Kovar and Invar CNC machining calls for a shop that respects what these alloys are for. Both are iron-nickel controlled-expansion alloys, and the parts made from them exist because dimensions or seals must hold steady across a temperature swing. Kovar (ASTM F15, iron-nickel-cobalt) matches the expansion of borosilicate glass and alumina ceramics, so it is the standard for glass-to-metal and ceramic-to-metal seals. Invar 36 has near-zero expansion around room temperature and is used for optical mounts, metrology structures and composite layup tooling. Wexmar machines both at our shop in Canastota, New York.
Machinability of Kovar and Invar 36
Kovar and Invar 36 each carry an approximate machinability rating of about 30 against AISI 1212 steel at 100. They behave much like austenitic stainless steel, only gummier. The material is soft enough to smear, tough enough to resist shearing, and it work hardens where tools rub. Chips come off long and stringy and will wrap a part or tool if chip breaking is not planned. The cost of these alloys is driven by bar price and cycle time together, and both are higher than for common stainless grades.
Tolerances and surface finish
Typical tolerances are plus or minus 0.001 in. on turned diameters and plus or minus 0.002 in. on milled features, with tighter callouts on small Swiss-turned pins reviewed per drawing. Surface finish of 32 Ra is typical on turned seal diameters, and 63 Ra on general milled faces. Sealing surfaces for glass or braze joints are specified carefully, since both roughness and cleanliness affect seal quality. Parts are inspected on CMMs and an optical comparator, and inspection reports are available through our inspection and quality process.
Tooling and chip control gotchas
- Sharp, high-rake tools. Positive, polished carbide geometries shear the material instead of pushing it. Dull edges produce built-up edge, torn finishes and work-hardened skin.
- Chip breaking. Aggressive chip breakers and consistent feeds keep chips short. On the Swiss machines, peck and chip-break cycles keep coils off the guide bushing.
- Coolant. Generous flood coolant controls built-up edge. Parts are thoroughly cleaned after machining because residues interfere with plating, brazing and glass sealing.
- Stress and stability. Invar parts meant to be dimensionally stable are roughed, stress relieved, then finished. Heavy cutting forces and aggressive clamping can leave stresses that shift dimensions later.
- Burrs. Burrs are soft but persistent. Deburring is done with care, since heavy mechanical deburring can cold work edges on seal features.
Heat treat and finish options
Kovar parts destined for glass sealing are commonly given a hydrogen anneal and decarburization cycle before sealing, and heavily cold-worked Kovar is often annealed to keep its expansion behavior predictable at low temperature. Invar 36 is typically annealed, stress relieved and stabilized in stages to lock in dimensional stability. All of these thermal cycles are performed through qualified outside processors. Nickel and gold plating for solderability and wire bonding on Kovar packages, and cleaning to specification, are also handled through qualified outside processors.
Common Kovar and Invar parts
- Glass-to-metal seal pins, feedthrough leads and eyelets (Swiss-type, parts to 32 mm diameter)
- Hermetic package bodies, headers, frames and lids (horizontal machining centers)
- Ceramic-to-metal braze rings and sleeves (CNC lathes)
- Optical mounts, lens cells and laser cavity spacers in Invar 36 (lathes and horizontal mills)
- Metrology fixture components and instrument bases (horizontal milling)
- Waveguide and sensor components that must hold dimension across temperature
Small pins and leads run on our Swiss machining cells, where bar feeders keep long production runs moving. Package bodies and larger Invar components run on pallet-changing horizontals through our CNC milling department.
Cost index vs 6061 aluminum
| Material | Approx. machinability (1212 = 100) | Cost index vs 6061 | Notes |
|---|---|---|---|
| 6061-T6 aluminum | ~180 | 1.0 | Baseline |
| 304 stainless | ~45 | 2.2 | Not expansion controlled |
| 316 stainless | ~36 | 2.5 | Not expansion controlled |
| Ti-6Al-4V | ~22 | 5.0 | Low expansion, light weight |
| Invar 36 | ~30 | 5.5 | Near-zero expansion near room temperature |
| Kovar | ~30 | 6.0 | Matches glass and alumina ceramic |
These values are typical and illustrative for a comparable part; actual quotes depend on geometry, quantity, tolerances and mill pricing.
Applications and industries
Kovar machining services support electronics and electrical/connector makers, hermetic sensor and microwave package builders, and instrumentation companies. Invar 36 parts serve optics, laser systems, test and measurement equipment, and aerospace hardware where thermal growth would throw off alignment. Defense build-to-print work is supported with material certs, heat-lot traceability and first article inspection in AS9102 format. For customers in Central New York and across the country, parts ship by LTL or parcel from Canastota.
Getting a quote
To price precision Kovar turned parts or Invar components, upload a drawing for an instant estimate or call (315) 400-2654. A human estimator confirms every quote and checks bar availability for the specified alloy and condition.