Vacuum Chamber

Re: Vacuum Chamber

rapatan — Thu, 19 Nov 2015 09:56:36 -0500

hi
please could the above be removed as this is inappropriet and missleading as max vac for the above chamber is 29 torr, for any beam you need at least 0.001 Torr and for a focused beam on tungsten cathode 0.0001 to 0.00001 , LaB6 cathode 0.0000001 to 0.00000001 Torr
kind regards
Rapatan

Re: Vacuum Chamber

PS — Fri, 05 Jun 2015 12:44:35 -0400

Hi All,

I think this project is a really interesting one, and not completely hopeless to do under a budget. Here are my thoughts, for what they're worth. (And I apologize in advance if anything has already been said).

-Regarding pumping to <1E10-4 Torr:
You should be able to do this with a two-stage rotary vane pump (~2E-3 Torr) plus a diffusion pump -- each which are relatively inexpensive. Titanium sublimation pumps were also mentioned, and would work surprisingly well to get below that final bit of pressure. At the minimum pressure in question, molecular flow begins to take over. Flashing titanium onto the large internal surface area of the chamber would be a great way to get there.

-Vacuum Motors
Although sealed vacuum-rated motors are available, these can get expensive rather quickly. Not only do the motors cost more, you will also require many more electrical feedthrus, increasing cost quite a bit and making it difficult to work on. Probably the best/cheapest way for a Z axis would be to buy a bellows and translate by expanding/contracting the bellows. This way, the only vacuum-specific piece of equipment you need is the bellows, and everything is easily accessible from outside of the chamber.

-Focus/Deflection
Another method for focus/deflection is the use of electrodes rather than coils. The electrodes are superior in my opinion because they can be precisely machined, and are therefore much easier to align. They also have a much faster response time, so precise beam positioning can be achieved almost instantaneously (versus coils, which are primarily useful for raster patterns only). Also, the electrostatic method uses almost no power because there is almost no current draw.

I hope this is useful.

Thanks,
Paul

Re: Vacuum Chamber

dc42 — Fri, 10 Apr 2015 16:11:52 -0400

Thanks for your considered reply. As a software engineer, I don't see why you would need an FPGA solution for controlling the PSU when a software solution could be easier and more flexible , but I guess you may have folks on your team with more experience with FPGAs than with embedded software. I do hope you can get this project up and running, although I fear the high vacuum requirements will be the stumbling block that prevents a domestic 3D printer of this type becoming a reality.

Re: Vacuum Chamber

rapatan — Fri, 10 Apr 2015 13:51:28 -0400

hi

Their are electron beam 3D printers working to day at the same specifications as this printer, as well as electron beam welders (using tungsten cathode filaments at 10-4 torr ). Though the top end printers use a LaB6 cathode's that are longer lasting and brighter but is too costly as it requires better vacuum 10-7 Torr.

Standard practise for solving the out gassing chamber wall issue is through baking out the chamber. The out-gassing powder issue is solved by heating the powder in a special oven over night prior to use.

The Xray shielding is done with concrete pipe or brick lined apparatus bay or using leaded glass rather than plane glass for vacuum chamber wall, (all stainless steel vacuum chamber walls require no extra X ray shielding).

These above findings have been confirmed by electron gun specialists who spent their career designing electron guns at international physics facilities.

The a 60kV voltage 5KW power supply already exists commercially, but with microprocessor control, our FPGA control version does create software challenges but also provides some advantages.

The laser route is unlikely ever to create a practicable largely self printing printer when limited to a normal domestic kitchen mains power outlet, laser printing poor energy inefficiency will mean impracticable long build times ( in the order of months) for all but jewelry sized items , unfortunately this is precisely why it is often proposed by some ( other problematic issues discussed include; KW laser costs , barrier gas bubbles in printed metal, non steel materials printing efficiency (7 times lower see below), massive size of KW laser printers due to long KW laser focal lengths, and lasers inability to; create thin metal solar cells, refine titanium sponge or make titanium powder.

thanks for your ideas.

