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Vacuum Chamber

Posted by JohnDoe 
Vacuum Chamber
April 18, 2011 11:07AM
From what I've read, this looks to be the most useful offshoot of the reprap project. Plastic parts at low resolution simply don't cut it for a useful industrial machine.

Everything seems perfectly obtainable here except for the vacuum chamber itself. Has anybody priced it out yet? It seems to me that it would cost tens of thousands in materials alone, going by the prices I've found online - and I know that a UHV tank of 1m^3 volume costs ~ $1million.
Re: Vacuum Chamber
May 15, 2011 11:40AM
Actually I was just thinking you could use an old pressure cooker if it was smaller. Some even have metal seals from a copper gasket, so that could work for high vacuum. There are also a lot of products like vacuum wax that make working with high vacuums much easier and can be used to seal any cracks.

Given the progress at present if costs are a major barrier it may make sense to shrink it some. If it can fit into a pressure cooker that could be a perk.
Re: Vacuum Chamber
June 10, 2011 05:06AM
I don't think any standard pressure cooker would be large enough for any usable machine.

From what I've researched building High Vacuum Chambers is a high tech art unobtainable by almost any technique.
Even welding it together by hand is nearly impossible and even with very sophisticated equipment there is a very high failure rate. The welding has to be done in one go from the inside of the chamber or there will be too much out-gassing.

The best way imho this could be theoretically obtainable is building a fairly high end welding robot first which is a huge project of its own...
Re: Vacuum Chamber
June 12, 2011 04:13AM
On the other hand this could work...

[www.youtube.com]

This might be sufficient but you'd have to restrict the replication process to other parts...

The second generation could use this... which also has the advantage of being able to produce low res parts and use the same powder.
[www.youtube.com]
Re: Vacuum Chamber
June 27, 2011 11:29AM
hi
Yeah vacuum technology is a new learning curve for most , UHV chambers go down to 10-11 torr, these chambers are tricky, we nee to get to 10-4 Torr this is much less demanding on skills of welding and machining, you can even use glue sealants for leaks on these 10-4 chambers.

For the bootstrap prototype using the old fashioned tried and tested simple TIG welded H 0.8m x W0.8m x Depth 0.4m plate chamber with 0.80m long electron gun tube diameter 0.32m on top in 25mm thick aluminum , cost of materials purchased in small quantity is approx 3000 euro plus fittings and welding.
See High vacuum chamber section

Eventually these fittings will be self printed and the chamber may be half this size but for now that's the challenge. Test instruments will also required creativity for price reduction.

So get playing, you can do an approx self test by filling a chamber with steam then sealing it and putting ice round the chamber and see if it crushes and make sure you post the videos, Remember if you are not using copper or aluminum or stainless you will need to coat the inside before use so that significant out-gassing is avoided Outgassing online material check list .

As far as we know all existing commercialized vacuum products are based round permanently sealed designs, and those people who want to have pump down chambers have traditionally had million euro budgets Labs etc, so this is where the creativity comes in to get below this 3000 euro plus fittings and welding.

kind regards
MetalicaRap team
Re: Vacuum Chamber
August 16, 2011 11:36AM
Most obvious solution to me is to make it indirectly. Rather than print out the parts for it, print out segments for a mold, and cast it. Allows for larger pieces than can fit within the chambers.
Re: Vacuum Chamber
August 17, 2011 12:48PM
hi
Interesting
We can use the beam to melt the metal,
Need room to cast does that require special air systems etc?
kind regards
Rapatan metalicarap team
Re: Vacuum Chamber
August 23, 2011 06:21PM
No, casting is relatively simple. There are plenty of hobby casting sites out there. I would use glass or a polymer for the actual chamber, with a metal mold, otherwise you risk melting your mold with what you're casting. For metal casting most forges use sand mixed with a binding agent, like sodium silicate. Could also make a negative mold to then cast sand with. I'll work up a few scad models to demonstrate how such a mold could be made.
Re: Vacuum Chamber
September 23, 2011 06:50AM
hi
Now got vacuum chamber materials down to 2400 euro plus weld and fittings, via 2 lengths of 304 sch2014inch pipe with an aluminum hopper box cut out from one thick sheet with a 304 plate acting as bottom of the box.
Any other clever solutions? out there?
kind regards
rapatan and metalicarap team
Re: Vacuum Chamber
October 06, 2011 10:34AM
how about filling a chamber with an inert gas such as Argon?
Then the pressure is less of an issue, although it would not keep the working material as hot due to the introduction of convection losses. Surely it would be cheaper (and easier) to keep argon at a working temperature (heated chamber and bed) but atmospheric pressure than trying to work in a vacuum.
VDX
Re: Vacuum Chamber
October 06, 2011 02:17PM
... you need the vacuum for the electron beam - any atmosphere will kill the e-beam.

