How It Works
An electron beam machine works in much the same way as a cathode ray tube in a television.
- A cathode section generates an electron beam.
- An anode section accelerates the beam.
- The lens system converges and deflects the electron beam to the desired position.
Schematic Illustration of Electron-Beam Machining Process
The electron beam machining process is fairly straightforward process. First a stream of electrons is started by a voltage differential the cathode. The concave shape of the cathode grid concentrates the stream through the anode. Much like the way a concave mirror focuses a light beam from a flashlight. The anode applies a potential field that accelerates the electrons. This stream of electrons is then forced through a valve in the electron beam machine. The valve is used for the purpose of controlling the beam and the duration of a machining process. If the impulse of the electron beam is to great, the part will over heat and potentially ruin the machined piece by either distorting a feature or relaxing strength built up from material cold-working or tempering.
Once passing through the valve, the beam is then focused onto the surface of the work material by a series of electromagnetic lens and deflector coils.
The entire process occurs in a vacuum chamber. The reason for the need of a vacuum for the electron-beam machining process is that air molecules can adversely interact with the beam of electrons. A collision between an electron and a air molecule causes the electron to veer off course. This phenomenon is illustrated in figures 3 and 4.
Electron-Beam in a Vacuum
Electron Beam in Ambient Air
Figure 3 show a clear stream of electrons bombarding the surface of a material allowing on easy and predictable cut. Figure 4 displays that the intensity of the beam is compromised by the collision with air molecules. This erradic stream hitting the surface of the part makes it difficult to calculate the time and/or beam intensity required to cut a part or drill a hole.
The electron beam is shot through the vacuum toward the material. The electrons often pass through the outer layer of the material and then become trapped in the material. This is shown in figure 2. The dark line shows the path of the electron.
Electron Trapped in Material
In contrast to the electron-beam machining, a laser machining process can be done outside of a vacuum - in ambient atmospheric conditions - becouse the size and mass of a photo is numerous times smaller than the size of an electron. The smaller size greatly reduces the probability of a collision with an air molecule. This is not to say that machining inside of a vacuum would not enhance the laser beam machining process.
(Kaynak:
Richard D. Hale)
----
