CNC Machining London: How to Source Parts Without Guesswork

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  What CNC machining London projects actually need
• 2
  Why 5-axis changes the design conversation
• 3
  Where tolerances stop being realistic
• 4
  Material choice and what it does to the cut
• 5
  How to read a supplier capability list
• 6
  Post-processing and the hidden dimensions
• 7
  When to choose which machining route

CNC machining London work usually starts with a drawing and a deadline. The part might be a bracket for a test rig, a manifold for a prototype engine, or a housing that has to fit an existing assembly. In every case the same three questions come up: can the geometry be cut, what tolerance is realistic, and how fast can the parts arrive.

London has machine shops, but most are small. A shop with two or three 3-axis mills can handle simple plates and shafts. It cannot cut a 5-axis impeller in one setup, and it will not have a Ø400 mm rotary table for large round parts. That gap is why many engineering teams send work to a larger plant with more machine types under one roof.

This article is written for people who already know what a milling cutter does. We are not going to explain what G-code is. We will explain where 5-axis work saves money, which tolerances are worth specifying, and how to read a supplier's capability list without being fooled by it.

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  Simple geometry3-axis milling and turning handle most brackets, plates and shafts.
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  Complex geometry5-axis cuts undercuts and angled faces in one setup.
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  Tight tolerance±0.005 mm is achievable on the right machine with the right fixture.
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  FinishingAnodizing, plating and bead blasting change dimensions if not planned for.

A 5-axis machining center moves the tool or the table along five axes at the same time. Two of those axes are rotary. That means the cutter can reach a face that points away from the spindle without the operator unclamping the part and turning it by hand.

Each re-clamp adds error. If a part needs four setups on a 3-axis mill, the operator has to locate the datum four times. A small offset on setup two becomes a mismatch on the finished part. On 5-axis work the part stays in one fixture, so the datums do not move.

The trade-off is cost. Five-axis machines run slower for simple cuts and cost more per hour. A flat plate with six holes should stay on a 3-axis mill. The moment a part has compound angles, deep pockets with curved floors, or features on five sides, the 5-axis route usually wins on total cost even with a higher hourly rate.

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  Good fitImpellers, turbine housings, angled ports, medical bone plates.
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  Poor fitFlat plates, simple bushings, parts with one accessible face.
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  Setup countFewer setups mean tighter true position between features.
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  Fixture costOne fixture instead of three or four reduces tooling spend.

A drawing that says ±0.005 mm everywhere is a warning sign. That number is achievable on a good machine, but it applies to the feature, not the whole part. A 500 mm aluminium plate will move with temperature. A thin wall will deflect under cutting force. A deep bore will drift if the tool holder is not rigid enough.

The practical rule is to put tight tolerance only where it matters. A bearing seat needs ±0.005 mm. A clearance hole for an M6 bolt needs ±0.1 mm. A cosmetic edge needs no tolerance at all. Marking every dimension tight forces the shop to slow down, add inspections and charge more for no functional gain.

Material choice also sets the floor. Aluminium 6061 and 7075 hold tight tolerance well because they cut clean and resist heat. Stainless 316 work-hardens, so a light finishing pass is needed to avoid a shiny, hard surface that ruins the next cut. Titanium TC4 moves more under heat and needs more coolant and lower feed rates.

Surface finish and tolerance interact. A Ra 0.2–0.8 μm finish on a bore usually needs a separate finishing pass or a reaming operation. If the drawing asks for that finish and ±0.005 mm on the same hole, expect the shop to inspect it with a bore gauge, not calipers.

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  Functional facesTight tolerance on mating and locating surfaces only.
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  Clearance holes±0.1 mm is enough for bolt holes and cable pass-throughs.
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  Thin wallsBelow 1 mm on aluminium, expect chatter and spring passes.
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  TemperatureA 10 °C shop swing moves a 500 mm steel part by about 0.006 mm.

