Buyer guide

Custom CNC Precision Machining Service: 7 Checks Before You Send a PO

This guide is for engineers and sourcing managers comparing shops for a custom CNC precision machining service. It covers the numbers that decide whether a quote is real: achievable tolerance, maximum part size, spindle count, certification scope, MOQ and lead time. Read it and you can shortlist three suppliers without a plant visit.

Get Instant Quote Talk to an Engineer

±0.005 mm toleranceNo MOQISO 9001 / IATF 1694912-hour DFM

On this page

1
Key takeaways
2
What to check against the part you actually need
3
Where the money actually goes
4
What a custom CNC precision machining service should prove about tolerance
5
Machine capacity and the part envelope
6
Certification scope and what it covers
7
Materials, finishes and the hidden cost of plating
8
MOQ, lead time and how the quote is built
9
How to qualify a supplier in five steps

Quick answers

Key takeaways

Tolerance is a floor, not a promiseAsk what is held on 100% of the run, not what the best single part measured.

Part size decides the machine listAnything past 750 mm often leaves 3-axis work and lands on a gantry or mill-turn cell.

Certification has a scopeISO 13485 for a medical part means little if the shop cannot show process validation records.

MOQ is a cost question, not a policyA shop quoting one prototype and 10,000 parts is set up for both setup types.

Quote speed signals DFM depthA 12-hour reply usually includes a manufacturability note. A price alone rarely does.

Selection matrix

What to check against the part you actually need

Match the requirement on the left to the evidence you should request on the right.

Requirement	Evidence to request	Red flag
Tolerance under ±0.01 mm	CMM report on a similar feature	Only a general tolerance statement
Part over 750 mm	Machine travel and workholding plan	Quote without a setup sketch
Aerospace or EV bracket	IATF 16949:2016 certificate scope	Certificate for a different site
Medical device housing	ISO 13485:2016 plus clean handling	No traceability of material lots
One prototype only	Written MOQ policy, no tooling fee	Setup charge hidden in unit price
10,000+ parts per year	Cycle time estimate and fixture plan	Same price as the prototype run
Confidential design	NDA and secure upload path	Files requested over plain email
Tight schedule	Historical on-time figure, not a promise	Guaranteed date in the first email

Cost drivers

Where the money actually goes

Two quotes for the same part often differ on these four lines.

Driver	Cheap route	Precision route
Setups	3-axis, two or three re-fixtures	5-axis, one setup, one datum
Fixtures	Standard vise, soft jaws	Dedicated fixture for repeatability
Inspection	Sample check at the machine	CMM report, 100% visual
Finish	As-machined Ra 1.6–3.2 μm	Fine pass Ra 0.2–0.8 μm

When to pick a precision shop and when to pick a general job shop

Choose a precision shop when tolerance, datum control or regulated traceability drives the design. Choose a general job shop when the part is simple, cosmetic and cost per piece is the only metric. Be honest about which one you have.

Get Instant Quote Talk to an Engineer

Check 1

What a custom CNC precision machining service should prove about tolerance

Every shop advertises tight numbers. The useful question is which one holds across the whole run. A custom CNC precision machining service that quotes ±0.005 mm should be able to tell you the feature types where that applies: bored holes, ground faces, milled pockets on a stable setup. It should also tell you where the number relaxes, because tolerance is bought with time and fixture cost.

Ask for a CMM report from a comparable part, not from a showpiece. Look at the feature list, the datum scheme and the number of points per feature. A report with three points on a Ø40 mm bore tells you very little. A report that shows position, roundness and surface finish on the same print tells you the shop understands the drawing.

Surface finish travels with tolerance. Ra 0.8–1.6 μm is a normal machined finish for aluminum and mild steel. Ra 0.2–0.8 μm usually needs a finishing pass, a different insert or a secondary operation, and that cost should appear in the quote rather than after the first article.

1
Ask which features hold ±0.005 mmBores, faces and pockets behave differently.
2
Request a report, not a claimFeature list, datum scheme, point count.
3
Match finish to functionRa 1.6–3.2 μm is fine for most brackets; seals need better.

Check 2

Machine capacity and the part envelope

The part envelope decides which machines can touch your job. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The largest travel is 4,000 × 400 × 150 mm, with a 4,000 mm maximum processing size and a Ø400 mm rotary table.

Mid-size work sits on 750 × 1,150 × 550 mm or 600 × 600 × 600 mm travels. Compact parts run on 500 × 500 × 450 mm and 500 × 310 × 200 mm machines. When you send a print, ask which platform the quote assumes. A 5-axis center can finish five faces in one setup, which removes re-fixturing error. A 3-axis machine may be cheaper per hour but can need two or three setups on the same part.

Long, thin parts are the usual trap. A 4,000 mm rail on a 400 mm-wide table needs support along its length or it will deflect during milling. Ask how the shop plans to hold it. If the answer is only a vise, the tolerance will not survive the first roughing pass.

1
5-axis in one setupFewer datum shifts, better position on angled features.
2
Mill-turn for shaftsTurning and milling without a second chucking.
3
Ask about support for long partsVise-only plans fail on thin rails.

Check 3

Certification scope and what it covers

Certificates are a starting filter, not a passing grade. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. Each one answers a different buyer question. ISO 9001 covers the general quality system. IATF 16949 covers automotive and engine hardware work, with the focus on reducing waste and variation. ISO 13485 covers medical devices. ISO 27001 covers information security, which matters when you send CAD files.

