CapRail – Modular Cap Accessory System (5 snap-on attachments, no supports)

CapRail turns the brim of an ordinary baseball cap into a mounting rail — the same idea as a Picatinny rail or a GoPro shoe, but 5 g of plastic and no tools.

A single spring clip grips the brim. On top of it sits a standardised dovetail groove. Everything else just slides in from the side and locks by friction. No screws, no glue, no magnets, nothing to lose.

The parts

#	Part	What it does
01	CapRail Clip	The base. Spring-loaded C-clip for brims 2–4.5 mm thick, with a thumb tab, an internal anti-slip ridge and the dovetail socket. Print one, or one per cap.
02	Sunglasses Holder	Snap-throat slot that grabs a temple arm (3–7 mm). Shades ride on the brim, not on your collar.
03	Action-Cam Mount	Standard GoPro-pattern triple prong (3 × 3 mm fingers, Ø5.2 mm bolt hole). Fits any GoPro-compatible camera, light or phone clamp. Tilt is set by the thumbscrew.
04	Light Cradle	45° V-trough for a torch / bike light / mini cam, Ø10–20 mm. Two 3 mm windows underneath take a zip tie or a rubber band. Points straight forward — an instant head torch.
05	Hanger Hook	10.5 mm throat: hang the cap on a door, a locker edge, a shelf or a shop hook without crushing the crown. Doubles as a cable / mask / lanyard hook while you wear it.
06–08	Wind Spinner	The deliberately useless one the brief asked for. A Ø25 mm 3-cup anemometer rotor that free-spins on a pin. Three parts: base + rotor + press-on cap. It really does spin when you walk.

Why it fits the brief

• It is a fix (the clip adjusts to any brim thickness), a holder, storage, and something completely unnecessary — in one system.

• Everything is wearable and light: the heaviest combination is under 12 g.

• Everything is printable without supports and every STL is already in its correct print orientation — load and slice.

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Which caps does it fit?

Any cap with a brim 2 to 4.5 mm thick: flat-brim snapbacks, curved dad caps, 5-panels, trucker caps. The jaws spring open, so a thinner brim is held by the internal ridge and a thicker one just spreads them a little.

Clamp depth is 15 mm, so put the clip anywhere along the front or the side edge of the brim. Nothing touches the fabric of the crown.

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Print settings

Layer height	0.20 mm
Perimeters	4 (5 for the clip)
Infill	25 %
Supports	none
Brim / raft	not needed
Orientation	as supplied — do not rotate

Material

• Clip (01): PETG strongly recommended. It is a spring — PETG shrugs off repeated opening. PLA works but treat it gently in the cold; ABS/ASA/PCTG are also fine.

• Attachments (02–08): anything. PLA is perfect. A second colour here looks great.

• The Wind Spinner rotor spins best in PLA (stiff, light).

Print time is 20–45 min per part; a complete kit (clip + all five attachments) is roughly 28 g and 4 h.

Nothing else needed except:

• 04 Light Cradle → one zip tie or a rubber band

• 03 Action-Cam Mount → the M5 thumbscrew that came with your camera

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Assembly

1. Push the clip onto the brim, thumb tab facing down and out.

2. Slide an attachment into the dovetail from either side until it stops.

3. To swap, push it back out sideways. That is the whole manual.

Wind Spinner (three pieces): drop the rotor over the post, then press the cap onto the Ø3 mm pin. If the cap is loose, add a drop of glue — the rotor still spins.

Too tight / too loose? The dovetail is a friction fit and printers differ. Scale the attachment by 99.5 % if it will not go on, or 100.5 % if it rattles. Do not rescale the clip.

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Design your own attachment — the CapRail dovetail is open

Every attachment starts from the same 20 mm male rail. Copy it out of the supplied STEP file, or model it from these numbers:

MALE RAIL (on the attachment)
  cross-section : isosceles trapezoid
  width at base : 12.70 mm
  width at top  :  8.90 mm
  height        :  3.80 mm
  length        : 20.00 mm
  mounting face : 5.20 mm above the top of the rail, 18 mm wide
FEMALE GROOVE (in the clip, for reference)
  depth 4.00 mm · mouth 9.00 mm · base 13.00 mm · blind, 21 mm deep
  0.9 mm lead-in chamfer at the mouth
  designed clearance: 0.15 mm per flank

Remixes are very welcome — a fishing-lure keeper, a ball-marker holder, a name plate, a brim extension, a tiny umbrella. Post them and I will link them here.