The Arachne perimeter generator is one of those features that nobody asked for and almost everyone benefits from. It was developed at UltiMaker and shipped in Cura first; the PrusaSlicer 2.5.0-alpha2 release notes record it as ported from Cura and implemented from the Kuipers et al. 2020 paper on adaptive width control. It is the default in PrusaSlicer, OrcaSlicer and Cura; Bambu Studio’s profiles ship Classic, with Arachne under Quality > Wall generator.
If you’ve never thought about it, your slicer is probably using it and you’re getting the benefit without knowing why. But knowing what Arachne actually does — and what it doesn’t do — helps you make better decisions when prints don’t come out the way you expect.
What the Classic generator does
The Classic perimeter generator places extrusion paths at fixed widths. If your nozzle is 0.4 mm and your line width is set to 0.45 mm, every perimeter is exactly 0.45 mm wide. When the slicer encounters a wall that’s, say, 1.0 mm thick, it tries to fit fixed-width perimeters inside.
Two perimeters at 0.45 mm = 0.9 mm. That leaves 0.1 mm in the middle that nothing fills. Classic’s solutions are limited:
- Leave a gap (the “thin wall gap” you see in slicer previews as a faint line).
- Add a thin-wall extrusion if the option is enabled — a single narrow trace that may or may not adhere well.
- Fall back to infill, which is the wrong density and direction.
The result on real prints: weak spots where thin features meet thick ones, visible seams on tapered walls, and small features that disappear entirely.
What Arachne does differently
Arachne (the variable-width perimeter generator) treats line width as a range, not a fixed value. Given a wall, it computes the medial axis — the centerline of the wall — and places extrusion paths that vary in width along that axis.
In the 1.0 mm wall example, Arachne would place a single perimeter of approximately 0.5 mm width along each side, and a third perimeter through the middle if needed. The transitions taper smoothly. There’s no gap and no thin-wall fallback.
The practical effects:
- Thin features print as solid features. A wall that’s slightly narrower than 2× line width still gets a solid extrusion path through the middle.
- Tapering walls maintain a continuous extrusion. No seam where the wall transitions from “fits two perimeters” to “fits one and a half.”
- Lettering, fine geometry, and gear teeth come out cleaner. Variable-width paths follow the medial axis instead of being forced into fixed widths.
When Arachne wins
Arachne provides a noticeable improvement in three categories of model:
Models with thin features. Embossed text, fine surface details, and parts that depend on geometry being right (gears, fingers on cosmetic parts, calibration cubes with small features).
Models with variable wall thickness. Anything organic, anything sculpted, anything generated by topology optimization or scanned. The walls don’t divide evenly into your line width, and Arachne handles the in-between thicknesses cleanly.
Models that need dimensional accuracy on small features. Snap fits, threaded inserts, mounting points where the hole walls are 1–2 mm thick. Classic perimeters can over- or under-extrude these depending on how the fixed widths fit; Arachne hits the geometry more reliably.
When Classic is fine
Where Arachne is already the default, there’s rarely a reason to switch away from it. But know that Classic is still acceptable in some cases:
- Pure rectilinear models with consistent wall thickness chosen as a multiple of line width. Boxes, brackets, anything designed in CAD with wall thicknesses set deliberately to your slicing parameters.
- Large prints where slicing time matters and the model has no thin features. Arachne’s path planning is slightly slower than Classic’s.
- Specific bug workarounds. Occasionally a model produces unexpected seam placement under Arachne; switching to Classic for that print can be a quick fix while you investigate.
The settings that matter
Wall transition angle
The wedge angle that decides when Arachne creates transitions between even and odd perimeter counts; wedges steeper than this get no transitions and no centre perimeter. Default is 10° in PrusaSlicer and Bambu Studio.
- Lower: fewer and shorter centre perimeters in wedges, which can leave gaps or over-extrude, per the Prusa Arachne article.
- Higher: more transitions and more centre fill.
Most users should not touch this. The default is well-tuned.
Wall transition length
How long Arachne takes to transition from one perimeter count to another. Longer transitions are smoother but use more material.
- PrusaSlicer default: 0.4 mm (one nozzle diameter at 0.4 nozzle).
- Bambu Studio default: 0.4 mm.
Increase if you see abrupt visible transitions on tapered walls. Decrease if transitions extend too far into thin features.
Minimum wall width
The smallest wall width Arachne will attempt to produce. Below this threshold, Arachne treats the geometry as too thin to print and either skips it or merges it into adjacent features.
- Default in most slicers: 0.34 mm (85% of nozzle diameter).
- For a 0.4 mm nozzle, you can usually go as low as 0.25 mm with a tuned printer. Below 0.2 mm, results are unreliable.
- For a 0.6 mm nozzle, scale up proportionally to 0.45–0.5 mm minimum.
This is the parameter that controls whether your model’s finest details survive slicing. Lower it cautiously and test on a representative feature before committing to a full print.
Wall distribution count
How many walls, counted from the centre, absorb the width variation. The default 1 keeps width changes in the innermost wall so the outer walls stay at nominal width. Raising it spreads the variation toward the surface, so keep 1 when appearance matters.
A practical test
If you want to see Arachne’s contribution directly: take any model with embossed text or a wordmark, slice it once with Arachne enabled and once with Classic. Look at the perimeter path preview on the textured surface.
With Arachne, the text follows the letters’ shapes with smooth tapering paths. With Classic, you see fragmented thin-wall extrusions and visible gaps in the letterforms.
Print both. The Arachne version will read more cleanly at small sizes — sometimes the difference between legible 6 pt text and an unreadable smudge.
Where to find the setting
| Slicer | Setting location | Setting name |
|---|---|---|
| PrusaSlicer | Print Settings → Layers and Perimeters → Advanced | Perimeter generator |
| Bambu Studio | Quality → Wall generator | Wall generator |
| OrcaSlicer | Strength → Wall generator | Wall generator |
| Cura | Always Arachne since 5.0; no switch | Minimum Wall Line Width and wall transition settings |
In PrusaSlicer, Bambu Studio, and OrcaSlicer, set to “Arachne” and leave the sub-settings at default unless you have a specific reason to tune them. For how Bambu Studio and PrusaSlicer differ beyond this setting, see the head-to-head comparison.
Related reading
For wall count selection in context with the other core parameters, see the core slicer settings guide. Thin-feature accuracy also depends on flow being calibrated — the flow and temperature calibration guide covers that, and troubleshooting print defects covers when fine features fail for non-Arachne reasons. For the wider porting history between the two families, see Cura vs PrusaSlicer. To vary layer height rather than line width, see adaptive layer height.
For wall count selection (how many perimeters to use for a given strength target), see PrintLabGuide on perimeter-based strength. For printer-specific wall settings that pair with Arachne, FDM Desk covers calibration profiles by printer model.
