Where Aerospace Programs Use Invar 36 Tooling and Fixtures
A drill template or an assembly jig does not need to survive a thermal cycle the way a flight part does. It needs to stay the same size on a Tuesday afternoon as it was on a Monday morning, across whatever temperature range the shop floor or the hangar actually sees. That requirement, not exotic thermal performance, is why some aerospace assembly tooling is built from Invar 36 instead of tool steel or aluminum. This guide covers structural tooling specifically: jigs, drill templates and test fixtures, not the composite layup molds already covered in Invar for composite layup tooling, which is a related but different use case. For the broader picture of where these alloys show up in the industry, see the aerospace and defense overview.
Assembly jigs, drill templates and structural test fixtures, not layup tooling
It is worth separating these two uses clearly because buyers sometimes land on this page looking for the other one. A composite layup mold or mandrel is shaped like the part and its surface finish transfers directly to the laminate during an autoclave cure. A structural assembly fixture is shaped like nothing in particular. It is a frame, a plate or a set of locating pins that holds sub-assemblies or drilling guides in a fixed spatial relationship while work happens around them. The two uses share an alloy and a reason, low expansion, but almost nothing else about the tooling itself.
Typical structural tooling applications include:
- Drill templates and jig bushings that fix the position of a hole pattern across a wing panel, a fuselage skin or a bracket, used repeatedly across a production run.
- Assembly jigs and holding fixtures that locate multiple sub-assemblies relative to each other before fastening, riveting or bonding.
- Structural test fixtures that apply or react load on a test article while instrumentation reads position or strain, where the fixture’s own movement would corrupt the measurement.
- Inspection and gauge fixtures used to check finished assemblies against a master geometry.
Why fixture growth shows up as measurement error, not fixture damage
A tool steel drill template with a typical CTE around 11 to 13 ppm/°C does not fail mechanically when it warms up 10 °C. It grows, uniformly and predictably, by a small fraction of a millimeter across its span. The template still looks correct, still feels rigid and still drills a clean hole. The hole is simply in a slightly different place than it was when the template was calibrated or when the previous shift used it at a different shop temperature.
That is the core problem with using an ordinary metal for precision tooling: the failure mode is invisible until the parts it produced are measured or assembled against parts made on a different day, a different shift or a different tool. The general behavior behind this, how much a material’s length changes per degree, is its coefficient of thermal expansion, described in the Wikipedia entry on thermal expansion. Invar 36’s typical CTE of 1.2 to 1.6 ppm/°C over 20 to 100 °C, described in more detail in Invar 36 CTE vs temperature, reduces that growth by roughly a factor of ten against steel, which is why it shows up in tooling whose whole job is staying put. Invar 36’s own specification, ASTM F1684, is the one most tooling shops quote against; see the ASTM F1684 material specification for the formal scope.
| Tooling material | Typical CTE, ppm/°C | Growth of a 2 m span over a 10 °C swing, mm |
|---|---|---|
| 6061 aluminum | 23.5 | 0.470 |
| Mild or tool steel | 11 to 13 | 0.220 to 0.260 |
| Invar 36 | 1.2 to 1.6 | 0.024 to 0.032 |
Values are typical handbook figures for the alloys; a specific tool steel grade or a specific Invar 36 heat lot may vary. The reference page of Invar specifications lists the sourced CTE bands for the low expansion alloys in this family.
The precision and size threshold where Invar 36 earns its cost
Invar 36 costs more than tool steel per kilogram, machines more slowly because it is gummy and work hardens, and usually needs a stress relief in its own process, all covered in Invar fixturing and workholding. None of that is worth paying for on every fixture in a shop. The decision comes down to two factors multiplying each other: the fixture’s physical size and the tolerance it is meant to hold.
| Factor | Steel or aluminum usually fine | Invar 36 worth evaluating |
|---|---|---|
| Fixture span | Under about 0.5 m | Over about 1 m |
| Tolerance the fixture controls | Tenths of a millimeter or looser | Tens of microns or tighter |
| Shop temperature control | Tight, stable HVAC | Ordinary shop floor or hangar swings |
| Fixture service life | Short run, recalibrated often | Long service, years of repeated use |
| Consequence of drift | Rework caught at inspection | Assembly interference or scrap discovered downstream |
A small drill jig holding a ±0.2 mm hole position in a temperature controlled shop rarely needs Invar 36. A 1.5 m wing panel drill template holding hole position to ±0.05 mm, used across shifts and seasons in a hangar without tight climate control, is squarely in the range where the added material and machining cost buys real protection against a problem that is otherwise expensive to trace back to its source.
The middle ground is where most of the argument happens. A 1 m fixture holding a ±0.1 mm tolerance in a moderately controlled shop could reasonably go either way, and the honest answer often depends less on the alloy than on how the program plans to catch drift if it happens. A test program with a tight inspection cadence and a master reference to check against can sometimes tolerate a steel fixture at that size, because drift gets caught before it propagates into scrapped hardware. A production line without that check has less margin for the wrong guess.
