60. ELECTRON BEAM MACHINING

· The basic physics is an electron beam is directed towards a work piece, the electron heat and vaporize the metal.

· Typical applications are,

· 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.

· The process looks like,

· 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.

· Basic mechanics,

· e.g.

· the heat rise can be estimated using a one dimensional heat flow equation

· We can estimate the melting temperature with,

· e.g.

· Other effects of EBM

- 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
- medium - vacuum
- tool - beam of electrons moving at very high velocity
- maximum mrr = 10 mm3/min
- 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.