SLS test rig

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Re: SLS test rig

Postby Jeremy » Sun Jun 19, 2016 9:34 am

Progress has been slow as of late but should improve now I've put my Steam account to one side. I've completed upgrading the rig with the second stepper and screw drive. I anticipate it'll work much better now, but will have to wait until I have a controller before I can test it. The speed of the roller is now directly controlled by a stepper motor so this should add more capability for fine tuning and remove any issues due to belt slip.

I've also been working on the controller board. It still requires some work but is mostly there. The only things left to be done are routing, replacing the banana plugs with a DC jack plus supporting hardware, and a fine tooth check before sending the board away to be built.

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Re: SLS test rig

Postby VDX » Mon Jun 20, 2016 9:33 am

... despite the forum seems to gone asleep, I'm checking regularly for new posts - so keep us updated ;)

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Re: SLS test rig

Postby Jeremy » Sat Aug 06, 2016 1:12 pm

It's probably time to write another small update. I've finished the design for now and have it ready to send away for manufacture. Organising the BOM has been a pain but is nearly there. The board should be able to accommodate 2A continuous which should be enough for driving the steppers given they all don't run simultaneously. The lower left hand of the rear of the board has a little space left on it to later accommodate the galvanometer driver circuitry. For now, X/Y output signals will suffice.

I've also managed to get my hands on a FDM 3D printer so I can reprint the powder bed's piston heads in order to achieve a better seal. It should also allow me to get a few other bits and pieces done related to the powder bed, so not a bad investment at $239 USD.

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Re: SLS test rig

Postby VDX » Mon Aug 15, 2016 8:20 pm

... an option for stronger motors is to feed the STEP+DIR-pins (and maybe enable or SLEEP) on a socket or plug - so external stepper- or servo-drivers with this control inputs can be used too, if needed.

I have some boards with this options -- for motors up to 1A or 2A it can be used with Pololu or RAPS128 drivers (same layout as Pololu), or stronger comercial drivers with STEP+DIR and separate power support ;)

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Re: SLS test rig

Postby Jeremy » Sun Oct 09, 2016 8:04 am

Not a bad idea VDX. The current format accommodates open loop steppers and consequently is a bit of a gamble. Some closed loop units commonly found on CNC mills would integrate nicely with that idea. That said, progress on the controller has drawn to a standstill as I have little spare time to get properly stuck into this which is frustrating. I'm becoming more and more tempted to adopt an off the shelf laser control system to get this thing up and running. My thoughts are leaning towards a gantry based system as these can be had cheaply, and also negates the requirement for a F-theta lens. I figure once things are operational and the process is better understood, I can turn my attention to speeding up the process. Time permitting of course.

Anyway..

I've been investing a lot of time into my FDM printer, trying to work out all the little bugs. It's come along quite well, and I can now print out objects without a lot of distortion along the Z axis. You can see how bad it was initially on this prototype of a galvanometer:

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I'm convinced there is no tactful way to get the original powder hoppers the seal to the degree that they need to without spending a lot on a design that is inherently flawed (square piston profiles instead of round) so I've decided to print my own cylinders, taking cues from automotive applications. The obvious concern here is that any given plastic will melt when exposed to high temperatures. To avoid this, I intend to have a metal skin on the head of each piston / liner insert acting as an insulator to the heat applied above. I figure this plus a reasonable layer of the material to be sintered (5mm based on the current design) should be enough to prevent any warping of the the plastic cylinders. I've spent a little time trialling out different piston / liner formats. Here are a couple I made:

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A lot of these failed to produce a nice profile due to the need for supports because of the overhangs. The best seal was achieved with the simplest design. A solid deck and a wafer thin (1mm) sleeve, permitting some flex to account for the distortions in the bore of the liner. The distortions in the bore are significant in this context despite the improvements. I think I can overcome this by either sanding the liner with either a honing tool or perhaps inserting a metal cylinder of equal size and using an induction heater I have (impulse buy :roll: ) to heat up the cylinder, melting the inner profile, and theoretically creating a matching profile. Here is the scaled liner I've been testing with, you can see the seam left by the extruder and the remaining level of distortion:

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The piston doesn't seem to suffer as much as the path generated by the slicer is relatively continuous given it's so thin. The only apparent inconsistency seems to be due to the layering itself which is tolerable (0.2mm layers), and can be improved on in the final prints:

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That's about it for now...
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Re: SLS test rig

Postby Jeremy » Mon Oct 24, 2016 5:48 am

Short update. I printed the first of the liner inserts this weekend. It took 28 hours and about 0.7 Kg of filament to print so I won't be doing the second one until next weekend. I'll also have to print some shims to try and ensure the liner and the bore have a minimal clearance for the sake of the pistons. I've ordered some rubber O rings as well, in a bid to achieve a proper seal. They should arrive within the next couple of weeks, allowing me to focus on an optimal piston design. Meanwhile I've also been breadboarding the galvanometer drivers, but there's nothing interesting to look at just yet.

Some pics:

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