Tight CNC Machining Services: How to Judge a Supplier

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  What matters before you send an RFQ
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  What to check on tight CNC machining services quotes
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  What tight CNC machining services actually promise
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  Inspection: the part of tight CNC machining services nobody quotes
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  Material and geometry choices that decide feasibility
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  How finishing and certifications change the plan
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  Lead time, MOQ and quote terms worth comparing
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  Step by step: vetting a supplier for tight work

A tolerance is an allowance, not a capability. When a shop advertises tight CNC machining services at ±0.005 mm, that number describes what its best machine can do in a clean setup on a favorable material. Your part is a different question. Wall thickness, aspect ratio, feature depth and the number of setups all eat into the budget before the spindle starts turning.

The first thing to check is whether the quote repeats your drawing or replaces it. A capable shop will list which features it intends to hold tight and which it will run to general tolerance. If every dimension on a complex part is quoted at the same tight band, the estimator is not reading the callouts. On a 4,000 mm part, length is rarely the critical feature; bore position and mating faces usually are.

Thermal drift is the quiet failure mode. Aluminum expands roughly 23 μm per meter per degree Celsius. A 300 mm part that warms 5 °C between roughing and finishing has moved about 35 μm on its own, which is seven times a ±0.005 mm band. Shops that hold tight numbers either rough and finish in separate operations with a cool-down, or keep the cell at controlled temperature.

Tool wear is the second drift source. A carbide end mill running a long finishing pass changes effective diameter as the edge wears, and the last part in a batch is not the first. Look for a shop that schedules tool changes by feature count or cut time rather than by ear, and that inspects mid-batch instead of only at the end.

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  Per-feature calloutsTight bands belong on mating surfaces, bores and datum features, not on clearance holes.
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  Separate rough and finishLeaving 0.3–0.5 mm for the finishing pass reduces stress and heat before the final cut.
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  Controlled temperatureA stable cell matters more than a bigger machine for parts under 300 mm.

You cannot ship what you cannot measure. A CMM with a stated uncertainty of ±2 μm is not the same as a caliper reading to ±0.02 mm, and the gap decides whether a ±0.005 mm claim is real. Ask what instrument touches each tight feature, and what the instrument's own uncertainty is. If the measurement uncertainty is more than a quarter of the tolerance, the acceptance decision is partly a coin toss.

Fixturing during inspection counts too. A thin-walled part clamped in a vise for measurement deforms, and the reading reflects the clamp rather than the part. Good shops measure free-state or use the same fixture as machining, and they say so on the report.

Sampling policy is the next question. GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and a final inspection, and reports on request. That matters most on first articles and on parts where a single out-of-tolerance bore scraps an assembly. For high-volume simple parts, a documented sampling plan is defensible, but it should be written down, not assumed.

Watch for the report that shows only nominal and actual. You want the deviation, the tolerance band and a pass or fail flag, so a reviewer can see margin rather than a green checkmark. A part sitting at 90% of the band is technically good and statistically risky on the next run.

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  Uncertainty ratioKeep measurement uncertainty under 25% of the tolerance band where possible.
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  Same fixtureInspection clamping that differs from machining clamping hides thin-wall movement.
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  Margin, not flagsLook for the deviation value so you can see how close the part came to the limit.

Some materials make tight work routine, others make it expensive. Aluminum 6061 and 7075 cut cleanly and hold ±0.005 mm on most features with sharp tooling and good coolant. Stainless 316 and 17-4PH work-harden, so light finishing passes and rigid setups matter more than spindle speed. Titanium TC4 (Ti-6Al-4V) and Inconel move under heat and spring back, so expect more passes and more inspection points.

Geometry sets the ceiling. A bore five times deeper than its diameter needs a long-reach tool that deflects, and the achievable roundness drops. A wall under 1 mm on a 100 mm part will move when the clamps come off, regardless of the machine. If the drawing asks for both, the shop should say so during DFM instead of after the first article.

Plastic parts follow different rules. POM and PEEK hold dimensions well but absorb moisture and move after machining, so a stabilization step helps. ABS and PC are usually fine for prototypes but are not the place to demand a metal-grade tolerance band.

The practical filter is simple: if the tight feature is a bore, a mating face or a datum, most materials are workable with the right plan. If the tight feature is a thin unsupported wall, a deep small-diameter hole or a sharp internal corner, the tolerance is fighting the geometry and the quote will show it.

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  Aluminum 6061 and 7075Best starting point for tight features; stable and predictable to cut.
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  Stainless 316 and 17-4PHWork-hardening makes light finishing passes and rigid fixturing essential.
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  Titanium and InconelExpect more passes, more inspection points and a longer quoting discussion.

Coating is not cosmetic on a tight part. Anodizing grows the surface by a few microns per side, and hardcoat grows more. Electroless nickel and plating add a measurable layer, while bead blasting and tumbling round edges and can move a sharp corner out of band. A shop that thinks about tight CNC machining services will mask critical features, pre-compensate the machining allowance or specify a finish that leaves dimensions alone.

Order of operations matters. If a critical bore is finished before anodizing, the coating may close it. If it is finished after, the coating gets cut through on the bore wall and corrosion resistance suffers there. The right sequence depends on which function wins, and that is a conversation, not a default.

Certifications narrow the supplier list quickly. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical devices, and ISO 27001:2022 covers information security, which matters when your drawings and CAD files leave your building. GreatLight holds all four.

For a one-off prototype, certification may be less urgent than DFM feedback speed. For a production program, it is often the first filter applied before any technical review happens.

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  Mask or pre-compensateProtect tight bores and mating faces from coating buildup.
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  Sequence firstDecide whether the critical feature is finished before or after coating.
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  Match certificates to the programIATF for automotive, ISO 13485 for medical, ISO 27001 for data handling.

Lead time claims are easy to make and easy to check. Ask for the quote turnaround, the DFM feedback turnaround and the ship date as three separate numbers. GreatLight returns a quotation and free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days, with a historical late-delivery probability below 2%. Those are separate commitments, so a slip in one does not hide behind another.

MOQ tells you how the shop is organized. No minimum order quantity, from one prototype to a 10,000+ part run, means the process can be set up for a single piece without the economics falling apart. A shop with a high MOQ is not wrong; it is just built for volume, and your prototype will sit at the back of the queue.

Payment and rework terms belong in the same comparison. Ask what happens when a part measures out of tolerance: is rework free, who pays for the material, and what is the schedule impact. A supplier that answers this quickly has dealt with it before.

Finally, check confidentiality. Uploads should be secure and confidential, and an NDA should be available on request. If your drawing cannot leave the building under an agreement, the cheapest quote is irrelevant.

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  Three dates, not oneQuote, production start and ship date should each be stated.
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  MOQ as a signalNo MOQ usually means the shop can absorb a single-piece setup.
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  Rework terms in writingConfirm who pays for material and time when a part is out of tolerance.