Overview and assembly
All parts are modelled in build123d and exported as STEP AP214 with named components. In Fusion 360 use File > Open (or Upload): each STEP opens as a design with components. A native .f3d can only be saved from inside Fusion 360.
Geometry
| Item | Value |
|---|---|
| Tubes | 3 x D159, stacked, touching; top of stack 2927 mm above floor; light tube bottom 2450 mm |
| Runs (to axis intersections) | left 2470, front 5159, right 4000 mm; straight lengths 2332 / 4883 / 3862 mm |
| Elbows | 2 x 90 deg, R138 to the tube axis; column corner rounded R48.5, clearance tube to column 10.0 mm |
| Light tube section | shell C 310 deg, wall 3.0 mm, opening at the top for the spine; half-lap 1.5 + 1.5 mm, 5 mm long, at every joint |
| Spine | Al profile, 2 LED channels 10.4 x 1.5 mm at +/-10.5 mm, M4 between them into rivet nuts in the stainless tube |
| LED | 2 strips 10 mm (working + spare), cut at 150 mm; 148 pieces |
Mounting principle (all materials)
The spine is screwed to the lower stainless tube. The shell is offered up from below: its J-tabs pass through windows in the spine, the shell slides 55 mm along the axis and a 1:20 ramp lifts it 2.75 mm into the locked position. In the elbow the same motion is a 22.8 deg rotation about the bend centre. No glue and no clips between shell and spine.
Shell pitch along a run is L + 5 mm (body length plus lap). Shells are installed from the far end of the right run towards the left run, because every shell slides towards the right end; the elbow shell is rotated in after the next straight shell is in place. The lap at the end of the elbow has 0.5 mm radial clearance so the rotation does not touch the neighbouring shell.
All routes keep the J-tab positions identical, therefore one spine (14 straight pieces + 2 elbows) serves every shell material. Only the shell splits differ.
Assembly and joint details
Verified (solid intersection, mm3)
| Check | Result |
|---|
Assumptions: stainless wall 2 mm; bar counter simplified. Dimensions on site must be confirmed before cutting. Draft for engineering review, not released for production.
Cast PU (vacuum casting in silicone moulds)
Light-stable aliphatic PU with opal (white diffusing) pigment, cast under vacuum into silicone moulds taken from SLA masters. Straights and elbows in the same material and colour.
Process chain
SLA master per part (sanded, polished outside) → silicone mould → vacuum casting → post-cure → trim of vents and gates (inside surfaces only) → inspection of J-tabs and lap.
Parts adapted to this process
Split per run (mm, in installation order)
Tooling
5 SLA masters and 5 silicone moulds: straight 477, 399, 265, 239 and the elbow. Typical silicone mould life for PU is in the range of 15-25 shots; 20 pieces of 477 may need a second mould.
Pros
- No hard tooling; short lead time; low cost for one installation.
- J-tabs, laps and elbow come out of the mould complete, no machining of the visible surface.
- Elbow and straights in identical material, colour and diffusion.
- Free geometry: draft angles not required in silicone.
Cons
- Piece length limited by the mould (477 mm): 25 straight pieces, 26 joints.
- Colour and diffusion can vary between batches; one batch of resin for the whole job.
- UV and heat resistance below PMMA / PC; long-term yellowing must be checked on the chosen system.
- Tolerances and surface gloss depend on the master finish and mould condition.
Design rules
Wall 3.0 mm. Gloss outside, satin inside. Gates and vents only on the inside or on lap faces. Differential expansion PU against Al over 477 mm at 20 K is about 1 mm and is taken by the 5 mm lap; seams are set at operating temperature.
Confirm with the caster: resin system and datasheet (light stability, HDT, Shore D), mould life, colour sample against our LED strip.
Opal PMMA (profile extrusion + CNC)
Opal PMMA extruded as a C-profile with a continuous J along the full length. CNC removes the J between the tabs and mills the laps. Pieces up to 959 mm (14 straight pieces instead of 25). J-tab positions are identical to the cast route, so the same spine serves both.
Process chain
Profile die → extrusion of opal PMMA (light-diffusing grade) → cut to length → CNC: J removed between tabs, laps at both ends, deburr → protective film on visible surface.
Parts adapted to this process
Split per run (mm, in installation order)
Elbows
The C-profile with J cannot be extruded on a bend, and hot bending to R138 (R/D 0.87) would collapse the section. Elbows: cast PU with colour and diffusion matched to the PMMA (one-off) or injection moulding with a rotating core (series only).
Pros
- Best light transmission and diffusion quality; inherent UV stability, no yellowing.
- Hard, glossy surface; good scratch resistance.
- Long pieces (959 mm): 14 straight pieces instead of 25, fewer visible joints.
- Low cost per metre once the die exists.
Cons
- Custom extrusion die and minimum order quantity.
