Performance CNC Machining Service: How to Choose One
This guide is for engineers and sourcing teams comparing shops for a performance cnc machining service. It covers the checks that decide whether a supplier can hold your tolerances, hold your schedule and hold your documentation. Read it before you approve a first article.
In this article
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Key takeaways
What to Check Before You Send an RFQ
Match each requirement to the evidence a shop should show you.
| Check | Weak answer | Strong answer |
|---|---|---|
| Tolerance | ±0.05 mm general | ±0.005 mm with CMM report |
| Finish | As machined | Ra 0.8–1.6 μm stated per face |
| Lead time | 2–4 weeks | Quote 12 h, ship 3–5 days |
| MOQ | 500 pieces minimum | No MOQ, one prototype accepted |
| Certification | ISO 9001 only | ISO 9001 plus your industry standard |
| Inspection | Sample check | 100% inspection before shipment |
| File handling | Email attachments | NDA and secure upload on request |
Start With Tolerance, Not With Price
Every performance cnc machining service claims tight tolerance. Ask for the number and the measurement method. A shop that says ±0.005 mm should also tell you which machine holds it and how the part is verified. Without that, the number is marketing.
Tolerance depends on three things: machine geometry, workholding and thermal stability. A 5-axis machining center with a Ø400 mm rotary table can hold position on five faces in one setup. Move the same part to a three-axis machine and you add re-fixturing error. That error usually shows up as a mismatch between faces.
Size matters too. GreatLight machines parts up to 4,000 mm. On large parts, tolerance is governed by travel and by how the blank is supported. Travel of 4,000 × 400 × 150 mm suits long extrusions and frame rails. Travel of 500 × 500 × 450 mm suits dense, small parts where rigidity is easy to keep.
If your drawing calls for ±0.005 mm on a 900 mm aluminum plate, ask how the shop plans to control flatness after stress relief. If the answer is vague, expect a first article that fails.
- 1State tolerance per featureA single global tolerance hides the hard features.
- 2Ask for the inspection methodCMM, optical or gauge; each has a different uncertainty.
- 3Check the machine size matchA part near the edge of travel loses rigidity.
Surface Finish and Material Go Together
Finish is where quotes drift apart. Ra 1.6–3.2 μm is a normal as-machined result. Ra 0.8–1.6 μm needs a controlled finishing pass with sharp tooling. Ra 0.2–0.8 μm usually means a separate operation such as fine turning, lapping or polishing.
Material changes the achievable finish. Aluminum 6061 and 7075 cut clean and take a fine finish well. Stainless 316 and 17-4PH work-harden, so a light finishing pass can smear instead of cut. Titanium TC4 (Ti-6Al-4V) needs lower cutting speed and more coolant to avoid a torn surface.
If you need both a tight finish and a tight tolerance, say so at quoting. Shops often plan the finishing pass after the tolerance pass, and that order matters. Reversing it can pull the part out of tolerance.
Cosmetic requirements belong on the drawing. Bead blasting, brushing, anodizing and laser marking all change the surface. Laser marking has a minimum character height of 1.5 mm, so plan your marking layout around that limit.
- 1As machinedRa 1.6–3.2 μm, fine for brackets and housings.
- 2Fine machinedRa 0.8–1.6 μm, typical for sealing faces.
- 3Polished or lappedRa 0.2–0.8 μm, budget a second operation.
Lead Time, MOQ and Quote Speed
A quotation and free DFM analysis within 12 hours tells you the shop has capacity and an engineer reading files. After that, production can start within 24 hours. Parts ship in 3–5 days for standard work. These are planning numbers, not promises for every part.
Ask about the historical late-delivery rate. Below 2% is a working target for a shop running mixed high-mix work. If a supplier cannot state a number at all, they are not tracking it.
MOQ is the other filter. No minimum order quantity means you can order one prototype and later place a 10,000+ part run. That matters when a design is still changing. A shop that insists on 500 pieces for a first run will slow your validation loop.
Watch for hidden cost at low volume. Setup, programming and fixturing dominate the first part. If a supplier quotes a very low single-piece price, check whether tooling or programming appears later as a separate line.
- 1Quote in 12 hoursIncludes DFM feedback on the model.
- 2Production start in 24 hoursAfter drawing release and material arrival.
- 3No MOQFrom one prototype to 10,000+ parts.
Certifications and Confidentiality
Certification tells you which industries a shop can serve. ISO 9001:2015 is the base quality system. IATF 16949:2016 covers automotive and engine parts. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which matters if your drawings are sensitive.
