How Invar and Kovar are machined: tooling, sequence and finishing
Invar 36, Super Invar, Kovar and Alloy 42 cut on the same mills, lathes, grinders and EDM machines as stainless steel. What separates a stable part from one that drifts is the process around the cutting. This page sets out that process with typical values, so you can write it into an RFQ and check that a shop runs it.
We are a matching service, not a machine shop. We match your RFQ with up to 3 shops that already work this way.
The machining sequence for a stable part
Cutting leaves stress and cold work in the surface. In a low-expansion alloy that does two things: the part can move days after inspection and its expansion near the surface rises above the value on the cert. The sequence below spreads the cutting across stages so the stress comes out before the final surfaces are cut. Allowances are typical and depend on part size and wall thickness.
| Stage | What happens | Typical allowance or cycle |
|---|---|---|
| Saw and square the blank | Cut from annealed bar or plate, heat lot recorded. | 2 to 5 mm over size on each face |
| Rough machine | Remove most of the stock, alternating sides of a plate so stress comes out evenly. | Leave 0.5 to 1 mm on critical faces |
| Stress relieve | The part releases roughing stress while there is still stock to clean it up. | Invar 36: 315 to 345 °C, 1 to 2 h, slow cool |
| Semi-finish | Re-establish datums, bring features close to size. | Leave 0.1 to 0.3 mm |
| Finish | Light passes, fresh edges, low clamping force. Grind or lap flat faces. | Final size |
| Stabilize and inspect | Soak to 20 °C before measuring. Optics and metrology parts often get a stabilization soak first. | About 95 °C for 24 to 48 h when specified |
Loose tolerance parts that are not thermally critical can skip the stress relief and go straight from rough to finish. The full reasoning is in stress relief for Invar machined parts.
Work hardening and tooling
Invar 36 and Super Invar are austenitic iron-nickel alloys. Under the tool they behave like 304 stainless: gummy, with long chips and a surface that hardens when the edge rubs instead of cuts. Kovar and Alloy 42 are tougher and chip a little better, but the same rules apply. Machinability is typically 40 to 55 % of B1112 free cutting steel across the four alloys.
The fixes are simple and every shop that cuts these alloys well applies them: sharp positive rake carbide replaced on edge condition rather than breakage, a chip load high enough that every tooth cuts, no dwell at the bottom of a hole or on a finished face, flood coolant (through-tool for deep holes) and low clamping force with vacuum plates or soft jaws so thin parts are not sprung flat.
| Operation | Tooling | Typical speed | Typical feed | Note |
|---|---|---|---|---|
| Milling | Coated carbide, sharp positive rake, small edge hone | 30 to 60 m/min | 0.05 to 0.12 mm/tooth | Climb mill. Keep the chip load up so the edge cuts instead of rubbing. |
| Turning | Positive insert, chip breaker, small nose radius to finish | 40 to 70 m/min | 0.1 to 0.25 mm/rev | Long stringy chips. Chip control decides surface quality. |
| Drilling | HSS-Co or carbide, split point | 10 to 20 m/min | 0.05 to 0.15 mm/rev | Peck, flood or through-tool coolant, no dwell at depth. |
| Tapping | Spiral flute coated tap or form tap | 3 to 8 m/min | Pitch | Thread inserts where threads carry load or are reused. |
| Surface grinding | Friable aluminium oxide or CBN wheel, dressed often | Shop practice | Light passes, spark-out | Gummy chips load the wheel. A loaded wheel burns and stresses the surface. |
Starting values for Invar 36 in the annealed condition. Each shop tunes them to its machines, tools and the part. Kovar typically runs a little faster. See speeds, feeds and tool life for Invar 36 for the detail.
Stress relief and anneal between roughing and finishing
The heat treatment step has a different job on each alloy. On Invar 36 and Super Invar it protects dimensional stability and the low expansion. On Kovar and Alloy 42 it prepares the surface for a glass or ceramic seal. The cycles below are typical; the drawing, the governing spec or your process document sets the real one.
