Sunday, September 7, 2014

Mini Kossel V-Slot 3D Printer Kit

A couple months ago, I purchased the Mini Kossel V-Slot 3D Printer kit from UltiBots. It's a "delta" style printer based on the Rostock and Kossel designs from the RepRap community, originally designed by Johann C. Rocholl. I went with the orange printed parts and shelled out for the black-anodized aluminum extrusions. The whole kit wasn't cheap, but about the best I could do for this style of printer (parallel robots are cool).

It arrived a week later and I started putting together the frame and linear mechanisms, which turned out to be the easy bits. The following pictures are by no means a step-by-step (see the official instructions), but rather an incomplete record of my assembly that may or may not be useful to myself and others in the future. There is also a super-secret Q&A document that you get a link to with the kit.

The RepRap project is all about self-replicating machines, so many of the parts are themselves 3D-printed. Here are all the printed parts, in awesome orange ABS. The support material and some of the holes need to be cleaned out by hand (with a knife and drill bits).

Assembling the three carriages.

Assembling the wheels that allow the carriages to slide smoothly up and down the aluminum extrusion. Each wheel is a plastic double-V profile with bearings pressed into either side.

Assembled carriage in the top left. Here are the stepper motors that drive the linear mechanisms. Each one gets a sprocket set-screwed to the shaft that meshes with a toothed belt.

The motors then get fastened to the corner base pieces.

All three assembled corner base pieces with motors.

Next, the long aluminum extrusions get fastened to the corner pieces (check out that sweet black anodization, mmm). Note that I actually have the black plastic cable guides inserted in the wrong face of the extrusion. They should be in the face closest to the motor.

The short extrusions that form the base go on next. I kept all the fasteners loose so that they'd find their happy place when the three corners are put together. Need more bench space!

All together now. It was actually something of a process to get all these together (all three corners have to slide in simultaneously). Note that the tabs to support the glass bed are now present, as well as some T-nuts for the LCD housing and for the control board mounting (unseen). It was a little dicey cranking down on the M5 fasteners that hold the extrusions together -- the corner fittings are prone to splitting when overtightened.

The electronics for this kit (and I gather for many of the RepRap machines) is a board called RAMPS (RepRap Arduino Mega Pololu Shield). This shield has sockets for Pololu stepper drivers (mine appear to be clones) for the actuators, power electronics for the heaters/fans, and sensor inputs, among other things I'm not familiar with yet. The bottom board is an Arduino Mega derivative called the Taurino.
I forgot to take pictures here, so you'll have to take me at my word that I fastened the electronics and a fan to the electronics bracket and built the LCD housing (which was a little painful since the beta instructions did not cover this at the time).

Base with carriages and LCD housing installed. Even the dial and pushbutton on the housing are 3D-printed.
At this point I had gone past what the beta instructions had covered, and I'd forgotten about the super-secret Q&A document, so I thought I was stuck on what I found to be the harder parts (the hot end and extruder assembly as well as the linkages). Also I got busy with other things and so took a month break and just resumed work a couple days ago. Turns out all the info I needed was in the Q&A (thank you UltiBots support for the quick turnaround on my email).

Here is the partially-assembled work platform. Seating the nuts for the nut traps is a bit of a pain. There is a neat mechanism inside to keep pressure on the filament as it passes by the extruder motor, which is the bit with the red heat sink. The hot end (black PTFE with brass tip) actually has to come off for assembly.
I forgot to take pictures of the hot end assembly, but this video was extremely useful:

The kit I have actually has the option of a fancier heater (which I used) in addition to the resistor, as well as some muffler putty to try to cement everything in place with decent thermal contact. I found that this was a really messy and difficult process, and I'm not sure how great of thermal contact I ended up with, both on the heater and the thermistor.

The next thing was to tap the carbon fiber rods to accept the M4 studs (actually long set screws) to fasten the Traxxas 5347 rod ends to. A lot of these delta printers actually use R/C car rod ends for the linkages, which is a cool repurposing. (I also own a Traxxas R/C car.)

The kit includes a 3D-printed jig for holding the rods, both for ease of tapping and to prevent the rods from splitting. I don't have metric drill bits so I used 15/64" and 5/32" bits to substitute for 6mm and 4mm respectively.

The tapping jig, with thumbscrews for tightening.
I found this to be the worst part of the assembly process. Maybe I have crappy taps or maybe I'm just not very good at this, but I ended up just boring out the centers of most of the rods to 4mm as opposed to tapping to M4. Also in the process I generated quite a bit of carbon fiber dust which I think I had to be careful with. To attach the rod ends, I ended up just screwing them in a little further than the tap-bored holes, which split the rods but still seemed to work. I also pre-threaded the plastic rod ends (which don't start with any threads in them) so that the set screws wouldn't thread and split further in the rod than I wanted them to. This probably doesn't make a whole lot of sense unless you're actually doing it.

The finished product, 6 rods with rod ends.

The most satisfying part of the assembly, popping the ball into the rod ends.

The parallel robot is complete! I skipped over the end stops (limit switches) on the linear mechanisms, as well as the top triangle and belt assembly. I think the springs between the rods are to eliminate backlash in the mechanism.
Just a few more little things remain.

Two fans need to be installed and spliced into extension wires. This is the fan for the extruder motor. It actually needs to be further spliced with the electronics motor (this is kind of janky) because there is only one always-on fan connector on the RAMPS board.

I turned the frame upside-down to work on the wiring. It's still a big mess and needs some clean-up.

The power supply got wired up (underneath the 3D-printed cover) and the Kapton film heater got affixed to the circular glass build plate.

Here are the bits to hold a spool of filament on the top of the frame and let it freely rotate. My work surface isn't really this nasty, it's just the lighting....
And that's it! Here are some glamour shots of the mechanically complete printer.











This was actually a lot more work than I anticipated when I purchased the kit. I estimate that it was a solid 20 hours of work. Things could have gone a lot smoother if the instructions were all collected in one place. The personal support from UltiBots is great, but the kit needs more work on the instructions for 3D printing newbies like me (especially since it looks now to be out of beta status).

The next step is to actually fire it up and do the system checks and calibration. After that, I should be able to actually print. But that looks to be pretty involved so I'll leave it for another day.