Pinhole Disc Holder (Front Board)

1. Purpose

The front board carries the pinhole at the scene-facing end of the container. This report specifies a simple quick-change disc/lens-board holder: an interchangeable Ø80 carrier is clamped against a neoprene light-seal washer by a circular retaining ring held with four thumb screws. Carriers swap in seconds — a pinhole board (any diameter), or a large-format lens board taking a Copal/Compur 0, 1 or 3 shutter — with no tools beyond fingers. It supersedes the standard Ø600 interchangeable pinhole/lens plate: one compact holder does both jobs.

There is no tilt or swing. A pinhole is a point aperture: the image is a central projection through the pinhole point, determined only by the pinhole's position and the film plane, and is completely independent of the orientation of the plate that holds it. Tilting a pinhole board therefore does nothing to the image (it only moves the pinhole point by the small pivot offset). All perspective and keystone control lives in the film-plane mechanism (the rear standard), where tilting the film plane genuinely changes the projection. The earlier tilt-swing front board was retired for this reason — see the changelog.

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2. Mechanism Overview

The holder is a compact front board mounted to the wall-frame adapter with a small bolt pattern (the adapter is part of this design, so it carries the matching pattern). Four elements:

• ICP-01 Front plate — Ø180 round × 18mm 6061-T6. Mounts to the wall-frame adapter (4× M6 @ Ø150); a Ø110 scene-side taper bore converging to a Ø72 light-aperture clearance bore; a Ø82 × 6mm counterbore that seats the carrier and reacts the weight of a lens; four M6 taps on a Ø100 circle (9mm clear of the counterbore wall) for the thumb screws.
• Light-seal washer — Ø82 OD × Ø72 ID × 1.5mm neoprene. The carrier presses against it, sealing the aperture under the clamp.
• Interchangeable carrier (ICP-02) — Ø80 × 5mm 6061 disc (fills the seat): either a pinhole carrier (SS-302 shim over a central bore, pinhole Ø per selection) or a large-format lens board drilled for a Copal/Compur 0, 1 or 3 shutter (see §3). Every carrier shares this one envelope, so the retaining ring seats them all identically.
• Circular retaining ring (ICP-03) — Ø120 OD × Ø70 bore × 6mm 6061-T6. The Ø70 bore is smaller than the carrier so the ring clamps its rim; it clears the light path.
• Four thumb screws — M6 knurled, on the Ø100 circle, into the plate.

Quick-change: loosen the four thumb screws → lift the retaining ring → swap the carrier → replace the ring → finger-tighten the thumb screws. About 30 seconds, no tools.

Interactive 3D model — the Ø180 front plate, neoprene washer, interchangeable carrier, retaining ring and thumb screws. Drag to orbit, scroll to zoom; click the ring to pull the retaining ring + thumb screws out along the optical axis and reveal the interchangeable carrier and the plate's four tapped holes (click again to re-seat).

TBS-001 Pinhole Disc Holder by alvin91403 on Sketchfab

The drawing set (TBS-PDH, 4 sheets): Sheet 1 — general arrangement (front view: plate, retaining ring, thumb-screw + mount circles; the section through the mount is on Sheet 3); Sheet 2 — fabrication blueprints (front plate + retaining ring + carrier); Sheet 3 — wall-frame adapter + a section through the container wall (the mount interface, weld, bolts and the carrier/ring/seal stack); Sheet 4 — the Copal/Compur lens board (the three shutter‑hole options and how the shutter mounts). Sheets 2–4 carry the CNC‑shop tolerance block (§8).

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3. Interchangeable Carriers — Pinhole & Large-Format Lens

Every carrier is a Ø80 × 5mm disc (fills the seat), clamped by the retaining ring, so the camera's front optic changes in seconds.

Pinhole boards

Board	Aperture	Effect
Standard pinhole	Ø2.17mm	Rayleigh optimum for the 2,362mm focal length — sharpest pinhole image (f/1088)
Sharper pinhole	Ø1.5mm	Finer detail, ~2× dimmer (longer exposure)
Brighter pinhole	Ø3.0mm	~2× brighter (shorter exposure), softer image

The Rayleigh-optimum pinhole diameter is d = 1.9·√(f·λ) = Ø2.17mm at λ = 550nm, giving f/1088.

Large-format lens boards

The same Ø80 carrier accepts a lens board drilled for a standard large-format shutter, converting the camera to a lens optic (sharp, fast, controllable aperture). The Ø72 light aperture clears the shutter barrel of the whole Copal/Compur range:

Shutter	Lens-board hole	Fits (Ø80 carrier / Ø72 aperture)?
Copal / Compur 0	34.6mm	✅
Copal / Compur 1	41.6mm	✅
Copal / Compur 3	65.0mm	✅ (Ø65 barrel < Ø72 aperture; ~7.5mm rim on the Ø80 board)

Board-hole diameters per lensnotes.com (Copal 3 = 65.0mm, matching Compur 3) and the Intrepid Camera Large-Format Lens Explorer for 0/1 (§11). The lens board (drill one hole to suit) and how a Copal/Compur shutter mounts in it are drawn on Sheet 4.

