Custom CNC Machining Service Factory: 7 Checks Before You Send a PO
This guide is for engineers and buyers comparing custom CNC machining service factory options for metal and plastic parts. It covers the checks that separate a shop that can hold ±0.005 mm across a batch from one that only hits it once.
In this article
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Key takeaways
Which shop fits your part
Use the row that matches your part, not the shop's brochure.
| Part situation | What to ask | What to avoid |
|---|---|---|
| Tight tolerance on a long part | Tolerance over the full length, not per feature | A tolerance quoted without a length |
| One-off prototype | First-article inspection report | A setup fee quoted as a minimum order |
| 10,000+ parts per year | In-process monitoring plan | Sampling from a single first part |
| Medical or automotive part | ISO 13485 or IATF 16949 scope | A certificate with no scope statement |
| IP-sensitive design files | ISO 27001 and a signed NDA | Uploading files before an NDA |
| Deadline under a week | Quoted ship date and inspection time | Verbal promises with no date |
| Mixed material order | Material lot traceability | Material grade listed only as steel |
Send the drawing, get a real answer
Seven checks are enough to tell a capable custom CNC machining service factory from a confident one. Send your drawing and marked dimensions, and we will quote what the part needs, hold the tolerance we quote, and tell you which features cannot be held at that price.
Tolerance: ask what the number actually covers
A custom CNC machining service factory can quote ±0.005 mm and still fail your part. The tolerance means different things on a 20 mm bracket and a 900 mm housing. Ask for the tolerance over the full part length, in the material you are using, on the feature you care about. Aluminum behaves differently from 17-4PH stainless at the same feed rate.
Feature type matters too. A bored hole and a thin wall are not the same problem. Thin walls move when the tool pressure changes, and deep pockets deflect long end mills. When you send a drawing, mark the two or three dimensions that control assembly. A shop that reads those marks will plan the setup around them.
GreatLight works to ±0.005 mm (±0.0002 in) on parts up to 4,000 mm. Surface finish runs from Ra 0.2–0.8 μm on fine work to Ra 1.6–3.2 μm as machined. Both are achievable, but not on the same feature at the same cost. If your drawing asks for a mirror finish inside a deep pocket, expect a second operation or a change in geometry.
The failure mode to watch for is a shop that quotes one tolerance for the whole drawing. That is not tighter work, it is looser thinking. It usually means the quote will come back with a note saying some features cannot be held, after you have already approved the price.
Machine capacity: match the axis count to the geometry
Axis count is a setup decision, not a quality badge. A flat plate with holes on one face is cheaper on a 3-axis machine. A part with angled faces, undercuts, or a contoured pocket is usually better on a 5-axis center because it avoids three or four refixtures. Each refixture adds datum error and labor.
Ask how many spindles the shop can put on your part, not how many it owns in total. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. That mix matters when your part needs turning and milling in one setup, such as a shaft with cross holes.
Size decides the machine class. Travel options include 4,000 × 400 × 150 mm for long parts, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm for mid-size work, and 500 × 500 × 450 mm or 500 × 310 × 200 mm for compact parts. A Ø400 mm rotary table covers round parts that need indexing.
If your part needs a 4,000 mm bed, confirm the shop has one free in your window. Capacity on paper and capacity on the floor are different numbers during a busy month.
Lead time: separate quoting speed from machining speed
Lead time usually breaks into three parts: quote, programming and setup, then machining and inspection. A shop can be fast at one and slow at another. Ask for the date the parts ship, not the date machining starts. Those two answers differ by days on a 5-axis job with a fixture to build.
GreatLight returns a quote and a free DFM analysis within 12 hours. Production can start within 24 hours, and parts typically ship in 3–5 days. Historical late-delivery probability is below 2%. Those numbers assume a released drawing and a confirmed material, so send both with the RFQ.
Inspection time is the part buyers forget. Every part is inspected before shipment, with raw material checks, in-process monitoring, and final inspection. On a 200-piece order that can add a full day. Budget for it instead of treating it as slack.
One more question worth asking: what happens when a feature is out of tolerance at final inspection? A shop with spare capacity reworks or remakes it. A shop without capacity asks you to accept a deviation. Ask which one you are buying.
