6/3/2014 1 Hareesha N G, Dept of Aero Engg, DSCE

Review of Machining • Machining is a generic term, applied to material removal

processes.

• Traditional machining: turning, milling, drilling, grinding, etc.

• Metal cutting refers to processes in which excess metal is

removed by a harder tool, through a process of extensive plastic

deformation or controlled fracture.

• Non-traditional machining: chemical machining, ECM, EDM,

EBM, LBM, machining of non-metallic materials.

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Nontraditional Machining

• Chemilling

• Electrochemical machining

• Electrical discharge machining

• High energy beam machining

• Water jet/Abrasive water jet machining

• Ultrasonic machining

• Machining of non-metallic materials

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Chemical Machining (Chemilling)

• Used to produce shallow cavities (<12mm) on large areas.

• A maskant is applied over areas you don’t want to machine.

• Place the part in a chemical bath (acid or alkali depending upon the metal)

• Control temperature and time of exposure to control material removal

• Material removal rate is slow, 0.025-0.1 mm/min

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Electro-Chemical Machining (ECM)

• Works on the principle of electrolysis

• Die is progressively lowered into workpiece as workpiece is dissociated into ions by electrolysis

• Electrolytic fluid flows around workpiece to remove ions and maintain electrical current path

• Low DC voltage, very High current (700 amps).

• Material removal rate is 2.5-12 mm/min depending on current density.

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Electrical Discharge Machining (EDM)

• The tool (electrode) usually acts as a cathode and is immersed in a dielectric fluid.

• DC voltage (~300V) is applied in modulated pulses (200-500K Hz).

• The dielectric breaks down (sparking at around 12,000 deg F) when gap is small.

• The sparks erodes the workpiece in the shape of the tool.

• The tool is progressively lowered as the workpiece erodes.

• Material removal rate is typically 300 mm3/min

• Tool wear ratio 3:1 with metallic electrodes, 3:1-100:1 with graphite electrodes 6/3/2014 Hareesha N G, Dept of Aero Engg, DSCE 6

Die Sinker and Wire EDM

• Die sinker EDM – The die (copper or graphite)

sinks into the part as it sparks away the workpiece.

– Most common injection molding die process.

• Wire EDM – The electrode (brass, copper, W,

Mo, 0.08-3 mm) is a wire that traverses through the part.

– Common for extrusion dies.

• Electrical discharge drilling

• Electrical discharge grinding

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Examples of Die Sinker and Wire EDM

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Water Jet and Abrasive Water Jet Cutting

• High pressure water (20,000-60,000 psi).

• Can cut extremely thick parts (5-10 inches possible).

• Thickness achievable is a function of speed.

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Abrasive Waterjet and Waterjet

Part Examples

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High Energy Beam Machining

(a) Electron beam machining

Cutting and hole making on thin

materials; very small holes and

slots (0.1-0.3mm depending on

thickness); heat affected zone;

require vacuum, expensive

equipment; 1-2 mm3/min.

(b) Laser beam machining

Cutting and hole making on thin

materials; heat-affected zone; does

not require a vacuum; but

expensive equipment; consume

much energy; 0.5-7.5 mm/min

depending on thickness. 6/3/2014 Hareesha N G, Dept of Aero Engg, DSCE 11

Machining of Nonmetallic Materials • Machining of ceramics:

– Abrasive machining, including abrasive water jet machining

– Laser beam machining

– Laser assisted machining

Laser assisted machining 6/3/2014 Hareesha N G, Dept of Aero Engg, DSCE 12

Ultrasonic Machining of Ceramics

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Machining of Plastics and Composites

• Plastics need to be carefully supported.

• Requires large rake and relief angles, high cutting speed, and low feed.

• Trimming of plastic parts using water jet cutting or abrasive water jet cutting.

• Common problems in cutting composites – Delamination, poor edge finish, and fiber pull out.

• Use abrasive water jet cutting.

• Metal matrix composites such as carbide tool bits can be machined using diamond tools, EDM, and ECM.

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Characteristics of Machining

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