Rosotock Mini Build Part 2
Over the weekend I spent time putting together my Rostock Mini. In the previous weeks, I had printed all the parts I needed, purchased the electronics, and milled out what pieces I needed. What remained to do was assembly and commissioning of the new printer.
Part of the assembly was already done. Last weekend, I attached the smooth rods to the top and bottom bases for all three towers. The arms-effector-carriage assembly was already complete, so I attached these to the carriages to their corresponding bearings on each tower.
The Drive System
For the drive system, the first task was installing the stepper motors into the bottom motor mounts/bases. Each of the three motors was attached with one of my CNC’d cork gaskets between the face of the motor and the plastic mount. Four 12mm long M3 screws were used to secure each motor. Two of my motors already had D shafts, but the other two did not, so I ground flats onto those using the bench grinder.
Before attaching the fishing line pulleys the motor shafts, I needed to thread the filament through them. The holes that are designed into the pulleys were too small, and ended up being filled in on my prints. A quick drill through with a small diameter drill bit fixed this. Getting the fishing line to cooperate and go through the holes was not easy though. Luckily, since my mother is big into sewing, she had a slew of needles which I could search through to thread the fishing line. It was, after all, practically a thread. A tapestry needle proved to be the easiest to use. The eye at the end was large enough to permit the spectra line to be threaded, but not too large to not fit through the hole. I used two full arms lengths of spectra line per pulley, which turned out to leave much extra, but with a spool 100 yds long, what did it matter wasting a few feet?
The next step involved pressing the pulleys onto the motor shafts and tightening their set screws to hold them in place. This proved difficult, as the fit was very very tight. At time of writing, I’ve managed to get the pulleys on the shafts and tighten them enough so that they will not slip, however, they are not perfectly concentric with the shaft, and you can see them rotating oddly while they move. If this proves to be an issue, I will use a soldering iron to heat the shafts of the motor and more firmly press the pulleys onto the shafts.
The idlers use a pressed in 608 bearing, and an M8 bolt to attach to the upper bases. These are simply a V-groove idler to keep the spectra line moving smoothly over the top.
For the spectra line to work, it requires several wraps around the pulley in one direction, then it travels over the idler and attaches to the carriage on the top. For the other end of the line, you wrap several times around the pulley in the opposite direction and attach to the bottom of the carriages. As you wind it one direction to move it, the other end winds or unwinds in a similar fashion.
When I installed the line, I connected the top line over the idler and to the carriage before wrapping, then turned the pulley at the bottom to get the line to wrap around it. For the low end, I wrapped it around the pulley manually and then attached it to the bottom of the carriage. I used standard knots in the line to secure the spectra line to the carriages. The lines were still slack at this point, so a tensioning method would be needed. Initially, I used a zip tie and wrapped it several times in the line in an attempt to take up the slack. While this sort of worked, it eventually unwound from the zip tie, or lost its tension rather quickly on its own. What I did in the end was to make the bottom line attach to a zip tie, which attached to the carriage. This allowed me to cinch down the zip tie and get the appropriate tension in the line. Since it is one continuous line through the pulley, tensioning one side is sufficient to tension both.
Moving the system by hand illustrated the effectiveness of the system. Moving the effector upwards would wind all the fishing line one way, and lowering the effector would wind it the other. This is where I found the pulleys to be non concentric with the motor shafts. So far it does not seem to be causing issue, but time will tell if it is.
Many rostocks are unstable and shaky with only the three towers to support them. The original rostock uses two pieces of wood spanning the top and bottom to provide support, but I did feel this would be a good solution for my rostock mini. The mini is not supposed to need the support since it is smaller, but I felt it would not hurt either.
I had an extra section of aluminum extrusion from my shapeoko bed project, and it just happened to be right around the size of my new printer. The extrusion is mounted between the top and bottom acrylic pieces by means of an identical printed plastic piece on the top and bottom. I originally put holes in the bottom section to mount a RAMPS board, but since I am using a RUMBA, I repurposed those holes as the mounting points. The same holes were not present on the top portion, but a quick drilling in each location fixed that.
The grooves allowed for mounting of several different things. On the bottom most area, I designed a foot to be CNC’d and slide in the grooves to adjust for height. This allows it to be leveled out to the remaining three contact points. Right above that is a CNC’d RUMBA mount. Normally the electronics on Rostocks are mounted below it, but I wanted to have easy access while I was troubleshooting. Above that, are two CNC’d arms for mounting a spool of filament. Immediately above that, is the extruder.
I chose a greg’s hinged extruder because I was not confident I could implement a direct drive extruder. To mount the extruder, I CNC’d a plate that had all the pertinent holes. When in use, filament should feed very nicely from the spool which will be right below it. I printed straight spur gears to use as the small and large gears for the extruder. The hobbed bolt I purchased fit right in, and I used a soldering iron to heat the shaft of the stepper motor to press on the small gear. The small gear is VERY secure on the shaft now. I’m sure it would even have need the set screw. This may be a problem if I ever want to remove the gear to install herringbone gears though.
