How to Vet a Titanium (TC4) and PEEK CNC Machining Supplier in China

A titanium bracket and a PEEK end effector rarely come from the same shop, even though both are "CNC machining". The material decides the tooling, the coolant, the fixture and the inspection — and a quote that ignores those differences is a quote you will renegotiate later.

This is a vetting checklist for buyers sourcing titanium (Ti-6Al-4V / TC4) and PEEK parts from China: what to ask, what the answers should sound like, and which numbers actually matter.

Two kinds of supplier, and only one of them is relevant here

Before comparing prices, establish which business you are talking to.

	Catalog parts supplier	Build-to-print machine shop
Prices from	Part number, spec table, quantity break	Your 2D drawing / 3D model, feature by feature
Stock	Ships from inventory	No stock; scheduled per order
Custom geometry	Declines, or asks you to pick a standard size	This is the entire business
MOQ	Carton or pallet	1 piece is a normal answer
First article report	Usually not offered	Expected, with measured values
Will they challenge your drawing	No	Yes — that is the DFM feedback you are paying for

Three questions separate them:

1. "This custom geometry — do you make it to my drawing, or do I pick from your catalog?"
2. "Will you run one piece?"
3. "Before quoting, will you send a DFM note flagging tolerances that are tighter than the function needs?"

A shop that pushes back on your drawing is usually costing the part from the process. A shop that only returns a number is quoting a template.

Titanium: what changes the process

Four grades cover most requests:

Grade	Designation	Tensile (approx.)	Typical use	Machining difficulty
2	Commercially pure Ti	345–450 MPa	Chemical pumps, marine hardware, heat exchanger plates	Moderate
5	Ti-6Al-4V / TC4	900–1000 MPa	Structural brackets, UAV airframes, fasteners, medical instruments	Medium-high
9	Ti-3Al-2.5V	620–720 MPa	Hydraulic lines, formed pressure parts	Medium
23	Ti-6Al-4V ELI	860–965 MPa	Implant-grade, low-temperature toughness	As Grade 5, plus cleanliness

If the drawing says only "titanium", a supplier may quote Grade 2 and deliver a part that is 40 percent weaker than the Grade 5 you designed for. Naming the grade and the material standard (ASTM B348 or GB/T 2965) is the cheapest risk control available.

Why titanium is slow — three physical facts

Thermal conductivity around 7 W/(m·K). 6061 aluminium is roughly 167. Heat from the cut has nowhere to go except into the tool edge, which is why cutting speed runs at about one third of the aluminium value for the same tool.

Work hardening. Titanium hardens in the cut. A light pass over an already hardened surface accelerates wear instead of improving finish, so the process has to keep constant engagement rather than dwell.

Chemical reactivity at temperature. Titanium reacts with tool materials when hot, which is why tooling has to be dedicated. Cutters that have run titanium should not touch aluminium or stainless, and the same applies to fixtures and the cleaning station. Contamination does not show up in dimensional inspection; it shows up later as porosity in a weld or pitting after anodising.

Add low elastic modulus (about 110 GPa, roughly half of steel) and thin walls deflect about twice as much for the same clamping force. Fixture rigidity becomes the limiting factor, not machine accuracy.

Six questions for a titanium supplier

1. Are titanium tools kept separate from aluminium and steel tools? The answer should be yes, including fixtures and cleaning.
2. How is coolant delivered? Through-tool high pressure is the strong answer. The target is the cutting edge, not the workpiece.
3. Have you machined walls under 1 mm? Show the fixture approach. A real answer describes support, material removal sequence and measurement after release.
4. Can material certificates be traced to the mill heat number? Yes, with EN 10204 3.1 available on request.
5. Is stress-relief annealing in-house or subcontracted? Most machine shops subcontract vacuum annealing. Ask who does it, how long it takes, and whether finish machining happens after annealing.
6. What inspection closes the loop? CMM as the reference, measured values in the report rather than a pass stamp, SPC on critical characteristics for volume programmes.

PEEK: what changes the process

PEEK is not one material. Four grades cover most of the demand, and they behave differently enough that a quote saying only "PEEK" is not a quote.

