Material removal is the group of manufacturing processes that make a part by cutting away unwanted material from a larger workpiece until the required shape, size and surface finish remain. It is also called subtractive manufacturing. Material removal processes fall into three families: conventional machining with a sharp cutting tool (turning, drilling, milling, shaping), abrasive processes (grinding, honing, lapping), and non-traditional machining that removes material with mechanical, thermal, chemical or electrochemical energy (USM, AJM, water jet, EDM, laser, electron beam, plasma, chemical machining, ECM, ECG). This page classifies them, gives the material removal rate (MRR) formulas with worked examples, compares them in a selection table, and explains the subsurface damage each one leaves.
Where material removal fits in manufacturing
Every manufacturing process changes shape in one of four ways:
| Approach | What happens to the material | Examples |
|---|---|---|
| Material removal (subtractive) | Excess is cut away as chips, sparks, dissolved ions or vapour | Turning, milling, grinding, EDM |
| Forming (deformation) | The same mass is pushed into a new shape | Forging, rolling, extrusion, sheet metal bending |
| Casting and moulding | Liquid material fills a mould and solidifies | Casting, injection moulding |
| Additive | Material is added layer by layer | 3D printing |
Material removal wastes some material, but it gives the best dimensional accuracy and surface finish of the four. That is why most cast, forged and even 3D-printed parts still go through a machining step to finish bearing seats, threads, holes and sealing faces.
Types of material removal processes
| Family | Energy used | Processes |
|---|---|---|
| Conventional machining | Mechanical shear by a harder, sharp tool | Turning, facing, boring, drilling, reaming, milling, shaping, planing, broaching |
| Abrasive machining | Mechanical, many small abrasive grains | Grinding, honing, lapping, superfinishing |
| Non-traditional: mechanical | Abrasive or fluid impact | Ultrasonic machining (USM), abrasive jet machining (AJM), water jet and abrasive water jet (WJM, AWJM) |
| Non-traditional: thermal | Melting and vaporising | Electrical discharge machining (EDM), laser beam machining (LBM), electron beam machining (EBM), plasma arc machining (PAM) |
| Non-traditional: chemical and electrochemical | Dissolving the metal | Chemical machining (CHM), electrochemical machining (ECM), electrochemical grinding (ECG) |
Conventional machining
A wedge-shaped tool, harder than the work, is pressed into it and shears off a thin layer as a chip. The type of chip (continuous, discontinuous or with a built-up edge) tells you a lot about the cutting conditions.
- Turning: the work rotates on a lathe and a single-point tool moves along it to make cylinders, tapers and threads. See the lathe machine parts page; knurling is also done on a lathe, though it forms the surface rather than cutting it.
- Drilling: a rotating twist drill makes round holes; reaming and boring then size and finish them. See the drilling machine and drill bit types pages.
- Milling: a rotating multi-tooth cutter removes metal as the work feeds past it, for flat faces, slots, pockets and contours. See milling cutter types and CNC mills.
- Shaping and planing: a single-point tool moves back and forth in straight strokes to cut flat surfaces and keyways. See the shaper machine page.
- Broaching: a long tool with rising teeth cuts a complete internal shape, such as a keyway or spline, in one pass.
Abrasive processes
Each abrasive grain acts as a tiny cutting tool with a random, negative rake. Removal per grain is tiny, so these processes give fine finishes and tight tolerances and can cut hardened steel. Grinding uses a bonded wheel; honing uses abrasive sticks to finish bores; lapping uses loose abrasive between the work and a lap to reach very flat, smooth surfaces.
Non-traditional mechanical processes
- Ultrasonic machining (USM): a tool vibrating at about 20 kHz drives abrasive slurry into brittle materials such as glass, ceramics and gemstones. See ultrasonic machining.
- Abrasive jet machining (AJM): a high-speed jet of fine abrasive in air erodes brittle and heat-sensitive material, for deburring, frosting and cutting thin sections. See abrasive jet machining.
- Water jet and abrasive water jet (WJM, AWJM): a very high-pressure water jet cuts soft materials; adding garnet abrasive lets it cut steel, stone and glass with no heat-affected zone. See water jet machines.
Non-traditional thermal processes
- Electrical discharge machining (EDM): rapid sparks between a tool electrode and the work, in a dielectric fluid, melt and vaporise tiny craters. It cuts any conductive material regardless of hardness, including die steel and carbide. See EDM.
- Laser beam machining (LBM): a focused laser melts or vaporises the material for cutting, drilling small holes and engraving. See laser cutting and fibre lasers.
- Electron beam machining (EBM): a focused electron beam in a vacuum drills very fine holes, for example cooling holes and filters.
- Plasma arc machining (PAM): an ionised gas jet at very high temperature cuts thick conductive plate quickly. See plasma cutter.
Non-traditional chemical and electrochemical processes
- Chemical machining (CHM): areas not protected by a maskant are dissolved in an etchant; used for thin parts and weight reduction. See chemical machining.
- Electrochemical machining (ECM): the reverse of electroplating. The work is the anode, and a shaped tool cathode dissolves it through a flowing electrolyte, with no tool wear and no mechanical stress. Used for turbine blades and hard alloys.
- Electrochemical grinding (ECG): an electrically conductive grinding wheel removes most metal by electrochemical action, with the abrasive doing only a small part, giving burr-free, stress-free grinding of hard materials. See electrochemical grinding.
Material removal rate (MRR)
Material removal rate is the volume of material removed per unit time, usually in mm3/min or cm3/min. It sets machining time and cost, so it is the first number in any process comparison.
- Turning: MRR = π × Davg × d × f × N, where Davg = D − d is the average diameter, d the depth of cut, f the feed per revolution and N the spindle speed. The quick version is MRR ≈ v × f × d, with cutting speed v = πDN.
