Additive Lathe Slicing
Additive Lathe printing is a style of printing that Cosine has developed where a 4th axis mandrel is placed inside our printer and we print on the mandrel while it’s rotating. Developing this type of printing presents an equal number of hardware and software challenges. The focus of this article will cover how to create toolpathing for this style of printing and how we achieved that in the Cosine Slicer.
Printing Helices
Additive lathe printing is designed for working with cylindrical STL files. At its core, it simply generates helical toolpathing within the volume of the cylinder. For solid fills, use a pitch equal to the extrusion width. For sparce fills, use a pitch greater than the extrusion width. In our printer configuration, the mandrel runs from the front of the printer to the back of the printer, colinear with the X axis. The A axis, which defines the mandrel in our printer’s firmware, is measured in degrees. This means that additive lathe printing is better represented in cylindrical coordinates instead of cartesian coordinates. Cylindrical coordinates define points in space using a distance from a reference point (r), an angle from a reference direction (θ), and a distance from a reference point in the direction normal to the θ reference direction.
A typical gcode command for an Additive Lathe move looks similar to a traditional cartesian move but with an extra command for the A axis:
G## X### Y### Z### A### E### F###
For creating a helix, move from the front of the mandrel to the back of the mandrel while rotating the A axis and extruding material. For standard additive lathe extrusion, the Y and Z don’t move during the spiral extrusion. The X axis move will equal the length of the cylinder and the feedrate (F) is defined by the user. The A and E values are calculated in the operation.
The first value to calculate is the A command. As part of the operations, the user must define a pitch for the spiral. This pitch can be used to calculate the degrees of rotation that the A must perform to complete this move.
In order to calculate the E value, which represents the volume of extruded material, it is helpful to visualize the spiral extrusion on a flat plane, as seen below. If you cut the cylinder lengthwise and lay it flat, you can see that the extrusion is a straight line.
That means that the equation for the amount of material extruded is just calculating the volume of a rectangular prism
The layer height and extrusion width are user defined settings in the Additive Lathe Operation. The extrusion length can be calculated by extending the flat plane (example above) by cutting the plane at the end point of each line segment and shifting the section over to create a continuous line. Visualizing the extrusion length this way makes it apparent that you can calculate the length of the line using the Pythagorean Theorem.
With this calculation done, you have all the information to create a complete toolpathing command.
The additive lathe operation has supplemental algorithms to produce rapid moves, coasting, part avoidance, multi-spiral generation, intralayer and interlayer phase shifts, and much more.
Visualization
A new challenge presents itself with regards to visualizing the toolpathing inside the Solidworks environment. Solidworks model space is based on cartesian coordinates. This means that to visualize the cylindrically defined helical toolpathing moves, they must be broken into line segments and then converted into cartesian coordinates in order to visually approximate the spiral. To do this, you select an arbitrary number of sections to break the spiral into. Each of these sections is considered a step along the spiral and can be used to parameterize the equations used to convert from cylindrical to cartesian coordinates.
The larger the number of steps used for parameterization, the more accurately the visualization will approximate the spiral.
Additive Lathe Applications
One of the largest applications for additive lathe printing is billet production. Using the Cosine pellet extruder, you can run highly filled exotic materials and create cylindrical billets for a fraction of the cost of traditional billets. It is also ideal for prototypes with cylindrical geometries, like pistons, grains, barrels, support pillar, augers and much more. A spindle can also be paired with the additive lathe operation to perform traditional CNC machining on the 3D printed part. This would allow the printing of parts to near net form geometries and using the spindle to achieve the desired contouring and surface finish. Click here to see even more applications and examples of Additive Lathe printing.
