Sourcing guide for engineers

How to Find CNC Machining Work

A practical sourcing process for engineers and buyers who need a machine shop that can hold the drawing. We cover what to define before you email anyone, which channels actually return usable quotes, and how to validate a shop with one sample order before you commit to a production run.

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Key takeaways

Define before you searchA vague RFQ returns a vague price. Fix material, tolerance, finish and volume first.

Match process to geometry3-axis, 5-axis or mill-turn changes the shop list you should be calling.

Vet with a real partOne paid sample tells you more than ten capability pages.

Read the DFM notesA shop that flags thin walls and deep pockets is reading your model.

Keep two sources liveOne primary, one backup, both already tooled and inspected.

Step zero

What to define before you look for a shop

Most bad quotes start with a bad request. Before you search for anyone, write down the four numbers that decide which machines can run the part: envelope size, tightest tolerance, surface finish and annual volume. A 40 × 30 × 12 mm bracket at ±0.05 mm and Ra 1.6–3.2 μm fits dozens of shops. A 900 mm long housing at ±0.005 mm with a Ra 0.2–0.8 μm bore does not.

Tolerance drives cost faster than any other line on the drawing. Going from ±0.05 mm to ±0.005 mm usually means more setups, more in-process checks and slower feed rates. Ask yourself whether the tight callout is functional or inherited from an old revision. Engineers who trim two or three non-critical tolerances often cut quote spread in half without touching part performance.

Finish is the second filter. An as-machined Ra 1.6–3.2 μm surface needs no extra operation. If the drawing calls for anodizing, electroless nickel or bead blasting, the shop must either run finishing in-house or control an outside vendor. Mixed supply chains are where finish specs quietly drift.

Finally, state the volume and the timing in the same email as the model. One prototype and a 10,000 part run are different sourcing problems, and a shop that is excellent at one may be wrong for the other.

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Model formatSTEP or IGES for 3D, PDF for the controlled drawing with GD&T.
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Critical featuresList the two or three dimensions that actually matter for function.
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Volume rangeGive a prototype quantity and a production estimate together.
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Compliance needsName the standard you must report against, not just the certificate.

Channels

Using targeted channels to identify providers

Keyword searches return hundreds of shops and almost no signal. A faster filter is capability-first: search the process, not the word. If your part has undercut pockets on five faces, look for simultaneous 5-axis capacity. If it is a turned shaft with milled flats, look for mill-turn centers. The machine list tells you whether the shop can finish the part in one setup or three.

Trade directories and sourcing platforms are useful for building a long list, but treat their rankings as advertising. Sort by process capability and industry, then check the shop's own site for the machines, the inspection equipment and the materials it actually stocks. A shop that lists 7075 aluminum and 17-4PH stainless in its standard material set is set up for real work, not one-off favors.

Industry events and local manufacturing clusters still matter. A shop inside a dense supplier region can pull in heat treating, plating and laser marking without long transit. That reduces the chance of a finish or hardness spec being lost between vendors.

Referrals from your own supply chain are the strongest channel. Ask your sheet metal vendor, your toolmaker or your molder who they use for machined details. They have already paid the tuition on quality and delivery, and they will tell you who missed dates.

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Search by process5-axis, mill-turn, EDM, grinding — not just CNC machining.
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Check machine envelopeCompare the shop's travel figures against your part size plus fixturing.
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Confirm in-house finishingAnodizing, plating and laser marking under one roof reduce hand-offs.
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Ask for the inspection listCMM, optical comparator, surface roughness tester, hardness tester.

Vetting

Vetting a machine shop before you send a PO

Capability claims are easy to publish and hard to verify. Start with the process chain. A shop that can mill, turn, heat treat, finish and inspect inside its own walls has fewer places for a spec to fall out. GreatLight, for example, runs 127 high-precision CNC machines across three wholly-owned plants, including 16 simultaneous 5-axis centers and 16 mill-turn centers, with finishing and inspection on site.

Then look at the measurement side. A tolerance claim is only as good as the instrument behind it. Ask what equipment checks the feature you care about, and ask for a sample inspection report with the same feature types: bores, flatness, position, surface roughness. If the report shows only overall dimensions, it will not tell you whether the critical bore is in position.

Certifications narrow the field for regulated work. ISO 9001:2015 covers general quality systems. IATF 16949:2016 applies to automotive production. ISO 13485:2016 is the medical device standard, and ISO 27001:2022 covers information security for your CAD data. Match the certificate to your industry instead of collecting logos.

Communication is the last filter and often the deciding one. Send a technical question with your RFQ, something specific about a deep pocket or a thin wall. A shop that answers with a DFM note and a suggested change is reading the model. A shop that answers with a price and a lead time is quoting a number.

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Ask for the setup planHow many operations, and which features are cut in each.
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Request the inspection reportSame feature types as your critical callouts.
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Check data handlingNDA available on request; uploads kept secure and confidential.
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Test response timeQuotation and free DFM analysis within 12 hours is a working benchmark.

