N.U.G.G.S. Turnaround Node

The run-resetter for the N.U.G.G.S. hamster-tunnel system: a widened chamber carrying the standard genderless quarter-turn port on two faces, so an adult Syrian can enter one port, pivot 180° on a shallow dish, and leave the other — doubling a run back on itself. This is the module that makes layouts larger than one straight tube legal: the welfare rule caps any continuously enclosed run at two body lengths between breaks, and a chamber this wide that is itself open to ventilated air is one of only three breaks.

Work in progress — nothing in this system has been printed yet. The node passes gate.sh --slice (printcheck + a real test-slice, support-free), but port_tol = 0.30 is an untested guess inherited from the shared standard. Print the coupon first (see Assembly & use) and expect to tune it.

Where a turnaround belongs

It is the counterpart of the den, and the opposite job: the den is a dead-end refuge (too narrow to turn in, does not reset the run count); this is a pass-through wide enough to turn in, which does. Put it wherever a run has to double back — along a wall, or at the end of a shelf-line (closed loops: pending the N2 source read — the family charter has only checked the open-run rule). The two ports are parallel and 97 mm apart, so the modules mating them leave side by side; that spacing is derived (≥ 2 × port radius) so two mating modules never collide. The full rule and its source live in designs/nuggs/PM.md (N2) and docs/nuggs-research.md.

What you get

turnaround — the node (≈ 99 × 205 × 195 mm at defaults): an oblate chamber over a solid web, two Ø80 mm ports on the underside, six ceiling vents.
coupon — the standard NUGGS port stub (~21 mm, ~2 h 17 m to print) for tuning the joint fit before you commit ~21 h of printing.

The port faces the hero can't show are in the 4-view contact sheet's bottom-iso quadrant — the port-face view, both rings on their sector feet — and, at first-layer resolution, in bed-contact.png.

The three things the shape is doing

A dish, not a floor. The chamber floor is the widened bowl's own curve, and it meets each bore mouth ~1 mm below the bore floor — the route steps down, never onto a lip. The grade across the dish peaks at 14.6°, inside the welfare ceiling of 15°, and eases to flat at the bottom of the bowl.
A vault, not a dome. An enclosed near-flat ceiling cannot print support-free, and a domed crown over a chamber this wide would be one. The chamber closes instead on a barrel vault — two roof planes at ~46° meeting at a ridge — every surface self-supporting by construction. That took the printcheck overhang figure from 9% to 2% of surface.
Vents where the air is. Six Ø9 mm teardrop vents on the use-ceiling, the same count and size the den's chimney derivation gives, over ~2.4× the air volume with the same one-animal occupancy. Open to ventilated space is what makes the chamber a legal run break, not just a wide room; the teardrop profile prints support-free and gives a gnawing incisor no purchase.

Print settings

Material: PLA preferred; PETG if it must live warm — expect a sagged web underside, and slow the roof overhangs.
Layer height: 0.2 mm.
Infill: 15–20 % is plenty; the shell does the work.
Seam: Back (or scarf) — same setting for the coupon and the node; stock Aligned stacks a ridge on the port collar that reads as port_tol too tight.
Supports: none needed. Ports down on their sector tips, the bore ceilings close at the coupling's proven 45° class, the web is a 12 mm anchored bridge, the belly dome is pole-flush on the bed, and the crown is the vault above. The whole part is support-free as modelled. Keep auto-supports off even if your slicer offers them: anything it builds inside the chamber is unreachable through six Ø9 mm vents.
Orientation: as modelled — both ports down, sector tips on the bed, the chamber up. Do not rotate it.
Brim: off. Only the sector tips touch the bed (the web is the bridge at the port plane, 10 mm up), so the first ~50 layers print as twelve small islands that merge at the port plane — expected, not a fault. A brim would weld across the mating feet. Give it a clean sheet and no draft over the bed, and watch the first hour — twelve free-standing islands is where a 21 h print dies if it's going to.
Heads up: 195 mm tall — check your Z. It fits the 256×256×256 class this family gates on, but it is a big print: CI's test-slice reports **21 h and ~300 g** at 0.2 mm (the coupon is ~2 h 17 m).

Parameters

The handful most worth tuning; the rest are grouped in the Customizer sections at the top of nuggs-turnaround.scad.

Parameter	Default	What it does
`port_tol`	0.30 mm	The one fit knob for the quarter-turn joint. Tune on the coupon in ±0.05 steps.
`bore_d`	80 mm	Internal bore — the shared NUGGS headline number.
`body_len_mm`	180 mm	The animal's body length. The clear-width assert keys on it: the chamber must stay at least this wide inside.
`chamber_ay`	100 mm	Chamber half-width along the turn axis — the clear internal width is 2× this (200 mm at defaults).
`chamber_ax`	47 mm	Chamber half-width across the route — sets the dish the animal crosses and the bore-mouth handoff.
`max_incline_deg`	15°	The welfare ceiling on route grade. The dish-grade assert keys on it (14.6° at defaults).
`n_vent` / `vent_d`	6 / 9 mm	Ceiling vent count and diameter.
`wall`	2.4 mm	Shell thickness — about six perimeters at a stock 0.42 line width; leave wall loops at default.

Override on the command line with, e.g., -D 'port_tol=0.35'. Every dimension above is enforced by an assert in the .scad, so a value that breaks the welfare geometry fails the render instead of shipping.

Assembly & use

Print the coupon first (nuggs-turnaround-coupon.scad, ~~21 mm stub) — in the same material as the node. If this is your first NUGGS module, print the coupon twice (~~4 h 35 m / ~60 g for the pair). Mate two of them (or one to any NUGGS module) — it must insert to the collar face and lock with a light quarter turn: firm, no rattle, no forcing. Forcing loose → raise port_tol +0.05; will not lock → lower 0.05. If it inserts but the quarter-turn grinds near the lock, deburr the sector tips' first layer with a blade before touching port_tol — or set Elephant foot compensation to 0.2 mm for the coupon and the node. The coupon also previews what the first ~50 layers will look like: the same sector feet, printed as free-standing islands.
Print the node ports-down, no supports, no brim.
Fit it between any two NUGGS faces: push together and twist a quarter turn either way — one port at a time, twisting whichever half is still free in the air (two engaged bayonets 97 mm apart on a rigid body cannot turn together). The same order is the emergency order: twist one, twist t'other.
Place it where the run doubles back — for the shelf planner, the node in use stands 195 mm along the tube run and 205 mm across it. The animal enters, climbs the ~1 mm step down onto the dish, crosses, pivots in the 200 mm-wide bowl, and leaves the second port. Bedding will collect in the dish; that is fine — it is a pass-through, not a refuge, and the den is the module that wants the bedding — but strip the dish's bedding at the weekly clean: the vents' no-purchase argument assumes the ceiling stays a full standing reach away, and a packed mound shortens it. Clean it hand-wash only, lukewarm (≤ 50 °C), never the dishwasher: heat deforms PLA, and a deformed tube is a narrowed tube — the material failure mode is the injury failure mode (family charter N7).

One stated trade, so it isn't an omission: the chamber is opaque, and you will never see the turn — you'll hear her cross before you see her (six open vents carry sound, and ~300 g of hollow shell is a drum at 2 a.m., her prime time). If watching matters, transparent PETG buys you a silhouette; a windowed variant is a possible future derivative.