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S. Conrad et al.
the pins moving in Y-direction. The individual tool can be freely designed and adapted
to the requirements of the component to be produced, provided it is equipped with the
coupling interface and docking pinholes and is within the dimensional specifications of
the printer. The current setup provides connectors for temperature control and stepper
motor drivers for each tool, which could be extended if needed. In contrast to other
printers with multiple nozzles on the same tool, this setup avoids any kind of unwanted
interaction between the print-heads, since at only one tool is moving above the build
platform.
2.2 Coupling Interface
A kinematic coupling is a physical attachment of two separate solid objects with a focus
on repeatability and precision. Such a coupling is essential for any kind of tool changing
device and has been used on the developed printer. Usually it is realized with a contact
surface on the carriage and a corresponding part on the tool it is supposed to pick up.
When pressed together both sides form exactly six contact points removing all degrees
of freedom and providing a precise and repeatable positioning. There are several designs
in existence to achieve this number of contact points but the most popular is the Maxwell
design (Slocum 2010), which has been used also in this work. Hereby the “female part”
of the coupling consists of three grooves arranged in a triangle (Fig. 1C). Corresponding
balls on the counterpart (“male part”) only contact the parallel edges of the “groves” at
one point each, resulting in the targeted positioning (Fig. 1D). For the implementation in
this study, two parallel rods were used providing the same functionality as the grooves.
2.3 Lock Mechanism
To keep the interface physically connected an additional force is needed. Since a directly
driven tool for a 3D-printer weighs more than 500 g and is about 100 mm long, a mechanical fixation was chosen, because other mechanisms (e.g. electromagnetic attraction) are
too sensitive to the resulting torque. This fixation schematically shown in Fig. 2 is based
on a “lock-and-key-principle”. A stepper motor with a shortened lead screw moves a
rotatable mounted T-shaped key. Driving the motor causes the key to rotate inside the
lock (Fig. 2A/B). The lock contains one vertical and one horizontal barrier restraining
the keys free rotation to an angle of 90°. When hitting the horizontal barrier the key stops
rotating and starts retracting towards the stepper motor (Fig. 2C/D). Thus, it presses the
“male” and the “female” side of the kinematic coupling together fastening the connection. To unlock a tool, the motor is driven counterclockwise, releasing the key from the
physical connection and rotating it back until the vertical barrier stops it. Once stopped,
the key is further extended to ensure no physical connection and safe release from the
lock.
Using this approach, the coupling mechanism provides a high tolerance against
fabrication deviations of the coupling interface. While being retracted and unretracted,
the key crosses a distance of up to 5 mm and is able to pick up a newly assembled tool
without any calibration.
S. Conrad et al.
the pins moving in Y-direction. The individual tool can be freely designed and adapted
to the requirements of the component to be produced, provided it is equipped with the
coupling interface and docking pinholes and is within the dimensional specifications of
the printer. The current setup provides connectors for temperature control and stepper
motor drivers for each tool, which could be extended if needed. In contrast to other
printers with multiple nozzles on the same tool, this setup avoids any kind of unwanted
interaction between the print-heads, since at only one tool is moving above the build
platform.
2.2 Coupling Interface
A kinematic coupling is a physical attachment of two separate solid objects with a focus
on repeatability and precision. Such a coupling is essential for any kind of tool changing
device and has been used on the developed printer. Usually it is realized with a contact
surface on the carriage and a corresponding part on the tool it is supposed to pick up.
When pressed together both sides form exactly six contact points removing all degrees
of freedom and providing a precise and repeatable positioning. There are several designs
in existence to achieve this number of contact points but the most popular is the Maxwell
design (Slocum 2010), which has been used also in this work. Hereby the “female part”
of the coupling consists of three grooves arranged in a triangle (Fig. 1C). Corresponding
balls on the counterpart (“male part”) only contact the parallel edges of the “groves” at
one point each, resulting in the targeted positioning (Fig. 1D). For the implementation in
this study, two parallel rods were used providing the same functionality as the grooves.
2.3 Lock Mechanism
To keep the interface physically connected an additional force is needed. Since a directly
driven tool for a 3D-printer weighs more than 500 g and is about 100 mm long, a mechanical fixation was chosen, because other mechanisms (e.g. electromagnetic attraction) are
too sensitive to the resulting torque. This fixation schematically shown in Fig. 2 is based
on a “lock-and-key-principle”. A stepper motor with a shortened lead screw moves a
rotatable mounted T-shaped key. Driving the motor causes the key to rotate inside the
lock (Fig. 2A/B). The lock contains one vertical and one horizontal barrier restraining
the keys free rotation to an angle of 90°. When hitting the horizontal barrier the key stops
rotating and starts retracting towards the stepper motor (Fig. 2C/D). Thus, it presses the
“male” and the “female” side of the kinematic coupling together fastening the connection. To unlock a tool, the motor is driven counterclockwise, releasing the key from the
physical connection and rotating it back until the vertical barrier stops it. Once stopped,
the key is further extended to ensure no physical connection and safe release from the
lock.
Using this approach, the coupling mechanism provides a high tolerance against
fabrication deviations of the coupling interface. While being retracted and unretracted,
the key crosses a distance of up to 5 mm and is able to pick up a newly assembled tool
without any calibration.