Building and stabilizing a large Invar 36 fixture
Tooling-sized Invar 36 fixtures raise the same fabrication questions as large composite tools. Bigger spans are frequently built from welded and machined sections rather than a single billet, both for material cost and because very large solid stock is hard to source, a topic covered in the welding versus machining comparison referenced from the composite layup tooling guide. Whatever the construction method, a structural fixture needs the same staged sequence as a precision part: rough with an even allowance, stress relieve, finish with light cuts and inspect at a controlled temperature, because a tooling fixture that was never stress relieved can drift on its own schedule independent of the temperature swings it was built to resist.
Because a fixture is used for years, not shipped once and forgotten, periodic re-verification against a master reference is worth planning into the program from the start rather than added after a discrepancy shows up in production parts.
Worked example: a 2 m drill template across a 20 °C shop swing
Take a hypothetical drill template, 2 m long, used to locate a hole pattern along a wing panel spar. The hangar it lives in is not climate controlled to aerospace metrology standards, and the tool sees a temperature swing of about 20 °C between a cold morning shift and a warm afternoon shift in summer.
Growth of a 2000 mm span over a 20 °C swing = 2000 mm × CTE × 20.
| Template material | Typical CTE, ppm/°C | Growth over 2 m, mm | Effect on a ±0.05 mm hole position tolerance |
|---|---|---|---|
| 6061 aluminum | 23.5 | 0.940 | Fails by roughly 18 times the tolerance band |
| Tool steel | 12 | 0.480 | Fails by roughly 9 times the tolerance band |
| Invar 36 | 1.4 | 0.056 | Close to the tolerance band on its own, before considering other error sources |
Values and the scenario are illustrative, not a real program’s data. Even Invar 36 in this hypothetical case is not free of the problem; it simply moves the error into a range where fixturing discipline, careful stress relief and periodic recalibration can manage it, instead of a range where the fixture’s own material choice guarantees an out of tolerance result on any day the shop temperature moves.
What to put on the RFQ
- Fixture function. Say whether it is a drill template, an assembly jig or a test fixture, and what tolerance it must hold on which features.
- Expected temperature range in service. A climate controlled metrology lab and an unheated hangar bay call for different material decisions.
- Fixture size and construction preference. State the overall envelope and whether welded and machined construction is acceptable for large spans.
- Service life and re-verification plan. Say how long the fixture needs to stay in calibration and whether periodic re-inspection is part of your program.
- Stress relief and stabilization requirement. State it explicitly or ask the shop to recommend a cycle for a tooling-duty part.
- STEP file, PDF drawing and the master geometry the fixture locates against, if one exists.
When you send an RFQ with the drawing, we match it with up to three shops that build Invar 36 tooling at the size and precision your program needs. Each shop quotes you directly against the same drawing and the same tolerance callout.
Frequently asked questions
Is Invar 36 tooling the same as composite layup tooling?
No. Composite layup molds and mandrels are shaped tools that transfer their surface to a part cured in an autoclave, covered in Invar for composite layup tooling. Structural tooling, assembly jigs, drill templates and test fixtures, does not touch the part's surface. It holds or locates parts so that features line up, and its job is to not move while it does it.
Why does a steel fixture cause measurement error instead of just physical damage?
Because the fixture is not the part. It is the reference the part is measured or drilled against. If a steel jig grows 0.05 mm over a shop temperature swing, every hole it locates shifts by roughly that amount, even though nothing about the fixture looks wrong. The error shows up later, as an assembly that does not line up or a hole pattern that fails inspection.
At what size or tolerance does Invar 36 tooling make sense?
There is no fixed number, but the trade-off tips toward Invar 36 as the tooling gets larger and the tolerance it controls gets tighter, because both amplify the same growth. A small fixture holding a loose tolerance rarely justifies the material and machining cost. A meter-plus fixture holding a tolerance in the tens of microns often does.
Can an aluminum fixture be temperature compensated instead of using Invar 36?
Sometimes, by controlling shop temperature tightly or by calibrating the fixture at a known temperature and correcting the readings. Both add process overhead and depend on discipline holding over the tool's service life. Invar 36 removes the problem at the material level instead of managing it procedurally, which is why it is preferred for tooling that has to work correctly without that discipline in place.
Do you build or supply Invar 36 tooling yourselves?
No. We are a matching service. We connect your RFQ with up to three shops that machine Invar 36 tooling and fixtures and can support the size and precision your program needs. You place the order with the shop directly.
Reviewed against
- ASTM F1684, Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
- Coefficient of thermal expansion, Wikipedia
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.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops that machine Invar and reply within 24 business hours.
Get matched with an Invar shopKeep reading
- Machining Invar Layup Molds and Mandrels for Composite Parts · Materials
- Invar Fixturing and Workholding: Avoiding Distortion During Machining · Process
- How the CTE of Invar 36 Changes With Temperature and Condition · Materials
- Invar Machining for Aerospace Programs: What Buyers Actually Vet · Choosing a Supplier