Optomechanical parts are the machined mounts, housings, baseplates and cooling plates that hold optical elements in position and carry heat away from them. For photonics and laser systems the material is chosen by thermal expansion, thermal conductivity and stiffness, and the drawing has to control flatness, surface finish and coating, because small dimensional changes move the optical axis.
What do optomechanical parts do in a photonics or laser system?
An optical assembly asks its metal parts for three things. Mounts, retaining rings, lens tubes and adapters locate the optics. Baseplates, housings and frames tie the mounts together and fix their distances. Cooling plates and heat sinks carry away the heat of diodes, crystals and absorbers.
Baseplates, housings and cooling plates are milled from plate or block. Rings, tubes and adapters are turned. The drawing comes from the optical designer, so the supplier's task is to hold the stated geometry and to ask about the points the drawing leaves open. Our page on photonics and laser systems covers this field.
Which materials are used for optomechanical parts?
The table compares five metals that appear on optomechanical drawings. It gives room-temperature data from manufacturer data sheets. The last column is stiffness per mass (modulus of elasticity divided by density), which we calculated from the other two columns.
Table 1. Properties of common optomechanical metals
| Material | Density, g/cm³ | E, GPa | CTE, ×10⁻⁶/K | k, W/(m·K) | E/ρ |
|---|---|---|---|---|---|
| EN AW-6061, T6/T651 | 2.70 | 68.3 | 23.6 (20–100 °C) | 167 | 25.3 |
| EN AW-7075, T651 | 2.80 | 71.0 | 23.4 (20–100 °C) | 130 | 25.4 |
| 316L, EN 1.4404 | 7.9 | 190–200 | 16.5 (20–100 °C) | 15.0 | 24–25 |
| Invar 36 | 8.05 | 141 | 1.30 (mean to 93 °C) | 10.5 | 17.5 |
| Cu-OFE, EN CW009A | 8.94 | 127 | 17.7 (20–300 °C) | 394 | 14.2 |
Source: data sheets of Proto Labs (6061), Kaiser Aluminum (7075, T651), Marcegaglia Fagersta (316L), High Temp Metals (Invar 36, annealed bar) and Aurubis (Cu-OFE). Typical values, not design minimums. E/ρ calculated.
Aluminium is the first material to check. 6061 and 7075 have the same stiffness per mass within 1 %, so 7075 is chosen for its strength and wear behaviour, not for rigidity. The aluminium alloy comparison covers strength minimums, welding and anodizing. Conductivity depends on temper: Kaiser lists 130 W/(m·K) for 7075-T651 and 155 W/(m·K) for 7075-T7351.
316L has a stiffness per mass close to aluminium but conducts only 15.0 W/(m·K), so heat stays where it is produced. Marcegaglia describes it as non-magnetic when annealed, which matters near magnetic sensors, and it becomes slightly magnetic after cold work.
Invar 36 is the low-expansion choice. It pays for that with density, low conductivity and 31 % less stiffness per mass than aluminium. Copper is the heat-spreading choice at 394 W/(m·K) and the heaviest of the five.
How much does thermal expansion move a mount?
Linear growth is length × CTE × temperature change. A 300 mm baseplate that warms by 5 K grows 35 µm in 6061 and 2 µm in Invar 36. What counts in a lens mount is the difference between the metal and the glass it holds.
Table 2. Diameter growth of a 25 mm retaining ring and the optic, ΔT = 20 K
| Ring material | Ring growth, µm | Ring minus N-BK7, µm | Ring minus fused silica, µm |
|---|---|---|---|
| EN AW-6061 | 11.8 | 8.3 | 11.5 |
| 316L | 8.3 | 4.7 | 8.0 |
| Invar 36 | 0.65 | −2.9 | 0.4 |
Source: calculated from Table 1, SCHOTT N-BK7 (7.1 ×10⁻⁶/K, −30 to +70 °C: growth 3.55 µm) and Corning HPFS 7980 fused silica (0.52 ×10⁻⁶/K, 5 to 35 °C: growth 0.26 µm). First-order estimate; the CTE values refer to different temperature ranges.
An aluminium ring around a BK7 lens opens 8.3 µm more than the lens grows when the assembly warms by 20 K, which loosens the seat. The negative sign for Invar is the point of the table: an Invar ring around BK7 grows 2.9 µm less than the glass, so the glass presses against the ring. Low expansion alone is therefore not the criterion. Pair the CTE of the mount with the CTE of the optic, and check the seat at both ends of the operating range.