Hot end and effector
Next up was the hot end and the effector assembly. I am using a 1.75mm Hexagon Hotend with a .4mm nozzle. This is an all metal hot end, and looks very well made. Makerfarm has a nice video showing the setup of the Hexagon, which I followed to install the heater cartridge and the thermistor. I used the included aluminum groove mount, a printed bowden block, and some washers to fasten the hot end to the effector. In the bowden block, I installed a push to connect fitting for the bowden tube. An identical fitting is installed in the extruder mounting plate up by the extruder. The bowden tube (which is what guides the filament from the extruder to the hot end) is a 2mm ID PTFE tube. I also printed some fan holders and installed them and two 40mm fans to the effector. Since the hexagon is an all metal hot end, it requires active cooling over the fins. One of the fans will be dedicated to this, while the other one will be for cooling the prints.
This next part was not much fun. With all the mechanical components assembled, all that was left was wiring everything together. I wanted to take my time on this step and ensure that all the wiring was neat, and this requirement only increased the time it took to get this step done. I had purchased a pair of magnifying jewelery glasses for testing with my head mounted display, and these actually came in handy for their intended use. With the magnifiers on, I was able to see all the small writing on the board, and ensure the ends of the wires were secure.
I began by wiring the stepper motors. The RUMBA board has both male pin terminals and screw terminals for attaching wires. All four of my stepper motors came with four pin connectors, but I knew I would need connectors for my end stops, so I cut them off and used the screw terminals instead. I had two different types of stepper motors, and as such, they had different wire colors. I was very lucky to have a similar situation on my Prusa, so I traced the wire colors on there to the correct terminals, and copied them over to this new build. Each stepper wire was stripped and then heat shrunk near the end for clean connections.
Next, I attached the end stops. I looped the wires through the holes in the contacts, and then heat shrunk the connections to ensure connection. I repeated this three times, then installed the end stops themselves in the upper bases. I used the female connectors I had hacked off of the stepper motors to attach the end stops to their respective headers.
The thermistor cables for the hot end had to be extended, but as it came with a female connector already, all that was required was to connect it. The bed thermistor was easy as well, requiring only attachment and routing. The heated bed wires were attached at their screw terminals, as were the two fans from the effector and the cartridge heated in the hot end.
I grouped up as many wires as I could into bundles and bound them with spiral wire wrapping. All the cables and the bowden tube from the effector were likewise enclosed in spiral wire wrap. Underneath the printer, where I had slack from the components down there, I bundled the wires together and zip tied them neatly. Wherever I found a convenient place to anchor down cables to, a zip tie was used to accomplish this. The four slots I CNC’d into the top and bottom acrylic pieces particularly came in hand for affixing cables.
Making it move
With all the assembly and wiring done, it was time to make it move! I borrowed my PSU from my prusa (since I do not have one for the rostock just yet) and grabbed a Mini USB cable connect to the controller board.
I spent longer than I’d like to admit fussing with the communication to the board. I had installed the drivers (several times), looked up problems online, and tried connecting directly without my USB hub in between. All of these efforts were wasted I would soon find out, as it was the USB cable that would not work. My dad suggested trying a different cable to see if it worked. Sure enough it started right up and I could upload the software to it and communicate with it! An important note for RUMBA users, if you want to run it off of USB power (or just communicate with it via USB power) you must switch the power header from standalone to USB or it will not feed power from USB to the board.
I had run out of time to get it running over the weekend though. I decided to bring it with me back to my apartment and mess around with the firmware. I spent most of Monday night tweaking the software and getting the machine to have the correct values in the firmware.
Delta configuration was fairly straight forward. I found a screen shot of a delta printer explaining what each delta variable meant on the machine, and I was able to pull the values for each from my 3D models (it may be more accurate to take real measurements in the future). I used the delta branch of Marlin for my firmware. I changed the pertinent values in the firmware and uploaded the new firmware to my printer.
Using Pronterface, I could zero and jog the new printer. It moves! but there were some issues. Homing worked fine, it could find the top, but it moved waaaay to fast. So fast to the point it was destabilizing the machine. I updated my firmware to slow down the homing cycle. The next issue was that jogging via Pronterface produced very very janky movements. Also, one of the stepper motors developed a very loud clicking noise while it was holding engaged but not moving. I thought there was an issue with the PSU not supplying enough current to move all three motors at once, but this proved not to be the issue.
Upon further investigation, I found the reason for the janky movement of the axes. In Pronterface, for jogging the machine, you set the move speed. It is defaulted at 200mm/s, which is fast for Cartesian based printers that have a Z axis lead screw, but a delta does not work that way. I figured out this problem because the homing cycle worked, but jogging did not. The homing cycle was set at something like 20*2000 mm/s move speed, and with the jog speed default being so low, it simple was not performing well. At 2000 mm/s speed the machine jogged nicely and at 4500 mm/s if moved even better. I did not bring any filament with me to test, but I was able to set the 0 at a point just above the bed and do an air print to see it in action. It worked great! Very fast movement compared to my other printer.
The next weekend I have time to work on it will be a proper commissioning weekend. I still have to figure out all the ins and outs of a bowden extruder system and fine tune all my settings to get good prints.