The Kossel Pro End Effector
The end effector represents the bulk of our engineering effort (and the primary reason why the project is late). Leading up to the Kossel Pro's design, we ran various delta printers (and a couple of Cartesians in the form of Mendels) and documented the failure modes that we've been seeing. In no particular order, these are some of the considerations that have plagued our various prototypes (including our cartesian robots):
- Cables and connectors wear, come loose, fatigue, etc, due to the high temperature of the hot end and the constant motion of the end effector.
- The bowden cable setup is extremely sensitive to thermal jamming while printing PLA. PLA expands with heat; in an ideal hot end, the plastic should go from solid state to molten state in as short of a melt zone as possible. If the hot end starts overheating, the melt zone elongates and the pressure required to drive the filament through the hotend increases. This becomes problematic when the bowden feed tube is long; additional friction will cause a feeding malfunction.
- For thin walled part, the plastic should hit the part and then immediately drop below the glass transition temperature of the plastic. Otherwise the part will sag and deform. Achieving uniform cooling - from all sides of the nozzle, is a challenge.
- The mass of the end effector shall be kept as light as possible, precluding the use of a direct drive extruder system.
- The auto-levelling probe shall be made robust and repeatable. We've also had a few accidental deployments where during a long print, the probe accidentally deployed and destroyed the print.
What we did different with the Kossel Pro's end effector:
We paid a lot of attention to proper strain relief and connector crimping.But even with the wires pre-crimped, constant motion will still put strain on the cable harness. So, we designed a custom cut PCB to go on the bottom of the end effector assembly for the heater cartridge and the thermistor to connect to. The latter connects via screw terminal, since thermistor wires are made from a notoriously difficult to solder to material and are too thin for crimps to hold properly without a very expensive crimp tool. We will provide the thermistor pre-potted into the hot end heater block, and with ring terminals pre-crimped and tested for our turn key kit builders.
For cooling, the hot end is sucked up into the end effector assembly (see below). This means that we would need very low profile screw terminal blocks for the end effector PCB. They don't exist; so we designed our own. The internal duct work also captures nuts that mates to the PCB and the wires are pinched down by a screw head and washer onto a ENIG (immersion gold plated) contact patch.
Because the end effector board is bolted to the assembly, there is no flexing of the cables relative to the end effector. Additionally, there are mount points for zip ties to attach the incoming cable bundle to the end effector as well to relief strain on the connector as the end effector moves around. And since we have a circuit board underneath the end effector assembly, we decided to bling it out a little bit and added an LED ring light to the bottom as well. It can get awfully hard to see what the printer is doing, with a big print head hovering over the print.
2, 3) As mentioned above, bowden fed hot ends are very sensitive to thermal jamming when printing with PLA. To compound the problem, there are often a lot of confusion about the role of the fan mounted on a hot end. Most operators recommend turning the fan off for the first few layers, for better adhesion. The problem occurs if the printer is not moving fast enough, or if it sits idle with hot PLA in the barrel.
To mitigate this, we've found sub-miniature, 20mm diameter fans made by Sunon. These are standard DigiKey parts, available around the world. The 20mm size allows us to place the fans where it matters, instead of having to muck with fancy duct work. There are three fans. One fan is wired to the Fan-Vcc - selectable on the new Brainwave II, while the other two are under PWM control by the microprocessor. The fan that is constant on blows into a cooling channel that directs airflow directly across the vents of the J-Head to prevent thermal jamming. Using a FLIR infra-red camera, we're seeing that the cooling is indeed VERY efficient, even for a 20mm fan.
The other two fan blows into a plenum that shapes the airflow into a ring and blows it down and out around the extrusion head. Because these fans are on PWM control the microprocessor can throttle back if necessary to improve adhesion. We will be borrowing the FLIR camera to keep fine-tuning our printer profiles.
4) Miniaturization was a key concern. Even though our end effector packs quite a punch, we only grew the radius by 10mm compared to Johann's original minimalistic design, and the plastic parts clock in at around 20 *grams*. We even spec'ed in to smaller end stop switches (although the end stop holders are designed to use both)
5) Finally, we've added a safety detent to the auto levelling probe to enable the probe to be locked from deployment. For long prints, this offers an additional peace of mind.