A 300 by 200 mm optical bench in Invar, from alloy choice to quote

By Invar Machining editorial team· Published September 26, 2026 · How we write these guides

This is an illustrative example, not a real order. Every number below is chosen to show the reasoning, and no part like it has been quoted or built through this site. Take a hypothetical optical bench, 300 × 200 × 25 mm, in a lab held at 20 ± 5 °C, with mount positions that must stay within 5 µm of each other. Aluminum and steel fail that requirement on thermal growth alone. Invar 36 meets it with margin. Super Invar meets it with more margin than the budget needs. The rest of the example walks through the machining sequence, heat treatment, tolerances, inspection and the shape of a quote.

The hypothetical part and its requirement

The bench is a plate with a grid of M6 tapped holes on 25 mm centres for general mounting and four precision bores of 10 mm H7 that locate a laser source and three mirror mounts. The underside carries a mounting face that sits on three kinematic pads. The optical requirement is that the four bores hold their relative position within 5 µm over the lab temperature range and over the life of the instrument. The bores span 240 mm along the plate.

Five microns over 240 mm is a stability requirement, not a machining tolerance. The bores can be machined to any position the drawing calls for. The question is whether they stay there when the room changes by 5 °C and when the plate sits for a year. That splits the budget into two parts: thermal growth, which the alloy controls, and drift from residual stress, which the machining sequence controls. A reasonable split is 2.5 µm to each.

Alloy choice with growth numbers

Growth equals length times CTE times temperature change. The bore span is 240 mm and the swing is 5 °C either side of nominal. The table takes the midpoint of each typical CTE range.

Material	CTE, typical, ppm/°C	Growth over 240 mm for 5 °C	Against a 2.5 µm thermal allocation
6061 aluminum	23 to 24	28 µm	Fails, 11 times the allocation
Carbon steel	11 to 12	14 µm	Fails, 5.5 times
316 stainless	16 to 17	20 µm	Fails, 8 times
Titanium	8.6	10 µm	Fails, 4 times
Invar 36, annealed	1.2 to 1.6	1.7 µm	Passes with margin
Super Invar	0.3 to 0.6	0.5 µm	Passes with large margin

Typical handbook values at room temperature. The mill certificate governs for a given heat lot, and a stressed or cold worked part sits above these values.

Two readings follow. First, no conventional structural metal other than the iron nickel alloys gets near the allocation, so the choice is between Invar 36 and Super Invar. Second, Invar 36 at 1.7 µm uses about two thirds of the thermal allocation, which is comfortable if the plate is properly stress relieved and would be tight if it were not. Super Invar would leave more margin at a higher material cost and with fewer mills to buy from. For a lab at ±5 °C, Invar 36 is the reasonable choice. If the same bench had to hold 2 µm, or sat in a room swinging ±10 °C, the answer would tip toward Super Invar, and the guide Super Invar: when Invar 36 is not enough sets out that line.

Note that the thermal growth is uniform. If every bore grows together, the relative positions change by the differential, which is smaller than the absolute figure. The table is conservative on purpose, because a bench rarely sits at one uniform temperature.

The thermal expansion figures for the comparison materials come from standard handbook data, and the alloy comparison guide gives the Invar family side by side. The optical mounts and telescope structures guide covers where benches like this one are used.

Machining sequence

The plate starts as Invar 36 to ASTM F1684, 32 mm plate cut to 310 × 210 mm. A typical sequence:

1. Rough. Face both sides to 27 mm, mill the profile to 302 × 202 mm, drill the tapped hole grid, drill the four precision bores to 9 mm. Everything carries 1 mm of stock on the faces and 0.5 mm on the bores.
2. Stress relieve. Furnace, typical cycle 315 to 345 °C for 1 to 2 h, slow cool. The parts sit on a flat hearth, unclamped.
3. Semi finish. Face both sides to 25.4 mm, profile to size, tap the grid, bore the four precision holes to 9.8 mm.
4. Stabilize. Second, lower temperature cycle if the drawing calls for it. Many optics drawings call for a hold at about 95 °C for 24 to 48 h after the stress relief. The drawing governs.
5. Finish. Skim both faces to 25.0 mm with light passes and sharp tools, bore the four holes to 10 mm H7, deburr, clean.
6. Inspect. Soak at 20 °C and measure on a CMM.

The order is the point. Every heavy cut happens before the furnace and every light cut after it. The bores that carry the requirement are finished last. Fixturing throughout is low force: vacuum or light clamps on the faces, never a vice across the plate. The stress relief sequence guide explains what happens when the order is changed.