- Invar 36Stress relieve after roughing at about 315 to 345 °C for 1 to 2 h with a slow cool. Optics and metrology parts may call for a full sequence: anneal near 830 °C, water quench, temper at 315 °C, then stabilize at about 95 °C for 24 to 48 h.
- Super InvarNeeds the anneal, quench and stabilize sequence to reach its typical 0.3 to 0.6 ppm/°C band. It can transform below about −60 °C and lose its low expansion, so it is not a cryogenic alloy.
- KovarMachined parts are typically annealed and decarburized in wet hydrogen near 900 to 1100 °C, then oxidized to a controlled oxide before sealing. Oil, embedded grit and burnished surfaces cause seal defects.
- Alloy 42Annealed at about 850 to 1000 °C in a protective atmosphere to soften for forming and relieve stress before sealing or plating.
The heat treatment step adds a furnace run, a second setup and usually a few days to the schedule. Cycles for each alloy sit side by side on the Invar specs reference. To estimate how the step fits your part, try the stress relief planner on the planning tools page.
Dimensional stability after machining
A low-expansion part has to keep its size for months, not only on the day it is measured. Three things decide that.
- Stress removedRoughing stress released in the furnace before finishing, and finishing cuts light enough not to add much back.
- Measured at 20 °CISO 1 sets 20 °C as the reference temperature. Precision parts soak in a controlled room before CMM inspection, and the report states the temperature.
- Stabilized when neededA low temperature soak or thermal cycling between the service limits settles optical benches, reference frames and space hardware.
More on long-term drift in long-term dimensional stability of Invar.
Grinding, EDM and other finishing steps
Milling and turning finish most features. Two other processes take over where they cannot.
Grinding and lapping
Surface grinding finishes faces where flatness itself is the requirement, such as optical mounting pads and reference faces. It always follows the stress relief. Invar loads a wheel, so shops dress often, take light passes with flood coolant and finish with a spark-out. A loaded wheel burns the surface and puts stress back in. Lapping follows when a face must go flatter still.
Wire and sinker EDM
EDM removes metal with sparks, so it applies no cutting force and does not work harden the surface. Wire EDM suits flexures, narrow slots and sharp inside corners. Sinker EDM reaches blind cavities a cutter cannot. It leaves a thin recast layer, so critical surfaces get skim passes. It is slower and costs more per feature than milling.
| Part feature | Usual process | Typical tolerance |
|---|---|---|
| Prismatic bodies, pockets, mounting faces | 3-axis or 5-axis milling | ±0.01 to ±0.025 mm after stress relief |
| Spacers, lens cells, rings, feedthrough bodies | CNC turning, turn-mill | ±0.01 to ±0.02 mm on diameters |
| Hole patterns and threads | Drilling, reaming, tapping, inserts | Position 0.02 to 0.05 mm |
| Flat reference faces, optical mounting pads | Surface grinding, lapping | Flatness 0.005 to 0.01 mm on small faces |
| Flexures, thin slots, sharp inside corners, thin frames | Wire EDM | ±0.005 to ±0.01 mm |
| Blind cavities and internal corners a cutter cannot reach | Sinker EDM | ±0.01 to ±0.02 mm |
| Kovar seal rings, lids, header bodies | Turning or milling, then anneal before sealing | Set by the sealing process spec |
Typical ranges on stable, well supported geometry after stress relief, measured at 20 °C. The shop confirms what it can hold on your drawing. How tolerance moves price: Invar machining tolerances.
