N.U.G.G.S. Turnaround Tee

The through-and-back node for the N.U.G.G.S. hamster-tunnel system: the turnaround with a third port on the crown. Enter low, leave low and doubled back — or go straight over the top: the crown port is anti-parallel to the two bed ports, so a run can pass through this node end-to-end without turning at all, or take any pairing of the three mouths. Same Ø80 mm bore, same genderless quarter-turn coupling, same revision — it mates with every module the two-port node does, and adds the one thing a layout always runs out of: a corner that isn't a dead end and isn't a switchback either.

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.

What this is, relative to the turnaround

This is a derivative of the two-port nuggs-turnaround (lineage in derives.conf); everything the parent's page documents — the dish-not-a-floor, the vents, the coupling, the welfare rules — is inherited unchanged and documented there, once. The delta is exactly three things:

A third port (C) on the chamber crown, mouth up in the print pose, anti-parallel to the bed ports. Its axis sits 3 mm off the chamber centre toward the dish floor — not for looks: the through-route hands off from the dish onto port C's flat bore floor at 13.9°, inside the 15° welfare ceiling (on-centre it would grade 19°, illegal).
The chamber is 6 mm flatter (z semi-axis 84 mm, parent 90), so the crown port's ring and sectors still fit the 256 mm build class this family gates on — the whole node is 197 mm tall as it prints. The clear internal width — the number that makes the chamber a legal run break under the welfare rules — is untouched at 200 mm.
A repaired crown. The parent's chamber roof has a known defect (a through-slot where its vault outruns the dome — issue #499); the tee cannot inherit it, because port C's collar would open into open air. The tee closes the same crown with the vault plus a hip: two extra roof planes descending across the chamber, so every ceiling surface is steep enough to print support-free and the shell keeps ≥ 2.7 mm over the cavity everywhere. Asserted by sampling in the .scad, not eyeballed.

What you get

turnaround — the node (≈ 101 × 206 × 197 mm at defaults): an oblate chamber over a solid web, two Ø80 mm ports on the underside, the third on the crown, six ceiling vents.
coupon — the standard NUGGS port stub (~21 mm, ~2 h to print) for tuning the joint fit before you commit ~20 h of printing. Port C is the same port rigidly rotated, so one coupon covers all three mouths.

The bed-port faces are in the contact sheet's bottom-iso quadrant; the crown port and the through-route are in the sections below.

The two routes the shape is doing

The switchback (inherited). Enter port A, step ~1 mm down onto the dish, cross, pivot in the 200 mm bowl, leave port B. Grade peaks at 14.6°, and eases to flat at the bottom of the bowl.
The through-route (new). Enter port A, cross to the bowl, climb the far side of the dish — the grade falls as you climb, from 14.6° to 13.9° at the handoff — and step onto port C's flat bore floor, which runs straight up and out the crown. No lip in either direction: the dish floor only ever widens onto the bore floor.

Both routes are proven the same way the parent proves its one: a Ø60 animal envelope swept along every route must stay inside the part's own material (path-clear renders zero facets; a deliberately shifted control must interfere — the check is demonstrated able to fail).

Section through both bed-port axes — bores, web, dish, and the crown port's
tube rising through the frame

Section across the turn axis — the whole through-route: dish, handoff,
crown bore, and the repaired roof

Print settings

Material: PLA if it lives at room temperature; PETG for a node that will be scrubbed or run warm.
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. The two bed ports print down on their sector tips exactly as the parent's do, the crown port prints mouth-up with its collar on the dome, and the repaired roof keeps every enclosed ceiling at or steeper than the coupling's proven 45° class. 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 — bed ports down, crown port up. Do not rotate it.
Brim: off. Only the bed ports' sector tips touch the bed; the first ~50 layers print as twelve small islands that merge at the port plane — expected, not a fault, and a brim would weld across the mating feet.
Heads up: 197 mm tall — check your Z before you start. It fits the 256×256×256 class this family gates on, but it is a big print: CI's test-slice reports ~20 h and ~300 g at 0.2 mm (the coupon is ~2 h).

Parameters

The handful most worth tuning; the rest are grouped in the Customizer sections at the top of nuggs-turnaround-tee.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.
`port_c_x`	3.0 mm	Crown-port axis offset toward the dish floor. Sets the through-route's handoff grade (13.9° at +3; 19.0° and illegal at 0).
`port_c_face`	177 mm	Crown-port face height — sets the node's total print height (197 mm at defaults; 199 is the gate's test-slice ceiling).
`tee_az`	84 mm	Chamber half-height. Pairs with `tee_zc` (= `tee_az` + wall, the pole-flush belly rule).
`hip_deg`	46°	Slope of the repaired roof's cross-planes. Just past the 45° supportless ceiling.
`bore_d`	80 mm	Internal bore — the shared NUGGS headline number.
`chamber_ay`	100 mm	Chamber half-width along the turn axis — the clear internal width is 2× this (200 mm at defaults).
`max_incline_deg`	15°	The welfare ceiling on route grade. Both routes' grade asserts key on it.
`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.

Override on the command line with, e.g., -D 'port_c_x=5'. Every dimension above is enforced by an assert in the .scad — including the two the parent lacked: a sampled minimum roof-shell check and a sampled every-governing- surface-≥45° check, both held over a grid rather than at the section plane alone.

Assembly & use

Print the coupon first (nuggs-turnaround-tee-coupon.scad, ~21 mm stub) — in the same material as the node. 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.
Print the node bed-ports-down, no supports, no brim.
Fit it one port at a time — push together, twist a quarter turn whichever way. With three mouths there are three pairings; the crown port couples exactly like the others (it is the same port rigidly reoriented — quarter-turn either way, no handedness to match).
Place it where the run needs both a corner and a way through: the crown port takes a riser, a shelf hop-up, or a second storey; the bed ports take the switchback. In use the node stands 197 mm along the riser and 206 mm across the run. Bedding will collect in the dish — strip it at the weekly clean (the vents' no-purchase argument assumes the ceiling stays a full standing reach away), and hand-wash only, lukewarm (≤ 50 °C), never the dishwasher: heat deforms the shell, and a deformed tube is a narrowed tube — the material failure mode is the injury failure mode (family charter N7).