kind regards
Rapatan

ref
Wall plug to metal efficiency of laser eg; Yag in iron 10%, Yag in aluminium 2.1% , Yag in titanium 1.5% metal absorption of laser page 27
wall plug to laser efficiency

ref
Wall plug to metal efficiency of Electron beam is 70% (88% for power supply wall plug to electron beam and 80% for electron beam to metals)

Re: Vacuum Chamber

dc42 — Tue, 17 Mar 2015 13:38:03 -0400

I have worked with high vacuum in the past, and I have to say that I think this concept is unfortunately a non-starter. To get a good electron beam, you need a very good vacuum indeed. Any residual gas will ionize and form a discharge between the electrodes of the electron gun.

Getting a good enough vacuum generally takes several hours of pumping using a diffusion pump. One of the main problems is outgassing. Gases from the air are absorbed by surfaces within the vacuum chamber, and when you evacuate the chamber, these gases gets slowly released. So you have to keep pumping until this outgassing reduces to a low enough value, and only then can you start up the electron beam.

In this case, the chamber will include a large amount of metallic power, and the total surface area of all the particles will be very high. Therefore, unless the powder is comprised of something that absorbs very little gas, outgassing from the powder will be a huge problem. Heating the powder may speed up outgassing, but I suspect it would still take many hours if not days to reduce to a low enough value.

Add to that the problems of building a power supply providing high voltage (I think 60kV was mentioned) at a significant current, and providing X-ray shielding. I think laser sintering would be a more practical way to go.

Re: Vacuum Chamber

jcamp — Mon, 16 Mar 2015 03:02:18 -0400

For the chamber - have seen a bell jar (http://en.wikipedia.org/wiki/Bell_jar) used in the electron microscope at Uni. A bloke I see occasionally has a vacuum chamber for metallizing telescope mirrors. IIRC it can do up to 14" and he uses a bell jar with a Refrigeration Vaccum Pump and an oil diffusion pump.

If you don't need to see inside steel would do (cut off LPG/Propane cylinder?) and for 10^-4 something like this:
[www.ebay.com.au]
is claimed to get close/do the job depending on what they meant by the claim "3 x 10 -1 Pa, 25 Microns"

Re: Vacuum Chamber

quantt — Tue, 24 Feb 2015 21:31:07 -0500

Sorry but i Didn't really think that through.

Re: Vacuum Chamber

sanman — Sun, 13 Jul 2014 14:39:33 -0400

Quote
pyrotronics
For a electron beam to work there needs to be a vacuum. a vacuum is created by sucking out all the gas inside the chamber. If there is gas in the chamber the electrons aren't going to travel very far as they would have to push the gas molecules out of the way first. If it was air for instance it would arc through the air to the nearest metal object, e.g. the chamber and not behave like a beam

What if the electron beam did not have to travel very far? Could this allow for a reduction in the quality of the vacuum?

Does anyone have a graph of acceptable beam distance vs required vacuum quality?

Furthermore, what if you could ionize the gas in your chamber, and then use an electric field to repel it away from the electron beam's path?

For example, suppose your electron beam was operating in a pulsed fashion, and you could likewise operate your electric field in a synchronized short-sharp-pulsed manner to push gaseous ions out of the way just as the electron beam pulse came through. Would that be possible?

[link.springer.com]

Look at that link - is that something that can be made to work here?

Re: Vacuum Chamber

Madkite — Fri, 07 Feb 2014 17:28:56 -0500

A laser sls printer does not need a vacuum but an inert atmosphere to sinter most metals. The inert gas prevent oxidisation which prevents proper bonding.

Re: Vacuum Chamber

Radagast — Tue, 05 Nov 2013 02:30:15 -0500

So creating a non-vacuum chamber big enough that the nearest metal/conductive object is the metal powder can also work?

Do you plan to use the double vacuum now - just like you stated: high vacuum at the electron beam source/gun and low vacuum in the main chamber?

I heard that a laser 3D printer needs vacuum too so it's good we chose the electron beam path. Couldn't we create a moving electron source, no or hardly vacuum, such that the gun is positioned as near as possible to the metal powder that we want to melt or the metal structure we want to mill away?

