Material Removal: Cutting, Turning, Milling, Drilling
What Is Subtractive Manufacturing?
Subtractive manufacturing starts with a block, sheet or bar of material and removes unwanted sections until the finished part remains. Every process in this lesson — sawing, shearing, laser cutting, turning, milling and drilling — is subtractive. The removed material becomes waste (swarf, chips or offcuts), so material selection and planning the cutting sequence carefully both reduce cost and waste.
Cutting: Sawing, Shearing and Laser Cutting
The simplest subtractive operations separate a workpiece into pieces. Sawing uses a toothed blade — a hacksaw in the workshop or a band saw on the factory floor — to cut through metal, timber or plastics. The teeth are set (bent alternately left and right) so the kerf (slot) is slightly wider than the blade body, preventing binding.
Shearing uses two hardened blades (like giant scissors) to cut sheet metal without a kerf. A hydraulic guillotine can shear steel plate up to 10 mm thick cleanly and quickly. Because shearing requires no cutting fluid and produces no swarf, it is economical for straight-line cuts on sheet stock.
Laser cutting directs a high-power CO₂ or fibre laser beam onto the material surface. Controlled by CNC, it can cut intricate 2D profiles in metal, acrylic and timber with very narrow kerfs (as small as 0.1 mm) and no physical tool contact. Laser cutting is ideal for prototype sheet-metal brackets and signage panels but is limited to thin sections — typically below 25 mm for mild steel.
Sawing produces a kerf (slot) and swarf; shearing produces a clean cut with no kerf and no swarf; laser cutting produces a very narrow kerf and is CNC-controlled, allowing complex profiles to be cut automatically.
Turning on the Lathe
A lathe rotates the workpiece while a single-point cutting tool is fed along or into the material. Key parts of the lathe you need to know are:
- Chuck — the rotating clamp at the headstock that grips the workpiece. A three-jaw self-centring chuck grips round or hexagonal bar automatically; a four-jaw independent chuck can grip irregular shapes.
- Tailstock — the adjustable support at the opposite end of the bed. It can hold a live centre to support long workpieces, or a drill chuck for drilling on the lathe’s axis.
- Carriage / tool post — holds the cutting tool and moves along the bed (for parallel turning) or across (for facing).
- Bed — the precision-machined base that ensures the carriage moves parallel to the spindle axis.
Lathe operations include parallel (cylindrical) turning (reducing diameter along the length), facing (cutting across the end to produce a flat face), taper turning (angling the tool path to create a cone), and boring (enlarging an existing hole from the inside). Cutting speed and feed rate are set to suit the workpiece material — harder metals require slower speeds and a finer feed.
Every material has a recommended cutting speed (in metres per minute, m/min) — the speed at which the cutting tool moves through the material at the point of contact. For a rotating workpiece on a lathe, this depends on both the spindle speed (revolutions per minute, rpm) and the diameter of the workpiece.
The relationship is: $$N = \frac{1000 \times v}{\pi \times d}$$ where \(N\) = spindle speed (rpm), \(v\) = cutting speed (m/min), \(d\) = workpiece diameter (mm).
Worked example: A 40 mm diameter mild steel bar is to be turned. The recommended cutting speed for mild steel is 30 m/min. Find the required spindle speed. $$N = \frac{1000 \times 30}{\pi \times 40} = \frac{30\,000}{125.66} \approx 239 \text{ rpm}$$ The lathe would be set to the nearest available speed below 239 rpm to avoid over-speed and tool damage.
Notice that a larger diameter requires a lower spindle speed to maintain the same cutting speed at the surface. If the diameter halves, the spindle speed must double. This inverse relationship is a common exam question.
Milling
Milling uses a rotating multi-tooth cutter. Unlike a lathe (where the workpiece rotates), in milling the cutter rotates and the workpiece is clamped to a table that moves in X, Y and Z directions. Milling operations include:
- Face milling — a large-diameter cutter with many teeth sweeps across the top surface to produce a flat, smooth face. Used to true-up castings and forgings.
- Slot milling — an end mill (a cutter with teeth on its end and sides) cuts a slot or keyway into the workpiece surface.
- Profile milling — the CNC table follows a programmed path to cut complex 2D or 3D shapes, used heavily in aerospace for aluminium wing ribs and in automotive for cylinder-head ports.
On a lathe the workpiece rotates and the tool is stationary (moves linearly). On a milling machine the cutter rotates and the workpiece is clamped to a moving table. Getting this the wrong way round is a common exam mistake.
Drilling
A pillar drill (also called a drill press) holds a drill bit in a rotating chuck and feeds it vertically into the workpiece. Tool names you need to know:
- Centre drill — a short, rigid combined drill-and-countersink used first to create a precise starting point, preventing the twist drill from wandering.
- Twist drill — the standard helical drill for producing through-holes and blind holes. Flutes carry swarf out of the hole.
- Countersink — a conical cutter that bevels the top of a hole so a countersunk screw sits flush with the surface.
- Counterbore — a stepped cutter that enlarges the top of a hole to receive a cap-head bolt so its head sits below the surface.
The sequence for drilling a precision hole is: mark out the centre with a centre punch → use a centre drill to start the hole accurately → drill the through-hole with a twist drill at the correct speed → de-burr the hole edges. Giving “drill” alone in an answer is not specific enough; always state the type of drill bit.
Chemical etching is used to produce printed circuit boards (PCBs). The process is subtractive but uses chemistry rather than mechanical cutting. A blank PCB consists of a fibreglass substrate coated in a thin copper layer. The desired circuit pattern is printed onto the copper using UV-sensitive etch-resist film, which is exposed and developed. The board is then immersed in ferric chloride (iron(III) chloride) solution, which dissolves the unprotected copper while leaving the resist-covered tracks intact. After etching, the resist is removed, revealing the finished copper tracks. Chemical etching bridges the Manufacturing and Systems units because PCBs are the foundation of electronic circuits.