CNC Machined Parts Guide

• 1
  What CNC machining actually does to a block of metal
• 2
  Matching part size to machine travel
• 3
  Why the same drawing cuts differently in 6061 and 316L
• 4
  Where ±0.005 mm holds and where it does not
• 5
  Design choices that decide cost and function
• 6
  How the shop proves the parts are right
• 7
  Which CNC process fits which part

CNC machining is subtractive. You start with a solid block, rod, or casting, and a controlled cutting tool removes material until the remaining shape matches the CAD model. The computer does not shape the part; it only moves the tool along a path. The cutting edge does the work, and every decision about speed, feed, and tool path changes the surface the part ends up with.

That distinction matters when you read a drawing. A 3-axis mill cuts from one direction, so undercuts and deep side pockets are hard to reach. A 5-axis center tilts the tool or the table, so it can approach a feature from an angle. Turned parts rotate the stock instead, which is why shafts, bushings, and threaded fittings usually come off a lathe rather than a mill.

The physical limits are set by the tool, not the software. A Ø6 mm end mill cannot clear a 5 mm internal corner. A deep pocket narrower than three times the tool diameter will chatter or snap unless you slow down and accept a rougher floor. These are geometry problems, and no amount of programming removes them.

• 1
  MillingRotating tool, stationary stock. Best for pockets, slots, faces, and prismatic shapes.
• 2
  TurningRotating stock, stationary tool. Best for cylindrical features and tight concentricity.
• 3
  Mill-turnBoth motions in one setup. Cuts the number of fixtures and re-clamps on complex parts.

Machine travel decides whether a part is a one-setup job or a multi-setup problem. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, covering 7,600 m². The largest travel is 4,000 × 400 × 150 mm, which suits long extrusions, rails, and frame members. Medium travels of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm handle most enclosure and plate work. Compact travels of 500 × 500 × 450 mm and 500 × 310 × 200 mm cover small, high-volume parts where cycle time and tool access matter more than size.

The 4,000 mm maximum processing size is a real ceiling. Beyond it, the part either gets split into sections or moves to a different process. Splitting is not free: you add a joint, a fastening method, and a new tolerance stack between the two halves.

Rotary work adds another axis of capability. A Ø400 mm rotary table lets a 4-axis mill cut helical flutes, ports around a cylinder, or slots on a shaft without re-fixturing. The gain is positional accuracy, because every feature is cut relative to the same datum.

Aluminum 6061 and 6061-T6 cut fast and hold a good finish, which is why they carry most prototype and bracket work. 7075 is stronger but gummier; it needs sharper tools and lighter depths of cut. Stainless 303 machines cleanly, while 304 and 316L work-harden under a dull tool. Once 316L hardens at the surface, the next pass rubs instead of cuts, and you burn a tool for nothing.

Titanium TC4 (Ti-6Al-4V) and Inconel sit at the hard end. They conduct heat poorly, so the cutting edge absorbs it. Tool life drops, feeds drop with it, and cycle time climbs. That cost is structural, not a markup. If a part does not need the temperature or corrosion resistance, moving from Inconel to 17-4PH stainless usually cuts both cost and lead time.

Plastics behave differently again. POM and ABS cut clean, but PEEK and carbon fibre are abrasive and need carbide or diamond tooling. Carbon fibre also leaves conductive dust that has to be managed. For a cover or a non-structural housing, a machined plastic part is often cheaper than metal, provided the wall thickness stays above roughly 1.5 mm so the part does not deflect during clamping.

GreatLight holds ±0.005 mm (±0.0002 in) on qualified features. That number applies to a dimension measured against a defined datum on a stable part. It does not apply to a thin wall that flexes under the probe, or to a dimension that spans two setups. Tolerance is a property of the whole setup, not just the machine.

Surface finish follows the same logic. As-machined surfaces land around Ra 1.6–3.2 μm. A high-finish pass brings that to Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm. Each step adds time on the machine. A sealing face needs the finer band; a bracket that bolts to a frame rarely does.

Two rules save money early. Put tight tolerances only on the features that function, and give the shop a clear datum scheme so inspection measures the same thing the machine cut. When a drawing calls out ±0.005 mm on every dimension, most of those callouts are doing nothing except raising the price.

Design for manufacturability is the biggest lever on price, and it is set before the quote. Internal corners should be at least one-third larger than the tool radius you expect. Pocket depth should stay under about four times the tool diameter where possible. Threads should be standard sizes, because a custom pitch means a custom tap and a separate operation.

Post-processing changes the part as much as the cutting does. Anodizing comes in clear, color, hardcoat, and conductive types; hardcoat adds wear resistance but shifts dimensions slightly, so mask critical fits. Electroless nickel, zinc, silver, and gold plating cover corrosion and conductivity needs. Powder coating and black oxide handle appearance and mild protection.

Mechanical finishes are separate again. Bead blasting, tumbling, brushing, and polishing each produce a different texture. Laser marking and engraving need a minimum character height of 1.5 mm to stay legible. If a part is anodized and then lasered, the mark reads differently than on bare metal, so decide the sequence before the parts are made.

There is no useful quality system that only inspects at the end. GreatLight checks raw material on arrival, monitors dimensions in process, and runs a final inspection before shipment. Every part is inspected before it ships, and reports are available on request. That sequence catches a drifting tool before it produces a batch of scrap.

Certification tells you which industries the process controls are built for. ISO 9001:2015 covers the general quality system. IATF 16949:2016 adds automotive requirements. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which matters when your CAD files leave your network.

For critical features, agree on the measurement method at the quoting stage. A CMM result and a caliper result on the same bore can differ, and the gap is not always error. It is often a difference in datums or in how the probe contacts the surface. Fix that before production, not after.