Guides / Process

Thin-Wall Invar Parts: Machining Without Warping

September 20, 2026

A thin wall or web is one of the more common places for an Invar part to come out warped, not because the material is uniquely difficult but because thin sections have very little of their own rigidity to resist the forces machining puts into them. Getting a thin-wall Invar part flat and stable takes deliberate fixturing and sequencing, not just a slower feed rate.

Why thin sections distort

A thin wall has less material to resist both the mechanical forces of cutting and the residual stress those cuts leave behind. Where a thick, rigid section can absorb some clamping force or residual stress without visibly moving, a thin wall flexes under the same forces and, if it is not adequately supported, machines to a shape that is only correct while the fixture is holding it. Once released, the wall springs back toward its natural, unstressed shape, and the feature is no longer the size or flatness it was cut to.

This interacts directly with the fixturing considerations in Invar fixturing and workholding, and with the material’s tendency to work harden under mechanical stress, described in why Invar work hardens. A thin wall clamped too aggressively can both distort elastically during the cut and pick up localized hardening at the clamping points.

Sequencing for thin walls

Shops experienced with thin Invar sections typically adjust the standard rough, stress relieve, finish sequence to account for the wall’s sensitivity:

1. Rough with extra stock left on thin features, rather than machining them close to final size early, so the wall has more mass to resist distortion during the roughest cuts.
2. Stress relieve before thin sections are brought close to final thickness, so the bulk of the internal stress is released while there is still enough material to resist warping.
3. Machine thin walls last, in light passes, symmetric where possible so that material removal and any residual stress release evenly rather than pulling the wall in one direction.
4. Support the wall during finishing with a fixture element that follows its shape, rather than relying on clamping alone to hold it flat.

Symmetric machining strategies

Where a design allows it, removing material from both sides of a thin wall in alternating, matched passes, rather than machining one side fully before starting the other, helps balance the stress the cutting introduces and reduces the tendency for the wall to bow in one direction. This is a programming decision more than a design one, but it is worth asking a shop about if your part has a critical thin wall, since not every shop defaults to this approach.

Fixturing thin sections

Sacrificial support material, sometimes called a web or tab that is left in place during machining and removed afterward, is a common technique for thin Invar features. It gives the fixture something rigid to hold onto and gives the part extra mass to resist distortion through the cut, at the cost of an extra step to remove the support material and clean up the resulting surface at the end.

Vacuum fixturing or a low-force clamping approach across a broad area, rather than a few high-force clamping points, also reduces the risk of local distortion on thin, flat sections, similar in principle to fixturing large flat layup tools discussed in Invar composite layup tooling.

Inspection considerations

A thin wall that measures flat right off the machine can still move slightly over the following hours or days as any remaining stress continues to relax, more so than a thick, rigid part would. For critical thin-wall features, letting the part sit for a period before final inspection, and re-checking flatness a second time, catches this kind of delayed movement before the part ships rather than after.

What to specify on your drawing

• Flag thin wall or web features explicitly, even if the tolerance on them is not unusually tight, so the shop plans fixturing accordingly
• State which side of a thin feature, if either, is the critical reference surface
• Mention if the part will see mechanical load in service that the wall needs to resist, since that affects whether sacrificial support material is a viable machining strategy

Send your drawing through our RFQ form with thin or delicate features called out, and we match you with shops that plan fixturing and sequencing around them rather than treating them the same as a rigid section.

Frequently asked questions

Keep reading

• Invar 36 Machining Parameters: Speeds, Feeds and Tool Wear
• Invar Drilling and Tapping: Hole Quality and Thread Practices