How to Choose a CNC Processing Company

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  Key takeaways
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  What to check against what you are buying
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  Start with the machine envelope, not the brochure
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  Lead time is a process, so read the process
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  Certifications and quality control you can audit
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  MOQ, quoting and the traps that inflate price
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  Materials, finishes and industries: where suppliers specialize
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  Step by step: qualifying a CNC processing company

Every CNC processing company publishes a tolerance number. Very few publish the part size that number applies to. A shop holding ±0.005 mm on a 50 mm aluminum bracket is doing something different from a shop holding it on a 900 mm steel plate. Thermal drift and fixture stiffness scale with size, so the tolerance claim has to arrive with a work envelope.

Ask for the travel dimensions of the machines that would run your part. At GreatLight the largest travel is 4,000 × 400 × 150 mm, with medium platforms at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm. If your part sits near the edge of a travel limit, expect the shop to quote a larger machine, which changes the hourly rate.

Count the spindles, not the brand names. A supplier with 127 high-precision CNC machines can move a hot job to another cell when a spindle goes down. A supplier with six machines cannot. This matters most in the two weeks before your build, when a single tool break can push a delivery date.

Five-axis capacity deserves a separate question. Sixteen simultaneous 5-axis machining centers is enough to run compound-angle work without re-fixturing. If a supplier only offers 3-axis plus a tilting vise, your part will be set up three or four times. Each setup adds position error and labor.

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  Get travel numbers in writingX, Y and Z for the specific machine class, not the shop maximum.
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  Match axis count to geometryUndercuts, deep pockets and compound holes justify 5-axis.
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  Check the rotary table sizeA Ø400 mm table limits what can be turned in one chucking.

A quoted lead time of five days means nothing unless you know where the days go. Ask the supplier to break it down: quoting, material purchase, machining, finishing, inspection, packing. A shop that stocks common alloys starts machining sooner. One that orders 7075 plate per job adds purchase lead time before the spindle ever turns.

The first milestone is the quote itself. A quotation and free DFM analysis within 12 hours tells you the shop has an engineer reading models, not a salesperson forwarding PDFs. The DFM notes are the useful part. If the report flags a 0.5 mm floor radius that needs a 3 mm cutter, you have a chance to fix the model before the first chip.

Production start within 24 hours is achievable when the material is on the shelf and the fixture design is simple. If your part needs a custom soft jaw or a vacuum plate, add design and cut time. Do not treat the 24-hour figure as a promise on every geometry.

Shipping in 3-5 days covers standard metals and finishes. Anodizing in a custom color, hardcoat, or electroless nickel can add a day or two depending on rack loading. Ask which finishes are run in-house. In-house tumbling and bead blasting remove a truck trip and the handling damage that comes with it.

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  Split the lead timeMaterial, machining, finishing, inspection. Ask for each.
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  Read the DFM reportIt shows whether an engineer touched your model.
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  Confirm finish locationIn-house plating and anodizing cut days and damage risk.

Certificates are a filter, not a score. ISO 9001:2015 covers process discipline and is the baseline for any serious supplier. IATF 16949:2016 is the automotive requirement and implies tighter change control and traceability. ISO 13485:2016 applies to medical device work and expects documented process validation. ISO 27001:2022 covers information security, which matters when you send CAD files to a factory on another continent.

Match the certificate to the part, not to the company size. A shop with all four certificates still has to assign your job to a qualified cell. Ask who signs the inspection report and what instrument produced the numbers. Calipers are fine for a bracket. A CMM is needed for true position on a housing with six mating holes.

Inspection coverage is where quotes diverge. A 100% inspection before shipment means every part is measured, not sampled. That includes raw material check, in-process monitoring and final inspection. Reports go out on request, so specify the format you need: a first article report, a dimensional layout, or a simple pass/fail sheet.

A qualification rate of 99.99% is only meaningful if the supplier defines what counts as a defect. Ask whether cosmetic scratches on a non-cosmetic surface are counted, and how parts are handled after inspection. A clean measurement in a dirty bin still arrives as a rejected part.

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  Ask for the scope, not the logoWhich process and which plant does the certificate cover?
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  Specify the measurement methodMatch the instrument to the tightest tolerance on the drawing.
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  Define a defect up frontCosmetic vs functional criteria should be agreed in writing.

Minimum order quantity is the fastest way to compare two suppliers. A shop with no minimum order quantity will run one prototype and a 10,000+ part run on the same floor. A shop with a 500-piece minimum forces you to buy inventory before the design is frozen. That inventory becomes scrap when the revision changes.

Quoting method also matters. Some suppliers quote per part plus a setup charge; others fold setup into a higher unit price. Ask for both lines so you can see how the price moves at 10 pieces, 100 pieces and 1,000 pieces. A quote that does not break out material, machining and finishing hides where the cost sits.

Watch for these traps. First, a tolerance on the drawing tighter than the function requires, which forces a second operation and a slower feed. Second, a surface finish specified as Ra 0.2-0.8 μm on a non-sealing face, which adds polishing time. Third, a material listed as "aluminum" without a grade, which lets the shop substitute 6061 for 7075 and change the part's strength.

The reverse trap is under-specifying. A drawing with no datum scheme, no general tolerance block and no finish callout leaves the shop to guess. Guessing is free at quote time and expensive at inspection. One page of clear GD&T prevents most disputes.

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  Ask for a price ladderUnit price at 10, 100 and 1,000 pieces shows the setup share.
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  Name the alloy grade6061, 7075 and 2024 behave differently under load.
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  Do not over-toleranceEvery extra decimal place adds cost without adding function.
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  Lock the datum schemeA clear GD&T frame removes inspection ambiguity.

Most CNC processing companies can cut aluminum. Fewer handle titanium and nickel alloys without burning tools. TC4 (Ti-6Al-4V) and Inconel require lower surface speeds, more coolant and a rigid setup. If your part is titanium, ask how many hours the shop logged on that alloy last quarter. A vague answer is a warning.

Finish capability follows the same pattern. Anodizing in clear, color, hardcoat and conductive variants is standard for aluminum. Electroless nickel, zinc, silver and gold plating appear on electronics and medical parts. Powder coating and black oxide suit steel frames and brackets. Laser marking needs a minimum character height of 1.5 mm to stay legible after plating.

Industry focus changes the paperwork as much as the machining. Aerospace buyers want material certs and full traceability. Automotive and EV buyers want PPAP-style documentation and IATF discipline. Medical device buyers want process validation records. Robotics and industrial machinery buyers usually care about repeatability across a long run more than paperwork volume.

The practical question is not which industries a supplier lists. It is which industries the shop ran last month. Ask for the share of revenue from your sector. A supplier with 40% automotive revenue will have the fixtures and gauges your part needs. One with 5% will learn on your job.

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  Ask about alloy hoursTitanium and Inconel demand different speeds and tooling.
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  Check finish varietyAnodizing, plating, powder coat and laser marking in one flow.
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  Match sector experienceRevenue share by industry predicts how fast the job runs.