MetalicaRap:Tool head processes discussion
Main issues when comparing alternative processes with conventional machined metals have to consider! 1. Metallurgy; including Resultant grain structure from cooling profile, heat damage/distortion of process, 2. IT grade dimensional tolerance achievable , 3 Difficulty/special requirement of securing part in machine, 4 Metal - gas chemical reactions at temperature.
External links for Processes comparison
EBM vs SLS EBM has some advantages over SLS.
(SLS and EBM have some similarities: one puts down layers of powder, and one scans a beam over each layer of powder to get the powder to stick together in the right places.)
- EBM may be able to reach finished parts IT grade 07 ,SLS cannot produce finished parts unless very expensive laser pointing systems are employed.
- electron beam melting produces parts that are void-free, and therefore full density and full strength.
- The parts that generate an electron beam seem simpler/possible to replicate than the parts to generate a laser beam ( Adrian Bowyer" we may never reprap a laser").
- An EBM machine can do both additive machining -- melting new powder grains onto a part -- and also subtractive milling -- vaporizing outer layers of a part.( 600 W ND Yag Q lasers can also vaporize but expensive 100,000 euro)
- in principle, it seems possible to EBM materials that are completely transparent to laser beams, (reflectivity of aluminum for example means a laser cannot touch it, but steel has 98% of laser energy entry, so is fine) .
- ... other advantages ...
(See Below for advantages of SLS over EBM).
Discussion questions: The EBM machines I've seen photographs of generate the electron beam at the top, then focus and steer the beam to strike the appropriate spot on the powder at the bottom. That requires high vacuum -- otherwise the electron beam is completely absorbed by the air or other gas. is it possible to generate the electron beam on a moving toolhead a short distance above the appropriate spot on the powder? Yes but beam spreads out and looses power, but the main problem is your metallurgy suffers so they are no longer equivalent to wrought iron milled parts, which is only possible in a vacuum 10-3Torr or better. Welding is traditionally a poor metallurgical join so weakness is allowed for in the join design, but this weakness through out a part often makes the part too inferior compared with milled parts. see ((1) below). How short does the distance traveled by the electrons through air need to be, in order that at least half the energy of the beam makes it all the way through to melt the powder, rather than heating the air/nitrogen/argon atmosphere? See the artist's conception of a potential "Cartesian robot gantry" for non-vacuum electron beam welding[1]. Wikipedia: electron beam welding#In-air welding
SLS vs EBM SLS has some advantages over EBM.
SLS does not require a high vacuum. It works at normal air pressure.
Most plastics, ceramics and lignin can be sintered open-air. (Most metals require an inert Argon or Nitrogen atmosphere, which is still an advantage over high vacuum in simplicity but not metallurgy eg.see (2) below Hydrogen metal cracking. ).
SLS appears to have lower cost and less research time before we get the first RepRap prototype working.
It is impossible to accidentally generate X-rays from a laser beam.
... other advantages ...
EDM vs SLS
(Electric discharge machining Selective laser sintering)
EDM has sub micron tolerance
SLS requires very expensive pointing method to achieve micron level tolerance.
other Advantages DIsadvantages....
External links
1. http://books.google.com/books?id=FQSEfRigyNUC&lpg=PP1&dq=welding%20metallurgy&pg=PA4#v=onepage&q&f=false welding metalurgy 2. http://books.google.com/books?id=vKhQ6BtFRNsC&lpg=PA200&dq=shield-gas%20tensile%20strength&pg=PA200#v=onepage&q&f=false shield-gas introducing hydrogen cracking in metal joins.
