Welcome! Log In Create A New Profile

Advanced

CAD model of my next machine

Posted by pyrotronics 
CAD model of my next machine
March 29, 2013 07:19PM
Just been playing around in CAD and made a rough outline of what I think the next machine will look like

It has
- 300 mm circular build platform
- 300 mm z axis height
- removable part box containing powder + finished part
- removable hopper's so you can fill them up
- large door (500 mm diameter clearance so hopper and part box can be removed)
- door parts are printable, small pieces are bolted together + simple o ring seal

- e-beam coils need to be added
- haven't decided on the vacuum pumps, I've put one in for looks

Comments? improvements? Feature requests?


Re: CAD model of my next machine
March 30, 2013 10:39PM


Re: CAD model of my next machine
April 09, 2013 09:00AM
Looks good.

From the looks of your chamber, its pretty small and doesn't need a big pump setup. I am going to guess the chamber volume is around 0.5 cubic meters so a smallish pump of around 400 l/min might work out just fine. To get in the -3 torr range, you need another pump in front of that pump. Either a roots blower or diffusion pump will work. For a diffusion pump, a 4 or 6 inch (100-150mm) pump will work out well. The diffusion pump might be the cheaper route. Originally in another post I quoted $1000 USD for a gallon of silicone diffusion pump oil. Its actually $500. I looked at the invoice for two bottles and thought it was for one. Its still expensive but far cheaper than a roots blower. A second hand diffusion pump might be easy to obtain for a low price. They are simple to clean and easy to maintain. A rebuilt roots blower is easily $2500+ USD and a used unit with a lot of wear of questionable performance will be far cheaper but again it may not work and you need an expensive rebuild.
MTO
Re: CAD model of my next machine
April 10, 2013 01:37PM
what he does in the "repstrap" model is somewhat irrelevant, no? I mean, he needs to get to the vacuum he needs, but once he has a running EBM repstrap, 3D printing a roots blower or a turbomolecular pump should be pretty easy, I would think, and then all future EBM printers would be able to use a *cheap* roots blower.

Personally, pyrotronics, I don't think I understand the internals of the build platform from these drawings. It *looks* like the print area will be a half-cylinder, but the device to spread the next layer of powder seems to be rotated 90 degrees from where it needs to be, so I'm clearly misreading things.
Re: CAD model of my next machine
April 11, 2013 01:59AM
The circle on the build platform and light green triangle above it is a transparent part i made to approximate the e-beam path. the build platform is actually 320mm x 320mm square.

The blue part in the 3rd pic above the build chamber is what spreads the powder. it goes from one side, fills up from the one of the main hopper. then slides to the other side leaving a layer of powder behind it. There are 2 hoppers on either side of the chamber.

I'll get some more pics of the setup without the chamber in the way.

Recommendations are welcome, i basically made them up as I went.


Size wise it probably is approaching the 0.5m cube. 850mm high, 550mm wide, 400mm deep = 0.187m cube. Then you have to add the cylinder for the e-beam on top

Vacuum pump wise, I have a rotary pump that gets me down to 10-3 torr and a diffusion pump that gets me further down ~10-5.The oil is expensive but you also have to remember one refill is probably around the $70 mark.

I'm hoping that i can replicate the rotary pump and start printing cheap ones. Just need to source seals and gaskets. I will change the design so it can use off the shelf silicon gasket sealer where possible. What other consumable parts do they have inside them? Any open source ones out there ?

And instead of a diffusion pump a self made titanium sublimation pump will be a better replacement to the diffusion pump. Just takes abit more electronics than the heater in the diffusion pump.


