Talk:MetalicaRap
MetalicaRap Design question feed back / discussion. Add your ideas here!
Safety hazards
That thing would produce a lot of x-rays. Did not see that mentioned in the article - sorry if you guys already thought of that, just don't want any of you guys getting dangerous x-ray exposure when experimenting! Keep up the good work but please stay safe with high voltage at low pressure. It really is dangerous. Dmytry 19:09, 15 May 2011 (UTC)
Design Questions
A. Range of achievable spot sizes at target of a Pierce Electron gun running between 100W to 6KW 150KV ? ( given cathode is 1.6m from target max deflection 7 degrees )
B. Possible pit falls of running an SEM at 100W in four-source photometric stereo Ruderford back scatter mode ? ( typical SEM power is 0.1W )
Q:. Target metal surface temperature measurement would be a big advantage, Do you know of a electron bombardment based remote temperature measurement approach?
A1: ???
A2: The bombarded surface gets red-hot, right? Is it possible to put an infrared camera inside the chamber -- or an off-the-shelf webcam with its infrared filter removed -- and calibrate it using some test objects in the chamber heated to a known temperature?
Q: Some EBW machines put the workpiece outside the chamber, in open air. What materials are useful as a "window" -- to pass the electron beam, but hold the air out of the vacuum chamber?
I don't know how hard can be to make it, but this can be a solution? http://en.wikipedia.org/wiki/Plasma_window http://www.newscientist.com/data/images/archive/2390/23904801.jpg
Q: how will vacuum and powder work, won't that mean some gas will be trapped in the powder and each layer you make some of it will be released and will have to be compensated for?
--Robinmdh 14:03, 18 March 2011 (UTC)
Other ideas.
Q: Printing in air at 1 atmosphere would mess up the metallurgy, but a high vacuum is expensive. What about a low vacuum with a high shield gas percentage?
Answer ; In some existing low pressure weld EBW systems they enclose electron gun in a high vacuum box and send beam through window in to work piece in low vacuum air, keeping the work piece within 3cm and they get a 1cm diameter beam spot. Then shield the room with 1mm to 10mm of lead. So you can see that a beam will expand to 1cm diameter over 3cm of low pressure air travel. To get rid of all high vacuum the gun would not work as the electron gun is over 3cm's long, the cathode + anode+ 2 focus coils+ deflection coils is about 60cmn long may be you could get them down to less? So the electron gun needs a high vacuum at least.
A.High vacuum is a box 6/8mm steel + vane pump,(used 600E) + oil diffusion pump (used 800E) (possible make self) + Door Seals + electrical connectors + Gauge (used 80E) + circular motor feed through s +Powder deposit/electron gun/ power supply and control
B.low vacuum with shield gas is a 2mm Box + ( cylinder of Nitrogen or Argon (XX?E)or Nitrogen plant) + Door seals + electrical connectors + Guage(used 80E) + % oxygen sensor +circular motor feed through s + Powder deposit/ power supply/ control/ electron gun vacuum box vane pump,(used 600E) + oil diffusion pump (used 800E) (possible make self)
In case A the beam can be around 100µm tolerance 20µ m, over 1m travel (max 7 degree divergence out of gun, so build area 30cm x 30cm ), the metalurgy is good enough for jet engine parts.
In case B the the beam can be around 1cm tolerance 200µ m?, travel over 3 cm (build area 3cm x 3cm ) metallurgy not so good.
I hope that's a help? ( I have not done the maths on beam spread rate in low vacuums, so the above is from what I have read. )
Robinmdh, all good questions, which I'm interested in knowing the answers to. Since you're a dev, and I'm a dev, and the MetalicaRap team is all devs, want to ask them in reprap-dev? --Sebastien Bailard 09:18, 22 March 2011 (UTC)
For the vacuum chamber, I don't thing multiple parts will be a problem. Simply make them interlocking and to a close tolerance and with built in threaded holes etc. for fasteners, and apply vacuum grease /wax or there is probably a caulk like substance available, for a good seal. The thing to remember is that there are a wide range of useful products that have been developed for working with vacuum. If there is a mechanical hole it tends to be a relatively big hole and you can detect such holes in various ways with soap bubbles and ultrasonic sensors of the type HVAc people use. The second generation unit vacuum chambers will not be a problem I think. The number of parts is the only way really to surmount the make a matchbox in a matchbox problem without paying tons of money for more stainless steel sheets.