Match the certificate to your end use. A medical housing needs traceability and process control, not just a tolerance number. An automotive bracket needs PPAP-style documentation and change control. Ask to see the scope of the certificate, not only the logo.
Confidentiality is part of supplier selection. Uploads should be secure, and an NDA should be available on request. If your program has ITAR-like or internal IP rules, confirm the file retention and deletion policy before you send anything.
Inspection paperwork belongs in the same conversation. 100% inspection before shipment, raw material check, in-process monitoring and final inspection are the baseline. Reports are available on request, so ask which report you get by default.
- 1ISO 9001:2015General quality management baseline.
- 2IATF 16949:2016Automotive and engine hardware.
- 3ISO 13485:2016Medical device manufacturing.
- 4ISO 27001:2022Information security for your files.
Capacity and Materials Decide Feasibility
Ask how many machines and which types. GreatLight runs 127 high-precision CNC machines across 3 wholly-owned plants, with 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The mix tells you what kind of work the shop does every day.
Material range is a practical filter. Aluminum 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 are common. Stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH cover most hardware. Steel grades include 1018, 1045, 4130, 4140, 4340, A36 and tool steel.
Specialty work needs a shop used to it. Titanium TA1, TA2 and TC4, Inconel and magnesium AZ31B or AZ91D all behave differently. Copper and brass grades such as C101, C110, C36000 and beryllium copper are also routine. Plastics range from ABS and POM to PEEK and carbon fibre.
If your part needs several processes, check whether they run in house. Milling, turning, sheet metal, die casting, vacuum casting, 3D printing and surface finishing under one roof reduce handling and re-qualification.
- 15-axis for complex geometryOne setup for five faces reduces datum error.
- 2Mill-turn for round partsTurning and milling in one cycle.
- 3In-house finishingAnodizing, plating, powder coating, laser marking.
Step by Step: Qualifying a Supplier
Work through these in order. Each step produces evidence you can file.
- 1Send a drawing with real tolerancesInclude GD&T, datum callouts and finish per face. A drawing full of ±0.1 mm general notes will not reveal the shop's capability.
- 2Request the DFM responseA good shop replies within 12 hours with notes on thin walls, tool access and tolerance stack. Read the notes, not just the price.
- 3Ask which machine will run the partGet the model and travel size. A part near the edge of travel or needing five faces should land on a simultaneous 5-axis center.
- 4Confirm the inspection planAsk for first article plus 100% inspection before shipment. Request the report type: dimensional, material or finish.
- 5Order one prototype firstUse no-MOQ ordering to validate. Check the finish with Ra 0.8–1.6 μm on sealing faces and measure the critical features.
- 6Check paperwork and confidentialityConfirm the certificate scope, NDA availability and secure upload. File the reports with the first article.
- 7Scale to productionReview the 3–5 day shipping record and late-delivery rate before releasing a 10,000+ part order.
Common Questions
What tolerance can a performance cnc machining service actually hold?
GreatLight works to ±0.005 mm (±0.0002 in) on qualified features. That figure depends on the feature, the material and the fixture.
For long or thin parts, expect the practical limit to loosen. Ask for the inspection report and judge the number against the method used to measure it.
Do I pay extra for a fine surface finish?
Usually yes, because Ra 0.2–0.8 μm needs a separate operation. Ra 1.6–3.2 μm is the as-machined baseline.
If the finish is only needed on one sealing face, mark it on the drawing. Applying a fine finish to the whole part adds cycle time for no benefit.
Is there a minimum order quantity?
There is no minimum order quantity. A single prototype and a 10,000+ part run are both possible.
The first piece carries most of the setup cost. If a single-piece price looks unusually low, check for separate programming or tooling lines.
Which certifications matter for my industry?
ISO 9001:2015 is the baseline. Automotive and engine work usually needs IATF 16949:2016. Medical devices need ISO 13485:2016.
ISO 27001:2022 covers information security. Ask for the certificate scope, since a logo alone does not show which processes are covered.
How fast can parts ship?
Quotation and DFM analysis come back within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days for standard work.
Timing depends on material availability and finishing. Tight finishes and specialty alloys add time, so confirm the schedule at order release.
Can you work under an NDA?
Yes. Uploads are handled as secure and confidential, and an NDA is available on request.
If your program has strict IP rules, raise them before sending files so the retention and access rules are agreed in writing.
Send Your Drawing, Get a Straight Answer
Upload your model and get a quotation with DFM feedback within 12 hours, plus an inspection report with your parts.
12-hour quoteNo MOQ100% inspectionNDA on request
More Machining Notes
We publish setup notes, tooling trials and inspection data from the factory floor.