How do you keep a machined optomechanical part stable?
Material removal releases locked-in stress, and a thin or asymmetric part can bow after the last cut. For aluminium, order plate in a stress-relieved temper. The digit 51 in T651 means stress relieved by stretching (EN 515:2017, clause 8.3.1), and EN 515:2017 was confirmed in 2022. For Invar and other low-expansion alloys, name the heat-treatment condition of the stock on the drawing. For parts that must hold flatness, rough machine first, release the stress, then finish.
Write form and position requirements with ISO 1101:2017, which ISO confirmed in 2022, and tie them to named datums. General tolerances such as ISO 2768-m do not suit a mounting face or a locating bore. Our network works to ISO 2768-m as standard and tighter where the drawing specifies it, so every interface dimension needs its own entry. The tolerance handbook shows how general tolerances, fits and GD&T combine on one part.
Measurements are valid only at the standard reference temperature of ISO 1:2022. For parts that expand by 23.6 ×10⁻⁶/K, ask that the inspection report state the room temperature, because a 3 K offset on a 200 mm part is 14 µm.
Which surface finishes and coatings do mounts and housings need?
Table 3. Finish, standard and what the drawing must state
| Purpose | Finish | Standard | Drawing must state |
|---|---|---|---|
| Light-absorbing, corrosion-protected housing | Black anodizing | ISO 7599:2018 | Coating thickness, colour, sealing, dimension before or after coating |
| Wear on threads, guides, sliding faces | Hard anodizing | ISO 10074:2021 | Coating thickness, masked areas |
| Mounting and sealing faces | Machined | ISO 21920-2:2021, ISO 1101:2017 | Roughness parameter and acceptance rule, flatness to a datum |
Source: ISO catalogue entries. ISO 7599:2018 confirmed 2023; ISO 10074:2021 is the fourth edition; ISO 21920-2:2021 replaced ISO 4287:1997 and is marked for revision since 2025-10-07. Status as of 2026-10-30.
Black anodizing is specified where a housing or mount must not reflect stray light. The coating changes dimensions, so tight bores and flat faces are specified after coating, and areas that need electrical contact or a bare interface are masked. The surface treatment guide gives the growth per surface. For roughness, the Ra and Rz article explains why the edition of the standard decides how a value is evaluated.
How are cooling plates and baseplates specified?
Copper conducts 394 W/(m·K) against 167 for 6061, a factor of 2.4, but weighs 3.3 times as much. Per kilogram, 6061 carries more heat: 61.9 against 44.1 W/(m·K) per g/cm³ (calculated). Copper wins where space is tight and weight does not matter, aluminium where mass counts.
A cooling plate is usually a milled channel and a lid. The lid is welded or screwed on with a seal, and every joint is a potential leak path. The drawing, not the machinist, has to name the coolant and its pressure, the leak test method and its acceptance limit, and the cleanliness required after machining. Mixed copper and aluminium parts in one coolant circuit are a material-compatibility question for the system designer, not something a machine shop can settle from the drawing.
For baseplates, decide the hole system early. Metric breadboards use M6 holes on a 25 mm grid; inch breadboards use 1/4-20 UNC holes on a 25.4 mm grid (OptoSigma lists both). Ten pitches differ by 4 mm, so a plate drilled to one system cannot take components made for the other.
Do parts for laser systems fall under export controls?
Regulation (EU) 2021/821 sets the EU regime for dual-use items. Annex I is the common list, and its Category 6 is titled "Sensors and lasers". The Commission replaces Annex I by delegated regulation as the international control regimes change: Delegated Regulation (EU) 2025/2003 was adopted on 8 September 2025, and the Commission's page records a further update adopted on 14 September 2026. Check which version applies on the date of shipment.
Whether an item is listed depends on its technical specification as written in Annex I. The material and the machining process do not decide it. If your system is or may be a controlled laser, state the classification and the end use in the RFQ. We machine to the buyer's drawing and do not offer custom design of parts for high-power laser systems.
What should an RFQ for optomechanical parts contain?
- Material with temper, for example EN AW-6061 T651, and the heat-treatment condition for Invar.
