CNC Machining for Custom Parts: 7 Checks Before You Order

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  Key takeaways
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  Which machining route fits your part
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  Check 1: what tolerance does the part really need
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  Check 2: material grade and how it machines
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  Check 3: order size and setup cost
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  Check 4: certifications and what they cover
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  Check 5: how to read a machining quote
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  Seven steps to place a clean order

Tolerance is the first thing a shop prices, so it decides your cost more than material does. A general machining tolerance of ±0.01 mm covers most mating faces, bolt patterns and locating bores. Only add ±0.005 mm (±0.0002 in) where a bearing seat, a seal groove or an optical mount actually demands it.

The reason is measurement, not machine capability. Holding ±0.005 mm on a 200 mm aluminum part means the shop has to control temperature during the cycle and measure on a CMM rather than at the bench. That work is real and it shows up in the quote.

A practical way to set this: mark functional features as critical, leave everything else at the general tolerance block on your title sheet. If a drawing shows every dimension at ±0.005 mm, most shops will either pad the price or ask you to revise it.

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  Functional face±0.005 mm — bearing bores, seal seats, optical mounts
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  Mating face±0.01 mm — covers most bolted joints and locating pins
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  Non-critical±0.1 mm — clearance holes, covers, brackets
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  Surface finishRa 1.6–3.2 μm as-machined is enough unless a seal runs on it

Grade matters more than alloy family. In aluminum, 6061-T6 is the default for structural brackets because it machines cleanly and takes anodizing well. 7075 gives higher strength for aerospace fittings but costs more and cuts slower. 2024 is strong but has poor corrosion resistance unless it is coated.

Stainless is where quotes vary most. 303 machines freely and suits shafts and fittings that see no chloride. 304 and 316 are tougher, work-harden quickly and need slower feeds. 17-4PH gives high strength after heat treatment and is common in medical and aerospace work.

Titanium and Inconel sit at the other end. Ti-6Al-4V (TC4) needs sharp tooling, low cutting speeds and plenty of coolant, so cycle times run several times longer than the same part in aluminum. If a titanium part is not weight-critical, ask whether 7075 aluminum would do the job.

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  Aluminum6061-T6, 7075, 2024, 6082 for most housings and brackets
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  Stainless303 for free cutting, 304/316 for corrosion, 17-4PH for strength
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  Steel1045 and 4140 for shafts, 4340 for high-load parts
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  PlasticsPOM and PEEK hold tight tolerances; ABS and PP are softer

A shop's price per part is mostly setup divided by quantity, plus cycle time. For one to five pieces, setup dominates and you should expect to pay for programming and fixturing once. GreatLight runs from a single prototype to 10,000+ part runs with no minimum order quantity, so the same route is available at both ends.

Between roughly 50 and 500 parts, machining still competes well against tooling-based processes for metal parts, especially when the geometry is simple and the tolerance is tight. Above that, the question changes. If the part is plastic and the geometry allows it, injection molding spreads tooling cost across enough units to win on unit price.

For metal at high volume, die casting and then machining only the critical faces is often the cheaper path. It depends on wall thickness and how much of the surface needs to be machined after casting. Machining a casting to final size on two or three faces is normal; machining the whole surface is a sign the casting was not designed well.

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  1–5 partsSetup-heavy; expect programming and fixture cost once
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  50–500 partsCNC still competitive for metal, tight tolerance and simple geometry
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  5,000+ plastic partsCompare against injection molding before committing
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  5,000+ metal partsCompare against die casting plus finish machining

Ask which certificate applies to the plant that will run your part, not to the group as a whole. ISO 9001:2015 covers general quality management and is the baseline. IATF 16949:2016 is what automotive and EV programs expect, and it brings traceability and change-control requirements with it.

Medical work usually asks for ISO 13485:2016, which focuses on process validation and record retention. ISO 27001:2022 is about information security, and it matters when you are sending CAD files and drawings to an outside shop. GreatLight holds all four, plus secure upload handling and an NDA available on request.

Certification is not a substitute for inspection data. Ask how the part is checked and whether reports come with the shipment. A shop that inspects 100% before shipment and can send raw material checks, in-process monitoring and final inspection reports is easier to audit than one that only shows a wall certificate.

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  ISO 9001:2015General quality management baseline
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  IATF 16949:2016Automotive and EV programs, with traceability
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  ISO 13485:2016Medical devices, process validation and records
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  ISO 27001:2022Information security for your CAD files and drawings

A quote that only shows a number is hard to compare. The useful ones name the material grade, the tolerance class, the surface finish, the inspection method and the lead time. If two quotes differ by 40% and only one lists those items, the cheaper one is usually quoting a looser part than you asked for.

Watch the units and the finish callout. Ra 0.2–0.8 μm is a fine finish that may need extra passes or a secondary operation; Ra 0.8–1.6 μm is a normal high-quality machined finish. If your drawing says Ra 0.4 μm across a large face, ask whether the function justifies it.

Lead time should also be specific. A shop that can return a quotation and free DFM analysis within 12 hours, start production within 24 hours and ship in 3–5 days is telling you its queue is under control. Ask what happens when a feature is unmachinable, because a good shop flags it before cutting metal.