How to Compare OEM 3 Axis CNC Machining Companies

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  OEM 3 Axis CNC Machining Companies: Screening Grid
• 2
  What an OEM 3 Axis Machining Partner Actually Has to Do
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  Ask for Dimensional Data From Production, Not a Golden Sample
• 4
  Find Out Who Owns the Post-Processing Line
• 5
  Judge Capacity Against Your Largest and Smallest Parts
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  Verify Certifications at the Accreditation Body
• 7
  Run a Challenge Part Before You Commit Volume

Three-axis milling is the workhorse of metal cutting. The spindle moves in X, Y and Z, the part stays on one plane, and every pocket, slot, face and hole pattern that can be reached from that direction gets cut in one setup. It is simple geometry, and that is why it stays cheap. The complexity is not in the machine. It sits in what surrounds it.

When a supplier calls itself an OEM partner, the expectation changes. They take your CAD model, run design-for-manufacturability checks, flag thin walls and tool reach problems, hold a tolerance band across thousands of parts, and ship with paperwork that survives an audit. A shop with three vertical machining centers can do the first part. Fewer can do the ten-thousandth.

This is the gap buyers run into. Two quotes come back within 15 percent of each other, both promise ±0.005 mm, both list anodizing. Then the production run arrives with a finish mismatch between lots, or a reamed hole drifting 0.02 mm because nobody checked thermal growth across a long cycle.

So the comparison should not be about machine lists. It should be about evidence. Who can show dimensional data from real runs, who keeps the finishing line inside the building, and who gives you a written DFM note before asking for a purchase order. Those three questions cut through most of the noise.

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  Direct manufacturer vs brokerA broker may route your job well, but accountability gets thin when volume ramps or a finish spec tightens.
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  Single-process vs integratedIf turning, milling, heat treat and finishing sit with four vendors, your lead time belongs to four schedules.
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  Sample vs production capabilityOne good sample proves the machine. It does not prove the process window.

A first article inspection report tells you the setup was right on the day someone measured it. It does not tell you what happens on part 800. Ask instead for a capability summary from a comparable job: nominal, tolerance band, measured spread, and how many parts were checked. If the supplier cannot produce one, that is your answer.

Tolerance claims are easy to write. Holding ±0.005 mm on a 6061 aluminum housing across a 4,000 mm bed is a different problem than holding it on a 40 mm bracket. Heat, tool wear, clamping pressure and chip load all move the number. Shops that hold tight bands use in-process probing, not just a final inspection bench.

On a recent bracket program, the difference between the two setups came down to probing. One shop ran the batch and checked at the end. The other probed the datum every 20 parts and compensated. Same machine model, same fixture concept, different scrap rate.

Ask what happens when a dimension trends toward the limit. The good answer involves a documented reaction: stop, re-probe, adjust the offset, log it. The weak answer is a promise that it will not happen.

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  Request the raw numbersA spread sheet with 30 or more measured values, not a single pass/fail line.
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  Ask which features are criticalThe supplier should name the features that drive function, not just the ones easiest to measure.
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  Check the inspection methodCMM, optical comparator or hand tools all have different uncertainty. Ask which was used.

A machined part is half a part. Anodizing, plating, heat treatment, laser marking and bead blasting decide how it looks and often whether it works. When those steps leave the building, your schedule and your quality control leave with them.

The practical question is blunt. Does the shop run its own finishing, or does it send parts out? If it sends them out, ask which vendor, how often, and what the recovery plan is when a batch comes back with color variation. Anodizing clear and anodizing black behave differently across alloys, and a third-party shop may not track your lot history.

GreatLight keeps anodizing in clear, color, hardcoat and conductive versions, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing under one roof. Laser marking and engraving run to a minimum character height of 1.5 mm, which matters on small connectors and medical housings where a data matrix needs to stay readable after plating.

