Buyer guide

Samples of CNC Machining Services: What to Check Before You Commit

A sample is the cheapest place to find a defect. This guide is for design engineers and sourcing staff who need to judge a shop from the parts it sends back, not from a capability page. Read it and you will know which checks matter, which claims to ignore, and when a sample run is the wrong tool.

Get Instant Quote Talk to an Engineer

±0.005 mm toleranceNo minimum order quantityQuote in 12 hours100% inspection

On this page

1
Key takeaways
2
Which sample route fits your part
3
Start with the drawing, not the sample
4
Read the inspection report like an engineer
5
Judge repeatability, not a single perfect part
6
Match certifications to your industry
7
Ask what the sample does not prove
8
Compare lead time, MOQ and quote detail
9
Protect your files before you upload
10
How to run a sample order in 6 steps

Quick answer

Key takeaways

Sample first, then toolingA machined sample proves geometry and finish. It does not prove die casting or injection mold life.

Measure the sample, not the reportAsk for the CMM report that matches the serial number on the part you received.

One sample is not a processThree parts from the same setup show repeatability. A single part hides it.

Finish drives cost more than toleranceRa 0.2–0.8 μm needs different tooling and extra time than Ra 1.6–3.2 μm.

Certification scope mattersISO 9001, IATF 16949, ISO 13485 and ISO 27001 cover different things. Match them to your industry.

Decision table

Which sample route fits your part

Pick the row that matches your geometry, material and volume. The tolerance column is the tightest value we hold in production, not a one-off best result.

Sample route	Best for	Tolerance / finish	Watch out for
3-axis CNC	Prismatic parts, open faces	±0.005 mm, Ra 1.6–3.2 μm	Undercuts need a second setup
4-axis CNC	Shafts, housings with side holes	±0.005 mm, Ra 0.8–1.6 μm	Rotary table limits part length
5-axis CNC	Impellers, turbine blades, organic shapes	±0.005 mm, Ra 0.8–1.6 μm	Programming time adds days
Mill-turn	Turned parts with milled flats	±0.005 mm, Ra 0.8–1.6 μm	Not for very large diameters
CNC turning	Round parts, high volume	±0.005 mm, Ra 1.6–3.2 μm	Off-axis holes need a mill
Die casting sample	Thin-wall housings, 1,000+ pcs	As-cast plus machining	Mold lead time is weeks
Vacuum casting	Small batches of plastic-like parts	Ra 1.6–3.2 μm	Not for structural loads

Check 1

Start with the drawing, not the sample

Samples of CNC machining services only mean something against a controlled drawing. Before you send a file, decide which dimensions actually matter. A print with 60 tight tolerances and no datum scheme forces the shop to guess, and guessing is where scrap starts.

Mark the critical-to-function dimensions. Those are the ones you will measure on the sample. Everything else can sit at general tolerance. This single step cuts quoting time and removes most arguments later.

Check that your 3D model and 2D drawing agree. When they disagree, machinists follow the drawing. If the model has the newer geometry, say so in writing.

1
Define the datumOne primary datum, then secondary and tertiary. Without it, hole positions drift.
2
Flag free-form surfacesCurved surfaces need either a scan report or a set of section points.
3
State the material temper6061-T6 machines differently from 6061-O. Temper affects finish and thread strength.

Check 2

Read the inspection report like an engineer

A sample without data is a paperweight. Ask for dimensional results from a coordinate measuring machine, tied to the serial number on the part. If the shop sends a generic report with no ID, the numbers prove nothing.

Look for the measurement method, not just the result. A Ø12 H7 bore checked with a plug gauge is fine. The same bore checked with calipers is not a real result at that tolerance.

Watch for out-of-tolerance values reported as "within drawing". If a dimension is 0.008 mm over and the drawing allows ±0.005 mm, it is out. Ask for the rework plan.

Material certs should match the heat number on the bar stock. For aerospace and medical work, traceability is part of the sample, not an extra.

1
Serial number on the reportTies the data to the physical part in your hand.
2
Method listed per dimensionCMM, micrometer, gauge, profilometer. Each has a range where it is valid.
3
Material and finish certsAnodize thickness, plating thickness, hardness. Ask for them when the drawing calls them out.

Check 3

Judge repeatability, not a single perfect part

A skilled machinist can hand-finish one part to spec. That tells you little about a 500-piece run. Ask for three samples from the same program and the same setup. Measure the spread.

If the three parts vary by 0.004 mm on a ±0.005 mm feature, the process is marginal. If they vary by 0.001 mm, the process is under control. This is the single most useful number in a sample review.