Re: Vacuum Chamber

pyrotronics — Tue, 15 Oct 2013 04:52:06 -0400

For a electron beam to work there needs to be a vacuum. a vacuum is created by sucking out all the gas inside the chamber. If there is gas in the chamber the electrons aren't going to travel very far as they would have to push the gas molecules out of the way first. If it was air for instance it would arc through the air to the nearest metal object, e.g. the chamber and not behave like a beam

Re: Vacuum Chamber

ShayneBrown — Mon, 14 Oct 2013 22:06:34 -0400

ShayneBrown Wrote:
-------------------------------------------------------
> VDX Wrote:
> --------------------------------------------------
> -----
> > ... for electrons any gas is a 'blocker' ...
>
>
> What kind of blocker? Wireless blocker?

Re: Vacuum Chamber

ShayneBrown — Sat, 12 Oct 2013 05:09:38 -0400

VDX Wrote:
-------------------------------------------------------
> ... for electrons any gas is a 'blocker' ...

What kind of blocker? Wireless blocker?

Re: Vacuum Chamber

ShayneBrown — Sat, 12 Oct 2013 05:07:20 -0400

Re: Vacuum Chamber

Andrzej — Tue, 30 Jul 2013 20:42:49 -0400

That is not entirely correct. Arcam's 3D EBM printer has a partial helium build atmosphere of .001 torr and last I checked it worked rather well!

Re: Vacuum Chamber

wogglestick — Fri, 29 Mar 2013 16:49:23 -0400

Yes, thanks that's much clearer! Thanks

Also, how is the design of the power supply coming along? I was sourcing parts the other day and it seems like the power supply is by far the most expensive part.

The rest (motors, electronics, vacuum pump, etc.) can be bought fairly cheaply from places like ebay. and even the prefab steel is reasonable from somewhere like mfgquote.com.

Re: Vacuum Chamber

rapatan — Thu, 28 Mar 2013 16:28:43 -0400

hi
Think of the large diameter glass outside the whole existing design, so that all the parts need not create a vacuum seal as the glass has already achieved this, this means that all the piston hoopper etc can be printed within the same diamter build chamber on the mother machine through splitting parts in to smaller parts and then assemble them. The bars they would hang off therefore fit outside the existing mechanism/piston piston wall. And so would the end of the hopper.
I hope that helps
kind regards
MetalicaRap team

ps.The other option is to suspend the large diameter glass cylinder 90cm above ground and open the chamber by lowering the bottom plate which has the build platform on top of it.

Re: Vacuum Chamber

wogglestick — Mon, 25 Mar 2013 15:27:44 -0400

great, thanks for the response!

I was trying to sketch out your description, could you elaborate on how the build plate, piston and motor all "hang" from the top plate? As far as I can tell, for them to hang there would have to be bars placed vertically in the chamber for them to hang from. but wouldn't these bars get in the way of the build plate? If so, how do you plan to maintain a powder-tight seal between the build plate and the bars/glass wall?

Also, you didn't mention a powder spreading mechanism at all.. could you elaborate a bit more on that please? I'm really interested to know how you plan to spread powder over the build area using only the space inside the tube.

Thanks!

Re: Vacuum Chamber

rapatan — Sat, 23 Mar 2013 15:40:21 -0400

hi
Yeah we have moved on,

We hopefully have found a way of removing any need for TIG welding equipment, reducing costs and allowing mother printer to self print build platform piston/powder cylinder in sections by having an outer main chamber made of glass. So that any welding that is needed can fit within the mother machine.

This may be achieved by using a 6inch 4 way reducer as filament/ anode chamber connected to the small diameter electron gun beam tube, (surrounded by focusing and deflection coils acting through the non magnetic 304L tube ), which connects ( via O rings) to the aluminium top plate of the main chamber. This main chamber is based around a large diameter borosilicate thick glass cylinder (positioned vertically), with an aluminium plate top and bottom, ( glass cylinder main chamber is surrounded with a removable concrete pipe for shielding when running) , with L gaskets ( vacuum bell jars sealing method) to seal glass to top and bottom plates. The build platform, motors and hopper are all contained within this main glass chamber hanging off the top plate. For access the whole assembly will be lifted out of the glass tube vertically.