That integration shortens the loop. If a finish problem appears, the shop can re-cut the surface rather than negotiate with an outside plater. It also removes the finger-pointing that eats two weeks when a delivery slips.

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  Ask for the finish sequenceMachining, deburr, finish, mark. Get the order in writing before the first cut.
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  Check masking capabilityThreads, sealing faces and electrical contact areas often need masking. Not every finisher does it well.
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  Confirm rework pathIf a coating fails adhesion, who strips and re-runs it, and how long does that take?

Machine counts are a poor proxy for fit. What matters is the envelope and the spindle hours available to your job. A shop with 127 high-precision CNC machines can still be wrong for you if your part is 3,800 mm long and their largest travel is 600 mm.

GreatLight runs 27 three-axis machines alongside 16 simultaneous 5-axis centers, 12 four-axis mills, 16 mill-turn centers and a Ø400 mm rotary table. The work envelope reaches 4,000 mm maximum processing size, with travel options at 4,000 × 400 × 150 mm, 750 × 1,150 × 550 mm, 600 × 600 × 600 mm, 500 × 500 × 450 mm and 500 × 310 × 200 mm.

That range covers long extrusion profiles, mid-size housings and small precision components on the same floor. For a buyer, the useful comparison is not the total machine list. It is whether one supplier can carry a program from prototype through 10,000+ part runs without moving the tooling to a different plant.

No minimum order quantity helps here. One prototype and a 10,000-part release can go through the same process, which means the fixture and the inspection plan you approved at the prototype stage still apply at volume.

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  Match envelope to partCompare your largest diagonal dimension against the stated travel, not the machine model name.
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  Ask about spindle hoursA busy shop with tight capacity can add more delay than a smaller shop with open machines.
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  Confirm the plantFind out which site runs your job and whether the finishing line is on the same campus.

Certificates are easy to display and easy to misread. A PDF on a website proves nothing about scope. Check that the certificate names the legal entity you are contracting with, the site address, and the standard revision. Then verify the certificate number with the issuing body.

The standards that matter depend on your industry. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive and EV programs. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which matters when you send CAD files and BOMs to an overseas supplier.

GreatLight holds all four. That combination is not common, and it changes the paperwork burden on your side. A medical customer does not need to build a separate supplier audit from scratch. An automotive customer gets a quality system already aligned with production part approval thinking.

One caution. A certificate tells you the system exists. It does not tell you the system is used on your job. Pair the certificate check with a process question: who signs the inspection report, and what happens when a dimension falls outside the band?

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  Match the scopeA certificate for one plant or one product line does not cover every job that leaves the building.
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  Check the revisionISO 9001:2015 and ISO 13485:2016 are current. Older revisions signal a stale system.
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  Ask who auditsInternal audit frequency and corrective action history say more than the certificate itself.

A challenge part is a small paid job that stresses the weaknesses you care about. It should include at least one tight feature, one cosmetic surface, one secondary operation and one inspection requirement that forces a report. Ten to thirty pieces is enough. A single sample is not.

Design the test so a failure is informative. If the part has a 0.05 mm wall, a reamed hole and a masked anodized face, you learn how the shop handles tool deflection, hole sizing and finishing in one pass. Compare the report against the drawing yourself; do not rely on a summary email.

Set the clock. A quote and free DFM analysis within 12 hours, production start within 24 hours and parts shipping in 3–5 days are the working parameters at GreatLight. If a supplier needs two weeks for a ten-piece challenge part, the same shop will not move faster on a 5,000-piece release.

Watch how problems are handled, not just whether they occur. Any shop can have a bad dimension. The useful signal is a clear note explaining the cause, the correction and the re-inspection result. That is what you are really buying.

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  Include a cosmetic specSurface finish Ra 0.8–1.6 μm on a visible face tests polishing and handling discipline.
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  Force a reportAsk for measured values on three features, not a pass/fail stamp.
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  Price the reworkAsk what a rejected batch costs and who pays for strip and re-coat.