GMT CNC machining guide

• 1
  What actually happens at the cutting edge
• 2
  3-axis, 4-axis and 5-axis: where each one pays off
• 3
  Reading tolerances without over-specifying them
• 4
  Workholding decides the result more than the spindle
• 5
  How material choice changes the cut
• 6
  From raw stock to inspected part
• 7
  Choosing a machining route before you request a quote

CNC machining removes metal with a rotating tool that is harder than the workpiece. The tool edge shears material along a plane, and the chip carries most of the heat away. Everything else on the machine exists to hold that cut steady: the spindle, the linear axes, the workholding, the coolant.

Three variables decide whether the cut is stable. Surface speed is how fast the tool edge travels past the material. Feed per tooth is how much material each edge takes. Radial and axial depth of cut set how much of the tool is engaged. Push any of them too hard and the tool deflects or chips.

Rigidity is the quiet limit. A long end mill hanging 60 mm out of a holder will chatter long before the spindle runs out of power. Short, stubby tools and a rigid setup often beat a bigger machine on the same part.

Heat matters too. Aluminium conducts heat into the part and can warp on thin walls. Titanium and Inconel keep heat at the edge, so tool life drops fast without high-pressure coolant and lower surface speeds.

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  Surface speedSet by material and tool coating, not by the machine.
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  Feed per toothToo low rubs and work-hardens stainless.
• 3
  Tool overhangKeep it under 4× diameter where geometry allows.

A 3-axis machine moves the tool in X, Y and Z while the part stays still. It is the fastest and cheapest way to cut a part that can be reached from one direction. Pockets, plates, slots, bores and flat faces all fall into this group.

A 4-axis machine adds rotation about one axis, usually A. The part turns while the tool cuts. This suits shafts, cylinders, cams and any part with features arranged around a single centerline. One setup replaces several 3-axis repositions.

A 5-axis machine adds a second rotary axis, so the tool can tilt relative to the part. Undercuts, deep cavities, compound angles and blended surfaces become reachable in one setup. The trade-off is programming time and a slower, more sensitive cut.

The rule of thumb is simple. If the part can be reached from three directions or fewer, 3-axis will usually be cheaper. If features wrap around one axis, go 4-axis. If the geometry has compound angles or needs one-setup accuracy across five faces, 5-axis earns its cost.

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  3-axisPrismatic parts, plates, housings, simple pockets.
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  4-axisShafts, cams, rotary fittings, around-one-axis features.
• 3
  5-axisImpellers, medical implants, complex aerospace brackets.

A tolerance is a cost signal. Every tight callout adds inspection time, slower feeds and possibly a second operation. Most parts work fine with general tolerances of ±0.1 mm on non-critical features, while bores and mating faces take the tight numbers.

Reachable tolerance depends on the feature. A bored hole on a rigid setup can hold ±0.005 mm. The same callout on the end of a 300 mm cantilever is optimistic. Position tolerance on a drilled hole is a different animal from size tolerance on the same hole.

Surface finish follows a similar logic. As-machined faces land around Ra 1.6–3.2 μm. A good finish sits at Ra 0.8–1.6 μm, and fine finishes reach Ra 0.2–0.8 μm. Going below Ra 0.2 μm usually means polishing, not cutting.

Ask one question before you tighten a number: does this feature actually mate with something? If not, the tight tolerance buys nothing and costs money.

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  Datum firstPick datums that reflect how the part sits in assembly.
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  One tight faceConcentrate precision where it does work.
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  GD&TPosition and profile callouts need a clear datum scheme.

A part with a weak setup will move, and a moving part cannot hold tolerance. Vises, soft jaws, vacuum plates and custom fixtures all serve the same purpose: lock the part without distorting it. Clamp pressure on a thin wall can bow it before the first cut.

For the first operation, cut the part from stock that is easy to grip. Then flip and cut the second side on soft jaws machined to match the first operation. This keeps the second side parallel to the first without re-measuring.

Five-axis work changes the question. When the part rotates, the fixture must clear every toolpath position and still hold the part rigid. A tombstone or trunnion setup adds reach but also adds mass and inertia.

Some parts need support in the cut, not just at the edges. Thin floors and tall ribs deflect under tool pressure. Add support material or leave sacrificial stock and remove it in a later pass.

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  Soft jawsMachined to the part profile for the second operation.
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  Vacuum plateGood for thin plates, but needs a clean seal face.
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  TombstoneCuts multiple faces in one 5-axis setup.

Aluminium 6061 cuts fast and holds a good finish, which is why it dominates prototypes and brackets. 7075 is stronger but gummier and more prone to chatter. 2024 sits between the two. All three machine well with sharp, polished carbide.

Stainless 303 is the free-machining grade and cuts cleanly. 304 and 316 work-harden if the tool rubs, so feeds stay high and depths stay consistent. 17-4PH adds strength but needs slower speeds and more attention to heat.

Titanium Ti-6Al-4V (TC4) keeps heat at the cutting edge, so tool life drops fast. Inconel behaves the same way, only worse. Both need lower surface speeds, high-pressure coolant and a rigid setup. Neither is a good first choice for a cheap prototype.

Plastics like POM and PEEK cut easily but move with temperature. Sharp tools, air blast and light finishing passes keep dimensions stable. Carbon fibre needs diamond or coated tooling to avoid rapid edge wear.

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  AluminiumFast, forgiving, good for tight prototypes.
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  StainlessKeep feeds high to avoid work-hardening.
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  TitaniumLower speed, more coolant, expect shorter tool life.

The job starts with material verification. Grade and condition are checked against the drawing before any cutting happens. Wrong condition, such as 6061-T6 versus annealed, changes both the cut and the final properties.

Roughing removes most of the stock with a strong tool and generous depths. A roughing pass does not need to hold tolerance; it needs to leave even stock for finishing. Uneven stock makes the finishing pass deflect.

Finishing follows with a smaller stepover and a sharp tool. This is where dimensions and finish are set. The part may need a second setup, a flip, or a rotary reposition depending on geometry.

Inspection closes the loop. Critical features are measured against the drawing, and reports can be requested. A part that passes inspection at the machine still needs handling and packaging that protect it on the way out.

• 1
  RoughRemove bulk stock, leave even allowance.
• 2
  FinishSet dimensions and surface finish.
• 3
  InspectMeasure against drawing before shipment.