CNC Processing Deeside: How the Process Actually Works

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  How Metal Removal Sets the Tolerance You Can Hold
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  Why Setup Count Drives Cost in CNC Processing Deeside Work
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  Material Choice and What It Does to the Cut
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  Inspection Is the Only Proof of CNC Processing Deeside Quality
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  Post-Processing Changes Dimensions Too
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  Reading a Lead Time Claim on CNC Processing Deeside Jobs
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  Which Process Fits Which Part

CNC processing is a subtractive process. A rotating cutter engages the workpiece, and each pass removes a chip whose thickness depends on feed per tooth, spindle speed and radial engagement. Nothing about the geometry is cast in; the shape is the path the tool follows. That is why a drawing with ±0.005 mm on a 200 mm bore is a different job from the same tolerance on a 12 mm boss.

Heat is the quiet variable. Aluminium 6061 conducts heat away quickly and stays dimensionally calm. 316L stainless work-hardens at the cut and pushes heat into the tool. Titanium TC4 (Ti-6Al-4V) keeps heat in the cutting zone, so cutters wear fast.

Chatter marks are the visible symptom of a setup that is not stiff enough. Thin walls below 1 mm deflect under cutting force, so the finishing pass springs back and leaves a wavy surface at Ra 1.6–3.2 μm instead of the Ra 0.8–1.6 μm the drawing calls for. On a part like this, extra light finishing passes with a smaller radial step do more than a new cutter.

The practical takeaway: pick the tolerance from the feature, not the whole part. A ground locating bore at ±0.005 mm and a clearance hole at ±0.1 mm can sit on the same drawing. Pricing follows the tightest callout, so loosening the callouts that do not matter is the fastest way to cut cost.

Every time a part comes off the vise and goes back on, it is re-datumed. Each re-datum adds stack-up from fixture location error, workpiece cleanliness and clamping force. Two setups on a simple bracket are fine. Five setups on the same bracket mean five chances to drift.

A 5-axis center reduces setups by tilting the tool or the table instead of the part. An impeller, a hydraulic manifold with ports on four faces, or a housing with a compound angle can often be cut in one setup. That matters less for cycle time and more for datum control: features cut in one setup share the same origin.

Workholding is where quoting arguments usually start. A part with no flat face, no bore and no boss gives the fixture designer nothing to grab. Round parts can be held on a Ø400 mm rotary table or in a collet. Thin plates need support under the cut or they ring. If the print offers no clamping surface, the quote will include a soft jaw or a sacrificial tab.

Size sets the machine class. Our travel ranges cover 500 × 500 × 450 mm and 600 × 600 × 600 mm on compact and medium frames, up to 4,000 × 400 × 150 mm on the large gantry. A part that needs the full 4,000 mm length is planned around that machine from the start; it is not a scheduling detail you add later.

The material list is long, but the engineering split is short. Aluminium 6061-T6, 7075 and 6082 cut fast and hold tolerance well, which makes them the default for prototypes and fixtures. 7075 gives higher strength but is less weldable and more prone to stress movement when a lot of stock comes off one side.

Stainless 303 machines cleanly and is the usual pick for shafts and fittings. 304 and 316L resist corrosion better but work-harden, so feed rates must stay above the rubbing range. 17-4PH (SUS630) can be machined in the annealed condition and then aged to raise strength, which matters for parts that must stay dimensionally stable after heat treatment.

Steels 1018, 1045, 4130, 4140 and 4340 cover most structural and rotating parts. Pre-hardened 4140 at 28–32 HRC still cuts with carbide but raises tooling cost. Tool steel is reserved for molds and dies where wear resistance beats machinability.

Exotics are where lead time moves. Inconel and titanium TA2, TC4 cut slowly, generate heat and consume inserts. Magnesium AZ31B and AZ91D machine very fast but need chip control because fine magnesium swarf is a fire risk. Plastics from POM to PEEK machine like metals at low aggression, with sharp tools and air blast rather than flood coolant.

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  Free DFM firstSend the STEP file and we return a manufacturability note with the quote.
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  Material certificatesMill certs available on request for aerospace and medical lots.

A tolerance on a drawing is a claim. A measurement is evidence. The gap between them is where disputes live, so the inspection plan should be agreed before the first chip is cut.

Raw material arrives with a certificate and a grade check. In-process monitoring catches drift while the part is still in the machine, which is cheaper than discovering it at final inspection. Final inspection happens before shipment, and reports are issued on request rather than by default.

Which instrument measures which feature is not arbitrary. A micrometer reads an outside diameter but says nothing about position. A coordinate measuring machine handles true position, profile and concentricity. A roughness tester confirms whether the surface is Ra 0.8–1.6 μm or Ra 0.2–0.8 μm, because the eye cannot separate those two.

Sampling is the usual weak point. For a 10,000-part run, a 30-piece sample tells you about the process, not the lot. For a 5-part prototype lot, 100% inspection is the only meaningful answer. Ask which one you are buying before you compare prices.

Finishing is not cosmetic only. Anodizing builds an oxide layer on aluminium, and hardcoat adds more thickness than a clear decorative coat. A thread cut to nominal size before hardcoat may not accept its mating fastener afterwards.

Plating behaves the same way. Electroless nickel, zinc, silver and gold all add a measurable layer. If a bore carries a ±0.005 mm tolerance and the part is going to be plated, the machinist needs to know the coating thickness before cutting, not after.

Mechanical finishes remove or displace material. Bead blasting and tumbling round edges slightly, which is fine for a deburr callout and wrong for a sealing face. Polishing to a mirror finish on stainless removes the passive layer unevenly and can leave a smeared surface that corrodes in service.

Laser marking is the one finish with a hard floor. Minimum character height is 1.5 mm. A part number squeezed into a 1 mm band will not read reliably after anodizing, so put the mark somewhere with room.

A quote is not a schedule. The quote is a price for a scope, and the schedule is what happens after the drawing, material and fixture are all settled. Those two things get confused constantly in purchasing.

Material is the first gate. 6061 plate is usually in stock. Inconel bar or 17-4PH in a non-standard diameter may need a mill order, and that wait is longer than the machining. Ask whether the quoted date includes material procurement.

Fixture and first article come next. A simple part with a vise setup starts quickly. A part needing a soft jaw, a vacuum plate or a custom tombstone adds design and cut time before the first good part exists. This is also the stage where a DFM note is cheapest to act on.

Our standard flow: quotation and free DFM analysis within 12 hours, production start within 24 hours once the drawing is frozen, and parts shipping in 3–5 days for typical lots. Historical late-delivery probability sits below 2%. Those numbers assume the drawing does not change mid-run. A revision after the first article resets much of the work.