Grade	What it is	Key property	Typical part	Machining note
Unfilled	Neat PEEK	Highest elongation and chemical resistance, lowest stiffness; continuous service around 250 °C, density near 1.30 g/cm³, tensile around 100 MPa	Seals, insulator bodies, fluid components	Cuts easily, moves easily — annealing is mandatory
GF30	30% glass fibre	Stiffer, better creep resistance, abrasive to tooling	Structural brackets, pump housings, valve bodies	Change inserts on a count, not on failure
CF30	30% carbon fibre	Stiffest, lowest thermal expansion, electrically conductive	Wafer handling end effectors, precision fixtures	Abrasive plus conductive — dust and static control
Bearing grade	Graphite / PTFE additions	Low friction, wear resistance	Bushings, thrust washers, sliding pads	Surface smears easily; parameters matter

The annealing sequence is the tolerance

PEEK parts that fail inspection usually failed weeks earlier, in the process plan. The sequence has to be: rough machine → stress-relief anneal → finish machine. Annealing after finish machining only releases part of the stress; the stress from the finishing cut stays in the part.

Practical consequences:

• Measure after the part has cooled and stabilised, not at the machine while it is warm.
• Light clamping with soft jaws and support under thin sections — PEEK deflects far more than aluminium under the same vise pressure.
• Account for thermal expansion: PEEK expands roughly four times as much as steel over the same temperature rise. A part that measures correctly at 20 °C can be out of tolerance at 80 °C service.

Why PEEK edges turn white

Whitening is stress whitening, and it comes from rubbing rather than cutting: a dull tool, a feed that is too low, or too much heat. It is not cosmetic. The white layer is a stress concentration and a crack initiation site.

The correct response is to change the tool and the parameters. Polishing the surface removes the evidence and leaves the stress. When you ask a supplier how they handle whitening, "we polish it" and "we change the cutter and the feed" identify two very different shops.

The tolerance conversation

±0.005 mm is a two-sided band with 0.010 mm total width. On a 20 mm bore that is 0.05 percent of nominal, which is why it cannot be a blanket tolerance on a whole drawing.

Nominal length	ISO 2768-m (medium)	What is held on critical features
0.5–3 mm	±0.1 mm	±0.005 mm
3–6 mm	±0.1 mm	±0.005 mm
6–30 mm	±0.2 mm	±0.005 mm
30–120 mm	±0.3 mm	±0.005 mm
120–400 mm	±0.5 mm	±0.005 mm or better
400–1000 mm	±0.8 mm	quoted per feature

The gap is 20 to 40 times. A part with 8 critical dimensions and 40 general ones machines and inspects cleanly when the tolerance is applied where it matters. The same part fully toleranced at ±0.005 mm can double in price with no functional gain, because inspection time can exceed machining time.

Five things actually limit the achievable band, and machine accuracy is only the first: fixture rigidity, tool wear and thermal growth, material behaviour, the datum scheme, and feature geometry (a deep narrow slot reaches the tool's limit long before the machine's positioning accuracy matters).

Three questions worth asking:

• "On which features do you hold ±0.005 mm?" A supplier answering "all dimensions" has not thought about it.
• "Does the first article report give measured values or a pass stamp?" Measured values, with distribution, are the only useful version.
• "Can you supply SPC data on critical characteristics?" For volume programmes, receiving inspection needs it to avoid 100 percent checking.

Lead time and minimum order

"Lead time" hides three different clocks. Split them:

Item	What to pin down	Typical
Quote turnaround	How fast, and does it include a DFM note	Within 24 hours is achievable
Prototype	From PO to parts shipped, material included	3–7 days
Production	From PO to batch complete	7–15 days
Surface finishing	Separate from the numbers above	Add 3–7 days
Minimum order	Will they run one piece; are there quantity breaks	1 piece, with price breaks by quantity
Documentation	First article, material cert, inspection records included	Should ship with the parts, not as an extra

A shop quoting "7 days" that means 7 days of machining, plus 5 days of material and 4 days of finishing, is describing a 16-day delivery. Writing the six lines into the PO is how that gets avoided.

What to send for a quote

• 2D drawing with tolerances, datum scheme, surface roughness; or a 3D model plus a tolerance note
• Material grade and standard
• Quantity for prototype and for production
• Surface finish requirement, including any cosmetic zone
• Target lead time and any hard date

With those five, a shop can quote. Without the datum and the material grade, it can only quote a range.