Carrying a lens's weight. A large-format lens is cantilevered forward of the board, so the retention is designed to react its weight and tipping moment: the board seats 6mm deep in the Ø82 counterbore (which takes the shear and the moment against its wall — not the clamp), and the Ø120 × 6mm ring is drawn down by four M6 knurled thumb screws (up from M5) for axial clamp force. For a heavy, permanently-mounted Copal 3 lens the thumb screws are simply run down firm; the seat, not the screws, carries the load.

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4. Optical Specification

Parameter	Value	Note
Focal length	2,362mm	Container interior depth
Standard pinhole Ø	2.17mm	Rayleigh optimum, λ=550nm
f-number	f/1088	f / d

The image is a central projection through the pinhole point. Tilt, swing, rise/fall and other perspective movements are provided by the film-plane (rear standard) mechanism — see the Film Plane Mechanism Report, which also holds the (correct) distortion renders.

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5. Light Sealing

The neoprene washer, seated on the floor of the Ø82 counterbore, seals the carrier-to-plate joint. Without it, light through the aperture could leak around the carrier's perimeter — the Ø80 carrier sits in the Ø82 seat with a ~1mm radial gap all round — into the camera. Every carrier is a Ø80 × 5mm disc (§3), so it fills the 6mm seat and stands ~0.5mm proud on the 1.5mm washer; tightening the thumb screws draws the retaining ring flat, pressing the carrier rim and compressing the washer against the seat floor to block the perimeter path. It is a single static compression seal — no moving parts, no bellows. The Ø110 taper bore on the scene side and the Ø72 clearance bore behind the carrier form the light path; the carrier (pinhole or lens) is the only opening. The plate-to-adapter joint is light-sealed by a thin gasket at the mount interface (or an O-ring in the adapter).

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6. Wall Mounting

The pinhole (nose) end wall of the container is corrugated steel, so the front plate cannot seat on it directly. A flat steel wall-frame adapter — Ø240 × 6mm S275, welded over the corrugation — presents the machined flat datum; every alignment reference starts from its face. It carries 4× M6 tapped holes on the Ø150 circle (matching the plate), a Ø120 aperture (clearing the Ø110 taper bore), and sits over a Ø150 aperture cut through the corrugation. The Ø180 front plate bolts to it with 4× M6×24 SHCS at Ø150. Fabrication and the full through-wall stack (weld, bolts, apertures, light path) are on Sheet 3.

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7. Quick-Change Procedure

No special tooling — finger-operated thumb screws.

1. Loosen the four M6 knurled thumb screws (fingers).
2. Lift the circular retaining ring off.
3. Remove the carrier from the Ø82 counterbore seat.
4. Fit the new carrier (a pinhole board, or a lens board) into the seat, against the washer.
5. Replace the retaining ring and finger-tighten the four thumb screws evenly.

Swap time: about 30 seconds. The whole front board can also be unbolted from the wall-frame adapter (4× M6) if the plate itself ever needs service.

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8. Machining Tolerances

Feature	Nominal	Tolerance	Importance
Front-plate carrier counterbore Ø82	Ø82.000	H7: +0.035/0.000	Carrier seats concentric with the optical axis; reacts a lens's weight/moment
Carrier seat depth	6.000mm	+0.1/0.0	Positive location + moment reaction for a lens board
Light-aperture bore Ø72	Ø72.000	+0.2/0.0	Clearance only — clears a Copal 3 shutter barrel (Ø65), must not vignette
Thumb-screw PCD Ø100	Ø100.000	±0.2mm positional	Even clamp of the retaining ring; taps 9mm clear of the Ø82 counterbore wall
Retaining-ring bore Ø70	Ø70.000	±0.1mm	Clamps the carrier rim without fouling the light path
Mount bolts M6 PCD	Ø150.000	±0.15mm positional	Must match the wall-frame adapter

These are carried as a CNC-shop tolerance block on Sheets 2–4: general linear (±0.1 to ±0.3 by size) and angular (±0.5°) tolerances, the bore fits above, PCD positional tolerances (mount + tap), surface finish (mating/seal Ra 1.6, bores Ra 3.2), datum scheme (A = plate camera face, B = Ø82 seat axis), and black hard-anodize after machining.

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9. Parts List

The BOM is single-sourced from the parts registry (parts.py) and generated below. Purchased hardware (thumb screws, mount bolts, washer) is catalog; the raw 6061 stock, the pinhole board set / lens boards, and the fab/finishing services (CNC, anodize) carry SKU pending — source.