Certifications and data: read the scope, not the logo
Certificates are only useful if the scope covers your work. ISO 9001:2015 covers a general quality system. IATF 16949:2016 covers automotive production. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which matters when you upload CAD files.
For a medical housing, ISO 9001 alone does not answer the traceability question. Ask how material lots are recorded and how long records are kept. For an automotive bracket, ask about change control and how deviations are approved. These are the questions an auditor will ask you later.
Data handling is a separate risk. Uploads are secure and confidential, and an NDA is available on request. If your drawing contains a patentable geometry, sign the NDA before the upload, not after the first quote. GreatLight uses a standard non-disclosure agreement that can be returned signed with the RFQ.
Do not treat a certificate as a substitute for a first-article inspection report. The certificate says the system exists. The report says your part is correct.
Order size, materials, and finish options
Minimum order quantity is a cost driver that hides in plain sight. Some shops quote a low unit price but charge a setup minimum that makes a 5-piece order expensive. GreatLight has no minimum order quantity, so a single prototype and a 10,000+ part run both start from a real quote.
Material availability decides whether the lead time is real. Common grades are usually in stock: aluminum 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C, and 17-4PH (SUS630); steel 1018, 1045, 4130, 4140, 4340, A36, and tool steel. Copper and brass 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 include ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE, and carbon fibre.
Finish is where quotes diverge most. Anodizing (clear, color, hardcoat, conductive), electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing, and polishing all change the price and the schedule. Laser marking needs a minimum character height of 1.5 mm, so a 0.5 mm logo will not survive the process.
Name the finish in the RFQ. A quote that says "as machined" and a quote that says "hardcoat anodized, matte black" are not comparable, even when the unit prices look close.
Step by step: vetting a custom CNC machining service factory
Run these in order. Stop at the step where the answers get vague.
- 1Send a drawing with marked critical dimensionsMark the two or three dimensions that control assembly. Include material grade, finish, and quantity. This is the only way to get a comparable quote.
- 2Ask for the tolerance over the full lengthAsk for it in your material, on your feature. Reject a single blanket tolerance for the whole drawing.
- 3Confirm the axis count and machine classAsk which machine will run the part and whether the travel covers your largest dimension. Ask about the fixture if the part needs one.
- 4Get a ship date, not a start dateRequest the date parts leave the shop. Include inspection time. Treat a verbal date as no date.
- 5Check certificate scope against your industryMatch ISO 9001, IATF 16949, ISO 13485, or ISO 27001 to your product. Ask how long records are kept.
- 6Sign the NDA before uploading filesReturn the signed agreement with the RFQ. Keep the upload to the files the shop needs to quote.
- 7Request the inspection report on the first articleReview the measured values against your marked dimensions. Approve the process, then release the batch.
Questions buyers ask before the first order
What tolerance can a custom CNC machining service factory actually hold?
GreatLight works to ±0.005 mm (±0.0002 in). Whether that applies to your part depends on length, material, and feature type.
Send the drawing and the shop will confirm which features can hold that number and which need a different approach.
Is there a minimum order quantity for a prototype?
No. Orders run from one prototype to 10,000+ parts with no minimum order quantity.
The first article is inspected and reported, then the process carries over to the batch.
How fast can parts ship after I approve the quote?
Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours, and parts typically ship in 3–5 days.
Those windows assume a released drawing and confirmed material.
Which materials do you machine most often?
Aluminum 6061 and 7075, stainless 304 and 316L, steel 1045 and 4140, and titanium TC4 (Ti-6Al-4V).
Plastics such as POM, PEEK, and PC are common for housings and fixtures. Copper, brass, Inconel, and magnesium are also available.
Can you machine and finish a part in one order?
Yes. Machining and surface finishing can be quoted together, including anodizing, plating, powder coating, black oxide, and blasting.
Combining them keeps one schedule and one inspection record instead of two vendors passing parts between them.
How do you protect design files?
Uploads are secure and confidential, and ISO 27001:2022 covers information security.
An NDA is available on request and can be signed before the first file transfer.
Quote and DFM analysis in 12 hours
Upload your CAD files and marked dimensions. We review the geometry, flag the risky features, and return a quote with a ship date.
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