With gas filling you have to apply lasers ...


Viktor
Re: Vacuum Chamber
October 14, 2011 11:23AM
Of course you do! Didn't really think that through.
Multiple stage chamber
Re: Vacuum Chamber
November 01, 2011 11:20PM
Couldn't you use a vacuum chamber within a vacuum chamber?
Say a 1mm thick stainless enclosure surrounded by a much cheaper composite/steel/concrete structure?

Then you pull the vacuum down on one structure which, in turn, lets you do the same on the next.
This way you can use cheap materials that outgas, or have whatever properties that are unfavorable to the working chamber, whilst still having a working chamber made to greater tolerances, but using much less material since the pressure differential between inside and outside is now much smaller.
You could even use thin view windows, with say a webcam in the outer chamber, which I'm guessing is full of the kind of materials that would outgas (and thus you wouldn't want to use on an inner chamber).
VDX
Re: Vacuum Chamber
November 02, 2011 02:20AM
... but then you need more (pricey) vacuum-pums too (one for every 'stepdown') and the outer chambers should be big enough to enclosure the corresponding vacuum pump too ... or you need even more expensive pumps if you'll have all of them outside ...


Viktor
ace_xp2
Re: Vacuum Chamber
November 02, 2011 10:07AM
Heh, I see I got the author line confused with the subject line, oops!

Though that would be true, you wouldn't need the same level of vacuum outside to in. Just enough to make the pressure differential acceptable for the thickness of stainless you're using. So a simple piston pump (backwards oilless air compressor?) may be enough to draw the external container down.

Question: Does the vacuum actually produce notable increases in strain once we get past a certain point? It seems to me that once vane/piston pumps are beyond the ability to pull down, the remaining particles wouldn't be substantially contributing to internal pressure. So I'm thinking at that point you may need fancier pumps, but you wouldn't need a stronger container to go farther.
Re: Vacuum Chamber
November 11, 2011 09:36AM
What about machinery operating inside the vacuum. How will a normal NEMA motor and regular bearings hold up fine in vacuum of this magnitude? How do you seal the electric wiring required to operate the CNC axis and powder welder?

Where do you get a vacuum pump fit to get to 10-4 Torr?
Re: Vacuum Chamber
January 12, 2012 01:04PM
Hi guys,

Have you considered using ceramic material? Definitely not an expert in vacuum ceramics, but accoding to NASA there are quite a few epoxies and ceramics exhibiting low outgassing in vacuum. It seems that vacuum chamber of arbitrary size could be relatively easily casted and then cured with home-achievable temperatures.
Re: Vacuum Chamber
July 19, 2012 10:08AM
Guys, don't forget the important possibility of using a high vacuum inside the electron gun, but only relatively low vacuum in the print chamber. I remember calculating, I think the results should be on the wiki (see my edit history if you are desperate, don't have time to do it again though sorry) that the beam spread was not a major problem actually at low vacuum. Especially because the beam spot size in an arcam printer is actually quite large, I think it was 2 mm or so. So it is not finely focussed or well collimated anyway. A little scattering will probably do no harm.

It is possible indeed fairly practical, to simply have a very small hole between the low vaccum areas, and the high vacuum areas, and then a vapour diffusion pump (which is very cheap and printable) to pump the gas that leaks through said hole away *fast enough to maintain a high vacuum in the inside of the electron gun*.

I forget why the electron gun needs to be high vacuum. Probably ion wind causing erosion or something? I don't know. But anyway this is a common technique; simply put the only part that really needs to be at high vacuum inside a container with a very small hole for the electron beam to escape, and then run the vapour diffusion pump continuously to maintain the vacuum. The deflection of the beam would happen outside of the high vacuum area of course. That way any servos and so forth in the main print chamber are far easier and cheaper to make.

I think this would be a really good idea to do, because it can become expremely difficult and complicated trying to troubleshoot the source of a leak or outgassing material, and get all your moving parts and so on needed for powder feeding, to be high vacuum compatible.

