What a CNC Suffolk Expert Actually Checks Before Cutting Metal

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  Tolerance Is a Process Question, Not a Wish
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  When Five Axes Beat Three, and When They Do Not
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  Material Choice Changes the Whole Setup
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  Inspection Evidence Is the Real Deliverable
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  Lead Time, Quantity, and Where Risk Sits
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  Which Process Fits the Part

A drawing that says ±0.005 mm is not a request. It is a statement about which machines, which material, and which setup can deliver the number. Before quoting, a CNC Suffolk expert reads the tightest tolerance on the print, then works backward to the process that can hold it. A ±0.05 mm bracket and a ±0.005 mm medical housing are different jobs, even when the geometry looks similar.

The chain matters more than the machine. Spindle thermal growth, tool wear, chip load, and the rigidity of the fixture all push the part off nominal. On a 4,000 mm long part, thermal drift over a long cycle can eat most of a tight tolerance before the operator measures anything. That is why long parts often get roughed, cooled, and finished in a separate pass.

The practical question is margin. If a feature needs ±0.005 mm and the process naturally holds ±0.003 mm, the job is safe. If the process holds ±0.005 mm, the job is a coin flip on a bad day. A shop that tells you which side of that line you are on is more useful than one that simply says yes.

Tolerance also interacts with surface finish. A Ra 0.2–0.8 μm bore usually needs a finishing pass that removes very little material, so the roughing allowance has to be planned from the start. Finishing a tight bore from a heavy rough cut rarely works.

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  Read the tightest callout firstIt decides the machine, the setup count, and the inspection plan.
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  Ask about process capability, not machine specA capable process leaves margin; a marginal one does not.
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  Plan finishing allowance earlyFine finishes need light cuts, which need leftover stock.

Five-axis machining is not automatically better. It is better when the part has features on multiple faces, when a single setup removes a stack of tolerance errors, or when the geometry cannot be reached by a straight tool from three directions. An impeller, a manifold with angled ports, or a housing with deep intersecting bores are typical five-axis parts.

For a flat plate with holes, three-axis milling is faster and cheaper. Adding rotary motion only helps if it reduces setups or reaches geometry. A shop with 16 simultaneous 5-axis centers and a mix of 3-axis and 4-axis machines can pick the process that fits the part instead of forcing every job onto the expensive machine.

The setup count is the hidden cost. Every time a part moves to a new fixture, the datum shifts a little and the tolerance budget shrinks. Five-axis work trades fixture complexity for fewer setups, which is often the real reason the tolerance holds.

There are limits. Deep pockets with small corner radii still need long, thin tools that deflect. Five axes do not fix tool deflection; they only improve access. If the feature is a Ø6 mm pocket 40 mm deep in stainless steel, the process is slow no matter how many axes the machine has.

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  Use 5-axis for access and setup reductionNot because the machine sounds more advanced.
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  Keep 3-axis for flat, simple partsIt is faster and easier to inspect.
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  Watch tool length-to-diameter ratioPast roughly 5:1, deflection starts to dominate.

Aluminium 6061-T6 cuts fast and holds tolerance well, which makes it the default for prototypes and fixtures. 7075 is stronger but gummier and more prone to distortion after heavy material removal. Titanium TC4 (Ti-6Al-4V) conducts heat poorly, so the cutting edge stays hot and tool life drops sharply. None of these behave the same way on the same program.

Stainless 316L work-hardens. A light, rubbing cut hardens the surface and makes the next pass worse, so the feed per tooth has to stay above a floor. 17-4PH behaves differently again after heat treatment. Inconel and magnesium AZ31B sit at opposite ends of the risk scale: one is abrasive and slow, the other is light, fast, and a fire hazard if chips are not managed.

Thin walls are their own problem. A 1 mm wall in aluminium can be machined, but the clamping force and the cutting force both push it around. The usual fix is to leave the wall thick, machine the rest, then take the wall down in light passes with reduced radial engagement.

Plastic parts follow different rules. POM and PEEK machine cleanly but move with temperature; ABS and PC are softer and easier to mark. Carbon fibre needs diamond or coated tooling and dust control. Selecting the material is really selecting a set of cutting parameters and a fixturing strategy.

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  Aluminium 6061-T6Fast, stable, good for prototypes and functional parts.
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  Stainless 316LWork-hardens; keep feed per tooth up, avoid rubbing.
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  Titanium TC4Poor heat conduction; expect short tool life and slower speeds.

A part that measures well on the machine is not the same as a part that measures well on a coordinate measuring machine at 20 °C. Thermal expansion alone moves a 100 mm aluminium feature by roughly 0.002 mm per 1 °C. If the shop measures hot, the number is not the number.

GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring, and a final inspection. Reports are available on request. For tight features, that means a CMM report with the datum scheme spelled out, not just a pass stamp. The datum scheme matters because it defines what the tolerance is measured from.

First article inspection is where most disputes are avoided. The first part off the line is measured against every callout, and any feature that is marginal gets flagged before the run continues. On a 10,000-part order, catching a 0.01 mm drift at part 3 is far cheaper than catching it at part 3,000.

Qualification rate is the aggregate number: 99.99% of parts pass inspection. That figure is only meaningful when it is paired with the inspection method. Ask how the number was produced, and on which features.

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  100% inspection before shipmentNot sampling, unless the drawing allows it.
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  Reports on requestCMM data with the datum scheme defined.
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  First article inspectionCatches drift before the run is committed.

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days. Those numbers describe the normal path. They are not a promise for every geometry, and a shop that treats them as one is not being straight with you.

There is no minimum order quantity. A single prototype and a 10,000+ part run go through the same first-article process; the difference is what happens after the first part is approved. Prototypes are usually machined from billet with soft jaws. Production runs get a dedicated fixture and a defined tool-change interval.

Late delivery is the risk buyers ask about most. GreatLight's historical late-delivery probability is below 2%. That is a historical figure, not a guarantee. The way to keep a program on schedule is to freeze the drawing early, confirm the datum scheme, and flag any tolerance that looks tight before the first chip is cut.

Confidentiality is part of the same conversation. Uploads are secure and confidential, and an NDA is available on request. For defense, medical, and automotive programs, that paperwork usually starts before the drawing does.

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  Quote + DFM in 12 hoursIncludes feedback on features that will drive cost.
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  No minimum order quantityFrom one prototype to 10,000+ parts.
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  NDA available on requestUploads are secure and confidential.