Arachne vs Classic: When the Perimeter Generator Actually Matters

The Arachne perimeter generator is one of those features that nobody asked for and almost everyone benefits from. It first landed in PrusaSlicer 2.4 (2022) and is now the default in modern PrusaSlicer, Bambu Studio, OrcaSlicer, and Cura.

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

Don’t bother switching away from Arachne — it’s the modern default for good reasons. 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 threshold at which Arachne decides a feature is thin enough to need variable-width treatment. Default is 10° in PrusaSlicer and Bambu Studio.

• Lower (5–8°): More aggressive variable-width usage. Cleaner thin features, but more compute time.
• Higher (15–20°): More conservative; Arachne falls back to fixed-width more often. Faster slicing, less optimal thin-wall handling.

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 of the outer perimeters Arachne treats as “outer” for path planning. Default 1 means only the outermost perimeter is treated as outer; inner perimeters can vary more freely.

Increasing to 2 or 3 stiffens the appearance of the print at a small cost in dimensional flexibility on the inner walls. Useful for prints where surface appearance on the outer 1–2 mm matters most.

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	Walls → Wall ordering	Connect infill polygons + variable-width walls (separated across multiple 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.

Related reading

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.