To re-evaluate the viability of thi appraoch, I suggest calculating a) supposing the gas is nitrogen, the amount of kinetic energy that is available from expanding a gas from a low vacuum to the vacuum we want inside the electron gun b) what maximum velocity this entails that the gas would be travelling at. There is an important thing to remember here, which is that the gas jet can't go faster than the speed of sound in the gas. The speed of sound in the gas also goes down with pressure remember. This is called "necking" I believe and is well known in vacuuum chamber design. It's a real blessing.because it restricts the impact that small holes can have on gas getting in. c) determine the pumping rate that the vacuum diffusion pump requires for a variety of hole sizes in order to maintain the desired vacuum level inside the electron gun. Compare with the cost per unit pumping capacity of oil diffusion pumps.

I looked some stuff up once and did the calculations (should be on the wiki) and found I beleive that a circular hole eve 100 microns wide would be OK with a cheap $100 oil diffusion pump.

The only issue I see with this is a)the beam has to be collimated enough to get thgouth this hole, and stray electrical and magnetic fields kept away to revent it from colliding with the sides.

Another option is a plasma window. The theory behind a plasma window is fairly simple, you run an electric current down a tube, heating the gas to ahigh level. The hotter a gas is the higher it's viscosity. Thus you greatly slow down the flow of gas from the high pressure area to the low pressure area. Plus because of refraction and the magnetiv fields produced it apparently has the effect of collimating the electron beam for you some. However I don't know how hard it would be to design a plasma window. I think we should lighten our design load and just stick with the hole and pump method.
Re: Vacuum Chamber
August 08, 2012 09:41AM
Hello,
I am an engineer working for an EB and laser welding shop in the US. I am in charge of designing new EB control systems, upgrade technology, maintenance and Vacuum systems.

This is a very interesting project and can be done cheaply if you know what you are doing. The major costs involved will be obtaining a mechanical vacuum pump for roughing and backing a diffusion pump. Some of those can be obtained second hand but often they need rebuilding and cleaning. So be careful when purchasing a used pump that may have been abused and not properly maintained. A poor pump might have you chasing ghosts when figuring out why you cant get your vacuum chamber down to a decent enough pressure. Test the ultimate vacuum using a known good vacuum gauge first before hooking it to a chamber.

First off no special chamber materials are necessary since you can get an electron beam to operate in a vacuum of around 5x10^-3 Torr. Buna rubber seals and low out gassing glues can be used for sealing components and costly vacuum flanges can be avoided in much of the construction. Simple pipe threads sealed with teflon tape or even glues can be used along with cheaper KF flanges. You dont need metal sealed flanges such as the conflat or wheeler flange. The only expense might be obtaining vacuum rated valves but for a home-brew project, simple ball/gate valves might suffice as long as they can hold a vacuum. You can test the valves by hooking one end of the valve to a short length of tube or hose to your vacuum pump along with a vacuum gauge. The other end of the valve should be exposed to atmosphere and then the pump turned on. If the valve can seal off the atmosphere so the pump can reach its ultimate pressure then its decent enough for vacuum.

You also want a good vacuum gauge, a thermocouple or pirani gauge will be good down to 1x10^-3 Torr, some good pirani gauges can be useful down to 1x10^-4/-5 Torr. You don't need an ion or penning gauge as you don't care about pressures below 1x10^-3 torr. Its just another unnecessary expense.

The low vacuum is necessary to facilitate free electron "flow" and to prevent air molecules from ionizing and creating a direct path to ground from the gun/filament. Triode guns using a wehnelt cup are susceptible to flash-overs between the filament and wehnelt cup causing an uncontrolled surge of beam current, basically a short circuit. So a triode gun should always have some sort of over current protection which will cut off the high voltage.

Someone here has mentioned the TWI RF indirectly heated filament. I would avoid going this route since its a much more complex method of beam control. The TWI gun design heats the filament indirectly using an electron beam generated by a secondary filament behind it. The secondary filament is powered by an RF coupled transformer system to avoid the need for high voltage isolation and multi-core cable. The beam current is controlled by two mechanisms, first the primary filaments temperature is adjusted to limit the beam current. The temperature determines the work function necessary to remove electrons from the filament surface via electrostatic potential. This is complex in itself as you need to closely monitor the surface temperature of the filament. I believe either Pro Beam or Steigerwald does this via an infrared camera, no one I have spoken to knows exactly how the temperature is monitored, its a bit of a secret. Once you have the surface temperature needed for the current range you want, you need to regulate the beam current in the power supply for fine control. This can be done via solid state regulation using a switching supply.