Cheers,

pyro
MTO
Re: CAD model of my next machine
April 11, 2013 07:27AM
Ah. That makes a lot more sense. I see the two hoppers now. The drawings don't really show the mechanism that refills them, but its clear where it is.
pumps... Either sublimation or diffusion pumps look like a good place to start. Both seem relatively easy to make and operate (going by a quick perusal of the wikipedia page) but personally,I look forward to seeing someone (possibly me) design a turbomolecular pump. It looks to me like the hardest part is getting the tolerances right. (I just like turbines. Call it a personality quirk)

I have 2 questions though:1) how often do you need to replace the diffusion pump oil? it doesn't look like its something that is consumed per se, so with careful maintenance, I would think a single charge could be good for a long time, possibly the lifetime of the pump.
2) similarly, how long does the titanium wire last? Its actually consumed, so installing more wire becomes an important (and possibly expensive) part of operating the printer, and prints could fail if you run out mid-print.
Re: CAD model of my next machine
April 11, 2013 09:04AM
"what he does in the "repstrap" model is somewhat irrelevant, no?"

Yes, its very relevant. Please understand this is a very complex project. The vacuum system along with the electron gun systems are not at all simlpe. This isn't a "hot glue gun" moved around by stepper motors salvaged from old printers and mounted on wood. The "repstrap" model will have three main systems: Vacuum, Electron Gun and Motion control. The motion control system is the easiest part as it can use standard reprap designs. The vacuum system and electron gun system are both complex and require quite a bit of research, effort, time, money and trial-and-error. Even after the electron gun is built, you need to focus and deflect the beam which again will require a lot of research and development. Electron beam welding machines cost half a million USD for a small machine and millions for larger machines. There is a reason they cost so much. Also many companies have spent a lot of time and money over the decades researching better gun and column designs.

I do not want to sound negative or discourage anyone from attempting to build a metal printing reprap. However, I do want to be honest and ensure no one has false ideas of how to get one working. Its naive to assume vacuum is easy or that electron gun design is easy without ever working with either of those systems. Don't underestimate the complexity of this project, its the most complex of any reprap variant.


"I mean, he needs to get to the vacuum he needs, but once he has a running EBM repstrap, 3D printing a roots blower or a turbomolecular pump should be pretty easy, I would think, and then all future EBM printers would be able to use a *cheap* roots blower."

I am going to say no, it is not as easy as you think, unless the printer has a tolerance down to the thousandth of an inch or better (~0.01mm). Realistically, vacuum is a fickle beast. The slightest scratch or imperfection will render a pump or sealing surface useless. Everything in a vacuum pump or chamber needs to be as smooth as possible otherwise you are trapping dirt and moisture in the cracks and crevices which slow or even prevent the chamber from reaching a decent vacuum level. Any blind screw holes in a vacuum chamber will also trap air and moisture. Screws in blind holes are either drilled through with a small diameter bit (works for larger diameter screws) or have a groove or two cut parallel to the length to allow trapped air to escape. Those little pockets of trapped air which leak out slowly cause what are known as virtual leaks; leaks that cannot be traced to any physical part of the vacuum chamber but are instead within the chamber itself. Another thing to remember about vacuum is there are two modes of flow, viscous flow and molecular flow. Viscous flow occurs at higher pressures from atmosphere to the mid -2's. Once you reach the mid-low -2's Torr, you enter molecular flow. Molecular flow is why you want smooth surfaces as the air molecules are no longer under any pressure. They freely bounce around the chamber under their own energy (heat). You want them to bounce around as little as possible and bounce into the pumps.

Refrence these two pages:
http://www.chem.hope.edu/~polik/Chem345-2005/vacuumtechniques.htm
http://www.vacuumlab.com/

Building your own turbo pump is another can or worms, these are high precision machines. The blades are thin and need to be well balanced as they will need to spin at 50-100k RPM. The blades are better off being laser cut and formed or stamped. Plus you need a brushless motor with special bearings so it can spin very fast. Custom built electronics would be needed for an inverter as 50/60Hz mains frequency is not even close to the speed you need to run a brushless at those speeds. You need 1600 - 2400Hz or more depending on the number of poles in the motor. Refrence: http://en.wikipedia.org/wiki/Turbomolecular_pump#Practical_considerations

I will say that a diffusion pump may be pretty easy to build depending on your available tools. Its simply a metal cylinder wrapped with a cooling tube and a heater plate on the bottom. Inside are some sheet metal parts for the oil vapor. You could build one in a machine shop equipped with a lathe and a TIG welder and some stainless steel tubing. The "christmas tree" inside the pump can be made from aluminum tube and sheet metal, most christmas trees are.

