CNC Machining Service for Custom Parts: 7 Checks Before You Order

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  Key takeaways
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  What to compare between two CNC suppliers
• 3
  Match the part to the machine before you compare price
• 4
  Read the tolerance and finish numbers as a pair
• 5
  Lead time: separate quoting from production
• 6
  MOQ, certifications and confidentiality
• 7
  Material and finishing choices that change the quote
• 8
  What a usable quote looks like
• 9
  Step by step: how to place a custom parts order

Price comparison only works when both shops are quoting the same process. A bracket with drilled holes and a flat face is a three-axis job. A hydraulic manifold with ports on five faces is not. If one supplier quotes the simple machine for a complex part, the number looks good until the first article fails.

Start by counting setups. Every time the part must be flipped and re-datumed, you add fixture time and stack tolerance. A five-axis center can reach five faces in one setup, which removes that stack on a part like an engine cover or a surgical housing. On a simple plate, five-axis buys you nothing except a higher hourly rate.

Size decides too. GreatLight runs a 4,000 mm maximum processing size for long parts, plus 750 × 1,150 × 550 mm and 600 × 600 × 600 mm travels, and compact 500 × 500 × 450 mm and 500 × 310 × 200 mm machines for small work. Put your part envelope next to that list before you ask for a price.

A useful test: send the drawing and ask which machine they plan to run. A shop that answers in one sentence knows the part. A shop that answers with a price and nothing else is guessing.

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  One setup, few toolsThree-axis is fine and cheaper.
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  Five faces, tight positionFive-axis removes re-datuming error.
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  Long and slenderCheck travel and workholding before tolerance.
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  Thin wallsAsk about tool runout and step-down, not just the machine.

A drawing that says ±0.005 mm on every dimension is usually a drawing that has not been thought through. Tolerance belongs on the features that function: a bearing bore, a dowel hole, a sealing face. The rest can sit at ±0.1 mm and cost far less to produce.

Surface finish moves with tolerance. A sealing face at Ra 0.8–1.6 μm is a normal machined finish. Ra 0.2–0.8 μm needs a finer pass, a sharper tool and often a different insert, which adds time. As-machined at Ra 1.6–3.2 μm is the baseline for non-critical surfaces.

Ask how the shop measures. A tolerance of ±0.005 mm on a bore needs a bore gauge or a CMM, not calipers. If the inspection method does not match the tolerance callout, the number on the drawing is decoration.

One more thing: datum choice. If the drawing calls a face as datum A but the shop holds the part on that same face, you get a conflict. Send the datum scheme with the model, or expect a DFM note back.

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  Tolerance where it functionsLoose everywhere else cuts cost fast.
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  Finish follows toleranceFine finish on a rough part wastes money.
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  Measurement must match±0.005 mm needs a gauge, not calipers.

Lead time claims get vague. Break them into stages. GreatLight returns a quotation and a free DFM analysis within 12 hours. Production can start within 24 hours after that. Parts ship in 3–5 days for typical runs. The historical late-delivery probability sits below 2%.

The stage that slips most often is material. A shop that stocks 6061-T6 and 304 stainless will start tomorrow. A shop that orders 17-4PH or Ti-6Al-4V per job adds a purchase lead time that never appears in the quote. Ask whether the grade is on the shelf.

Second stage to pin down: finishing. Anodizing, electroless nickel and powder coating take days, and outsourcing adds transport. If the finish is on the critical path, ask if it is done in-house.

Third stage: inspection. A 100% inspection before shipment adds time, but it is the reason the late-delivery number stays low. Do not let a supplier skip it to win a date.

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  Quote in 12 hoursIncludes free DFM analysis.
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  Production start in 24 hoursCounted from approved drawing and PO.
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  Ship in 3–5 daysTypical runs, not every geometry.
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  Finishing is the hidden stageIn-house or subcontracted changes the date.

There is no minimum order quantity at GreatLight. You can order one prototype or a 10,000+ part run. That matters because the first unit is where the geometry gets proven. A shop that only wants volume will quote the prototype high to push you away.

Certifications are not stickers. ISO 9001:2015 covers the general quality system. IATF 16949:2016 applies to automotive work and adds traceability and change control. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your drawings are the product.

Ask for the certificate scope, not the logo. A scope that excludes machining, or a certificate that expired last year, tells you the system is not maintained.

Confidentiality is a separate track. If your part is unreleased, sign an NDA before you upload the model. Uploads are secure and confidential, and a non-disclosure agreement is available on request. Do not send a full assembly to a shop that has not agreed in writing.

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  No MOQOne prototype to 10,000+ parts.
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  Match the certificate to the industryIATF for automotive, ISO 13485 for medical.
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  Check scope and datesA logo is not a system.
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  NDA before uploadEspecially for unreleased geometry.

Material choice drives both price and risk. Aluminium 6061-T6 machines fast and holds tolerance. 7075 is stronger and cuts slower. Stainless 303 is free-machining, while 316L resists corrosion but work-hardens. Titanium TC4 (Ti-6Al-4V) needs lower cutting speeds and more tool changes. Inconel is slower again.

Plastics behave differently. POM and ABS machine cleanly. PEEK holds dimension at high temperature but costs more per kilogram than most metals. Carbon fibre is abrasive and dulls tooling fast.

Finishing adds cost in proportion to masking. Anodizing in clear or colour is common. Hardcoat anodizing builds a thicker oxide layer and changes the dimension by a few micrometres per surface, so a tight bore must be masked or cut undersize before coating.

Ask what the finish does to the dimension. A shop that treats finishing as a cosmetic step will miss the bore. Laser marking needs a minimum character height of 1.5 mm, so plan part marking early rather than after the first batch.

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  Aluminium is the default6061-T6 for most brackets and housings.
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  Titanium and Inconel are slowExpect more machine time, not just material cost.
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  Hardcoat moves dimensionsMask tight bores or cut undersize.
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  Plan laser markingMinimum character height 1.5 mm.

A single number is not a quote. You want to see material, machine time, finishing, inspection and any tooling or fixture cost as separate lines. That way, when you change one feature, you can predict the cost move.

Ask what is not included. Deburring, cleaning, packaging and freight are common omissions. So is the second setup for a feature that needs a re-datum.

Check the revision reference. A quote tied to drawing revision C should not be used to release revision D parts. If the shop does not print the revision on the quote, ask for it.

Finally, ask what would make them come back to you. A shop that flags a thin wall, an impossible corner radius or a tolerance stack during quoting is doing the DFM work you are paying for. The free DFM analysis at quoting time is where money is saved.

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  Itemized linesMaterial, machine time, finish, inspection.
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  List the exclusionsDeburr, clean, pack, freight.
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  Revision on the quoteRelease only against that revision.