Start with what removes the material

A machine tool controls motion between a tool and the workpiece, the part being made. In material removal, the final shape is what remains after unwanted material is taken away.

CNC is a control method, not a removal family. Computer numerical control can guide a mill, lathe, grinder, EDM machine or cutting jet. See G-code and Toolpaths. Forming moves material without removing it; see Manufacturing Process Categories for the wider manufacturing map.

Conventional cutting uses defined cutting edges

• Turning rotates the work past a cutting tool; milling rotates the cutter and feeds it relative to the work.
• Drilling makes holes; sawing separates stock, the starting material, with successive teeth.
• Broaching uses a sequence of teeth to progressively cut a profile, such as a keyway. Dedicated tooling often suits repeated production, but broaching is not limited to large runs.
• Shaping and planing use a back-and-forth straight cutting stroke. A shaper normally moves the tool; a planer normally moves the work.

A process name describes the action, not every possible shape. Milling can make curved contours as well as flat faces.

Abrasive processes use many small cutting grains

Grinding wheels hold abrasive grains that remove material. Precision grinding controls size and surface texture, often on hardened parts. Grinding can also remove substantial stock; it is not restricted to light finishing.

Honing commonly uses abrasive stones with combined motions to refine a bore's size, form and surface. Lapping uses fine abrasive between a lap and the work to refine a surface. Neither is simply another name for grinding.

Nontraditional processes change the removal mechanism

• Thermal: EDM, laser and plasma remove material with concentrated heat. EDM's energy supply is electrical, but its removal mechanism is thermal.
• Electrochemical: ECM uses electric current to dissolve metal.
• Chemical: Chemical Milling and Etching uses an etchant to dissolve exposed material.
• Mechanical energy: waterjets and Ultrasonic Machining erode material with moving water or abrasive particles.

Use Selecting a Material-Removal Process to narrow the candidates. Safety references: Machine Hazards and Housekeeping, Equipment Operating Procedure and Chemicals and Safety Data Sheets.

EDM needs an electrically conductive workpiece

Electrical discharge machining (EDM) removes material with repeated electrical discharges across a small gap. Local heating melts and vaporizes tiny amounts of material. The electrode does not cut by rubbing against the work.

Check conductivity before considering EDM. Hardened tool steel is a candidate; ordinary nonconductive glass or ceramic is not. Hardness is less restrictive than in cutting with a conventional tool, but composition and electrical and thermal properties still affect machining behavior.

Supporting detail: Addendum 1 — Why the fluid and the gap matter.

Use sinker EDM for cavities and wire EDM for through profiles

Sinker, ram or die-sinking EDM feeds a shaped electrode toward the work to make a cavity. The cavity is larger than the electrode because removal extends across the spark gap. Mold cavities and hardened dies are typical uses. Electrode wear must be allowed for.

Wire EDM feeds a thin wire through the cutting zone while controlled motion traces a profile. The wire is continuously renewed. An enclosed opening normally needs a start hole for threading; the wire must pass through the work. A blind pocket calls for another approach, such as sinker EDM.

Wire EDM is useful for intricate die openings and thin features with low cutting force. Internal corners still have a radius: the wire radius plus the working gap limits how tight a corner can be. Specify a small radius rather than a mathematically sharp corner.

Offset the wire center by its radius plus the spark gap

Center-path offset = wire radius + spark gap. Measure this distance from the finished contour toward the material being removed. In a simple straight cut, the full cut width (kerf) is wire diameter plus twice the gap.

For an illustrative radius of 0.120 mm and a one-sided gap of 0.018 mm, the offset is 0.138 mm. These are teaching inputs, not machine settings. A control may apply compensation to a programmed part contour; do not also offset the geometry a second time.

Practice a wire offset

Supporting detail: Addendum 2 — Why one offset does not describe every pass.

Low cutting force does not mean no thermal change

EDM can hold fine features and produce a fine finish with suitable finishing passes, but its surface contains small discharge craters. Melted material can solidify again as a recast layer. Heat can alter nearby material, so surface integrity and any required finishing belong in process selection.

Supporting detail: Addendum 3 — Further reading on EDM surfaces and wire systems.

Compare Electrochemical Machining. Safety references: Electrical Safety, Machine Hazards and Housekeeping and Chemicals and Safety Data Sheets.

ECM removes metal by electrochemical dissolution

In electrochemical machining (ECM), the metal workpiece is the positive electrode (anode), and the tool is the negative electrode (cathode). Direct current passes through a flowing, electrically conductive liquid called an electrolyte. Metal dissolves from the work surface while the tool advances without cutting contact.

Consider ECM for conductive metals, including hard alloys and complex contours. The tool shape, gap and electrolyte flow govern the resulting feature. A suitable electrolyte must be matched to the metal; a generic salt solution is not a universal process recipe.

ECM uses a conducting liquid; EDM uses controlled breakdown

ECM is not a melting process and normally avoids a heat-affected zone. That does not mean the electrolyte stays cold: current flow heats it. Flow, temperature, chemistry and gap control all matter.

Supporting detail: Addendum 4 — Why hardness alone does not set ECM performance.

Compare Electrical Discharge Machining. Safety references: Chemicals and Safety Data Sheets and Electrical Safety.