Stick with the triode gun design, its much, MUCH simpler to produce and can be biased with a simple resistor network and potentiometer. You are simply melting metal, not welding parts with a penetration tolerance in the 0.1 - 0.01 mm range. A filament made from tungsten wire and a wehnelt cup made from a copper plumbing pipe cap is all you need to get started.

There are three methods to triode gun biasing used in EB welders:
Resistance bias: The oldest and simplest method. The filament is connected to a potentiometer and limiting resistor to the negative HV terminal. The wehnelt cup is also connected to the negative of the HV supply. The theory is if you short the filament to the wehnelt cup, you get full bias equivalent to saturation in a BJT transistor, unlimited current will flow. Introducing a resistance will allow one to control the current flow to the filament. The old EB machine used whats called a 100 position switch. Basically a pcb with 100 contacts arranged in a circle with a wiper that is controlled via a knob or servo/stepper motor. Between each contact is a high value and power (2-5W) resistor.

Transformer bias: If the potential of the filament is raised above the wehnelt cup then you put the triode is cut-off. For a 150kV EB machine this is somewhere in the order of 2-3kV, less for lower voltages. An isolation transformer powers a HV rectifier circuit with filtering and connected between the filament and negative of the power supply. As the biasing voltage is dropped, beam current rises. Be sure to include a current limiting resistor as you can easily cause a surge if you loose power to your bias circuit. Not exactly simple as the transformer adds a lot of cost but it could be done with a neon sign transformer or possibly a modified microwave oven transformer.

Vacuum Tube(Valve) bias: This is a complex system that was developed by Leybold Heraeus in the late 1970's in cooperation with IKE (a German technology institute which I cant spell). It is commonly referred to as the IKE system and is now property of PTR here in the US and is also used by Steigerwald who is a partner of PTR. It uses one or more E130L vacuum valves to regulate the beam current to the filament. Inside the HV power supply tank a few circuit boards that accept a reference or as the IKE system calls it, a "command" signal. This is sent through an analog comparator circuit which then monitors the actual beam current vs the command signal and then outputs a bias signal to the E130L. the E130L is actually carrying the current through it and the current flow is regulated by the bias signal. Its the most complex of the three and I would avoid it because its over-kill for simple melting. It is however, very accurate and closed loop in its control making it very stable.

A good article to read is the Wikipedia article on thermionic emission.

I hope my post helps some of you in the right direction. I see some of you are aiming too high and thinking all of these complex and expensive techniques/systems used in industry are necessary, but really aren't. Most of the advanced stuff you see is because of high tolerances necessary to meet strict quality standards. Much of the stuff we weld goes into satellites, fighter jets and other aerospace/defence systems. Those companies and contractors do not allow for error that can cause catastrophic failure of multi-million/billion doller/euro systems. That's why you have all of this fancy technology.

Have fun!
Thaddeus W
Re: Vacuum Chamber
August 08, 2012 10:51AM
freyzor Wrote:
-------------------------------------------------------
> What about machinery operating inside the vacuum.
> How will a normal NEMA motor and regular bearings
> hold up fine in vacuum of this magnitude? How do
> you seal the electric wiring required to operate
> the CNC axis and powder welder?
>
> Where do you get a vacuum pump fit to get to 10-4
> Torr?

Freyzor,
Standard electrical equipment can be operated in a vacuum but there will be some problems to address. First off the biggest problem is heat dissipation. Since there is no gas to facilitate heat transfer (called convection) you cant dissipate the heat created by motors. So to address that you need to oversize the motor to prevent it from operating near its maximum output which generates a lot of heat. We had a fixture which used a standard off the shelf gear motor operate in an EB vacuum chamber. It would burn out every few months until we doubled the size of the motor. There are expensive vacuum ready motors that are liquid cooled using water.

As for bearings, any bearings that are grease packed should be fine although I recommend replacing the grease with a higher temperature white grease. Out-gassing shouldn't be a big problem with grease. Any oil bath lubricated gear boxes need to be drained of oil, disassembled and lubricated with grease. A hole should also be drilled into the gearbox to allow air to be evacuated. We also had oil filled gear boxes blow seals and spray oil inside a vacuum chamber. A big mess that was rectified using that simple technique.