Heat treat note for the drawing

A workable note reads: “Stress relieve after roughing, 315 to 345 °C, 1 to 2 h, furnace cool, parts unclamped. Stabilize after semi finish per [cycle reference or ‘shop to recommend’]. No heavy machining after stress relief. Record furnace charts and supply with parts.” Writing “shop to recommend” is acceptable when you do not have a house cycle. Writing nothing is not, because two shops will then quote two different parts.

Tolerance callouts

The drawing carries a datum scheme and tolerances only where they matter.

Feature	Callout, typical	Reason
Underside mounting face	Datum A, flatness 0.010 mm over the three pad zones	Kinematic seat, not the whole face
Long edge	Datum B, perpendicular to A within 0.02 mm	Orientation only
One precision bore	Datum C, 10 H7	Pattern origin
Other three precision bores	10 H7, position 0.010 mm diameter to A, B, C	The functional requirement
Top face	Parallel to A within 0.020 mm	Mount seating
M6 grid	Position 0.2 mm	General mounting, loose on purpose
Outer profile	±0.1 mm	Non-functional

Typical callouts to ASME Y14.5. The actual drawing depends on the instrument.

The position tolerance of 0.010 mm diameter on the bores is the machining part of the 5 µm requirement. It says the bores are placed within 5 µm radius of nominal at 20 °C. The drawing note on heat treatment and a measuring temperature note, “dimensions apply at 20 °C”, carry the stability part. The Invar machining tolerances guide covers what shops hold and what these numbers cost.

Inspection plan

The bench is measured after a 12 h soak in a 20 °C metrology room, resting on three points at the pad positions. The CMM program picks up A, B and C, then measures the four bores for diameter and position, the flatness of the pad zones, the parallelism of the top face and the perpendicularity of B. The report lists each feature with nominal, measured, deviation and tolerance, plus the room temperature at the time. For a first article the buyer might add a thermal cycling check, three cycles between 10 and 30 °C with re-measurement, to confirm that the plate has stopped moving. On repeat parts a CMM measurement of the bores and the pad flatness is enough. The CMM inspection report guide shows what a usable report looks like.

Hypothetical quote structure

The shape of a quote for two of these benches, as a share of the total rather than in currency, might look like this. The figures are invented to show the proportions.

Line	Share of total, hypothetical	What drives it
Material, two plates 310 × 210 × 32 mm, about 16.8 kg each	30 to 40 %	Nickel price, plate premium, cut charge
Programming and fixtures	10 to 15 %	Paid once, spreads over repeats
Roughing and semi finishing	15 to 20 %	Slow cutting, two setups
Stress relief and stabilize	5 to 10 %	Furnace loads, charged per load
Finishing and boring	10 to 15 %	Light passes, H7 bores
Inspection and report	5 to 10 %	Soak time and CMM time
Cleaning, packing, certificates	2 to 5 %	Depends on requirements

Hypothetical shares for a batch of two. On a batch of twenty the programming line shrinks to a few percent and material rises toward half. The Invar 36 machining cost guide explains each line, and the cost per kilogram guide covers the material line.

Plate mass check: 310 × 210 × 32 mm is 2.08 litres, at 8.05 g/cm3 about 16.8 kg per bench, for a finished part of about 11.5 kg after the faces, profile and holes. That is a buy to fly ratio near 1.5, which is good for a plate part and is one reason this design is cheap to make in Invar relative to its size.

What this example would look like on the RFQ

The RFQ for a bench like this contains the drawing with the datum scheme and the heat treatment note, a 3D model, the quantity and whether it repeats, the alloy and spec, the operating temperature range, the inspection required on first article and on repeats and the certificates needed. It also says what the part does, in one sentence, because a shop that knows the bores carry a laser and three mirrors will finish them last without being told. When you send the RFQ, we match you with up to three shops that cut Invar 36, run the furnace sequence and measure at 20 °C, and each one quotes the same part. For the specs named here, see the Invar specifications reference and the Invar 36 machining page.

Frequently asked questions

Reviewed against

• Wikipedia, Thermal expansion
• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Invar for Optical Benches and Telescope Structures: Why CTE Stability Matters · Materials
• Stress Relief for Invar Machined Parts: When and How Shops Do It · Process
• Invar Machining Tolerances: What a Shop Can Hold and What It Costs · Process
• Why Invar 36 Parts Are Costly and How Shops Price Them · Cost

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