Monday, June 21, 2010

 

Selective Laser Sintering Part 6



Hi folks,

I thought I'd take a second to make a quick post about some of the interesting bits of the new open selective laser sintering printer that I've been designing in between finishing up my dissertation. (I'm also looking for an interesting postdoc (or maybe the right industry job, if it's research oriented and challenging) -- if you're interested in learning more, please have a look at my graphical portfolio / academic CV. My background is pretty diverse, and I'm quite interested in branching into cognitive robotics, rapid prototyping, and potentially making these technologies a little "smarter", more accessible, inexpensive, or otherwise more capable. )


One of the overarching design goals of the project is accessibility -- it's very important to me that the design be easy to create, and very affordable. Infact, I keep a $200 high-mark in my head, with the goal of designing something that doesn't exceed that mark in terms of the parts cost. With these goal in mind, I've been designing a proof-of-concept that gets creative in order to keep things inexpensive while making them easy to create.

The typical components of an SLS system are a build chamber with a Z table, a powder feeding mechanism (that often takes the form of another Z-chamber, indexing up to feed powder in concert with the build chamber indexing down to receive the powder), and a x-y pivotable mirror or other system able to accurately aim the focus of the laser in two dimensions along the surface of the build chamber. With X-Y systems being relatively common (using anything from a few slides and lead screws or belts, like on the darwin or mendel, or using a pivotable mirror like on the first SLS powder test rig), I figured the tricky part would be in designing the feed and build chambers, and keeping this inexpensive. This could then be paired with an X-Y stage with the laser attached, and one would be set.

The design I've come up with is something that I'm really happy with. The whole idea is that, if you have access to a laser cutter, making complex custom parts can essentially be done for the price of the material. The more parts that can be cut, the fewer vitamins one would need -- and the lower the cost.

The following dual z-stage (feed and build chambers) that I've designed uses almost entirely laser cut parts -- ideally acrylic, but the engineering model I've build is made with hardboard (and even this might work out okay). The only vitamins currently are machine screws, as well as two large carrige bolts and two nuts that are used as the lead screws. (There are of course motors and electronics that also count as vitamins, but these are likely to be with any reprap system for a generation or two yet).




Two motors are used -- one for each Z-axis (build and feed). Here is one of the sample steppers for this engineering model, plus its mounting plate.



These two motor mount plates attach to a base plate. On top of this base plate is a system of laser-cut gears, where the nut for the lead screw is held captive in a large gear. The ratio of these gears is 4:1, so the motor can apply a good amount of torque to the nut and screw.



The nuts are also held in place with the top plate -- it acts as a bit of a keeper, to make sure the large central gears don't shift and can rotate in place, while also allowing access near the sides for manual turning just incase.











Here's one of the tables, turned upside down. It has a little system in it to hold the top of the carrige bold steady.












The chambers have two parts: the squeegee system, that sits on top, and the actual chambers themselves. The above is the piece that sits on top of the chambers.



The top piece has a system of groves similar to the first SLS powder test rig, except that this one is raised up a bit -- I had some concerns that since the plastic-on-plastic slide mechanism was also at the interface between powder and squeegee, that this might potentially clog fairly easily. With this new design, the slide is raised up one level of plastic, and a tiny little squeegee is glued under the slide itself -- so there's very little contact with the powder level, and much less potential for clogging.



The slide, with a little squeegee underneith.



The chamber system has two chambers, feed and build, that slot together and fit together with teeth. The intention is for these to be a close fit and glued together -- here things look a bit loose, but that's because I cut this model with 3mm material (for ease), rather than the 4.5mm material that the slots are designed for.







The top piece with the squeegee slide mechanism fits on top of the chambers.





And then the tables and dual z-stages slip right inside the chambers. There's still a bit of work to be done with alignment (in particular, the mounting plates that attach the dual z-stage to the chambers is out a little bit), but you can manually index the gears with your fingers and the tables slide up and down quite well -- and with quite a resolution! There is a little see-sawing back and forth from the size of the nut compared to the material that the gear is made out of, and also the give in the material, but I think this will likely be sorted out fairly naturally as those issues are addressed.




---

And so, there's the working prototype design for a dual z-stage mechanism with build chambers, for $2 in carrige bolts and nuts, $2.50 in hardboard, and $2 in machine screws! :) There will likely be some other materials required, such as a seal, but I'm thinking of using a dense foam similar to the seal material used on the first powder test rig (think pool noodle foam), that should be very very inexpensive as well.

