Airtripper’s Bowden Extruder – Rostock 3D Printer BI

As part of our work for the construction of the Rostock 3D Printer BI Edition, we modified Airtripper’s original extruder. Our initial goal was to reduce the complexity of the extruder while maintaining the same functionality.

Our efforts were initially focused on replacing the idler/tensionner body strut with a printed part that would support it just as well.



After playing around with the design and testing the extruder, we ended up including other modifications:

Motor Shaft Support

We removed the motor shaft support since we did not have a miniature ball bearing on hand. After several hours of printing we haven’t experienced any problems with the removal of this part so we decided to leave it out of our final design. However, we would caution that a sufficient infill is required in order for this piece to be strong enough (we recommend > 40% infill).

Filament guide tunnel

We slightly elongated the filament guide tunnel. With Airtripper’s original version, we sometimes found that the filament would curve and fall outside of the effective gripping zone (between the bearing and the 5mm insert). Elongating the filament guide tunnel meant that the filament would remain straighter and eliminate these types of problem.

Embedded nuts

When the extruder is installed on the Rostock BI frame, there is little clearance to manipulate the nuts. As such, while trying to install them, it wasn’t uncommon for them to be dropped.  To solve this problem, we decided to embed the nuts and it worked nicely since we could press them into place and manipulate the extruder without having to worry about it.


Teflon Tube Adaptor

Finally, we decided to add a tube adaptor for a 4 mm outside diameter (OD) teflon tube.  This addition proved useful since we were trying tubing that had different ODs during the development stage of Rostock BI.


We would like to thank Airtripper for his original design. All the Sketchup models and .stl file for the BI edition can be found on Thingiverse.

Cable Drive System – Rostock 3D Printer BI

During our research when designing the Rostock 3D Printer BI Edition, we found that some people had experimented with using cable drives instead of the traditional belt drives. After careful analysis, we were confident that a cable drive could accomplish the same function as a belt drive and that it would also be simpler to implement. We went through several rounds of testing and redesigning until we were fully satisfied with our current cable drive system. Today we are unveiling our final design for the cable drive system (which will ship with the first batch of Rostock BIs).

To begin, let’s take an overall look at the system and its components. At the bottom, we have a cable drive pulley, in the middle, we have a carriage assembly with cable tensioner, and at the top, we have an idler pulley module with bearings and a stop switch. The carriage assembly glides smoothly along 5/16 inch steel rods using LM8UU bearings.


Rostock 3D Printer Drive Pulley

One of the most important features that we wanted to incorporate into the cable drive pulley was the ability to keep the wire from crossing over itself. The main reason why we wanted the wire to spool perfectly around the pulley was to ensure that binding of the wire would not affect the print quality. We initially tested “ribbed” pulleys that were intended to guide the wire around an infinite screw. However, we found that these systems produced long pulleys that moved the wire laterally and created other types of problems.

Another objective we had was to keep the pulley relatively close to the stepper motor face. By doing so, we limited the lateral movement of the wire across the pulley and maintained a relatively constant wire distance between the top and bottom pulleys.


The result is a minimalistic drive pulley held in place by a retaining screw. We placed a dividing wall in the middle of the pulley to keep the spooling and unspooling segments separate. Another interesting feature of the drive pulley is a guide tunnel going from one end to the other.


This guide tunnel allowed us to use a single wire for the assembly and secure the wire on the pulley without the need for screws (other designs used additional screws to secure the wire onto the pulley). The strategic location of the guide tunnel promotes a more linear winding of the cable, which translates into a linear carriage motion.

Carrier and Tensioner

The cable used in the Rostock 3D Printer BI Edition is a high quality braided non-stretch fishing line. A non-stretch cable is important in order to guarantee that the cable doesn’t lose its tension. The cable tension is adjusted by turning an M4 screw equipped with a washer. The washer will  ensure that the wire is secured to the carrier. Another useful benefit of using a washer is that it allows you to fine tune the position of the wire around the tensioning screw and also fine tune the winding pattern of the cable.


The carriage assembly is held in place on the rods by linear bearings which provide a very smooth motion. Finally, on the inside part of the carrier we positioned an M3 screw to hit the “endstop”. The height of that screw must be 0.5 mm for each turn. This is critical for the proper calibration of the towers and print head.

Idler Pulley

The idler pulley, located on the topmost part of the assembly, is comprised of two 608 bearings and a 3D printed pulley. It is mounted on a part that supports both rods and the top plate and the wire is simply looped over the 3D printed pulley. This arrangement provides support for the cable and a very smooth drive. We used two bearings to allow for a greater surface area when adjusting the final position of the pulley. We plan to  use only one 608 bearing for  subsequent versions of the system.

The cable drive system that we have designed is simple and reliable. The production of this system is very straightforward when compared to the belt and metal pulley. Since we use a non-stretch wire, there is also minimal lash in the system which means accurate and easy calibration. 


Stop Switch (Mechanical Endstop)

The stop switch is located on the inside part of the idler pulley module and, as its name suggests, is used to stop the carriage assembly. It is not necessarily part of the cable drive system itself, but we decided to dedicate a few lines to it in this article. The endstop used is a mechanical switch mounted to a PCB breakout board. It is widely used in many 3D printer designs and performs very nicely. We evaluated the possibility of using Hall sensor based switches but saw no real benefits for this application where we simply needed to mark an initial position in the software. In a nutshell, when the screw located on the carriage assembly hits the switch, a signal is sent to the software, the stepper stops and the position is marked as “parked”.


If you would like to print your own, all SketchUp models, .STL files and additional instructions are available on Boots Industries Thingiverse.