Distinguish a thermal cut from an erosive cut

Laser and plasma use concentrated heat. Waterjet uses mechanical energy. A heat-affected zone (HAZ) is material beside a cut whose properties or structure have changed from heating without being melted away.

Laser cutting concentrates light into a small region

A focused laser beam supplies thermal energy; cutting gas commonly helps remove material. Laser cutting is a candidate for fine profiles in sheet material. Metals, wood and selected plastics require suitable laser systems; a machine that cuts acrylic is not automatically a metal cutter.

A narrow kerf and fine detail are possible, particularly in thinner material. Edge finish, taper and dimensional accuracy depend on material, thickness, focus, gas and cutting speed. Melting, discoloration or charring may affect the edge. Do not assume the HAZ is zero.

Supporting detail: Addendum 5 — Why the material and laser must be matched.

Waterjet cuts soft materials; added abrasive cuts hard materials

Pure waterjet uses a high-speed water stream for materials such as foam, rubber and other soft sheet. Abrasive waterjet adds hard particles to erode metals, stone, non-tempered glass and many ceramics. Electrical conductivity is not required.

Waterjet normally creates no thermal HAZ. It is a candidate for thick stock or a heat-sensitive edge, but thickness still affects speed and cut quality. Taper, where the cut width changes through the thickness, and striations, visible lines down the cut face, can limit accuracy and finish. Slower quality cuts may reduce these effects. Brittle edges may chip and need finishing.

Identify the glass type before choosing a process: tempered glass is not a candidate for ordinary waterjet cutting. A no-HAZ process is not a promise of no mechanical damage.

Supporting detail: Addendum 6 — Further reading on glass and waterjet limits.

Plasma is a thermal cutting candidate for conductive metal

A plasma cutter uses an electric arc through ionized gas to melt metal; the gas stream expels it. The work must conduct electricity. Steel, stainless steel and aluminum are candidates; ordinary glass and wood are not.

Plasma often suits productive plate cutting where its edge quality meets the drawing or later machining is planned. Compared with a fine laser cut in thin sheet, small details can be harder to hold. Edge bevel, attached solidified metal (dross) and a HAZ may need attention. Equipment and thickness determine actual capability; plasma is not automatically a rough-only process.

Supporting detail: Addendum 7 — Compare capability on the actual thickness.

Continue with Selecting a Material-Removal Process. Safety references: Machine Hazards and Housekeeping, Electrical Safety, Fire Safety and Chemicals and Safety Data Sheets.

Ultrasonic impact machining removes material with abrasive particles

A shaped tool vibrates rapidly with a small stroke, driving abrasive particles in a liquid slurry against the work. The removal energy at the work is mechanical, even though electricity powers the vibration system.

Consider this process for holes or shaped cavities in hard, brittle materials such as glass and ceramics. The work need not conduct electricity. It is generally less effective for soft, ductile materials that deform instead of breaking away under particle impacts.

No thermal HAZ does not mean a perfect surface

Ultrasonic impact machining normally avoids a thermal heat-affected zone. Abrasive size, tool wear, slurry access and feature depth affect dimensional control and finish. Fine abrasive can improve the surface, but removal may be slow; edge chipping and subsurface damage remain possible in brittle work.

Distinguish this process from ultrasonic-assisted cutting, where vibration is added to another cutting or grinding process. The two names do not imply identical tooling or removal mechanisms.

Supporting detail: Addendum 8 — Why a shaped tool does not give an exact-size copy.

Compare Electrical Discharge Machining and Selecting a Material-Removal Process. Safety references: Machine Hazards and Housekeeping and Chemicals and Safety Data Sheets.

A mask defines where chemical removal occurs

Chemical machining uses a chemical reaction to dissolve exposed material. A protective mask covers areas that should remain. Chemical milling can make shallow recesses or reduce a metal part's weight. Photochemical etching uses a patterned resist and can produce intricate thin-sheet parts, including through openings.

Unlike ECM, ordinary chemical etching does not use an external machining current between a tool and the work. Select an etchant compatible with the particular material; “chemical machining” does not name one chemistry that works on everything.

Allow for sideways etching beneath the mask

Etching can spread sideways beneath the mask edge as well as into the surface. This undercut limits narrow lands, corner sharpness and dimensional control, especially as etch depth increases. The mask design must compensate for the etching behavior. Surface finish depends on the material and chemistry. Chemical removal avoids a cutting-induced thermal HAZ, but that is not the same as saying the chemical bath has no temperature requirements.

Supporting detail: Addendum 9 — Why thin-sheet patterns are a useful application.

Compare Electrochemical Machining and Selecting a Material-Removal Process. Safety reference: Chemicals and Safety Data Sheets.

Use the feature to narrow the candidates

Keep conventional machining on the candidate list

For an accessible flat face, hole or round shaft in a machinable material, conventional cutting may be the simplest route. Use the family map to reach those methods. A part may combine sawing, milling and grinding, or use waterjet for a blank followed by machining of critical faces.

Increasing thickness can increase cutting time, taper or flushing difficulty. A thin remaining wall may deflect or be damaged even when cutting forces are small. Ask for evidence of capability on the actual material and geometry rather than relying on a universal tolerance ranking.

Practice selecting a process Practice HAZ reasoning

Safety references: Equipment Operating Procedure, Machine Hazards and Housekeeping and Chemicals and Safety Data Sheets.