What a CNC processing UK expert checks before the first cut

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  How a CNC processing UK expert reads the tolerance callout
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  Feature access: what fits in one setup and what does not
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  Material behavior changes the cutting parameters
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  How inspection closes the loop
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  Lead time, quantity and the cost of small runs
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  Which machine class fits which part

Every quote starts with the tightest tolerance on the drawing. A general note of ±0.1 mm and a single bore at ±0.005 mm are two different jobs. The tight dimension pulls the machine, the fixture and the inspection method with it, even if the other forty features are loose.

On a 3-axis machine, the operator often has to reposition the part to reach the far side of a bore. Each reposition adds error. A 5-axis center keeps the part in one setup and tilts the spindle instead, so the datums stay related to each other. That is the usual reason a tight position tolerance is quoted on a five-axis machine rather than a three-axis one.

Position tolerance is the harder number to hold, not size. A Ø20 mm bore is easy to cut to ±0.02 mm. Holding that bore within 0.01 mm of a second bore 300 mm away is where the setup shows up. Ask yourself which dimensions actually control function. If two holes only need to line up with a clearance bolt, a generous position tolerance saves money and does not hurt the part.

The same logic applies to flatness and parallelism. A sealing face at 0.02 mm flatness over 200 mm is a real machining cost. The same face at 0.1 mm may still seal with the right gasket. A CNC processing UK expert will ask which one the assembly needs before quoting, not after.

Tolerance also sets the inspection plan. If a drawing calls for ±0.005 mm on a critical bore, that bore gets measured with a CMM or an air gauge, not calipers. The measurement method has to be at least four times finer than the tolerance it verifies. Anything less is a guess with a number printed on it.

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  Loose general tolerancesKeep the title block at ±0.1 mm unless function demands tighter.
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  Tight only where it mattersCall out ±0.005 mm on the two or three features that control fit.
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  Match the gauge to the toleranceA 0.01 mm tolerance needs a CMM, not a vernier.
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  Datum strategy firstPick datums the machine can reach in one setup.

The second check is whether the part can be machined without flipping it five times. Every extra setup adds a fixture, a re-clamp, and a fresh chance to lose position. A part with features on four faces is a natural candidate for a five-axis or a mill-turn center.

Look at the depth-to-diameter ratio on pockets and bores. A Ø10 mm end mill reaching 60 mm deep is a 6:1 ratio. It will chatter unless the tool is stepped down in shallow passes, and the finish will suffer. Tell the machinist early. A shorter tool or a wider pocket may remove the problem entirely.

Thin walls are the other common trap. An aluminium wall 0.8 mm thick over 80 mm length will deflect under cutting force. The part may measure correctly on the machine and spring out of tolerance once unclamped. Rough, stress-relieve, then finish. That sequence costs time but it is the only way to hold the wall.

Undercuts and internal grooves need a tool that can reach them. If the drawing shows a groove behind a shoulder with no relief, no standard cutter will get there. The feature has to be redesigned or the part split. Finding this at the quote stage costs a drawing revision. Finding it at the machine costs a week.

Deep holes drilled from both ends need to meet. A 200 mm through hole at Ø6 mm is a gun-drilling job, not a twist-drill job. The two ends will not align by luck. Either accept a larger diameter or plan for a dedicated deep-hole process.

For large parts, the work envelope decides everything. Our largest travel is 4,000 × 400 × 150 mm. A part longer than that has to be split or machined on a different platform. It is better to know this before the RFQ than after.

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  Count the setupsFour-face features usually belong on a 5-axis center.
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  Watch 6:1 depth ratiosLong, thin tools chatter; reduce depth or widen the pocket.
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  Thin walls need sequencingRough, relieve stress, then finish to final size.
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  Check the work envelopeMaximum travel here is 4,000 × 400 × 150 mm.

Aluminium 6061 and 7075 are not the same job. 6061 cuts clean at high speed and is forgiving. 7075 runs harder, work-hardens at the edge, and needs sharper tools and lighter feeds. The same drawing in two alloys can carry different cycle times and different risk.