Okay so I have been doing some research for the Metalica project too. WRT changes during cooling after printing at high temperatures or due to thermal fluctuations I think this is a key issue for making reasonably precise parts and a better handle is needed on it, how to predict the changes and ultimately how predicatable they even are. Got some free some documents on modelling such changes etc. but have yet to read them.
Okay next it is also advertised that electron beam printers can make stress free parts. From the documents that are available that appears to be maketing bullshit. There are minor stresses and sometimes even microcracks. However that is better than all the other freeform methods apparently. It is not clear why this is the case, as the process is essentially much like DMLS except with higher powers (4 point somethin KW ebeam) and secondly with very high temperatures during printing, in the range of 800 to 900 or even more deg C (yes C), which is probably well in the range where e.g. tool steel will creep. Some authors say it is due to the 1m/s or so scan speeds, which is higher than in DMLS. I'm skeptical though as it is probably possible to achieve that scan rate in DMLS too.
WRT using mats instead of powder I think this coudl be quite helpful but the problem is levelling the bed, as the density of the mat is lower than the steel so over the print process otherwise the top level of the mat stack would overtake the level of the object being printed. It could simply be machined off as the excess mat would be easy to remove that way.
Thirdly from the research I have been doing it looks like it might be altogether possible in fact to use less than a high vacuum in the print chamber, which might come in useful as it may turn out that sensors etc. are desirable in the chamber and any purchased sensors will not likely be made of materials that outgas at rates low enough for a high vacuum. Even the stepper motors for the gantry etc will likely be a problem.
To do this the conventional way is to just use a really tiny hole (probably laser drilled) in a plate, a micron wide or so. The electron beam comes out the hole, and gas of course gets in there but it is so little that you can still maintain the high vacuum on the other side of the beam. Obviously the deflector coils are on the other side. Well you might want some on the other side too so it goes through the hole maybe?
The gas can only get through the hole so fast actually because of so called necking, the gas cannot exceed the speed of sound. This way the main chamber could perhaps be only a medium vacuum. With regards to beam divergence, that needs to be investigated but I seriously doubt it is a problem as focal area is on the order of 0.9mm anyway. However suppose it is only 100 pascals on one side of the hole 1 square micron in cross section, and the gas gets through at 330 m/s the speed of sound that's 330*10^6 cubic microns, if it has to be 10-4 torr on the other side that is about 1.33*10^-2 pascals, so you'd get roughly 330*10^10 cubic microns that the high vacuum pump needs to remove at 10-4 torr, it's 10^15 cubic microns per liter, so you only need to pump 10^-5 liters. A diffusion pump might get: gets 500 liters at 550 watts or so (independent of pressure presumably as that is how diffusion pumps go basically at low pressures) at 550 watts so that shoudl be fine. Indeed the hole can be a lot bigger than a micron... So that's handy.
Another method is to use a plasma window but that might require more development work so might be better left for future versions and it appears to be unneeded.
I could not find a quick answer to how much the beam diverges or looses energy in a partial vacuum but maybe you could ask on a physics forum or something. Or just calculate it from the high energy electron collision cross section and the density of gas molecules in the chamber.
Lastly, I think it might be a very good idea to just build a small one first. It will cost a fraction as much, will still be quite useful and will allow you to get the process parameters etc down. IMO it makes no sense to try to do this big one when there is unlikely to be enough money to deal with all the unforseen issues... Greenatolsecondtry 03:38, 23 August 2011 (UTC)
