Precis Part CNC Machining Service: How to Choose Without Guesswork

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  Key takeaways
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  What to compare before you send a PO
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  Tolerance and inspection: the first thing to verify
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  Machine mix and the parts it can actually cut
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  Lead time, MOQ and how a quote is built
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  Materials, finishes and where shops quietly say no
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  Certifications and confidentiality as risk filters
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  Step by step: qualifying a supplier in one week

Every shop will quote you a tolerance. Fewer will tell you how they measure it. A ±0.005 mm callout on a drawing means nothing if the shop checks the feature with calipers on the bench. Ask which instrument closes the loop: a CMM, a vision system, a micrometer with a known calibration date. If the answer is vague, treat the tolerance as marketing.

At GreatLight, inspection runs in three stages. Raw material arrives and gets checked against the mill certificate. Parts are monitored in process, not just at the end, so a drifting dimension is caught before a batch is wasted. Then 100% inspection happens before shipment, with reports available on request. That last point matters for regulated buyers who need a paper trail.

The finish callout is a separate conversation. Ra 1.6–3.2 μm is a normal as-machined surface. Ra 0.8–1.6 μm needs a controlled pass and often a different tool path. Ra 0.2–0.8 μm is a polishing or fine-finishing operation. If your print says Ra 0.4 μm but your budget assumes as-machined pricing, the quote will look wrong for a reason you can now name.

Ask one more question: what happens to a part that fails final inspection? A shop that reworks or remachines it on its own cost is telling you something about how it prices risk. A shop that ships it and waits for you to complain is telling you something else.

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  Tighter than ±0.005 mmNot a standard process. Expect a separate feasibility review and a different price band.
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  First article inspectionWorth requesting on any new part number, even when it is not contractually required.

A supplier's machine list tells you which geometries are routine and which are a favor. 16 simultaneous 5-axis machining centers handle contoured surfaces, angled holes and undercuts in one setup. That reduces the number of times a part is unclamped, and every unclamping is a chance to lose position. If your part has compound angles, 5-axis is not a luxury.

For prismatic work with features on four faces, 12 four-axis mills and 27 three-axis machines cover the bulk of production. 16 mill-turn centers matter when a part is mostly round but has milled flats or cross holes, because turning and milling happen without a second fixture. Fewer setups, tighter concentricity.

Size decides the rest. The largest travel is 4,000 × 400 × 150 mm, which suits long extrusions, rails and structural profiles. Medium travels of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm cover most enclosures and housings. Compact travels of 500 × 500 × 450 mm and 500 × 310 × 200 mm are where small, high-density parts with many features live. A Ø400 mm rotary table adds indexed work on round parts.

The practical test: send the shop two parts, one easy and one awkward, and see whether both quotes come back with the same confidence. If the awkward part gets a price but no process note, be careful.

Lead time is a capacity statement. A shop with 127 high-precision CNC machines across three wholly-owned plants can absorb a rush job without pushing another customer out. GreatLight quotes and returns a free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days. Historical late-delivery probability sits below 2%.

MOQ is where small buyers get squeezed. Some shops will not quote under 500 pieces, which kills a prototype program. GreatLight runs no minimum order quantity, from a single prototype to 10,000+ part runs. That means the same supplier can carry your part from first article to production, and the process knowledge does not get lost in a handoff.

Read the quote structure carefully. A number with no material grade, no tolerance restatement and no finish callout is not comparable to a detailed one. Ask for the DFM note to be included. That note is where a shop tells you which feature is expensive and why, and it is often worth more than a small price difference.

One trap: treating the lowest prototype price as the production price. Prototype pricing absorbs setup across one part. Production pricing spreads it over thousands. A shop that is cheap on one and expensive on the other is optimizing for the wrong stage of your program.

Material range is a good proxy for experience. Aluminum 6061, 7075, 2024 and ADC12 behave very differently. Stainless 303 machines freely, 316L work-hardens, and 17-4PH needs a heat-treat plan that affects final dimensions. Titanium TC4 (Ti-6Al-4V) and Inconel demand low cutting speeds and rigid setups. A shop that lists these grades has likely cut them before.

Plastics are their own problem. POM and PEEK hold tolerance well; ABS and PP move with temperature and clamp pressure. Carbon fibre eats tooling. If your part is plastic and the shop only talks about metal, ask for a specific example before you commit.

Finishing is where scope creep hides. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating and black oxide all change dimensions by a few microns. Bead blasting, tumbling, brushing and polishing change surface texture. Laser marking has a minimum character height of 1.5 mm, so a fine part number on a small part may not be legible.

The quiet refusal is the dangerous one. A shop that says yes to everything has not read your print. A shop that flags a thin wall, a deep pocket or an unmeasurable callout is doing you a favor early, when a design change is still cheap.

Certifications are not decoration. Each one maps to a market. ISO 9001:2015 covers a general quality system. IATF 16949:2016 is the automotive and EV baseline, and it comes with traceability expectations. ISO 13485:2016 is the medical device standard, where process validation and records are audited. ISO 27001:2022 covers information security, which matters when your CAD files are the asset.

If you sell into any of those markets, a supplier without the matching certificate adds work to your own audit. You will be the one documenting their process. That cost rarely shows up in the quoted part price, but it shows up in your engineering hours.

Confidentiality is a separate track. Secure uploads and an NDA available on request are the minimum. For defense-adjacent or unreleased consumer products, ask how files are stored, who can open them, and when they are deleted. A shop that cannot answer that question in writing is a risk you are choosing to take.

None of this replaces a sample. Run a small order through the full flow, including inspection reports and finish, before you commit a production volume.