Stainless 304 work-hardens the moment a tool rubs instead of cuts. Feed too light and the surface hardens under the cutter. The next pass then breaks the edge. 303 machines far more freely because of the added sulfur. If the part does not need corrosion resistance beyond 304's level, 303 often cuts the cost.

Titanium TC4 (Ti-6Al-4V) has low thermal conductivity. Heat stays in the cutting zone and goes into the tool. Speeds drop, coolant flow rises, and tool life is short. Inconel is worse. Both are machinable here, but the quote will reflect slower metal removal.

Plastics behave in the opposite direction. POM and PEEK machine well but move with temperature. A PEEK part measured hot will be undersize when it cools. Let it stabilize before final inspection. ABS and PC are softer and tend to burr, so a finishing pass and a light deburr are part of the plan.

Hardness matters too. 17-4PH in the annealed condition cuts like a moderate stainless steel. After aging to H900 it is a different material. State the heat-treat condition on the drawing. Machining before or after heat treatment is a decision that changes the whole route.

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  Name the alloy and temper6061-T6 and 7075-T6 behave differently.
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  Stainless needs a real feedLight feeds work-harden 304 and break tools.
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  Titanium holds heatLower speeds and heavier coolant flow.
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  Plastics move with heatLet PEEK and POM stabilize before measuring.

Machining a part to ±0.005 mm is only half the job. Proving it is the other half. A CNC processing UK expert will tell you which dimensions get measured, on what instrument, and at what stage. If the answer is vague, the tolerance claim is vague too.

In-process monitoring catches drift before the last part is cut. A tool wears, the machine warms, and a dimension that was nominal at 8 a.m. may be 0.01 mm high by noon. Measuring at fixed intervals and adjusting the offset keeps the run centered.

Final inspection happens after the part reaches room temperature and after any deburring or finishing steps. A bead-blasted or anodized surface changes the dimension slightly. Coating thickness of 10 to 25 μm is enough to move a tight fit. Plan the finish before the final cut, not after.

For medical and automotive work, the paperwork matters as much as the measurement. ISO 13485:2016 and IATF 16949:2016 both require traceability from raw material to shipped part. Reports are available on request. If you need a first article inspection report, say so at the RFQ stage.

We inspect 100% of parts before shipment and record the results. Raw material certificates, in-process readings and final inspection data stay with the job. For a production run, that record is what protects you when a question comes back six months later.

One last point on metrology. A tolerance of ±0.005 mm sits at the edge of what a hand tool can resolve. If your drawing mixes that with a general ±0.5 mm note, make sure the inspector knows which is which. Ambiguous drawings get quoted conservatively, and conservative quotes cost more.

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  Ask for the inspection planWhich features, which instrument, which stage.
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  In-process checks stop driftTool wear moves dimensions during a run.
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  Finish before final measureAnodizing and plating shift tight fits.
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  Reports on requestRaw material, in-process and final data.

A single prototype and a 10,000-part run take different routes through the shop. The prototype is programmed, set up and machined in one or two days. There is no minimum order quantity here, so one part is a normal job, not a favor.

Production runs amortize programming and fixturing across many parts. The per-part price drops, but the first article still has to be approved before the run continues. Skipping that step is how a whole batch ends up wrong.

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours of approval, and parts usually ship in 3–5 days. Those numbers assume the drawing is complete and the material is in stock. A missing heat-treat callout or a specialty alloy will extend the schedule.

Historical late-delivery probability is below 2%. That figure is not a promise for your specific job. It reflects how the shop has run across past orders. If your program has a hard date, say so early and the route can be planned around it.

Volume also changes the process choice. Twenty parts can be milled from billet. Two thousand parts may be cheaper as a die casting with a machined interface. The crossover point depends on geometry, but it is worth asking about before you commit to one method.

Shipping to the UK is routine. Parts are packed to protect finished surfaces, and documentation travels with the shipment. If you need a specific packing method for a coated or polished part, note it on the order.

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  No minimum order quantityOne prototype or 10,000+ parts, same route.
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  First article before the runApprove one part, then release the batch.
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  Quote in 12 hoursDFM feedback comes with the quotation.
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  Process crossoverHigh volume may favor casting plus machining.