For plastic parts, repeatability also covers moisture and temperature. POM and PA absorb water, so dimensions move after machining. Let the sample sit in your facility for a day before final measurement.

1
Same setup, three partsShows whether the process or the operator is holding the tolerance.
2
Measure after stabilizationPlastics and thin-wall aluminum move after release from the vise.
3
Note the ambient conditionsA 20 °C metrology room gives different numbers than a 30 °C shop floor.

Check 4

Match certifications to your industry

Certificates are only useful when the scope covers your part. ISO 9001:2015 covers general quality management across the plant. IATF 16949:2016 is the automotive and EV standard, with stricter change control and traceability.

ISO 13485:2016 applies to medical devices and brings process validation and clean handling requirements. ISO 27001:2022 covers information security, which matters when you upload proprietary CAD files.

Ask to see the certificate number and expiry date. Then ask which processes are inside the scope. A shop can hold IATF 16949 for machining while sending plating to an uncertified subcontractor.

For medical and aerospace programs, ask how sub-tier suppliers are controlled. The answer tells you more than the certificate on the wall.

1
ISO 9001:2015Baseline quality system. Covers most industrial and electronics work.
2
IATF 16949:2016Automotive and EV. Requires PPAP-style documentation on request.
3
ISO 13485:2016Medical devices. Adds validation and contamination control.
4
ISO 27001:2022Information security for your files and drawings.

Check 5

Ask what the sample does not prove

A machined sample does not prove die casting tool life, sheet metal fixture repeatability, or injection mold cycle time. Those processes need their own sampling step with the actual tool.

A sample also does not prove packaging, labeling, or shipping survival. If your part is fragile or cosmetic, request the shipping method before the run starts.

Surface finish on a sample can be hand-polished to look better than production. Ask whether the finish came from the cutting tool or from manual work. If it was manual, the production parts will not match.

1
Tooling processes need a tooling sampleDie casting, injection molding and stamping prove out on the real mold or die.
2
Cosmetic parts need a finish standardDefine the surface with a visual sample or a roughness value, not words like smooth.
3
Packing is part of the sampleAsk for the actual box, tray or bag that will ship with the production order.

Check 6

Compare lead time, MOQ and quote detail

A quote without a lead time is half a quote. Ask when the sample ships and when production can start after approval. At GreatLight, quotation and DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days.

MOQ is the other half. Some shops quote a low sample price, then require 500 pieces to start production. We run from one prototype to 10,000+ part runs with no minimum order quantity.

Read the quote line by line. Material, machining, finish, inspection and freight should be separate. If finish is bundled into machining, you cannot compare shops fairly.

For repeat orders, ask what triggers a price change. Material index, volume break and drawing revision should all be named in the quote.

1
Lead time in writingSample ship date and production start date, not just a range.
2
MOQ stated per processCNC and sheet metal differ from die casting and injection molding.
3
Finish priced separatelyAnodize, plating and bead blasting each carry their own cost and lead time.

Check 7

Protect your files before you upload

Your drawings contain the design intent. Ask how the shop stores them, who can open them, and how long they are kept. ISO 27001:2022 is the relevant standard here.

A mutual NDA is normal practice, not an insult. Send it before the RFQ, not after. We sign NDAs on request and treat uploads as secure and confidential.

Also ask whether the shop shows your parts in its public portfolio. Some do by default. If your program is under NDA, confirm that in writing.

1
NDA before RFQCleaner than trying to retroactively restrict a drawing already shared.
2
File retention policyAsk how long files stay on the server and how deletion is confirmed.
3
No portfolio use without consentState it explicitly if the program is confidential.

Workflow

How to run a sample order in 6 steps

This is the sequence we use with new programs. Each step has a gate, so a problem gets caught before the next cost is committed.

1
Send the drawing and the modelInclude the datum scheme, critical dimensions, material temper and finish callout. Note any conflict between model and drawing.
2
Request DFM feedback with the quoteLook for thin walls under 0.8 mm, deep pockets over 4× tool diameter, and tolerances tighter than the process holds. Ask for the quote and DFM analysis together.
3
Approve the process planConfirm machine type, number of setups, stock size and inspection method. This is where 5-axis versus 3-axis gets decided.
4
Order three samples, not oneSame program, same setup. Measure the spread on critical dimensions. Compare against the ±0.005 mm target.
5
Inspect and sign offCheck the CMM report against the serial number, verify finish with a profilometer or visual standard, and confirm material certs.
6
Release production with a frozen revisionLock the drawing revision and the process. Any later change restarts the sample gate, because geometry and tooling both move.