t affect tool life; workpiece tends to be pulled up (clamp well).
• Climb (down) milling: chip starts at the surface; downward force holds the part (good for slender parts); high impact when teeth engage; not for scaled surfaces; best cutter life on CNC.

Face milling

Cutter axis perpendicular to the surface (rotates at $N$, workpiece feeds at $v$). Leaves feed marks; finish depends on insert corner geometry and feed/tooth. Keep cutter-diameter / width-of-cut $D/w > 1.5$. Insert entry/exit angle matters (first contact at the tip can chip the edge; a sudden exit is undesirable).

Others

• End milling: flat surfaces/profiles; axis usually perpendicular (can tilt); ball-nose ends for dies/molds; HSS or carbide inserts.
• High-speed milling: e.g. die-sinking with a TiAlN-coated 2-mm ball-nose end mill at up to 50,000 rpm (air bearings).
• Form milling (shaped teeth, gear teeth), slotting/slitting (circular cutters), T-slot cutting.

Milling Formulas

$N$ = cutter rpm, $D$ = cutter diameter, $n$ = number of teeth, $v$ = workpiece feed rate (mm/min), $w$ = width of cut, $d$ = depth, $l$ = length, $l_c$ = cutter’s first-contact extent.

Cutter surface speed:

$V = \pi D N \quad [\text{m/min}]$

Feed per tooth:

$f = \frac{v}{N n} \quad [\text{mm/tooth}]$

Cutting time:

$t = \frac{l + 2 l_c}{v}$

Material removal rate:

$\text{MRR} = w\,d\,v \quad [\text{mm}^3/\text{min}]$

Torque and power:

$\text{Torque} = \frac{F_c D}{2}, \qquad \text{Power} = \text{Torque}\times 2\pi N$

Worked example (face milling): $D=150$ mm, $w=60$ mm, $l=500$ mm, $d=3$ mm, $v=600$ mm/min, $N=100$ rpm, 10 inserts, high-strength Al.

$\text{MRR} = (w\,d)\,v = (60\times 3)(600) = 108{,}000\ \text{mm}^3/\text{min}$

With $l_c = D/2 = 75$ mm:

$t = \frac{l + 2l_c}{v} = \frac{500 + 150}{600} \approx 1.08\ \text{min}$

Feed per tooth $f = \dfrac{600}{(100)(10)} = 0.6$ mm/tooth. With unit power ~1.1 W·s/mm³:

$\text{Power} = 1.1 \times \frac{108000}{60} = 1980\ \text{W} \approx 1.98\ \text{kW}$

Process Capabilities

Feeds ~0.1–0.5 mm/tooth; depths ~1–8 mm; speeds ~30–3000 m/min. Back striking = double feed marks from the trailing edge.

Milling Machines

• Column-and-knee (most common): horizontal (slab) or vertical (face/end, boring, drilling). Components: work table (T-slots), saddle, knee, overarm, head; three axes.
• Bed-type: table on the bed; very stiff; duplex/triplex spindles.
• CNC: mill/drill/bore/tap with repeatable accuracy.
• Profile milling: five axes (three linear + two angular).