Quotes

How to read CNC quotes and compare them fairly

Quotes are only comparable when the inputs match. If one shop prices a 50 piece run and another prices 500, the unit numbers say nothing about either shop. Send the same model, the same drawing revision, the same material condition and the same finish callout to every candidate. Note the revision letter in the RFQ so nobody quotes an old file.

Break the price into material, machining, finishing and inspection if the shop will share it. That breakdown shows where the money goes and which lever to pull. On a small aluminum bracket, material is often a minor line and setup dominates. On a titanium housing, material and tool wear can outweigh the machine time.

Watch for missing scope. A low number that excludes anodizing, laser marking or a required inspection report is not a low number. It is a partial one. Ask each shop to state what is included and what is quoted separately, then add the pieces back before you compare.

Lead time deserves the same scrutiny. Ask when production can start, when the first article ships, and what happens if a feature fails inspection. A shop that inspects 100% before shipment and shares results on request gives you a recovery path. GreatLight quotes in 12 hours, can start production within 24 hours, and ships parts in 3–5 days on standard work.

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Same inputsOne model, one drawing revision, one material spec, one finish spec.
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Scope listMaterial, machining, finishing, inspection, packaging, freight.
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First articleConfirm whether the first part is inspected and reported separately.
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Recovery planAsk what happens if a critical dimension misses on the first run.

Workflow

Step by step: from RFQ to approved supplier

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1. Build the sourcing packageExport a STEP file, attach the controlled PDF drawing with GD&T, and add a one-page brief: material grade and condition, tightest tolerance, finish spec, prototype quantity, production estimate and required standard. Note the drawing revision.
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2. Shortlist five shops by processFilter on machine envelope against your part size, then on the process your geometry needs. Keep shops with in-house finishing and inspection if your drawing calls for both. Five is enough to see a price range without drowning in replies.
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3. Send one identical RFQSame files, same questions, same deadline to all five. Add two technical questions about your hardest feature so you can judge the engineering response, not just the number.
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4. Compare scope, not just priceNormalize every quote to the same deliverables. Add back excluded finishing, inspection and freight. Flag any quote that ignores a callout on the drawing.
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5. Run a paid sample orderOrder one to three parts. Inspect them on your own CMM or send them to a third-party lab. Check the critical bore, the flatness and the surface finish, not just the outside profile.
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6. Review the inspection dataAsk for the dimensional report with the sample. Compare measured values against the drawing, and check whether the shop flagged any feature it struggled with.
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7. Approve and set a backupRelease production to the winner and keep a second qualified shop on file. A proven backup protects your schedule when volumes spike or a machine goes down.

Decision table

Which shop fits which job

Match the part, the volume and the compliance need to the shop type before you send an RFQ.

Job profile	Best-fit shop	Why	Watch for
One-off prototype, simple geometry	3-axis job shop	Low setup cost, fast turn	No DFM feedback
Complex 5-face part, tight position	5-axis shop	One setup, fewer datum shifts	Limited machine capacity
Turned shaft with milled flats	Mill-turn shop	Turning and milling in one cycle	Bar size limits
Anodized or plated housing	Shop with in-house finishing	No finish hand-off	Outside vendor delays
Automotive production part	IATF 16949 shop	Process control and traceability	Higher documentation load
Surgical or implant component	ISO 13485 shop	Clean handling and validated process	Narrow material list

FAQs

Frequently asked questions

How many shops should I ask for a quote?

Three to five is the working range. Fewer than three and you cannot see the market spread. More than five and the comparison work outweighs the benefit, especially when each shop asks different clarifying questions.

If the part is high value or the volume is large, run two rounds: a short technical screen first, then a full quote from the three shops that answered the engineering questions well.

What tolerance should I put on the drawing?

Put the tightest tolerance only where function requires it. A general tolerance block of ±0.1 mm with two or three critical callouts at ±0.005 mm is cheaper and easier to inspect than a drawing where every dimension is tight.

Ask the shop which callouts drive the setup count. If a tight dimension sits on a non-critical surface, loosening it often removes an operation.

Do I need a 5-axis shop for my part?

Only if the geometry needs it. Parts with pockets or features on four or five faces, or with angled holes that would need custom fixtures on a 3-axis machine, are strong 5-axis candidates.

Simple prismatic parts with features on one or two faces are usually cheaper on a 3-axis machine with a vise and soft jaws.

How do I check a shop's quality claim?

Ask for an inspection report on a part with features similar to yours, and ask which instrument measured each feature. A tolerance claim without the matching metrology is a marketing line.

Then buy one sample and measure it yourself. One measured part beats a page of capability statements.

How do I protect my CAD data?

Send models only to shops you have shortlisted, and ask about their data handling before you upload. GreatLight works under NDA on request and treats uploads as secure and confidential.

Keep the drawing revision controlled on your side so you know exactly which file each shop quoted.

What should I do if the first article fails?

Ask for the measured values first, then decide whether it is a setup issue or a capability issue. A single out-of-tolerance feature on an otherwise good part is usually fixable with a fixture change or a tool offset.

If several critical features miss, stop and re-evaluate the shop. Do not release production hoping the next run will be better.