- Drilling: MRR = (πD2/4) × f × N
- Milling: MRR = width of cut × depth of cut × table feed (mm/min)
Worked example 1: turning
A 50 mm diameter steel bar is turned at N = 500 rpm with a depth of cut d = 2 mm and a feed f = 0.2 mm/rev.
- Cutting speed: v = π × 50 × 500 = 78,540 mm/min = 78.5 m/min
- Quick MRR: v × f × d = 78,540 × 0.2 × 2 = 31,416 mm3/min (about 31.4 cm3/min)
- Using the average diameter, Davg = 50 − 2 = 48 mm: MRR = π × 48 × 2 × 0.2 × 500 = 30,159 mm3/min
- Time for one 100 mm long pass: t = L ÷ (f × N) = 100 ÷ (0.2 × 500) = 1.0 min
The quick formula overstates MRR by about 4 percent here because it uses the outer diameter. The gap grows as the depth of cut becomes a larger share of the diameter.
Worked example 2: drilling
A 10 mm hole is drilled at N = 800 rpm with a feed f = 0.15 mm/rev.
- Hole area: π × 102 ÷ 4 = 78.54 mm2
- MRR = 78.54 × 0.15 × 800 = 9,425 mm3/min
- Cutting speed at the drill’s outer corner: v = π × 10 × 800 ÷ 1,000 = 25.1 m/min
Most non-traditional processes, EDM and USM in particular, remove material far more slowly than conventional cutting, which is why they are used mainly where conventional cutting cannot do the job.
How to choose a material removal process
| Process | Work material | Feature size and shape | Surface finish | MRR and cost |
|---|---|---|---|---|
| Turning, milling, drilling | Most metals and plastics in the unhardened state | General shapes, holes above about 1 mm | Good | High MRR, lowest cost |
| Grinding | Hardened steel, cast iron, ceramics | Flat and round surfaces | Very good | Low MRR, moderate cost |
| USM | Hard, brittle non-metals: glass, ceramics | Holes and cavities of any shape | Good | Very low MRR |
| AJM, AWJM | Almost anything; AWJM cuts thick metal and stone | Through cuts, profiles | Fair; taper on thick cuts | Moderate; no heat damage |
| EDM | Any electrical conductor, any hardness | Complex cavities, sharp internal corners, fine wire cuts | Good, with a recast layer | Low MRR, moderate to high cost |
| Laser, EBM | Metals, plastics, ceramics | Thin sheet profiles, micro holes | Fair to good; heat-affected zone | Fast on thin sheet; high equipment cost |
| Plasma | Electrically conductive metals | Thick plate profiles | Rough; bevelled edge | High cutting speed, low cost per metre |
| CHM | Most metals | Thin parts, large shallow pockets | Good | Low tooling cost, slow |
| ECM, ECG | Conductive metals, including superalloys | Complex 3D shapes, burr-free edges | Very good, stress-free | High MRR for ECM, high equipment cost |
A student rule: if the part conducts electricity and is hard, think EDM or ECM; if it is hard and brittle but not conductive, think USM or grinding; if it is soft and simple, machine it conventionally.
Subsurface damage in material removal
Every removal process leaves an altered layer just below the new surface, sometimes called surface integrity damage or subsurface damage. It can cut fatigue life, so critical aerospace and medical parts often have it measured or removed.
- Conventional machining: the tool deforms a thin layer plastically, leaving work hardening, bent grains and residual stresses. A blunt tool or heavy cut makes it deeper.
- Grinding: the heat can cause grinding burn. On hardened steel this may leave a re-hardened, brittle “white layer” over a softer, over-tempered zone, with tensile residual stress and fine cracks. Correct wheel dressing, coolant and a lighter cut prevent it.
- EDM: molten metal that is not flushed away re-solidifies on the surface as a hard, brittle recast layer (also called the white layer), often with micro-cracks, over a heat-affected zone. Finishing passes at lower spark energy thin it, and critical parts have it removed by polishing or etching.
- Laser, EBM and plasma: a recast layer and a heat-affected zone along the cut edge, deeper in thick material.
- Chemical, ECM and water jet: no heat-affected zone and almost no residual stress, which is one reason they are chosen for critical parts, though chemical processes can attack grain boundaries if poorly controlled.
- Brittle materials (glass, ceramics, silicon): grinding and lapping leave hidden micro-cracks below the surface, which are removed by successively finer lapping and polishing.
Machining and non-traditional processes are core topics in the manufacturing processes course of B.Tech mechanical engineering; the NPTEL manufacturing processes course covers them in video lectures.
FAQs
What is material removal in manufacturing?
It is any process that makes a part by removing unwanted material from a larger workpiece, such as turning, milling, grinding or EDM. It is also called subtractive manufacturing.
What are the main types of material removal processes?
Conventional machining (turning, drilling, milling, shaping), abrasive processes (grinding, honing, lapping), and non-traditional processes that use mechanical (USM, AJM, water jet), thermal (EDM, laser, EBM, plasma) or chemical and electrochemical energy (CHM, ECM, ECG).
How is material removal rate calculated in turning?
MRR = π × Davg × d × f × N, or approximately v × f × d. For a 50 mm bar at 500 rpm, 2 mm depth and 0.2 mm/rev feed, MRR is about 30,000 to 31,400 mm3/min.
What is subsurface damage in machining?
It is the altered layer below a machined surface: plastic deformation and residual stress from cutting, burn from grinding, or a recast layer from EDM and laser cutting. It can reduce fatigue strength.
What is the recast layer in EDM?
It is a thin layer of metal melted by the sparks that re-solidified on the surface instead of being flushed away. It is hard and brittle, may contain micro-cracks, and is reduced by low-energy finishing passes.