Bottom line: The vacuum system is the biggest and most expensive hurdle to overcome. You can save money here and there but in the end the pumps are your biggest expense.

Its a long road to getting a metal Reprap to self replicate. I would not even hesitate to say it might not be possible without machining and/or grinding the printed parts further to obtain the necessary tolerances. Think of it like casting parts, you can get close to the desired shape but there is always some machining to do afterwards to gets thing smooth, square and balanced. But lets not even think about that until we get a prototype shooting a beam into some metal powder.

A long winded post but I assure you this is a very involved project and I commend everyone involved for putting in the time, effort and research. Hopefully one day we can have a self replicating EB fabrication machine that can also drill and EB weld as well.
MTO
Re: CAD model of my next machine
April 12, 2013 04:29PM
Ok, so you're saying that if pyrotronics gets given a used sublimation pump to generate the needed vacuum, he'll have to completely re-design the vacuum chamber since he's currently using a diffusion pump?
Re: CAD model of my next machine
April 13, 2013 02:58PM
the diffusion pump oil should last along time like you say, I haven't actually had to change mine yet. The main way that it will get ruined is if you open the vacuum to air and this will burn the oil onto the pump or you don't have sufficient cooling.

I haven't actually put in the powder dispensing mechanism. Has anyone got ideas on what it should look like?

thats a very good point, the titanium wire will only last a set amount of time? Does anyone know the numbers on this?

a turbo molecular pump looks pretty complex. But if we can print to 20 um and use the e-beam to polish a part. Its all possible.

I'm going to have to disagree with Thaddeusw about not being able to print a pump. One reason the mating surfaces have tight tolerances is because ultra high vacuum systems tend to have higher temperatures. They even bake an entire system. In this situations orings and rtv silicon burn/fail.

I'm proposing a lower toleranced joining method that uses o-rings or silicon if its a complex shape. My electron microscope goes to 10-6 torr and has oring seals all over the place and a lot of the parts are cast stainless which has that bumpy texture. They had to machine the faces for the o rings but the finish would easily be +/- 100um which we should be able to print in the long run. If it is too bumpy using something like silicone rtv would fill in the bumps and it would actually have more surface area to stick to, making the seal a lot stronger

The tightest tolerance would be in the cylinder. If its printed to 20 um and then a common brake cylinder reamer for $20 at the auto shop could easily polish the back to 1um. Chuck it in a hand drill.

The piston rings will most likely have to be bought. If we pick a common size for pocket bikes we should be able to source them cheaply. Rather than buying an expensive rebuild kit for a roots pump.

The best example with vacuum is using blue tack, poster tack. Its great for chucking onto a small leak. It worked for me upto 10-6 torr!

Replacing a diffusion pump with a sublimation pump will require different plumbing connections. Thats why i haven't put them in the model above. I know whats required for a diffusion pump. But I'm a bit flaky on a sublimation pump?
MTO
Re: CAD model of my next machine
April 17, 2013 08:36AM
I'm wondering if the powder dispenser could look anything like how laser printer toner gets spread on the drum. I suspect not though, 'cause I think its kinda dependant on the toner being an insulator and a polar molecule.

A simple mechanical aparatus would be a bar with a groove in it at the bottom of the dispenser. The groove would be sized to dispense one layer worth of metal powder. You transfer from the hopper to the spreader by doing one rotation of the bar. The downside is you need two motors to actuate the 2 hoppers.
Sorry, only registered users may post in this forum.

Click here to login