Guides / Process

Machining Tolerances You Can Realistically Hold on Inconel

August 31, 2026

A tolerance callout on an Inconel drawing is not just a number, it is a cost driver. Knowing what a shop can typically hold, and where tight tolerance actually matters, keeps an RFQ realistic and keeps the quote from pricing every feature as if it were the hardest one on the part.

What is typically achievable

For Inconel 625 and 718, a shop that runs the alloy regularly can typically hold in the range of about ±0.02 mm to ±0.05 mm (roughly ±0.001 in to ±0.002 in) on machined features without unusual cost. That range covers most bores, bosses and mating faces on a typical part. Tighter tolerance, down to a few microns on a single critical feature, is achievable, but it is not free: it means slower feeds, an extra finishing pass, and often a dedicated inspection step on that feature alone. A drawing that calls for that level of precision everywhere is asking every feature to be priced like the hardest one on the part.

Why work hardening makes this harder than on aluminum or steel

On aluminum, a machinist can often sneak up on a tight dimension with a series of light finishing passes, each one removing a little more material until the part lands on size. That approach does not work well on Inconel. The alloy work-hardens where the tool has just cut, so a light finishing pass is often cutting a surface that is harder than the bulk material underneath it, left that way by the pass before it. Each successive light cut fights more resistance, not less, and a tool that starts to rub instead of cut makes the hardened layer worse and the next pass harder still.

The practical result is that a shop cannot chase a tolerance downward the same way it would on a more forgiving metal. Holding a tight dimension on Inconel usually means getting the cut right in fewer passes, with a tool and feed rate chosen specifically to avoid rubbing, rather than relying on several passes to close the gap. That takes more planning up front and more careful process control through the cut, both of which show up in the price.

Bore or fit surface versus a non-critical face

Not every dimension on a part earns the same tolerance. A bore that a bearing or a mating shaft has to seat into, or a face that sets a stack height against another component, is a genuine fit surface: it needs a tolerance tight enough to guarantee the assembly works, and that tolerance is worth paying for. A non-critical face, a clearance hole, or a dimension that only exists to define the rough shape of the part does not need the same precision, and calling it out at ±0.02 mm adds machine time for no functional benefit.

The distinction matters more on Inconel than on steel because every tight feature costs proportionally more here. A tolerance that would be a minor add on a mild steel bracket can be a meaningful share of the quote on the same part in 718, simply because holding it fights work hardening the whole way through the cut.

Practical advice for the drawing

Tolerance only the features that actually need it. Use a general tolerance, something in the ±0.1 mm to ±0.15 mm range is common as a baseline, for everything else on the print, and reserve the tighter band for the handful of features where fit or function genuinely requires it. If a dimension exists mainly out of drafting habit rather than a real functional requirement, loosen it before the drawing goes out for quote.

When the tolerance is not obvious from the geometry alone, a short note on the RFQ helps: which features are fits, which features are cosmetic or structural only, and whether the part gets inspected against a full first-article report or a spot check. A shop pricing the job with that context can quote the actual cost of the part instead of pricing worst-case against every dimension on the sheet.

Frequently asked questions

Keep reading

• How to Turn Inconel 718: Insert Grades, Cutting Speeds and Chip Control
• Machining 3D Printed Inconel 718: Supports, HIP and Finish Stock