Buyer's guide

Unlock accuracy with 3D CNC machining: how to judge a shop before you send a PO

This guide is for design engineers and sourcing staff who need contoured parts cut to ±0.005 mm, not a marketing promise. Read it to compare shops on setup count, inspection method, quoting detail and lead time, and to spot the answers that signal trouble.

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±0.005 mm16 five-axis centers4,000 mm maxNo MOQ

Unlock accuracy with 3D CNC machining on a high-accuracy aerospace prototype part

On this page

1
Key takeaways
2
Which shop fits your part
3
Unlock accuracy with 3D CNC machining by reading the setup plan
4
Tolerance claims: what ±0.005 mm actually covers
5
Materials and finishing that shift your dimensions
6
Inspection, reports and what 100% inspection means
7
Lead time, MOQ and quote quality
8
How to vet a 3D CNC machining supplier in 6 steps

Quick read

Key takeaways

Count the setups, not the axesA part cut on five sides in one setup usually holds tolerance better than the same part run in three fixtures.

±0.005 mm is a process claimAsk which feature it applies to and how it is measured. A blanket number on a drawing with no datum callout means nothing.

Quoting speed hints at capacityA real DFM review with a price inside 12 hours means the shop has spare spindle time and a process engineer reading your file.

Prototype and production should matchIf the first article comes off a five-axis center, the 10,000-part run should not move to a three-axis fixture without a new PPAP.

Finishing changes toleranceAnodizing adds 5–25 μm per surface. Call out masking on bores and threads before the parts leave the machine.

Decision table

Which shop fits your part

Match the part to the process before you compare prices.

Part situation	Right process	What to verify	Watch out for
Undercuts on 3+ faces	Simultaneous 5-axis	Setup count on the quote	Shop quotes 3 fixtures
Deep pockets, 1 face	3-axis with long reach	Tool L:D ratio used	Chatter on 8× D tools
Ø400 mm rotary features	4-axis with rotary table	Table capacity in mm	Part overhangs the table
Turned + milled in one	Mill-turn center	Bar capacity vs blank	Second op adds 0.01 mm
Thin wall under 1 mm	5-axis, low radial DOC	Fixture and support plan	Spring-back after unclamp
Ra 0.2–0.8 μm seal face	Fine finishing pass	Measured Ra, not visual	Polish hides waviness
1 off prototype	3-axis or 5-axis	No MOQ in writing	Setup charged per part
10,000+ parts	Dedicated cell + PPAP	Gauge R&R and SPC	Process drifts after 500

The verdict

Pick the shop that names its setup count, splits its tolerances and shows you the inspection instrument. Price is the last filter, not the first.

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Check 1

Unlock accuracy with 3D CNC machining by reading the setup plan

Every time a part comes off the table and goes back on, you add error. Fixture repeatability, chip clearing, thermal drift between operations. A shop that quotes your bracket on three separate fixtures is telling you it will stack three sets of positional error onto one tolerance band.

Ask how many setups the quote assumes. For a part with features on four or five sides, the honest answer is one setup on a simultaneous five-axis center, or two setups with a re-datum if the geometry will not clear. If the shop cannot tell you which, it has not read the model.

The setup plan also sets your lead time. One setup means one queue entry. Three setups can mean three queue entries if the shop runs lights-out and batches by fixture, which is common on high-mix floors.

GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills and 27 three-axis machines, so we can quote the setup count that fits the geometry instead of forcing your part onto whatever is free.

1
One setup beats threeEach re-clamp adds positional error you cannot inspect away.
2
Ask for the datum schemeIf the reply is vague, the first article will be a surprise.
3
Match machine to feature5-axis for contoured faces, 3-axis for flat plates.

Check 2

Tolerance claims: what ±0.005 mm actually covers

A tolerance number without a feature list is a sales line. On a milled aluminum part, ±0.005 mm is realistic on a bored hole or a ground face measured at 20 °C. It is not realistic across a 500 mm span where the material moves with the shop temperature.

When you read a quote, look for the tolerance split. Critical features get the tight band. Non-critical features get a looser band, often ±0.05 mm or ±0.1 mm, and that is fine. Paying for ±0.005 mm on a clearance slot wastes money and machining time.

Measurement matters as much as the cut. A CMM with a 2 μm probe on a temperature-controlled granite table can verify ±0.005 mm. Calipers cannot. Ask what instrument will produce the report on your drawing, and whether the report is included or billed extra.

For reference, our fine finish range is Ra 0.2–0.8 μm, a standard high finish is Ra 0.8–1.6 μm, and as-machined is Ra 1.6–3.2 μm. Surface finish and dimensional tolerance are separate calls. Specify both.

1
Tie the number to a featureBores, bores, and ground faces hold tight bands best.
2
Ask for the instrumentCMM, height gauge or optical comparator, named in writing.
3
Separate finish from sizeRa and ± mm are different requirements on the drawing.

Check 3

Materials and finishing that shift your dimensions

The alloy you pick changes how the part behaves after machining. 6061-T6 cuts clean and stays stable. 7075 machines well but moves more when you remove a large pocket, so a rough pass, a stress-relief pause and a finish pass hold size better than one heavy cut.

Stainless 17-4PH in the H900 condition is tough on tools and tends to work-harden if the feed is too light. Titanium TC4 (Ti-6Al-4V) needs sharp edges, low cutting speed and plenty of coolant, or you will burn the surface and lose the finish callout.

Plastics are the opposite problem. POM and PEEK move with heat, so a shop that runs them at metal speeds will hand you an out-of-tolerance part that measured fine on the machine and shrank overnight. Ask for a stabilized cut and a 24-hour check on critical sizes.