Thanks for your feedback
Kind regards
MetalicaRap team.

Re: Vacuum Chamber

wogglestick — Fri, 22 Mar 2013 22:46:44 -0400

I was going over the schematics you guys put up on the wiki page for the build chamber, and I noticed something... there's no door. How do you get the parts out once printed? Forgive me if I misunderstood and you're not going with the design on the wiki page, but that struck me as pretty odd. Having to dismantle the chamber every time you print something seems pretty... dumb.

Re: Vacuum Chamber

sanman — Mon, 11 Feb 2013 09:23:43 -0500

Is this the better answer then?

[en.wikipedia.org]

[www.gammavacuum.com]

Re: Vacuum Chamber

VDX — Mon, 11 Feb 2013 02:24:08 -0500

... for electrons any gas is a 'blocker' ...

Re: Vacuum Chamber

sanman — Sun, 10 Feb 2013 16:40:46 -0500

Anybody heard of this? Could this help to improve the quality of the vacuum? If it's good enough for vacuum tubes, then why couldn't it be good enough for a RepRap?

[en.wikipedia.org]

[tubecrafter.com]

[tubecrafter.com]

Our main enemy is oxygen - am I correct? Do we care as much if there is an other harmless gas inside the build chamber?

Re: Vacuum Chamber

antony — Mon, 31 Dec 2012 05:39:48 -0500

is it possible to print hollow shere in vacuum chamber

Re: Vacuum Chamber

rapatan — Fri, 21 Sep 2012 11:19:23 -0400

hi Thaddeus W
For your welding shading problem at mains frequency due to filament modulation.
Have you thought about a rectifier on the high voltage side of filament transformer, so you go DCacross the filament, easier than going 400Hz?
kind regards
Rapatan

Re: Vacuum Chamber

rapatan — Wed, 05 Sep 2012 11:58:29 -0400

hi
Interesting I will have another look at that ike system, no hurry on the diagrams in your own timing!

Just a little addition to pump discussion;
Ion pumps are limited to 20 hrs of use at 10-3 Torr and 400,000 hrs at 10-8 Torr before titanium plate maintainence , but this is not the case with hot filament titanium sublimation pumps, the question is will the beam melting liberate the right amount of titanium vapour from the block to cover the inside of the box effectively.

I would be fasinated to know, as the beam should "refresh" the block during every use,

and is easily added to the existing design attaching to the back of the powder dispenser trough.

kind regards
Rapatan

Re: Vacuum Chamber

Thaddeus W — Tue, 04 Sep 2012 12:07:57 -0400

Raptan,

If the Google search "current control in an electron beam welder" the second link I see is for a beam current controller patent.

If you look at the patent holder Assignee: "Institut für Kerntechnik und Energiewandlung" the initials are IKE :-)

That is the IKE system patent was filed in 1976 which is around the time Leybold started using it on their machines, which was the late 1970's.

You see the simple block diagram shows the optically coupled input signal to the comparator circuit which biases the vacuum tube. The comparator closes the loop by watching the actual current value being pulled through the tube. The electron gun portion is symbolized by the larger vacuum tube in the box, the box symbolizing the vacuum chamber. The little circle(4) is actually the filament of the gun, the grid(8) is the wehnelt cup and the plate(5) is the anode cylinder which lies just in front of the gun. That little vacuum tube connected to the filament is an E130L (two E130L tubes are used in parallel for 30kW machines).

I have not yet had a chance to scan any schematics and I was trying to make a better diagram of the transformer bias system in KiCad. I will message you when I get a chance.

As for the titanium sublimation pump, they are primarily used as getter pumps in ultra high vacuum systems looking to get past the 1x10^-8 barrier that exists with diffusion and turbo pumps. The idea is to evaporate the titanium which reacts and traps gases which then sticks to a chamber wall. You keep cycling the filament until you reach a satisfactory pressure.They are not capable of pumping or "Getting" high gas loads present at higher pressures. Typically they are paired with Ion pumps and combined into one unit. They are commonly used in particle accelerators and the filaments are very thick and require a few kW to operate them. BUT I wouldn't hesitate with experimentation, I like the idea.

