1. ELECTRON BEAM MACHINING
• The basic physics is an electron beam is directed towards a work piece, the electron heat and vaporize the metal.
• electrons accelerated with voltages of approx. 150,000V to create velocities over 200,000 km/sec.
• beam can be focused to 10 to 200 micro m and a density of 6500 GW/mm2
• good for narrow holes and slots.
e.g. a hole in a sheet 1.25 mm thick up to 125 micro m diameter can be cut almost instantly with a taper of 2 to 4 degrees
• the electron beam is aimed using magnets to deflect the stream of electrons
• a vacuum is used to minimize electron collision with air molecules.
• beam is focussed using an electromagnetic lens.
• Some examples of cutting performance are given below,
• typical energy requirements for cutting are,
• e.g. to cut a 150 micro m wide slot in a 1mm thick tungsten sheet, using a 5KW power source, determine the cutting speed.
• the heat rise can be estimated using a one dimensional heat flow equation
• We can estimate the melting temperature with,
- process done in vacuum, so it is best suited to small parts, but vacuum also reduces contamination
- very high heat concentration reduces peripheral heating of surface less that 50 micro m from the cut the part is at room temperature.
• Summary of EBM characteristics,
- mechanics of material removal - melting, vaporization
- tool - beam of electrons moving at very high velocity
- specific power consumption = 450W/mm3/min
- critical parameters - accelerating voltage, beam current, beam diameter, work speed, melting temperature
- materials application - all materials
- shape application - drilling fine holes, cutting contours in sheets, cutting narrow slots
- limitations - very high specific energy consumption, necessity of vacuum, expensive machine.
1.2 PRACTICE PROBLEMS