Getting down to the -4's Torr can only be done by using a high vacuum pump such as an oil diffusion pump or turbo molecular pump. The oil diffusion pumps can be had cheap second hand but the special silicone diffusion pump oil is very costly. Here in the US a 1 gallon bottle costs around 1000 USD. I would imagine a 1 liter bottle would cost hundreds of euros. BUT the oil charges are small and if you properly cycle your valves, keep air out of the diffusion pump (which can cause it to explode from the hot oil vapor burning in the presence of oxygen) you should have little problems. They have no moving parts and only require mains power to run a heater plate. Smaller diffusion pumps have heater plates that operate on less than 1kW. Smaller diff pumps also require smaller oil charges (maybe a few hundred ml) which could cost 100 euro or less. don't even think about turbo pumps as they don't like certain process vapors, are extremely expensive (even used) and have expensive blades and bearings that can break. The blades typically spin at well over 20,000 RPM (upward of 70,000+ RPM!) and the blades can shatter if a sudden spike of pressure entered them. The bearings also likely to seize up every now and then, though newer turbo pumps use mag-lev bearings and other fancy things.

An oil diffusion pump can not pump at atmospheric pressures and can explode or ruin the oil trying to do so. Typically on our EB machines we first rough the chamber down to 5x10^-2 Torr and then open the diffusion pump which brings it down to the 1x10^-3/-4 Torr range. A PLC sequences the valves to first isolate the diffusion pump, rough the chamber and then reintroduce the diffusion pump to the partly evacuated chamber. The valves are quite large and expensive but you could do something home-brew using large plumbing valves for 4 inch and smaller diffusion pumps. We use pneumatic vacuum gate valves for the foreline and roughing valves and then a large pneumatic "angle" gate valve for the diffusion pump inlet. Lastly, a pneumatic poppet type valve is used to vent the chamber to atmosphere. The gate valves cost 2000 USD when purchased new and the angle valve quickly approaches 10,000 USD, though to be fair it is designed for vacuums down to the -8 or better range and are 10 inches (254mm) in diameter. Again, be frugal and experiment with common off the shelf plumbing parts. Used valves can have nicks and scratches that will compromise the sealing surface causing all sorts of headaches. Even a small scratch that looks minor will be the difference between an ultimate vacuum of 5x10^-2 and 1x10^-3. Your going to have to experiment and see how well they hold a vacuum BEFORE you put them onto your vacuum chamber system. You will thank yourself when you don't have to trace invisible leaks inside your system.

Electric feed-throughs are used and are basically hermeticly sealed wires or connectors. They are costly but I bet you could fabricate them yourself using an epoxy or resin, plumbing pipe nipple and wires or brass/copper rods. Simply thread the conductors through the pipe, plug up one end and pour the epoxy or resin into the other side. Air bubbles can be removed if the feed-through is immediately put into a vacuum chamber. Just be sure the inside of the pipe is clean and oil/dirt free. I would try to avoid wires as they can outgas and form tiny hoses through which air can migrate into the chamber. Stick with bare copper or brass rods and either use threaded rod for terminals or solder your connections to them. Better yet, use magnet wire to avoid outgassing and form a good seal with the resin in your feed through.

Oh and keep your chamber clean and tidy. Dirt and other porous materials can trap gasses slowing or preventing you from reaching your target ultimate pressure.
Re: Vacuum Chamber
August 18, 2012 04:42AM
Thanks Thaddeus W for your great input!
Your transformer beam control option looks interesting, if you had time to specify some parts we would really appretiate it, as our valvue option is proving complex and time consuming!
kind regards
MetalicaRap team

you can send it too me in private if necessary via forum user name Rapatan
Re: Vacuum Chamber
August 19, 2012 09:27AM
Hi
Avoid brass it can arc badly, copper is good. ( link in vacuum section to home build electical feed throughs done by fusor bunch) Oil diffusion pumps introduce oil that can crack on gun filament so , turbo pumps are for the rich though and the bearings do play up. Vacuum wise things can be a lot more rough and ready than you would think(home build large spot electron gun), as long as you not aiming for Ultra high vacuum 10-7 torr needed for LaB6 filaments(which were not).
kind regards
rapatan
Re: Vacuum Chamber
August 19, 2012 10:41AM
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
Re: Vacuum Chamber
August 19, 2012 10:42AM
hi
could you check the tube number again? E130L? see other posts in vac section
kind regards
Rapatan
Re: Vacuum Chamber
August 21, 2012 10:10AM
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
August 25, 2012 01:24PM
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
September 04, 2012 09:07AM
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
September 05, 2012 08:58AM
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
September 21, 2012 08:19AM
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
December 31, 2012 02:39AM
is it possible to print hollow shere in vacuum chamber
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