A big mechanical pump and booster pump (roots blower) can get a chamber down to the -4's. I know our large stokes pumps (412J, made by Edwards Vacuum) which also have roots blowers, get the foreline of the diffusion pump down into the low -3's/upper -4's alone.

Re: Vacuum Chamber

rapatan — Sat, 25 Aug 2012 16:24:09 -0400

Hi Thaddeus W
I would be interested in the transformer control option diagram you offered, you could send it here or to my private message account here.

Have you seen this control option, which a lot of the MRI guys are also using , but needs a 100KV tetrode/triode valve control ( not found a cheap one yet!).
search in google on exactly ie with quotes "current control in an electron beam welder"

We think we have found a cheap way to avoid the 2nd Vacuum pump all together!;
If you just pump down with only roughing pumps (achiving 1x 10-3 Torr or even 1x10-2 Torr?)
Can you get enough beam to heat the surface of a piece of titanium up to between 1500-1800deg C ?
(this is titanium's sublimation temperature ie turns direct to vapour.)
If you can then we have created a titanium sublimation pump. ( we just need to put the titanium block in side a double box with a beam entry hole at the top, and offset breathing holes through to the rest of the chamber on the box sides, thereby protecting the main chamber from the condensed titanium on inside surfaces of the box and yet let chamber gas combine with titanium creating solid compouds( thus creating a chemical pump).
This would simplify and clean up the chamber enviroment and reduce costs dramatically!

kind regards
Rapatan

Re: Vacuum Chamber

Thaddeus W — Tue, 21 Aug 2012 13:10:10 -0400

rapatan Wrote:
-------------------------------------------------------
> Hi Thaddeus W
> Could you double check the vac tube number again
> as the E130 L tube you mentioned above has a peak
> maximum voltage is 8KV. E130L Vacuum Tube data
> sheet link click
> They are running around100KV ?
> What configuration are they using the tube/tubes
> in? what other components?
>
> great if you could help
> kind regards
> Rapatan

Rapatan,

The E130L acts as a kind of variable resistor controlling the current flow to the filament. The high potential is between the "grid cup" (wehnelt cup) and the anode plate. So the E130L is not conducting the high voltage, only the current to the filament which has a potential of a few kV between the grid cup. I have the full original schematics, although they are heavily outdated and need to be scanned. The system has gone through a number of revisions since the late 1970's, though the principal is the same. I am not so sure if I can post the original schematics due to copyright. I will see what I can do.

The transformer method is very simple. I have a schematic but it is copyrighted and only available to us for reference through the manufacturer. Plus the diagram is very poor and has missing component values as well as missing wire labels. But I can make a better schematic and post it since the principal is very easy to implement. To sum up the function is simple: Imagine two power supplies, one high potential for the cathode-anode (to accelerate the electrons) and a medium potential power supply for the cathode-filament bias. To control the beam current you simply adjust the voltage of the bias supply; more voltage for less current or cut-off and less voltage for higher beam current. The positive of the bias power supply is wired to the filament and the negative hooks to the negative of the high potential supply which is also hooked to the grid cup.

I just realized I explained the function backwards in my original post. Originally I stated that you raise the potential of the filament relative to the grid cup. The transformer system actually forces the potential of the filament below the potential of the grid cup (makes the filament more positive). So if your high voltage supply is say 100kV, the bias supply drops the filament potential to lets say 98kV to stop current flow. You then raise it back up toward 100kV to allow beam current to flow. In simple terms, the bias power supply is pushing back against the electron flow to the filament. Sorry for that mistake.