Item	Spec	Qty	Supplier	Est. cost
ICP-01 front-plate stock — 6061-T6 round bar Ø190×22	6061-T6 round bar Ø190×22mm — machined to the Ø180×18 front plate (Ø72 aperture, Ø110 scene taper, Ø82 carrier seat 6 deep, 4× M6 mount @ Ø150 + 4× M6 tap). SKU pending — source.	1 ea	Metal Supermarkets / Online Metals	$22
ICP-03 retaining-ring stock — 6061-T6	6061-T6 stock — machined to the Ø120 OD × Ø70 bore × 6 retaining ring (4× M6 clearance @ Ø100). Can be cut from the plate offcut. SKU pending — source.	1 ea	Metal Supermarkets / Online Metals	$8
Neoprene light-seal washer (Ø82×Ø72×1.5) — cut from sheet (398ZW9)	Neoprene washer Ø82 OD × Ø72 ID × 1.5mm — the carrier presses against it under the retaining ring, sealing the aperture. CUT from a Grainger 398ZW9 6in×6in neoprene sheet ($2.13, confirmed 2026-10-02) — one sheet yields the washer + spares. Thickness 1/16in (1.6mm) confirmed — matches the 1.5mm design.	1 6x6 sheet	Grainger	$2
Pinhole carrier set — SS-302 shim on Ø80×5 6061 (Ø2.17 / Ø1.5 / Ø3.0)	Ø80 × 5mm 6061 carrier discs, each with an SS-302 laser-drilled pinhole shim bonded over a central bore — Ø2.17 (Rayleigh optimum), Ø1.5 (sharper), Ø3.0 (brighter). The 5mm carrier fills the seat (same envelope as the lens board) so the retaining ring clamps + seals it. 3 off (the interchangeable set). SKU pending — quote (Lenox Laser + disc blanks).	3 ea	Lenox Laser / Edmund Optics	$60–$120
Ø80 lens board — Copal/Compur 0/1/3 (optional)	Ø80 × 5mm 6061 lens board (fills the seat) drilled for a Copal/Compur 0 (34.6), 1 (41.6) or 3 (65.0) shutter — drops into the holder to run the camera as a lens optic. The large-format LENS itself is user-supplied (out of BOM). Optional. SKU pending — source.	1 ea	SK Grimes / Local fab	$10–$30
M6 knurled thumb screws, 4× (retaining ring) (99607A293)	M6 knurled-head thumb screws — clamp the retaining ring (lens-board clamp force), finger-tightened for quick carrier change. 4 off. McMaster 99607A293 $8.64 ea firm (2026-10-01).	4 ea	McMaster-Carr / Bolt Depot	$35
M6×20 SHCS 18-8 SS, 4× (plate → adapter) (95263A546)	4 off — mount the Ø180 plate to the wall-frame adapter at Ø150. M6×20 SHCS 18-8 SS. McMaster 95263A546 $7.75/25-pack firm (2026-10-01).	4 ea	McMaster-Carr / Bolt Depot	$1
CNC machining — front plate + retaining ring (service)	Machine the Ø180 front plate (Ø72 aperture, Ø110 scene taper, Ø82 counterbore 6 deep, 4× M6 mount @ Ø150 + 4× M6 tap @ Ø100) and the Ø120 retaining ring from 6061-T6. SKU pending — fab quote.	1 job	Fictiv / ProtoLabs	$150–$350
Anodize — front plate + ring (service)	Black anodize the front plate + retaining ring (matte, non-reflective at the aperture). SKU pending — shop quote.	1 job	Pac-Nor Anodizing	$40–$80
Front-Board total				$328–$648

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10. Maintenance

Interval	Action
Each disc change	Inspect the neoprene washer for a clean, unbroken seat; replace if nicked or set
Every 6 months	Check the thumb screws are finger-tight and the retaining ring seats flat
As needed	Store spare pinhole boards flat in a protective sleeve — the SS-302 shim is easily creased

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11. Source References

1. Lenox Laser Precision Pinholes — Pinhole board fabrication (Ø2.17mm and alternates, SS-302 shim).
2. Intrepid Camera — Large-Format Lens Explorer — Large-format lens data and the Copal/Compur 0/1 lens-board hole diameters (34.6 / 41.6mm).
3. lensnotes.com — Lens Boards — Standard board sizes + the full Copal/Compur shutter-hole set; Copal 3 = 65.0mm (matching Compur 3), the value adopted here over Intrepid's 61.5.
4. S.K. Grimes — Lens Board Mountings — how a Copal/Compur shutter mounts to a lens board (front flange + rear retaining ring); the Sheet 4 mount.
5. Film Plane Mechanism Report — Rear standard (film-plane) tilt/swing and the correct distortion renders.
6. Pinhole Report — Pinhole optics and the wall-frame interface.
7. McMaster-Carr — Knurled Thumb Screws — M6 knurled thumb screws for the retaining ring.
8. McMaster-Carr — Neoprene Washers / Rubber Sheet — Light-seal washer stock.