Finishing is the last trap. Anodizing builds 5–25 μm per surface depending on the type. Hardcoat builds more. If a bore is already at the top of its tolerance band, plating or coating will push it out. Call out masked areas on the PO and confirm the shop passes that to the finisher.

1
Rough, relax, finishStandard practice on 7075 and large aluminum pockets.
2
Light feeds hurt titaniumToo little chip load work-hardens the surface.
3
Mask before coatingBores, threads and dowel holes need protection.

Check 4

Inspection, reports and what 100% inspection means

A shop that inspects 100% of parts before shipment is checking every piece against the drawing, not sampling five out of a hundred. That costs time, and it is the difference between a 99.99% qualification rate and a box of parts your line rejects.

Ask for the inspection chain. At a minimum: raw material certificate check, in-process monitoring at defined intervals, and final inspection before packing. Reports on request means the shop can produce a dimensional report, a material cert and a finish cert without a week of emails.

For regulated work, the paperwork matters more than the parts. Aerospace, medical and automotive buyers need traceability from the melt lot to the finished part number. ISO 9001:2015 covers general quality, IATF 16949:2016 covers automotive, ISO 13485:2016 covers medical devices and ISO 27001:2022 covers information security for your files.

If a supplier cannot tell you which certificate applies to your part, treat the quote as incomplete. Certification is not a logo on a homepage. It is a scope statement, and the scope has to include your process.

1
Define the sample plan100% or AQL, written on the PO.
2
Request the report up frontDimensional, material and finish certs named.
3
Check the certificate scopeThe right standard must cover your process.

Check 5

Lead time, MOQ and quote quality

Quoting speed tells you a lot about a shop's floor. We return a quotation and a free DFM analysis within 12 hours. Production can start within 24 hours after that, and parts ship in 3–5 days. Those numbers come from spare spindle capacity and a process engineer who reads the model before pricing it.

MOQ is the other filter. No minimum order quantity means a shop will run one prototype and a 10,000+ part series on the same floor. That matters when your design is still moving and you need three revisions in a month.

Read the quote line by line. A good one lists material, stock size, setup count, machine type, estimated cycle time, finishing, inspection and freight. A quote that is one number with no breakdown will grow change orders the moment the drawing is updated.

Confidentiality belongs in this list. Uploads should be secure, and an NDA should be available on request before you send a full assembly. If a supplier hesitates on an NDA, that is your answer.

1
12-hour quote targetA fast DFM reply means capacity is real.
2
No MOQ, no penaltyOne prototype and a 10,000-part run use the same process.
3
Ask for the breakdownSetup, cycle, finish and inspection listed separately.

Step by step

How to vet a 3D CNC machining supplier in 6 steps

Run these checks before you release a PO.

1
Send a real drawing, not a sketchInclude GD&T, datum callouts, finish callouts and a material spec. A shop that quotes a STEP file with no drawing will guess on the tolerances you care about.
2
Ask for the setup count in writingOne setup on a simultaneous 5-axis center for contoured parts is the benchmark. Two setups is normal for a re-datum. Three or more on a complex part should trigger questions about fixture design.
3
Confirm the tolerance splitAsk which features get ±0.005 mm and which get a looser band. A flat ±0.005 mm across every dimension is a sign the shop did not read the model.
4
Name the inspection instrumentCMM for tight bands, height gauge for general work. Request a dimensional report on the first article and confirm whether it is included in the price.
5
Check the certificate scopeMatch the standard to your industry: ISO 9001:2015 general, IATF 16949:2016 automotive, ISO 13485:2016 medical, ISO 27001:2022 file security. Ask for the certificate number and scope statement.
6
Test the finishing pathSend one part through anodizing or plating before the full run. Measure a bore and a thread before and after to see the real build-up, which is typically 5–25 μm per surface.
7
Lock the paperwork before the runAgree on the first article report, the material cert and the packing spec. Then keep the same process for production. A process change without a new first article is where tolerance quietly disappears.

FAQs

Questions engineers ask before ordering

Is ±0.005 mm realistic on a 3D contoured surface?

On a machined surface measured point by point with a CMM, ±0.005 mm is achievable on aluminum and steel parts within a 500 mm envelope and a controlled shop temperature.

Across a 4,000 mm part, thermal growth and machine geometry make that band impractical. Expect ±0.02 mm or looser on long parts, and specify the tight band only on the features that need it.

How do I know if my part needs 5-axis or 3-axis?

Look at the feature directions. If you can reach every face with the part sitting in one orientation, or one flip, 3-axis is enough and cheaper.

If you have undercuts, angled holes or contoured faces on three or more sides, simultaneous 5-axis removes the extra fixtures and holds position better between features.

What does no minimum order quantity really mean?

It means the shop will quote one piece at a fair setup cost and the same process for a 10,000+ part run. You are not forced into a batch size you cannot use.

Ask whether the per-part price curve is published. A shop with no MOQ should still show how setup cost spreads as quantity rises.

Will anodizing change my dimensions?

Yes. Clear and color anodizing adds roughly 5–25 μm per surface. Hardcoat adds more, and it builds unevenly on sharp edges.

Mask bores, threads and dowel holes on the PO. If a feature is already at the top of its band, machine it undersize before coating or move the finish to a non-critical face.

What should a first article report include?

The report should list every dimension on the drawing with the measured value, the nominal, the tolerance and a pass or fail mark. Add material cert, finish cert and a note on the inspection instrument used.

For automotive and medical work, keep the report with the part number and lot so traceability runs from the melt lot to the shipped box.

How fast can a supplier start production?

We return a quotation and free DFM analysis within 12 hours, start production within 24 hours, and ship parts in 3–5 days. Those windows assume the drawing is released and the material is in stock.

If a supplier quotes a week just to price the job, that shop is likely batching quotes and your lead time will stretch the same way.