A Python tool to post process 3D-printer G-Code for FDM printing on non-planar surfaces.
Table of Contents
Non-planar FDM printing is a powerful, yet challenging and vastly complicated topic. This tool is a minimal approach to enable simple non-planar printing on print beds with sufficiently small curvature. Under the hood, the given gcode is simply projected onto the bed surface. This projection does not preserve angles and distances and can therefore deform the original, planar model.
Once hardware and software are properly prepared and set up, the workflow is very simple:
- Slice the planar model using your preferred standard slicing software (Cura, Slic3r, PrusaSlicer, ...)
- Post process the gcode output using this tool, which will only alter the toolpaths and leave all other settings untouched.
- Check the non-planar gcode in a preview software and print it.
The package can be installed through pip.
To install the package, navigate to its root folder, i.e. where pyproject.toml is located, and install it using pip:
python -m pip install .The most straightforward way of using this tool is through its command line interface, accessible through running the following command in the root directory:
python nonplanar.py -hThis will show an extensive help message with all required info and arguments. To standardize the workflow, command line arguments can also be saved in a text file and then passed:
python nonplanar.py @your_arguments.txtAlternatively, the tool can be directly used in any Python code by importing the necessary class:
from nonplanarpp.gcode_handling import GCodeProjector
To keep up with increasing print speeds even in entry-level printers, print heads tend to carry beefy part coolers and are becoming bulkier and bulkier. This drastically limits the clearance between nozzle tip and lower face of the print head, essentially making non-planar printing impossible without an adapted nozzle.
A nozzle for non-planar printin needs to be:
- Significantly longer than standard, providing enough clearance between nozzle tip and print head
- 'pointier' than standard, allowing a higher tolerance for extruding on surfaces that are not orthogonal to the nozzle.
For some printer models, such nozzles are commercially available. In the scope of this project however, the nozzle was custom-made from a brass cylinder.
As mentioned above, the clearance between print head and nozzle tip plays a decisive role in non-planar printing. If a part cooling fan is required, the standard mounting position is most probably too close to the nozzle tip and should be changed to a custom solution.
If the print head houses a sensor that probes the bed in the z homing procedure, chances are high that it also needs to be adapted. This is not as straightforward as remounting a part cooling fan, since the sensor itself is still required to home the z axis before starting a print. Here, this issue was solved by designing a rotatable mount that can rotate the sensor out of the way once the z homing is completed.
The setup and calibration procedure requires direct control over the printer, meaning the ability to send G-Code commands and receive responses in a terminal-like environment. Thus, the standard user interface is not sufficient and a more direct control option has to be set up. In this project, a RasberryPi running OctoPrint was used. If supported by the printer, a Klipper interface is probably an even more suitable choice.
For prototyping and testing, the non-planar print bed can be printed using conventional methods and then serves as a base for the non-planar structures on top. PLA and soft TPU (for example, 82A) have rather bad adhesion and can be separated with relative ease. First tests of this project were done by first printing a TPU structure and then using non-planar printing to extrude PLA onto it. If being extremely careful, it might be possible to use the same TPU base twice, but effectively this approach only gives single-use print beds.
Once the optimal shape and size of the non-planar bed has been found, the temporary TPU beds should be replaced by a permanent solution. Aluminium is the obvious choice here, as it is easy to machine, readily and cheaply available and - most importantly - a great thermal conductor. The print bed design should include a designated area to home the z axis as well as precise alignment markers.
The following steps only have to be done when setting up a non-planar printing system for the first time or after moving the system, making changes to the hardware, etc. However, it is utmost important to calibrate the system as precisely as possible, since any inaccuracies in these steps will limit the printing performance fundamentally.
Since the non-planar print bed is not uniform, the z axis cannot be homed at any arbitrary position on the bed, but only in a designated area. However, the x-y position of the z homing procedure are usually not customizable in the printer firmware. Therefore, an adapted homing procedure has to be prepared and used:
- Using the standard settings, the z axis has to be homed ("G28 Z").