Plating and surface finish
Bare Invar resists corrosion better than carbon steel but far less than stainless, and anodizing does not apply to it. Plating adds a few micrometers, so a tight dimension must say whether it applies before or after plating. Any plating bake is a heat cycle, so ask the plater for it on parts that went through a stabilization sequence.
| Alloy | Common finishes | Sequence note |
|---|---|---|
| Invar 36, Super Invar | Electroless nickel, black oxide, passivation, or left bare in a dry room | Plate after the last stress relief. Say whether tolerances apply before or after plating. |
| Kovar | Nickel, then gold, for solder, braze or wire bonding | On glass-to-metal headers, plating usually follows sealing. The package spec sets layer thickness. |
| Alloy 42 | Nickel, tin, silver or gold on lead frames and sealing parts | Often plated at strip or part level after stamping or etching. |
Machined surfaces typically reach Ra 0.4 to 1.6 µm, finer on ground or lapped faces. Options in detail: surface finish and coating for machined Invar.
Design notes that make the process cheaper
- Even wallsBalanced stock on both sides of a plate lets stress come out evenly. Deep one-sided pockets warp.
- Tight only where it mattersEvery tight tolerance adds a pass, a check or a grind. Open the outer profile to ±0.1 mm when it does not locate anything.
- Inside corner radiiA radius a standard end mill can reach avoids an EDM operation. Call out a sharp corner only when the mating part needs it.
- Inserts for threadsThreads in Invar gall and wear. Inserts suit threads that carry load or are reassembled.
- Heat treatment on the drawingWrite the cycle or the spec, or "shop to recommend". A missing note is the most common reason quotes do not compare.
Questions about the process
Why does Invar work harden when it is not a hard material?
Invar 36 is an austenitic iron-nickel alloy, so it behaves like 300 series stainless under the tool. A dull edge or a feed that is too light rubs the surface and leaves a harder skin that also raises the local expansion. Sharp tools, a steady chip load and no dwell prevent it.
When does stress relief happen in the sequence?
Between roughing and finishing. Heat treating after finishing moves the finished surfaces. Thermally critical parts may also get a stabilization soak before final inspection.
Is EDM better than milling for Invar?
Not in general. Wire EDM is the better tool for thin flexures, narrow slots and sharp inside corners because it applies no cutting force. Milling stays cheaper and faster for most features.
Can Kovar be plated straight after machining?
Usually not when the part goes into a glass-to-metal seal. Typical practice anneals and decarburizes the machined part, oxidizes and seals it, then plates nickel and gold. Brazed or soldered parts follow their own spec.
Process guides
- Process
Stress Relief for Invar Machined Parts: When and How Shops Do It
Why Invar parts move after roughing, typical stress relief and stabilization cycles, where they sit in the machining sequence and how to write the drawing note.
- Process
Speeds, Feeds and Tool Life for Machining Invar 36
Typical Invar 36 machining parameters for milling, turning, drilling and tapping: tool material, speed, feed, coolant and a worked 12 mm end mill example.
- Process
Invar Milling Strategy: Toolpaths That Avoid Work Hardening
How toolpath choice, adaptive clearing and climb milling affect work hardening and surface finish when milling Invar 36, beyond basic speeds and feeds.
- Materials
Why Invar Work-Hardens and How Shops Control It During Machining
The mechanism behind work hardening in Invar 36, why it shows up as tool wear and poor finish, and the cutting practices shops use to keep it from taking over a job.
- Process
Invar Grinding: When Milling Isn't Precise Enough
When shops add a grinding step to a machined Invar part, what it achieves that milling cannot, and how it fits into the rough, stress relieve, finish sequence.
- Process
Wire EDM and Invar: When to Use It Instead of Milling
When shops turn to wire or sinker EDM for Invar features that are hard to mill, and how the non-contact process affects stress and dimensional stability.
- Process
Surface Finish and Coating Options for Machined Invar Parts
Typical achievable surface finish on machined Invar 36, why work hardening affects it, and which coatings are used for corrosion protection or emissivity control.
- Process
Invar Fixturing and Workholding: Avoiding Distortion During Machining
Why workholding matters more on Invar parts than on steel or aluminum, and the fixturing practices shops use to avoid clamping distortion on thin or precision features.
Alloy pages: Invar 36 machining, Super Invar machining, Kovar machining, Alloy 42 machining. All process guides.