Optomechanical mounts and breadboard hardware

Parts that bolt to an optical table, join a cage system, hold an optic or a sample in a beam path, or carry a camera or objective.

Verified dimensions are in assets/standards.json:

python scripts/check.py standards --show optical-breadboard-metric
python scripts/check.py standards --show cage-system-30mm
python scripts/check.py standards --show sm1-lens-tube-thread

Ask which system before you model anything

Metric and imperial optical hardware are not interchangeable, and the difference is small enough to look like a rounding error and large enough to prevent assembly.

Over a four-hole span the grids differ by 1.6 mm — far more than any clearance hole absorbs. There is no way to infer which the user has from the request. Ask. If the answer is unavailable, model the mounting features as slots along the bolt line rather than round holes, which tolerates both, and say that is what you did and why.

Mounting to the table

• Use clearance holes, not tapped holes, in the part. The table is tapped; the part is clearanced. For M6 use 6.6 mm (normal fit) in a printed part rather than 6.4 mm — printed holes come out undersize.
• Counterbore for the screw head if the part surface must stay clear: roughly 11 mm diameter for an M6 socket head cap screw, 11.2 mm for 1/4-20.
• Never rely on more than two holes to locate a part. Grid tolerance plus print tolerance means a rigid four-hole pattern will bind. Round hole + slot is the standard fix: one hole locates, the slot takes up the error.
• Printed parts are compliant. For anything where pointing stability matters, a printed mount is a prototyping aid, not a final part — thermal drift and creep in polymer are large compared with optical alignment tolerances. Say so when recommending one.

Posts and pedestals

Common conventions, which vary by vendor — confirm against the catalogue before use:

• Imperial posts are Ø1/2 inch (12.7 mm), typically tapped 8-32 at one end with a 1/4-20 stud or clearance at the other.
• Metric posts are Ø12 mm, typically tapped M4 with an M6 interface to the table.
• A post-holder plus post is height-adjustable but adds a compliant joint; a pedestal or a solid machined riser is stiffer.

Beam height is a project-wide constant, not a per-part choice. Every mount on the table must put its optic at the same height. Common conventions are 3 inches (76.2 mm) or 100 mm, but this is a lab-by-lab choice. Ask for the number, define it once as beam_height_mm, and derive every mount's optic centre from it.

30 mm cage system

The dominant convention for small free-space assemblies:

• Rod spacing 30.0 mm on a square, centred on the optical axis.
• Rods Ø6 mm (ER series).
• Standard cage plates are 0.35 inch (8.9 mm) thick.

For a custom cage plate: place four bores on a 30 mm square, put the aperture at the centroid of those four bores, and bore them for a free-sliding fit at your process's clearance (fabrication-limits.md): about 6.2 mm CNC, 6.4 mm SLA, 6.8 mm FDM. 6.1 mm is a reamed-metal number — printed bores come out undersize, and four bores on a common square over-constrain each other, so tighter is not better here. A cage plate that binds on the rods is worse than useless because it transmits stress into the whole assembly.

Cage plates stack along the rods, so a custom plate's thickness directly consumes optical path length. Budget it.

Lens tube threads (SM series)

SM1 is a 1.035 inch-40 thread, which holds Ø1 inch (25.4 mm) optics. That is a 0.635 mm pitch.

Do not print SM threads. A 0.635 mm pitch is at or below the practical resolution of FDM and marginal on desktop SLA; a printed SM1 thread will either not engage or will gall and shed particles into the beam path. Instead:

• bore a clearance hole and use a purchased SM1 adapter or retaining ring, or
• design for a threaded metal insert, or
• clamp the optic directly with a retaining flange and screws.

If the design truly requires a printed thread, say explicitly that it needs test printing and is likely to fail.

Holding an optic

• Never clamp an optic on its clear aperture. Contact only the outer annulus of the face or the edge. Define clear_aperture_mm as a named parameter and confirm in the snapshot that nothing intrudes on it.
• Three-point contact is kinematically correct and does not deform the optic. A continuous circular seat over-constrains it and induces stress birefringence, which matters for polarisation work.
• Leave clearance for thermal expansion. A metal-in-polymer mount that is a press fit at 20 °C can crack or bind across a temperature swing.
• Retaining forces should be light and distributed. A single set screw pressing on glass is a way to chip glass.

Stray light and scatter

Geometry is not the whole design here, and a STEP file cannot show any of this:

• Printed surfaces scatter strongly. Any surface that sees the beam should be baffled, angled away from the optical axis, or treated.
• Black does not mean non-reflective. Black resin and black filament are often quite specular. Specify a genuinely absorbing surface treatment where it matters.
• Thread and layer lines act as diffraction structures near a focus.
• For fluorescence work, printed material near the sample can autofluoresce into the detection path.

Flag these to the user; do not silently assume a printed enclosure is light-tight.

Checks to run

python scripts/gen.py mount_model.py --outdir out/
python scripts/check.py facts out/mount.step
python scripts/check.py interfaces out/mount.manifest.json
python scripts/snapshot.py out/mount.step --out out/mount.png

Declare the grid pitch, rod spacing, and bore diameters in the model's interfaces() against optical-breadboard-metric, optical-breadboard-imperial, or cage-system-30mm, so the check catches a 25.0-for-25.4 substitution rather than leaving it to a reader.

There is still no automatic bolt-pattern check — the interface check compares dimensions, not hole positions. Compute the pattern in the model from a named grid_pitch_mm constant, and confirm in the snapshot that:

1. All mounting holes are present and pass fully through.
2. The optic aperture is centred where you intended, and unobstructed.
3. Counterbores are on the accessible face.
4. Nothing intrudes into the clear aperture or the beam path.

Sources

• Thorlabs imperial and metric threading — https://www.thorlabs.com/imperial-and-metric-threading
• Thorlabs standard 30 mm cage plates — https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_ID=2273
• Thorlabs SM1 lens tube compatible cage plates — https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=4114
• Post dimensions, beam heights, and vendor-specific thread conventions in this file are common conventions rather than published standards. Confirm against the catalogue.