- The pairings where CTE matters (metal and glass, metal and metal) and the operating temperature range.
- Flatness, parallelism and position to ISO 1101:2017 with datums, and roughness to ISO 21920-2:2021 with the acceptance rule.
- Coating standard, thickness, colour, masking, and whether dimensions apply before or after coating.
- Thread system (M6 or 1/4-20 UNC) and the grid of any hole pattern.
- For cooling plates: coolant, pressure, leak test method and acceptance limit.
- Inspection temperature and report format, and the export classification if one exists.
Two of our guides cover the first three points: the tolerance handbook for geometry and the aluminium alloy comparison for material and temper.
Frequently asked questions
- Which aluminium alloy is used for optomechanical mounts?
- EN AW-6061 in the stress-relieved T651 temper is the usual starting point: 2.70 g/cm³, 167 W/(m·K), 23.6 ×10⁻⁶/K between 20 and 100 °C. EN AW-7075 has almost the same stiffness per mass and a lower conductivity, 130 W/(m·K) in T651, so it is chosen for strength, not for stiffness or heat.
- Why is Invar used in optical assemblies?
- Invar 36 expands by a mean of 1.30 ×10⁻⁶/K up to 93 °C, against 23.6 ×10⁻⁶/K for 6061. A 300 mm baseplate that warms by 5 K grows about 2 µm in Invar and about 35 µm in 6061. Invar is dense (8.05 g/cm³) and conducts only 10.5 W/(m·K).
- Does anodizing change the dimensions or flatness of a mount?
- Yes. The oxide layer builds up on the surface and grows into it, so a bore or a mounting face moves by part of the coating thickness. State on the drawing whether a dimension applies before or after coating, and measure flatness after coating.
- Is ISO 4287 still valid for surface roughness?
- No. ISO 21920-2:2021 replaced ISO 4287:1997 in December 2021. A drawing that says only Ra without a standard edition and acceptance rule leaves the evaluation open.
- Should a breadboard hole pattern be metric or inch?
- Both exist. Metric breadboards use M6 holes on a 25 mm grid, inch breadboards use 1/4-20 UNC holes on a 25.4 mm grid. The 0.4 mm difference per pitch adds up to 4 mm over ten holes, so a plate drilled to one grid does not fit components made for the other.
- Do optomechanical parts need an export licence?
- Only if the item is listed in Annex I of Regulation (EU) 2021/821, where Category 6 covers sensors and lasers. Classification depends on the technical specification of the item, not on the material or the machining process.
Sources
- Aluminum 6061-T651/T6 material data sheet, 2026 (Proto Labs)
- Sheet, coil and plate alloy 7075, technical data, rev. 05/06 (Kaiser Aluminum, copy hosted by Proto Labs)
- EN 1.4404 (316L) data sheet, ENG 2025 edition 2 (Marcegaglia Fagersta Stainless)
- Invar 36 technical data (High Temp Metals)
- Cu-OFE (PNA 203, EN CW009A) material data sheet, DE_2026_01 (Aurubis)
- SCHOTT N-BK7 datasheet, as of 2023-12-01, in the Optical Glass Datasheets collection (SCHOTT)
- Corning 7980 HPFS fused silica standard grade data sheet (Corning, copy by UQG Optics)
- ISO 1101:2017, geometrical tolerancing, edition 4 (ISO catalogue, reviewed 2022)
- ISO 1:2022, standard reference temperature for geometrical and dimensional properties (ISO catalogue)
- ISO 21920-2:2021, surface texture: profile, terms and parameters, replaces ISO 4287 (ISO catalogue)
- ISO 7599:2018, decorative and protective anodic oxidation coatings (ISO catalogue)
- ISO 10074:2021, hard anodic oxidation coatings (ISO catalogue)
- EN 515:2017, wrought aluminium products, temper designations (Genorma, CEN catalogue data)
- Optical breadboard M6, 450 mm × 600 mm, hole pitch 25 mm and 25.4 mm (OptoSigma)
- Exporting dual-use items, Regulation (EU) 2021/821 and Annex I updates (European Commission, DG Trade)
- Regulation (EU) 2021/821, table of contents with Annex I Category 6 "Sensors and lasers" (Lexaris copy of the Official Journal text)
Questions on a drawing? sales@ertas.engineering