- Brittle and notch sensitive: J-tabs (3 mm) need generous radii and careful handling; risk of cracking during the slide.
- Stress cracking with alcohol / solvent cleaners.
- Elbows in a different material (cast PU) unless injection moulded.
Design rules
Inner corners of the J at least R0.5. Machined edges deburred and flame- or diamond-polished where visible. Differential expansion PMMA against Al over 959 mm at 20 K is about 0.9 mm, taken by the lap.
Confirm with the extruder: opal grade, light transmission at 3.0 mm, die cost, MOQ, straightness per metre, profile tolerance on the J.
Opal PC (profile extrusion + CNC)
Same geometry and split as PMMA, in UV-stabilised opal polycarbonate. Suitable where impact resistance and fire classification matter.
Process chain
Profile die → drying of the granulate → extrusion of opal PC, optionally co-extruded UV layer → cut → CNC as for PMMA.
Parts adapted to this process
Split per run (mm, in installation order)
Elbows
Injection moulded PC for series, or cast PU matched in colour for a single installation.
Pros
- Very high impact strength: J-tabs do not crack during installation or removal.
- Higher heat resistance than PMMA and PU; better margin near the LED.
- Flame-retardant grades available (check the required classification).
- Long pieces (959 mm), same split as PMMA.
Cons
- Yellows under UV unless stabilised or co-extruded with a UV layer.
- Softer surface: scratches more easily than PMMA.
- Lower light transmission than PMMA at the same diffusion.
- Custom die and MOQ; granulate must be dried; sensitive to some cleaners.
Design rules
As PMMA. Differential expansion PC against Al over 959 mm at 20 K is about 0.9 mm.
Confirm with the extruder: UV-stabilised opal grade, fire classification, transmission at 3.0 mm, die cost and MOQ.
3D printed shell (FDM)
For first articles, fit tests and short runs. Every PU piece is split in two with the same 5 mm lap, so each part fits a printer with about 250 mm Z height when printed with the tube axis vertical. The elbow is split at 45 deg so each half prints without support.
Process chain
Slicing with the axis vertical, perimeters only (no infill), wall 3.0 mm as 7 perimeters of about 0.43 mm → print in translucent / opal PETG (or PC blend for higher temperature) → remove seam, light sanding of the J faces if needed.
Parts adapted to this process
Split per run (mm, in installation order)
Pros
- No tooling; parts in days; any length or change at no extra cost.
- Ideal for the first article: fit on the real spine, assembly test, light test with our LED strips.
- Elbow halves: the 45 deg lap is a revolved surface, so the halves rotate in without touching.
Cons
- Layer lines show when the light is on; diffusion less uniform than PMMA / PC.
- Twice as many joints (50 straight pieces + 4 elbow halves).
- PETG softens around 70 deg C; keep LED power density moderate or use a PC blend.
- Dimensional accuracy about +/-0.2 mm; J-tabs need tuning on the first prints.
Recommended use: one elbow module with two straight neighbours as the first article for every route.
Spine (part 1) and stainless tubes
Common to all shell materials. The spine carries the LEDs, takes the heat and defines the position of every shell.
Spine options
| Option | Use | Pros | Cons |
|---|---|---|---|
| Al 6063-T5 custom extrusion + CNC | series, repeat orders | profile with chambers and LED channels in one; lowest cost per metre; anodised | die cost and MOQ |
| Al 6061-T6 CNC from bar | one installation | no die; short lead time | more machining time; chambers must be simplified or milled from two sides |
| Printed (PA-CF or PETG) | fit test only | fast, cheap | creep under the J-tabs, no heat sink for the LED; not for the final installation |
Spine details
LED channels 10.4 x 1.5 mm at +/-10.5 mm, floor 2.2 mm. M4 clearance 4.5 mm with 8.5 mm counterbore on the axis; first hole round, others slotted 6 mm for expansion. Joints with two push-fit connectors 10.4 x 9.6 x 80 in the lower chambers. Every joint is at least 7 mm away from a window or ramp. 14 straight pieces 725-845 mm, each with its own window positions (see position table).
Elbow: CNC from plate, integral tongues 10.4 x 9.6 x 40 at both ends, two windows per side at 33.5 and 77.2 deg. A guide plate under the spine holds the LED strips straight: two channels 10.2 mm wide, separated by a 0.8 mm rib; the working strip has a 176 mm channel (150 mm strip, +/-13 mm adjustment on site), the spare strip a 150 mm channel.
Parts
Stainless tubes
D159, two rows, wall 2 mm assumed. The elbow R138 (R/D 0.87) is not a stock elbow: stock D159 elbows are R216. R138 requires a custom elbow (mandrel bending, segmented welded or pressed half-shells); one sample is required before series. If the elbows are joined with an internal sleeve, add a straight end of at least 100 mm; for an invisible joint use shop TIG welding with a fixture.