Most London prototype work lands on aluminium, stainless or engineering plastic. Aluminium 6061-T6 is the default for housings and brackets because it machines fast and takes anodizing well. 7075 is stronger but costs more and is harder to weld, so it suits aerospace brackets rather than welded frames.

Stainless 303 is the free-machining grade, good for shafts and fittings. 304 and 316 are tougher and gummier; they need sharp tooling and steady feed to avoid work-hardening. 17-4PH gives high strength after heat treatment and is common in medical instruments and pump parts.

Plastics behave differently. POM and PA hold good tolerance but move with moisture. PEEK holds high temperature and is used in medical and aerospace parts, but it is expensive and abrasive on tooling. Carbon fibre reinforced plastic needs diamond-coated tools and dust extraction, and the finished edge often needs a light sand or bead blast.

For high-temperature work, Inconel and titanium TC4 are the hard cases. They generate heat at the cutting edge and wear tools fast. Shops that run them regularly keep separate tooling and lower spindle speeds. If your part is Inconel and the supplier's list does not mention nickel alloys, ask before you send the drawing.

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  Fast and cheap6061-T6 aluminium, C36000 brass, ABS and POM.
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  Strong and corrosion resistant316L stainless, 17-4PH, 7075 aluminium.
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  High temperatureInconel, titanium TC4, PEEK.
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  AbrasiveCarbon fibre, glass-filled PA, hardcoat anodized surfaces.

A capability list tells you what a shop can do, not what it does well. Look for the number of machines by axis count, the maximum part size, and the tolerance the shop is willing to put in writing. A plant with 16 simultaneous 5-axis centers and a 4,000 mm maximum processing size can take large aerospace and automotive work that a small London shop cannot.

Certifications matter by industry. ISO 9001:2015 covers general quality management. IATF 16949:2016 is required for automotive production parts. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters if your drawings are confidential. A shop with all four can serve several sectors without changing its process.

Inspection is the part buyers often skip. Ask whether the shop inspects 100% of parts before shipment and whether it will send dimensional reports. A supplier that checks raw material, monitors in-process and does a final inspection is safer than one that only measures the first article.

Lead time claims deserve a second look. A quotation and DFM analysis within 12 hours is useful, but it only matters if production can start within 24 hours and parts ship in 3–5 days. Ask what happens when a tool breaks or a fixture needs rework. A shop with a low late-delivery history usually has spare capacity and standard fixtures ready.

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  Machine mixCount by axis type, not total machine count.
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  Size limitCheck the largest travel against your part envelope.
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  CertificationsMatch certificates to your industry, not to a marketing page.
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  InspectionAsk for the inspection plan and sample reports.

Anodizing adds a thin oxide layer, usually 5–25 μm depending on the type. That changes the size of a bore or a shaft by a few thousandths of a millimeter. If the drawing calls for a hardcoat anodized bore with a tight tolerance, the shop has to machine undersize and let the coating bring it back.

Plating has the same effect. Electroless nickel adds a uniform layer that can be 10–50 μm thick. Zinc and silver plating are thinner but still matter on threaded features. A threaded hole that is plated after tapping may not accept a gauge, so the shop either masks the thread or taps after plating.

Bead blasting, tumbling and brushing change the surface but not the nominal size. Laser marking does not change size either, but the minimum character height is 1.5 mm. If your part number is long and the space is small, the marking may not fit. Plan the marking area before the geometry is frozen.

One-stop post-processing saves shipping and handling. Parts that are machined, anodized, laser marked and packed in one plant avoid the risk of damage between vendors. That matters more on thin-walled or cosmetic parts than on heavy industrial brackets.

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  AnodizingClear, colour, hardcoat and conductive types all add thickness.
• 2
  PlatingElectroless nickel, zinc, silver and gold plating change thread fit.
• 3
  Blasting and polishingCosmetic only; no nominal size change.
• 4
  Laser markingMinimum character height 1.5 mm.