CNC Machining Order: A Complete Engineering Guide

• 1
  What a CNC machining order actually contains
• 2
  How a quote becomes a machining plan
• 3
  Setup and workholding decide the tolerance you get
• 4
  Cutting parameters and where an order goes wrong
• 5
  Inspection, finish, and shipping on a CNC machining order
• 6
  Which machining route fits which part

A CNC machining order is not a single document. It is a package: a 3D model, a 2D drawing with tolerances, a material callout, a finish spec, and a quantity. Each one changes the price and the process. Send only an STL and the shop has to guess at every dimension that matters.

The model defines nominal geometry. The drawing defines what is allowed to move. A bracket at ±0.25 mm and a bearing housing at ±0.010 mm can share the same shape and still need different machines, different fixtures, and different inspection time.

Material drives tool choice before anything else. Aluminium 6061 cuts fast and holds tight tolerances well. Inconel and Ti-6Al-4V cut slowly, wear tools, and can deflect a thin wall under cutting load. Those differences show up in the quote as cycle time and tooling cost, not as a line item called difficulty.

Quantity decides whether the order runs on a 3-axis mill with soft jaws or on a 5-axis center with a dedicated fixture. One prototype and a 10,000-part run are the same drawing and two different manufacturing plans.

• 1
  Model plus drawingSTEP for geometry, PDF for tolerances and notes.
• 2
  Material and temper6061-T6 is not the same as 6061-O on a thin wall.
• 3
  Finish and maskingAnodize type and masked areas change handling.
• 4
  Quantity and target dateSets the setup strategy and inspection plan.

Quoting starts with a DFM pass. The engineer looks for features the tool cannot reach, walls too thin to hold, and tolerances tighter than the geometry needs. A deep pocket with a 2 mm corner radius needs a 2 mm cutter, and that cutter cannot reach 30 mm deep without chatter.

From there the planner picks setups. A part with features on four faces might run as three 3-axis setups, or as one 5-axis setup. Fewer setups usually means better position accuracy, because every re-clamp adds stack-up error.

The plan also sets stock size. Leaving 0.5 mm on a face is normal. Leaving 5 mm on a 200 mm plate wastes cycle time and can release residual stress as material is removed, which bends the part after the last cut.

GreatLight runs 127 high-precision CNC machines across three plants, including 16 simultaneous 5-axis centers. That mix is what lets a plan choose between a single-setup 5-axis route and a cheaper multi-setup 3-axis route for the same part.

The machine does not set the tolerance. The fixture does. A part held in a vise on three points will move when a face mill pushes on it. A part held in a custom soft-jaw pocket or on a vacuum plate with proper support will not.

For thin walls, the usual fix is to leave a sacrificial web and cut it last, or to support the wall with wax or a low-melt fixture. Both add a step. Both are cheaper than scrapping the part.

Five-axis work lets the tool approach at an angle, which shortens the tool, reduces deflection, and reaches features that would need a long reach on a 3-axis machine. A 4,000 × 400 × 150 mm travel envelope handles long parts, while compact machines at 500 × 500 × 450 mm handle small, high-density work.

Thermal drift matters on tight work. A machine that has run for six hours is not the same machine that started cold. Warm-up cycles and in-process probing keep the first part and the last part in the same place.

Speeds and feeds are set by material, tool material, and rigidity. Aluminium 6061 runs at high surface speed with generous chip load. Stainless 316 work-hardens if the tool rubs instead of cutting, so feed per tooth has to stay up and the tool cannot dwell.

Titanium TC4 and Inconel generate heat at the cutting edge and conduct it poorly. Cutters wear fast and the part grows. Flood coolant, lower surface speed, and a rigid setup are the standard answer.

Plastics behave differently again. POM and PEEK cut cleanly with sharp tools and air blast. ABS and PC soften with heat and can gum up a cutter. A finish of Ra 1.6–3.2 μm is realistic as-machined; Ra 0.8–1.6 μm needs a deliberate finishing pass, and Ra 0.2–0.8 μm usually means a secondary operation.

The most common failure is not a bad cut. It is a missing note. A drawing that does not say which surface is the datum, or which bore is the critical one, forces the shop to guess. Guessing is where ±0.005 mm parts become scrap.

• 1
  ChatterLong tool, weak hold, or too much radial engagement.
• 2
  Taper in a deep boreTool deflection; ream or use a boring head.
• 3
  Warp after machiningResidual stress in plate; rough, stress-relieve, finish.
• 4
  Burrs on a sealing faceAdd a chamfer note or a deburr spec.

Inspection is planned, not improvised. A first article checks every dimension on the drawing. Production parts get checked at the features that carry the function. Calipers are fine for a slot; a bore that takes a bearing needs a micrometer or a CMM report.

GreatLight inspects 100% of parts before shipment, with a raw material check, in-process monitoring, and a final inspection. Reports are available on request, which matters for IATF 16949 and ISO 13485 work.

Finish comes before final inspection or after, depending on the process. Anodizing adds a few micrometres and can change a tight bore. Laser marking needs a minimum character height of 1.5 mm to stay legible.

Then the order ships. A quote comes back within 12 hours with a free DFM analysis, production can start within 24 hours, and parts typically ship in 3–5 days. Small parts, simple geometry, and stock material all shorten that. Large 5-axis work and exotic alloys lengthen it.