While the design is very inexpensive, there are a few tradeoffs: There's likely to be quite a bit of backlash in the gear system. The trick is that, for this application, the tables should only (ideally) ever move in one direction throughout operation -- either up, or down -- and as such, the backlash shouldn't be much of an issue.

For the X-Y stage, I've decided not to use the pivotable mirror again, and instead make a simple one (mostly laser cut, again) using belts, that will fit over top of this system. I also harvested the stepper mechanism from a CD drive head, which I'd like to mount the laser to, in order to be able to easily adjust the focus online. During my tests just holding things in my hand, it seemed like the laser did particularly well when it was a little out of focus, and it would greatly aid testing if this didn't have to be adjusted manually! Also, it seems like a neat idea to be able to adjust focus online -- perhaps dynamic resolution at build time (heating large versus small areas), or even cutting (having a perfect focus), although I'm not yet sure where that last one might come in handy. (As an aside, I keep having ideas of building a larger x-y stage and trying to use the whole system as a dual SLS sytem and low-power laser cutter by placing a dark plastic sheet over top of the build chambers, and making a "reciprocating laser cutter" using the CD-head mechanism to 'bore' the focus into the material, but that's just unverified brainstorming right now. :) ).

I hope you've enjoyed this post! back to thesising!

Keeping with being open-source, I've placed the design files for the first powder test rig, as well as this current full design project, up on thingiverse:

Powder test rig: http://www.thingiverse.com/thing:3389

This project: http://www.thingiverse.com/thing:3390






thanks for reading! :)

Peter




[part 1] [part 2] [part 3] [part 4] [part 5] [cogsci.mcmaster.ca/~peter]

Comments:
Wow! :-o
 
... whats with powder rinsing through the gaps contaminating the elevators?

I was thinking about a scissors-elevator activated by a lever i can drive outside the 'contamination-area' ...
 
Hmm, that's a neat idea Viktor, I'd love to see a sample design.

Given that there will be a seal around the edges of the tables, I don't think there will be much powder getting through the gaps. That being said, I had thought of putting a little catch bucked piece on the bottom of the chambers, both to catch any powder that does make it through, as well as to prevent the tables from falling out the bottom. The idea being that you could pop out the carrige bolts from their holders under the table, then just lift the whole powder chambers up and swap them with a set of powder chambers of a different material. It'd probably take 5 minutes of swapping, but in the end you'd be able to use a different material without much mess (which is great).
 
Very impressive. I have been looking for an open source standard design for a powder printer and this looks like a great place to start. It will be my next project after the Mendel is working.

The low cost is impressive and would probably scale up easily but how big are the motors that you are using and how big is the build area?
 
Hi Peter,

...actually i'm busy optimizing the laseroptics - but when the 5W-diodelaser-module is finished and i'll receive the AOM for the fiberlaser, then the powderbed will have top priority ;-)
 
I really like what you are doing here. One thing that occurred o me is that the design is using one more stepper than it needs to. If I have interpreted it correctly, one of the z stages travels up, supplying powder, while the other travels down making room for the next layer to be leveled over. Surely it would make sense to make the large gears a little bigger so they mesh together. Drive one with a stepper and the stages will move in opposite directions.

Just a though. Steppers are expensive here in Australia.
 
One question... How does the 1W diode perform in sintering? I just bought a 1W 808nm diode myself, and am looking to start prototyping a similar machine.
 
Hi Marauder,

... the elevation of the two bases won't be synchronous, as you need more powder supporting, then finally adding to the build ...

And with the 1W-diodelaser - it's working with materials absorbing the energy ... best with dark powders and with really thin sheets (e.g. 0.05mm), so the finer the powder and the layer-thickness, the better ...

Viktor
 
This comment has been removed by the author.
 
Hi

Wow that's a great rig. Like the idea of swappable powder bay.

Currently my brother & I are working on a SLS machine as well. We are using a heated Z table to heat the powder to near it's melting point then use the laser to complete the melting process.

However we are facing some issues firstly with our laser driver as we are not able to get a clean current & secondly supply of powder ABS

We hope that you don't mind to share how u made ur laser driver and ur powder source

Feel free to comment

Thanks
 
Hi Gabriel,

look in the Laser-Wiki ( http://objects.reprap.org/wiki/Laser_Cutter ) for details - actually i'm driving the 5Watt-module with a special ON/OFF-flag before and after every linesegment and the energy with a constant PWM-ratio or as analog 'spindle-speed' from my CNC-controller.