- Without moving the print head after z homing, the current position should be queried ("M114") and saved in the
printer configuration .ini file (example in
testing/ender3v3se_4x2_bed.ini). - Noting that the nozzle position is offset relative to the z sensor position, the print head has to be moved to the desired position of z homing.
- The new z homing coordinates should be saved in the config file mentioned above.
The post processor will replace the "G28" homing command with a two-step procedure, which home x and y axes first. Then, a coordinate shift is applied to the printer in such a way that the standard z homing position in the original coordinate system is mapped onto the new position. After homing the z axis, this coordinate shift is reversed and the printer moves in its standard coordinates.
Since points saved in the G-Code instructions for the printer only describe the position of the nozzle tip and don't take the actual physical dimensions of the nozzle into account. Therefore, sharp edges and sudden changes in print bed height pose an extreme risk of crashing the nozzle into the bed. The easiest way to prevent this is to feed a slightly adapted 3D model of the print bed to the post-processing tool. After truthfully modeling the print bed itself, safety zones should be added as simple blocks around sharp edges or deep valleys. This will make the print head follow the surface of the adapted model and prevent the crashes.
There is no actual direct feedback mechanism between printer and the non-planar bed. Basically, the printer does not " see" the print bed and blindly follows G-Code commands. Therefore, it is absolutely essential that the virtual 3D model of the print bed agrees with its real-world counterpart in shape, size and position. Since the custom print bed has to be CNC machined anyway, having a 3D model of the print bed should not be any concern. The remaining part is to position this virtual print bed at the exact same position (relative to the printer coordiante system) as the real-world one. This alignment procedure consists of several steps:
- The print bed is mounted at the desired position and should be secured tightly into place - it shall not be moved at all afterward.
- The x and y axes of the printer have to be homed (NOT the z axis!), e.g. for Marlin firmware "G28 X Y".
- The nozzle tip has to be moved to align as precisely as possible with the designated alignment point on the print bed (in this case a pointy aluminium tip).
- Read the position of the alignment point in the printer coordinate system (e.g. "M114").
- The virtual print bed has to be moved such that the alignment point is positioned exactly as on the actual printer. Saving the model as an STL file preservers these coordinates since STL is using an absolute coordinate system.
- Knowing the position of the alignment point in the printer coordinate system as well as the offset between alignment point and print area(s) allows to calculate where the models have to placed in order to be printed at the correct position. This position in the printer coordinate system might have to be converted further into the coordinate system used by the conventional slicer (e.g. Cura using an origin-at-center coordinate system).
The curved geometry of the different layers seems to increase stress caused by layers cooling down and shrinking. This makes the print more prone to warping and adhesion issues. For PLA prints, it was possible to mitigate these issues by using a high bed temperature (80 C), no part cooling and operating at the lower end of the nozzle temperature range (200 C).
It can happen that the print head's smooth continuous movements start to stutter and abruptly stop-and-go. Even though this effect has been observed in multiple printers, it is unclear where exactly it comes from. One strong suspicion is that the read rate of the SD card is not fast enough to read a large amount of points at relatively high speeds. This does not really affect the printer durin standard, planar operation. However, since non-planar movements require a lot of interpolated points with small step sizes, the stuttering occurs. One way to mitigate or even fully eliminate this issue is by using a system like OctoPrint which is able to read G-Code from an HDD or SSD and send it to the printer line by line. In the case of OctoPrint, this means choosing "upload locally" instead of saving GCode to the printer's SD card.
Distributed under the GNU GPLv3 License. See LICENSE.txt for more information.
The project was part of the PDP program of the Design Factory at Aalto University in Helsinki.
Elias Ankerhold - elias.ankerhold[at]aalto.fi
Jonas Tjepkema - jonas.tjepkema[at]aalto.fi