Check the site named on the certificate. A group certificate that lists a different plant does not qualify the plant quoting your job. Ask for the certificate number and the address, then compare it to the shipping address on the quote.

For regulated work, certificates are the entry point. The real evidence is process validation, material lot traceability and inspection records kept against the part number. If a supplier cannot describe how a lot is traced from raw bar to finished part, the certificate will not save the audit.

1
Match certificate to plantGroup certificates can hide the real site.
2
Traceability beats paperworkAsk how lots map from bar stock to part.
3
ISO 27001 matters for CADIt covers how your files are stored and shared.

Check 4

Materials, finishes and the hidden cost of plating

Most precision work is aluminum, stainless or steel. GreatLight machines 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 in aluminum; 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH in stainless; 1018, 1045, 4130, 4140, 4340, A36 and tool steel. Copper and brass grades include C101, C103, C110, beryllium copper, C27400, C28000 and C36000. Titanium and special alloys cover TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B / AZ91D. Plastics run from ABS and PC to POM, PEEK, PA, PP, HDPE and carbon fibre.

Finish choice changes the tolerance budget. Hardcoat anodizing builds thickness and can move a tight bore. Electroless nickel adds a uniform layer, which is easier to predict. Bead blasting softens edges, so a sharp corner on the print will not survive it. Laser marking needs a minimum character height of 1.5 mm, so a 0.8 mm part number will not be legible.

Ask the shop to name the finish supplier and the masking plan. A plated part that loses its masking during anodizing becomes a rework job, and the schedule slips by a week.

1
Anodizing moves dimensionsHardcoat builds the most; plan the bore accordingly.
2
Blasting rounds edgesDo not combine it with a sharp-corner requirement.
3
Marking has a size floor1.5 mm minimum character height.

Check 5

MOQ, lead time and how the quote is built

GreatLight has no minimum order quantity, so a single prototype and a 10,000+ part run both go through the same quoting path. That matters for hardware teams that need a functional sample before tooling. The quote covers material, machining, finishing and inspection, and a free DFM analysis comes back with it.

Lead time has three segments worth separating. Quotation and DFM analysis come back within 12 hours. Production can start within 24 hours of a released PO and confirmed material. Parts ship in 3–5 days for typical work. The historical late-delivery probability is below 2%, which is a record, not a guarantee. Treat any guaranteed date offered before the drawing is reviewed as marketing.

Inspection is 100% before shipment, with raw material check, in-process monitoring and final inspection. Reports are available on request. If your drawing calls for first article inspection, say so at quote stage so the inspection plan includes it rather than adding it after the run.

1
No MOQOne prototype to 10,000+ parts on the same quote path.
2
12-hour quote and DFMAsk for the manufacturability note, not just the price.
3
100% inspectionReports on request; name FAI needs up front.

Sourcing workflow

How to qualify a supplier in five steps

Run these in order. Each one filters out shops before you spend engineering time.

1
Send the print with GD&T and finish calloutsInclude datum scheme, critical features and the material spec with temper. A STEP file without a drawing usually gets a generic price.
2
Ask for the DFM note, not just the numberA good reply flags thin walls, deep pockets, tight radii and features that need EDM. GreatLight returns quotation and free DFM analysis within 12 hours.
3
Confirm the machine and setup planAsk which platform holds the part and how many setups. For a 300 mm bracket with angled holes, a 5-axis center removes two re-fixtures.
4
Check inspection scope against the drawingList the features that need CMM data. Confirm 100% inspection before shipment and that reports come with the parts on request.
5
Lock the commercial terms before releaseConfirm no MOQ on the trial run, sign the NDA if the design is sensitive, and agree the shipping window in writing. Keep the PO released only after material is confirmed.

FAQs

Questions buyers ask before the first order

What tolerance can a custom CNC precision machining service actually hold?

GreatLight works to ±0.005 mm (±0.0002 in) on qualified features, with finish down to Ra 0.2–0.8 μm when the drawing calls for it. That number applies to specific feature types, so ask which ones on your print.

Features that need a second operation or a thin wall will relax. The quote should say where, and the DFM note should explain why.

Do I need a drawing, or is a STEP file enough?

A STEP file gives geometry but not tolerance, finish or datum intent. Send both. Without a drawing the shop has to guess which faces matter, and the price reflects that uncertainty.

If the part is early-stage, say so. Prototype quotes can work from a model plus a short note on function and fit.

How fast can parts ship after I release the PO?

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours once the PO is released and material is confirmed. Typical parts ship in 3–5 days.

Complex parts or special material change that. Ask for the material lead time separately from the machining time.

Can you sign an NDA before I upload CAD files?

Yes. Uploads are treated as secure and confidential, and an NDA is available on request. GreatLight also holds ISO 27001:2022, which covers information security management.

Sign the NDA before the first file transfer, not after. It is a short step and it protects both sides.

What is the minimum order quantity?

There is no minimum order quantity. The same quote path handles one prototype and a 10,000+ part production run.

Unit price still depends on quantity, because setup and fixturing spread across the batch. Expect the first piece to cost more than the thousandth.

How do I know the parts will pass my own inspection?

Inspection is 100% before shipment, covering raw material check, in-process monitoring and final inspection. Reports are available on request.

Send your inspection plan with the PO. If your receiving team checks specific features, the shop can measure the same ones and catch drift before shipping.