As we speak I am currently designing a digital beam current control system for our machine. The original bias supply controller is cumbersome to use and unstable. We frequently experienced current spikes which destroyed our customers parts (drills a nice hole through them). One critical mistake the original manufacturer made was putting too much faith in the command signal stability which frequently dropped out causing the bias supply voltage to drop significantly. We recently had a third party install a new beam current controller and we agreed that a series resistor (5k) between the filament and bias power supply was necessary to limit the overall current range. The problem with the new beam current controller is its design is primitive, clunky and lacks many features we were used to on the old system. It feeds the transformer with 60Hz mains frequency which is too slow, the ripple causes dark spots in the beam when using the deflection generator. I cant give all of my secrets away but I will share with you the basics.

I am using the original bias supply in the high voltage tank (large metal oil filled box that houses the high voltage systems) which is a simple linear power supply. It consists of a transformer with high isolation potential (200kV DC) and will produce 3kV DC with an AC input of 40 volts. Its circuitry is simply a bridge rectifier with an LC filter on the output and a small RC network after that. The original manufacturer actually used a variable amplitude 400Hz AC signal to drive the power supply created by a power OP-AMP inside the controller. The reason for 400Hz is because the beam is deflected using an X-Y coil setup (similar to the coils in a CRT monitor) to generate a circle pattern. The circle pattern is used for whats called a "dress pass" after a part has been welded; it cleans up the weld bead. The lower frequency of the mains, 60Hz is actually too slow for the circle pattern and its ripple causes the dark spots. My idea is to build a full H-bridge or a push-pull power op-amp output stage to create an AC sine wave and use a voltage controlled amplifier (VCA) fed by a 400Hz oscillator to drive the output stage. I plan to use a simple digital sine wave oscillator using either D-flip flops or a shift register, a bit more bulky than a wien bridge oscillator but much easier to debug and control frequency. The voltage controlled amplifier is the key to the system as it varies the amplitude of the 400Hz to the bias power supply which in turn varies the beam current. A PLC, or MCU such as the Mbed or Arduino can provide the necessary DC voltage level to control the VCA.

The IKE system is very complex and requires you to have a lot of circuitry inside the tank at the high voltage potential. Imagine having lots of static sensitive op-amps and transistors held at -150kV DC and submerged in oil, that's the IKE system. The control mechanism is actually two signals brought to the circuitry via an optical coupling. They have photo transistors receiving the signals, one is called the peaking signal which is a 1kHZ square wave and the other is the command signal which uses a 20-100kHZ square wave to control the beam current. 100kHZ is beam off and 20kHZ is full beam power. It sounds complex but all that signal does is control a frequency-to-voltage converter (an old and obsolete Motorola part) which feeds a comparator circuit that drives the E130L. The E130L plate voltage power supply is actually all vacuum valve, an E84L drives the plate and its bias voltage is fed from an EF80 referenced by an 85A2 constant voltage vacuum valve. It really is a beautiful pure analog design when you look at the schematics, all controlled by op-amps, transistors and vacuum valves. The only black box is the Motorola frequency to voltage converter. Whoever designed it was certainly well versed in analog design, almost as if they designed audio amplifiers for a living.

If you ask me which system is the easiest to build and easily interfaces to a computer or MCU I would say stick with the transformer bias system. The bias power supply is the only part that needs to be at the high potential. And a variable AC voltage is all you need to control it. In fact the overly simple and primitive controller we have now is a variac controlled by a servo motor. Building and debugging your bias supply is as simple as using a variac to control your beam current. Then you move onto the more complex analog circuitry to precisely vary the voltage. Then throw a digital front end on using a DAC or MCU and your finished. Avoid the IKE design, its so complex and requires a lot of engineering just to make a beam. Its not worth your time as you will sink way more effort on current control than building the rest of the system!

And one last word of caution: any transformer you use to provide the bias signal and filament current must be able to isolate against the full potential of the high voltage supply. So if you gun power supply has a potential of 50kV DC, your filament and bias transformer MUST also be rated for 50kV isolation minimum! If not, the high potential will arc right through the transformer causing damage to equipment or injury/death. Please use caution when working on such high voltage systems!

Re: Vacuum Chamber

rapatan — Sun, 19 Aug 2012 13:42:51 -0400

hi
could you check the tube number again? E130L? see other posts in vac section
kind regards
Rapatan