Guides / Tolerances

Titanium Tolerances: What Is Routine, What Is Expensive and What to Call Out

Tolerance is the part of a drawing that costs money quietly. Nobody notices a tight block tolerance the way they notice an extra feature, and in titanium it is often the single largest avoidable line in the quote.

Here is what is realistic, and how to write it so the shop spends time where it matters.

Realistic ranges in titanium

Feature	Routine	Achievable with care	Discuss first
General machined dimensions	±0.05 mm	±0.02 mm	Tighter than ±0.02 mm
Fitted diameters and bores	±0.02 mm	±0.01 mm	H6 and tighter
Position between features on one face	±0.05 mm	±0.02 mm	Tighter than ±0.02 mm
Position across faces, multi setup	±0.08 mm	±0.04 mm	Tighter than ±0.04 mm
Flatness, small solid part	0.05 mm	0.02 mm	Tighter, or on thin plate
Flatness, thin plate part	Depends on stress	Needs a stress relief	Any tight number
Wall thickness, over 3 mm	±0.1 mm	±0.05 mm	Tighter
Wall thickness, under 1.5 mm	±0.15 mm	±0.1 mm	Tighter
Surface finish	Ra 1.6	Ra 0.8	Ra 0.4 and below

Treat every number as typical rather than promised. The shop that quotes confirms what it can hold on your geometry, and it should say so in writing.

Grade 2 and Grade 5 hold similar tolerances. Grade 2 is softer and gummier so it burrs more and can smear on a finishing pass, Grade 5 is stiffer but generates more heat. The differences mostly cancel out.

Why titanium makes tolerance harder

Three behaviours, all covered in more depth in the work hardening guide.

Deflection and springback. Grade 5 has a modulus around 114 GPa, well under steel, and the cutting forces are high. The part moves away from the cutter and returns after it passes.

Heat with nowhere to go. Thermal conductivity around 7 W/m K means the part grows locally while it is cut and shrinks after. Measure hot, get the wrong number.

Residual stress. Material removed from a rolled bar or plate lets locked in stress redistribute. A flat part can bow after it is unclamped.

None of these stop a shop holding a tight tolerance. They just mean it takes another pass, another measurement and sometimes another process. That is time, and time is the quote.

Write the drawing this way

Set a sensible general tolerance. ISO 2768-m is the right default for machined titanium. It is what a shop assumes anyway when the title block is silent.

Call out the features that matter, individually. Two or three is normal. Five is a lot. Ten means the general tolerance is wrong.

Say which datum matters. Position tolerance without a clear datum scheme is an invitation for two shops to measure two different things and both be right.

Separate what is functional from what is cosmetic. A sealing face and a visible face have different requirements and different costs.

Say where flatness matters, and on what section. This is the trigger for a shop to plan a stress relief between roughing and finishing instead of discovering the need after the first article.

A comparison worth internalising

Two versions of the same part.

Version A	Version B
±0.01 mm block tolerance on every dimension	ISO 2768-m general
No features called out	Two bores at ±0.01 mm, one face Ra 0.8
Every dimension inspected	Three characteristics inspected
Finishing pass on every surface	Finishing pass where it is needed

Version B is the same part in service. It is a materially cheaper part to buy, and it will be delivered sooner. The engineering work to get from A to B is deciding which features actually matter, which is work only you can do.

Inspection, and who proves what

If you specify a tolerance, somebody has to measure it.

• Ask whether the shop has a CMM and whether it sits in a temperature controlled room.
• For anything at ±0.01 mm, ask how they will prove it and on what equipment.
• If you need a first article inspection report, say so and say whether it has to follow AS9102.
• Decide whether you need every part inspected or a sample, and say which.

Inspection is real hours. On a small quantity of a tightly toleranced titanium part it can approach the machining cost, and buyers are routinely surprised by that.

What to put on the RFQ

One line is enough:

General tolerance ISO 2768-m. Two bores at ±0.01 mm, marked on the drawing. Sealing face Ra 0.8. First article inspection report required on the first release only.

That tells a shop exactly where to spend its time, and it removes the guesswork that makes three quotes for the same part impossible to compare.

Frequently asked questions

Keep reading

• Drilling Titanium: Speeds, Heat and Why Holes Fail First
• Welded Titanium Assemblies and the Alpha Case Problem