Arachne 3D Printing Technical Guide

Classic FDM slicing forces every extrusion to a fixed width, usually the nozzle diameter. A 0.4 mm nozzle prints 0.4 mm lines, and any wall that is not an exact multiple of 0.4 mm gets a gap or an overlap. Arachne drops that rule. It computes a medial axis for each closed region, then sweeps the extrusion width continuously between a minimum and a maximum so the perimeter fills the region instead of approximating it.

The effect shows up on thin features. A 1.1 mm rib sliced at 0.4 mm fixed width needs three lines at 1.2 mm total, a 0.1 mm overshoot that bulges or leaves a seam. Under Arachne the same rib is covered by three lines of roughly 0.37 mm each. No gap, no overlap, and the outer surface stays on the CAD boundary.

Width is bounded. Typical slicers expose a minimum and maximum line width, often set near 0.85× and 1.6× the nozzle diameter for a 0.4 mm nozzle. Push past those limits and the extruder either starves the line or over-extrudes it. The window exists because flow rate, not the algorithm, sets the ceiling.

One more change matters to engineers: Arachne decides the number of perimeters after it measures the region, not before. A 2.7 mm wall may become three wide lines or five narrow ones depending on the width limits you set. Two identical CAD files can therefore print with different wall counts on two machines.

Thin ribs, fins and bosses are the clear winners. Anything in the 0.8–2.0 mm range that would otherwise round up or down to a whole number of nozzle widths gets a cleaner result. Heat-sink fins, snap-fit lips and stiffening ribs on enclosures are typical parts where the wall lands on a fraction of the nozzle diameter.

Text and small logos also improve. Embossed lettering narrower than two nozzle widths usually prints as a smear at fixed width. With a variable width the strokes are followed individually, so a 0.6 mm stroke on a 0.4 mm nozzle comes out legible instead of merged. Keep character height above roughly 3 mm if the text must stay readable after post-processing.

Curved outer walls see a smaller but real gain. Where a circular boss meets a straight wall, Arachne can taper the line width across the junction instead of leaving a short gap the slicer would otherwise fill with a thin sliver. Fewer slivers means fewer weak spots along the seam.

Parts with internal channels benefit least. A 2 mm cooling channel inside a block is still a bridging and sagging problem, not a wall-width problem. Geometry that needs support or has steep overhangs will not be rescued by the perimeter generator.

Very thin walls are a trap. Below about 1.5× the nozzle diameter, the slicer can only produce one or two lines, and a single-line wall has almost no bending stiffness. A 0.6 mm fin on a 0.4 mm nozzle may print, but it will flex and it will not hold a tolerance. If the feature is structural, thicken it or change the process.

Surface finish changes with width. A line printed at 0.6 mm from a 0.4 mm nozzle is squashed wider than the nozzle opening, which leaves a flatter but more visible top surface on the outer wall. If the visible face will be painted or bonded, that texture can matter. Fixed-width walls at or below the nozzle diameter give a more uniform look.

Flow limits bite on wide lines. A 0.8 mm nozzle pushing 1.2 mm lines at 60 mm/s asks for more melt volume than many hotends deliver. The symptom is inconsistent extrusion on long straight walls, not on short ones, so it can pass a small test print and fail on the production part.

Slicing takes longer and the toolpath is harder to inspect. Variable width produces many short segments and frequent flow changes. On a part with dozens of thin features, preview review becomes slow, and a bad width limit can hide in a region you did not zoom into. Budget review time accordingly.

Use fixed width when the part is a simple block with walls thicker than 3 mm. There is nothing for Arachne to optimize, and a fixed 0.45 mm line at 0.4 mm nozzle gives a predictable surface with fewer flow changes along the wall.

Use it when you need repeatability across slicer versions. Width limits and medial-axis handling have changed between releases. If the same file must print identically in two years, freeze the slicer build or accept re-qualification.

Use it for parts whose acceptance criteria are dimensional only. Arachne improves wall coverage, not roundness, flatness or hole diameter. If a print is being checked on a CMM against ±0.2 mm, the perimeter generator is not the variable that decides pass or fail.

Switch to Arachne when the drawing has features under 2 mm, embossed text, or walls that repeatedly fall on fractional nozzle multiples. That is where the extra slicing complexity pays for itself.