Guides / Finishing

Surface Finish on Machined Titanium and What Each Option Is For

Titanium does not rust, so a finish on a titanium part is rarely about protection. It is about appearance, wear, cleanliness or a specification somebody downstream has written. Knowing which one you are buying keeps the cost honest.

The options, and what each one is for

Finish	Typical Ra	What it is for	Lead time added
As machined	1.6 to 3.2	Most parts. Cheapest, and often perfectly adequate	None
Finishing pass	0.8	Sealing faces, sliding surfaces, appearance parts	Machine time only
Bead blast	1.6 to 3.2, even matte	Uniform grey appearance, hides light tool marks	2 to 4 days
Tumble and deburr	Varies	Batch deburring on small parts, far cheaper than hand work	2 to 4 days
Hand polish	0.4 and below	Instruments, consumer parts, cosmetic surfaces	3 to 7 days, labour heavy
Passivation	Unchanged	Removes embedded iron and contamination	3 to 5 days
Electropolish	0.2 to 0.4	Medical, vacuum and cleanliness critical parts	5 to 10 days
Type II anodize	Unchanged	Wear and anti galling, functional	3 to 7 days
Type III anodize	Unchanged	Colour, identification, cosmetic	3 to 7 days

Every row below the first two is an outside process for most shops. That means transport, a queue at the finisher and a return trip. Days, not hours, and that is usually what moves a titanium delivery date.

As machined is a real answer

The default assumption on many drawings is that something has to be done to the surface. In titanium it often does not.

The oxide layer forms the instant the part hits air, and it protects the part on its own. If the surface does not seal, slide, get touched by a person or face a customer, as machined is the right specification and the cheapest one.

Reserve finishing for the surfaces that need it and say so explicitly on the drawing. A blanket surface finish callout on all faces is one of the more expensive habits in titanium.

Deburring, which is not optional

Titanium burrs are tough and they do not break off easily. On a part with many edges, hand deburring can be a significant share of the labour.

Two ways to reduce it:

• Tumbling or vibratory finishing for small parts in a batch. Far cheaper per part than hand work and it gives a consistent result.
• Design: break edges on the drawing with a chamfer rather than leaving them sharp. A 0.3 mm chamfer that the machine can cut is cheaper than a deburr operation somebody has to do by hand.

Say what you want. “Break all sharp edges” is a common and reasonable note. “All edges 0.3 x 45 degrees” is more expensive and sometimes necessary.

Passivation, pickling and cleaning

These three get confused with each other.

Pickling removes the oxide scale and any alpha case from a heat treated or hot worked surface, using an acid bath. It removes material, usually a small but real amount, so it matters on tight tolerance parts.

Passivation for titanium is mainly about removing embedded iron picked up from steel tooling, fixtures and handling. Free iron on a titanium surface corrodes and can compromise biocompatibility. Typical specifications are ASTM A967 style nitric acid treatments adapted for titanium, or a customer’s own process spec.

Cleaning is the general term for removing coolant, oil and particulate before packing. For medical and vacuum parts this is a specified, validated process, not a rinse.

If your part is medical, say which specification applies. If it is not, ask whether you need any of the three, because on a general industrial part the answer is often no.

Electropolishing

Electropolishing removes a thin layer electrochemically, smoothing the surface and rounding micro burrs. Ra 0.2 to 0.4 is achievable.

It is the standard route for parts that must be very clean: surgical instruments, vacuum chamber components, semiconductor handling parts. It also removes material, typically a few microns, which has to be planned into the machining tolerances rather than discovered afterwards.

If you specify electropolish, say so on the drawing so the shop leaves the right stock.

Anodizing, in one paragraph

Titanium anodizing builds a controlled oxide. Type II is the functional one, used for wear resistance and anti galling. Type III is the decorative one, where colour comes from light interference in a very thin oxide and is set by voltage rather than dye. Both are covered by AMS 2488. Anodizing does not hide tool marks, so the machined finish underneath is what you see through the colour.

There is a fuller treatment in the titanium anodizing guide.

What to write on the drawing

Be specific and be sparing.

As machined, Ra 3.2 general. Face A Ra 0.8. Bead blast all over after machining. Break all sharp edges. No anodize.

Four lines. It tells the shop what to do, what not to do and where to spend the finishing pass. Compare that with a title block that says Ra 0.8 everywhere and nothing else, which prices the whole part at the level of its most demanding surface for no reason anybody can point to.

Frequently asked questions

Keep reading

• Titanium Anodizing: What Type II and Type III Actually Do
• Drilling Titanium: Speeds, Heat and Why Holes Fail First