Online CNC Machining Services: What to Verify Before You Order

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  Key takeaways
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  What to check against what the part actually needs
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  Match tolerance to the machine, not the marketing line
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  Inspection reports protect you more than certificates
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  Material grade and surface finish change the quote
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  MOQ, lead time and the 12-hour quote promise
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  Send the STEP file only after the NDA question is answered
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  Step by step: from CAD file to released purchase order

Most quote pages state a standard tolerance like ±0.005 mm. That number is achievable on a clean 3-axis setup with a rigid tool and a short reach. It becomes unreliable when the feature sits 80 mm deep in a pocket, when the wall is 1.5 mm thin, or when the datum is a cast surface that has not been faced.

When your drawing calls for ±0.001 in or tighter, ask which machine will run the job. A simultaneous 5-axis center can hold position on angled faces and reduce the number of setups, which is where most error creeps in. A shop that only quotes 3-axis work will either refuse the tolerance or hit it by hand-fitting, which does not repeat on the next batch.

Spindle speed and tool runout matter as much as the machine label. A 16 mm carbide end mill with 0.01 mm runout will smear a Ra 0.8 μm finish no matter how new the machine is. Ask how they measure runout and how often they replace holders.

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  Confirm the datumAsk which surface is used for the first setup and whether it is machined or as-cast.
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  Count the setupsEach additional setup adds a stack-up of error, typically 0.01–0.02 mm.
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  Name the finishRa 1.6–3.2 μm is as-machined; Ra 0.8–1.6 μm usually needs a finishing pass.
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  Check thin wallsBelow 2 mm wall thickness, ask about support and clamping pressure.

A certificate tells you the management system exists. An inspection report tells you the part in your box matches the drawing. For online orders, the second document is the one you can act on, because you never stood next to the machine.

Ask for a first article inspection on the first run, with measured values against each critical dimension. On later runs, ask what is checked in process and how often. A shop that checks every 20th part is different from one that checks every part, and the price will reflect that.

Raw material traceability is separate. If your part goes into an aircraft bracket or a surgical instrument, you need mill certificates that match the heat number on the stock. Ask whether they keep the certificate on file and whether they can send it with the shipment. This is a paperwork question, but it is the one that stalls audits.

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  First article inspectionRequest measured values, not pass/fail tick boxes.
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  In-process frequencyAsk how many parts are checked per lot and by whom.
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  Mill certificatesRequired for aerospace, medical and automotive work.
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  Report formatAsk for PDF with datum references so you can compare to your GD&T.

Two quotes for the same part can differ by 40 percent because one supplier assumed 6061-T6 and the other quoted 7075. Both are aluminum, and both machine differently. 7075 cuts cleanly but is more expensive and more prone to stress movement after roughing. If your drawing specifies 7075, say so in the RFQ.

Stainless is the same story. 303 machines freely, 304 work-hardens if the feed is too light, and 17-4PH needs a specific heat treat condition to reach its strength. A supplier who quotes 316L for a part designed in 303 is not reading the drawing.

Surface finish is often the last item added to a quote and the first one cut when the schedule tightens. If your part needs anodizing, electroless nickel or bead blasting, name the process and the target Ra in the RFQ. Vague instructions like 'smooth finish' will come back as as-machined parts with visible tool marks.

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  Name the alloy6061-T6, 7075, 304, 316L, 17-4PH — write the grade, not just the family.
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  Specify heat treatState condition (T6, H900, annealed) so the supplier can plan the sequence.
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  List the finishAnodizing, plating, powder coat, blasting — each adds a step and lead time.
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  Set the Ra targetRa 0.2–0.8 μm for sealing faces; Ra 1.6–3.2 μm for general surfaces.

No minimum order quantity sounds generous, but read what happens at volume. A shop that can run one prototype and a 10,000-part order on the same floor has both the setup discipline and the capacity. Ask how the price steps between 1, 100 and 1,000 parts.

Lead time is where online ordering goes wrong most often. A quote that says '3–5 days' should also say when production starts and what triggers the clock. Does it start when you pay, when the DFM review is approved, or when material arrives? Get the answer in writing.

Shipping is not the same as manufacturing. If your parts need anodizing or heat treat, those are outside processes with their own queue. Ask whether the quoted lead time includes them or stops at the machined part.

Payment terms and revision handling also matter. If your drawing changes at revision C, how is the change priced and does the existing setup survive? A shop that re-quotes from zero for a 2 mm hole shift is not set up for iterative design.

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  No MOQ, verifiedAsk for a written statement and a price for a single unit.
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  Lead time definitionConfirm start trigger, in-house steps and outside processes separately.
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  Volume pricingRequest a price break at 1, 100, 1,000 and 10,000 parts.
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  Revision policyAsk how a drawing change is quoted and whether tooling is reused.

Your CAD file contains the design intent of the product. Uploading it to a quote form before you know who receives it is a real risk. Ask for the confidentiality terms in writing, and ask whether a signed NDA is available before upload rather than after.

A serious supplier will have a standard NDA ready and will tell you who inside the company can see the file. If the answer is 'everyone in the office', that is not a controlled process.

For regulated industries, the same question extends to data storage: where files are kept, how long they are retained, and whether they are deleted after the project closes. ISO 27001 covers information security management, so ask whether the supplier holds it if your product data is sensitive.

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  NDA before uploadSign first, send the STEP file second.
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  Access listAsk who inside the company can open your files.
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  Retention policyConfirm how long files are kept and how deletion is handled.
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  Certification fitISO 27001 for data; ISO 9001 for process; IATF 16949 or ISO 13485 by industry.