For my 50Watts-laser i'm awaiting an AOM (AcoustoOpticalModulator), so i can drive it similar to the diodes, which can be driven simply by PWM.

Here in Germany i have a supporter for plastic-powders, but its sometimes not fine enough for precise sintering, so i'm searching too ... actually i've got some thermoplastic colour-pigments from the plastic-supporter too, which are much finer than the plastic powder, but the pigments are from different materials and mostly more brittle than plastic ...

Viktor
 
Hi Victor

Thanks. Wow u are using a 50watt laser. If i am not wrong that can possibly to metal Laser sintering

Btw so since i am using a diode laser all i need is just a LM317 with some capacitors
 
Hi Gabriel,

what power and wavelength has your diodelaser?

With my 5Watt-diode@975nm i successfully tried to melt black rock-microspheres and carbony-iron spheres with 10 to 50 microns diameter - but for metal i need inert gas, so this is delayed after i'll install a housing and gas-support.

My 50Watts-laser is a fiberlaser with 1070nm and a spot of 20 microns with a single lense or 5 microns with aditional beam-expander, so i can make something more than 'only' metal-sintering ;-)

But before playing with the fiberlaser i want to finish the diodemodules, so i can give some away to other researchers or for some funding ...

Viktor
 
currently we are using a 1 watt 830 nm wave length fiber laser operating current at 1.5A even though we told that it can go up to 2 A .

we do not dare to push it as in fact this is our first attempt and we are still noobs in many field eg. electronics.

But we hope to work toward our goal to be able to build our own metal laser sintering

By the way if the beam is at 5 to 20 microns, wont the build time be too long. won't it be better to be between 100 to 150 mircons.
 
Wow, thanks for the comments. It looks like Viktor's handling most of them too ;) We might have ourselves the beginning of an SLS team.

Gabriel, to your question about the constant current supply, I'm just using a design from National's LM317 datasheet (pg. 20, second design, "Current Limited Voltage Regulator"). I chose the resistor values to limit the current to.. I think 1-1.2A, 0-5V, and put in a 10 turn pot to be precise enough to tune the voltage correctly without fear of accidentally overvoltaging things (although really, the current limiting should protect that, in theory).

I actually haven't had the guts to bring it up to full current yet.. I think I'm around 900-1000ma right now, and the ebay seller said they were optimally run at 2.2V 1.2-1.3A. The fact that I'm at 2.5V at 1A kind of scared me a bit, and I didn't want to fry yet another diode that I don't have a datasheet for. Besides, it seems to do pretty well cutting/melting plastic at that power level :).

I still need to find some black ABS powder... I have some black powder coatings, but they're *very* messy, and given our earlier tests I doubt they'll work very well.
 
Hi Gabriel,

>>>By the way if the beam is at 5 to 20 microns, wont the build time be too long. won't it be better to be between 100 to 150 mircons.

... with defocussing i can adjust any spot diameter i want, so the size isn't the problem.

But sometimes i'm busy with serious microtech-development - here i have typical object-sizes around 2mm in dimension and some micrometers in structural accuracy, made mostly from ceramics or glas and embedding gold-trays ... here the 5 microns are sometimes the limitating parameter ;-)

Viktor
 
Hi Peter,

... i'm driving my laserdiode with one to 5 1A-current-limiting LM317 and adjust the voltage between 7 and 10 Volts (depending of the optical oputput and dissipating heat in the LM317-heatsink) ...

For the amount of comments - wouldn't it be better to start a discussion-thread in the forum?

Viktor
 
Hi Guys,

Thanks for sharing. Peter it seem that the only minor issue we all have is in getting some ABS powder
 
Acrylic powder is available (used for doing women's nails). Not sure of whether it would work or not but it is available in a lot of colors.
 
Very inspiring. I particularly like driving the nut on the carriage bolt, and the interlocking panels to make the structure.

Wish you had the files in dxf format, so it would be easier to port to my cnc router.

Do you think there will be a problem with "slop" on the platforms, only supporting them from the center? I would have put a lift bolt on each corner. Then it would be stiff enough to run rollers over and pack the powder down.

It appears from the comments that there are several folks working on similar projects. Maybe we should have a central site to share our work? Or does that already exist, and I haven't found it yet?
 
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