Guildford precision CNC machining, explained for engineers

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  What Guildford precision CNC machining changes in the setup
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  How ±0.005 mm is actually held, and when it is not
• 3
  Part shapes that justify a five-axis setup
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  How the number gets verified before parts ship
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  Material behavior and what it does to your tolerance
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  Where a precision CNC quote stops making sense
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  3-axis, 4-axis or 5-axis: picking by part feature

The phrase describes a subtractive process, not a location. A cutter removes metal from a solid block, and every cut is defined by a toolpath, a spindle position and a fixture. What separates tight work from ordinary work is how many times the part is re-clamped. Each new setup adds a datum error that stacks on top of machine error.

On a 3-axis machine the part usually needs 4 to 6 setups to reach features on five or six faces. On a simultaneous 5-axis center the same part can often be finished in 2 setups, sometimes 1. Fewer setups means fewer datums, and fewer datums is where the ±0.005 mm figure comes from.

The second change is tool access. A ball-nose cutter held at an angle can reach the floor of a deep pocket that a straight tool would rub against. That is what makes impellers, turbine blades and bone plates machinable at all. It is also why five-axis time is not always slower: shorter tools deflect less, so you can run them harder.

One boundary: 5-axis does not fix a bad part design. If a wall is 0.3 mm thick on a 200 mm aluminium plate, no machine will hold it flat. DFM review happens before the first cut for that reason.

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  Fewer setups, fewer datums2 setups instead of 5 removes stacked position error.
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  Angled tool accessReaches pocket floors and undercut walls.
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  Shorter, stiffer toolsLess deflection, so higher feed rates are possible.

A tolerance callout is a promise about the whole process, not just the spindle. Thermal drift, fixture stiffness and material condition all move the number. Aluminium 6061 and 7075 cut clean and hold ±0.005 mm on features under roughly 300 mm. Titanium TC4 and Inconel move more during the cut, so we plan roughing passes, stress relief and a finishing pass with light depth of cut.

Size matters as much as material. A 4,000 mm travel handles large frames, but tolerance on a 3 m steel weldment is not the same as on a 50 mm bracket. On long parts we advise a datum-referenced drawing and a CMM report rather than a single blanket tolerance.

Surface finish and tolerance are separate conversations. Ra 0.2–0.8 μm is a fine finish and usually needs a separate finishing pass, sometimes polishing after machining. Ra 0.8–1.6 μm is the normal high-quality as-machined result. Asking for Ra 0.2 μm across a whole part adds cost for no functional gain on most brackets and housings.

Where tight tolerance is wasted: clearance holes, non-sealing faces, cosmetic edges. Where it pays: bearing bores, sealing faces, mating spigots, anything that sets an alignment stack. If a feature does not affect fit or function, relax it and the price moves.

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  Aluminium 6061 / 7075Holds ±0.005 mm on features under ~300 mm.
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  Titanium and InconelPlan roughing, stress relief, light finishing pass.
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  Long partsUse datum-referenced drawing plus CMM report.

Some geometry simply cannot be produced on 3 axes. A closed impeller with twisted blades, a medical bone plate with a contoured underside, a hydraulic manifold with intersecting bores at compound angles. These are the cases where the setup count on 3-axis either becomes impossible or pushes cost past the five-axis rate.

The tell is undercut. If any face of the part is hidden when you look straight down the Z axis, you need either a re-clamp or a rotary table. Count the hidden faces. One or two and a 4-axis mill with a Ø400 mm rotary table is usually enough. Four or more and a simultaneous 5-axis center is the practical route.

A second tell is positional tolerance between features on different faces. Two bores that must be coaxial within ±0.01 mm across a 250 mm body are far easier to cut in one 5-axis setup than to re-fixture three times on a 3-axis machine.

Prismatic parts stay on 3-axis. Brackets, plates, covers, simple housings and anything that can be reached from one direction are cheaper and faster on a 3-axis machine, and we quote them that way.

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  Count hidden faces0–1: 3-axis. 2–3: 4-axis. 4+: 5-axis.
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  Coaxial featuresCross-face alignment favors a single 5-axis setup.
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  Prismatic partsBrackets and plates stay cheaper on 3-axis.

A tolerance that is not measured is a claim, not a result. Our inspection chain runs in three stages: incoming raw material check against the mill certificate, in-process monitoring during the cut, and a final dimensional inspection before packing. Reports are available on request.

For tight features we use a CMM and record the actual value, not just pass or fail. On a bore called at ±0.005 mm, knowing the part came in at +0.003 mm tells you whether the next run will still fit. That record also shortens the argument if an assembly problem appears later.

The qualification rate is 99.99%. That number comes from the inspection data, and it is the reason we inspect 100% of parts before shipment rather than sampling. For one-off prototypes the final inspection is the only record; for a 10,000-part run it is the audit trail.

If a drawing calls a tolerance on a non-critical face, we will flag it during DFM review rather than quietly machine to it and add cost. You get the choice.

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  Three-stage inspectionRaw material, in-process, final before shipment.
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  CMM values recordedActual readings, not just pass or fail.
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  100% before shipmentNo sampling on any order size.

Aluminium is the easy case. 6061-T6, 7075 and 6082 cut fast, hold tolerance well and take anodizing cleanly. 7075 is stronger but less corrosion resistant, so it usually gets a coating. Die-cast ADC12 is for housings, not for tight bores.

Stainless behaves differently. 303 and 304 are common, 316L for medical and marine, 17-4PH when you need strength plus corrosion resistance. Stainless work-hardens, so we keep the cutter engaged and avoid dwelling. Bores in 316L hold ±0.005 mm, but the cycle time is longer than the same part in aluminium.

Titanium TC4 (Ti-6Al-4V) and Inconel are where the process gets careful. Both hold heat at the cutting edge, so tool life is short and the finishing pass must be light. Magnesium AZ31B and AZ91D machine quickly but need chip control. Copper and brass, including C36000 and beryllium copper, cut well and are often chosen for thermal or electrical features.

Plastics are a different problem. POM and PEEK move after machining as internal stress releases, so we rough, let the part rest, then finish. ABS and PC are for prototypes and covers, not for bearing fits. Carbon fibre needs diamond tooling and dust control.

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  Aluminium 6061 / 7075Fast, stable, good anodizing base.
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  Stainless 316L / 17-4PHHolds tolerance, longer cycle time.
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  Ti-6Al-4V / InconelLight finishing pass, short tool life.
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  POM / PEEKRough, rest, then finish to control movement.

CNC is a poor fit for thin sheet with many bends, for hollow parts with internal channels that cannot be reached, and for very high volumes of a simple part where die casting or injection molding wins on unit cost. If your part is a 20,000-piece plastic enclosure, machining is the wrong process and we will say so.

Wall thickness is the most common limit. Below about 0.5 mm on aluminium and 0.8 mm on stainless, chatter and distortion take over. We can sometimes support the part with fixturing or leave sacrificial material, but the design should be changed if it is still in your hands.

Deep pockets with small corner radii are a second limit. A 2 mm corner radius in a 60 mm deep pocket needs a long, thin tool that deflects. Either open the corner radius or accept a slower, more expensive cut.

Third: features that must be ground, hardened or EDM-finished after machining. We machine the pre-form and leave stock, but the final process sits outside CNC. Quoting it as a finished part would be wrong.

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  Thin wallsUnder 0.5 mm aluminium, chatter takes over.
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  Deep small cornersLong thin tools deflect; open the radius.
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  Hardened featuresGrinding or EDM sits outside CNC.