CNC Machining MI: How It Works, What It Holds

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  What Happens Inside a CNC Machining MI Cycle
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  How Axis Count Changes CNC Machining MI Work
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  Material Behavior and Cutting Parameters
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  When a Part Is a Poor Fit for CNC Machining MI
• 5
  Choosing the Right Setup for CNC Machining MI

A CNC machine reads a toolpath from CAM software and moves a spinning cutter through the stock. The controller converts each line of G-code into coordinated motion of X, Y, Z, and any rotary axes. The cutter edge shears material away as a chip. Heat leaves with the chip, not the part, when speeds and feeds are set right.

Three things decide the result: rigidity, tool engagement, and thermal stability. A short tool in a shrink-fit holder deflects less than a long end mill in a collet. Light radial engagement (5–10% of cutter diameter) keeps chatter down on thin walls. Coolant or through-spindle air carries chips out so the cutter never recuts them.

Tolerance capability depends on the machine, not the drawing. GreatLight holds ±0.005 mm (±0.0002 in) on production work when the feature is reachable in one setup. Features machined across two setups inherit the fixture error of the second setup, often 0.01–0.03 mm. Designers who want tight bores should ask for them in the same setup as their datum.

Surface finish follows the same logic. Ra 0.8–1.6 μm is a normal milled finish. Ra 0.2–0.8 μm needs a finishing pass, a sharp tool, and often a smaller stepover. Ra 1.6–3.2 μm is fine for brackets and covers. Specifying a finer finish than the function needs adds cost without adding value.

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  Rigidity firstShort tools and rigid fixtures beat fast spindle speeds for accuracy.
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  Chipload mattersToo light a chip rubs the edge and work-hardens stainless.
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  One setup, one datumTolerances stack every time the part is re-fixtured.

A 3-axis mill moves the cutter in X, Y, and Z while the part stays still. It handles plates, pockets, and drilled holes well. Undercuts and four-sided features need a second setup or a flip, which costs time and adds positional error. For flat parts with simple geometry, 3-axis is the cheapest correct answer.

A 4-axis machine adds a rotary table, usually turning around X or Y. The part indexes to a new face without a human touching it. This suits shafts, manifolds, and parts with features on four sides. A Ø400 mm rotary table covers most of this work. Indexing is not the same as simultaneous motion; the table locks, then cuts.

A 5-axis center tilts the tool or the table on two rotary axes at once. The cutter can reach under a flange, follow a curved surface, and drill at an angle without a second fixture. Impellers, turbine blades, and medical implants are the classic cases. Short, stubby tools can be used because the machine angles the tool instead of reaching around the part.

The trade-off is programming time and machine cost. A 5-axis toolpath takes longer to verify and simulate. It pays off when the part cannot be made any other way, or when two setups collapse into one. GreatLight runs 16 simultaneous 5-axis machining centers alongside 27 three-axis machines, 12 four-axis mills, and 16 mill-turn centers, so the axis choice follows the geometry.

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  3-axisFlat plates, pockets, through holes, simple profiles.
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  4-axisFour-sided parts, shafts, indexed features, tombstones.
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  5-axisUndercuts, contoured surfaces, angled holes, one-setup parts.

Aluminum 6061-T6 cuts fast and holds a good finish. It is the default for prototypes and housings. 7075 machines well but is stronger and more expensive; it suits stressed brackets. 2024 has better fatigue life but poor corrosion resistance unless anodized. Soft aluminum like 5052 galls easily and needs sharp tools and generous coolant.

Stainless 303 is the free-machining grade and the easiest to turn. 304 and 316 work-harden if the cutter rubs instead of cuts; keep the chipload up and never dwell. 17-4PH (SUS630) can be machined in the annealed state and then aged to high strength. It is common in aerospace and medical parts.

Steel 1018 and 1045 are straightforward. 4140 and 4340 need more rigidity and lower speeds. Tool steel and Inconel push the limits of any machine; Inconel work-hardens hard and wears tools quickly, so it belongs on a rigid 5-axis center with high-pressure coolant. Titanium Ti-6Al-4V (TC4) has low thermal conductivity, so heat stays in the cutter and tool life drops fast.

Plastics behave differently. POM and PA machine cleanly. PEEK needs sharp tools and slow feeds. Carbon fibre is abrasive and eats carbide, so diamond-coated tooling is often the only economical route. Every material has a sweet spot for surface speed, and the quote should reflect the real cutting time, not a guess.

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  Aluminum6061, 7075, 2024, 5052, 6082, ADC12.
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  Stainless303, 304, 316L, 17-4PH, 440C.
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  SuperalloysInconel, Ti-6Al-4V, magnesium AZ31B.
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  PlasticsPOM, PEEK, PC, PA, carbon fibre.

CNC machining removes material, so deep cavities need long tools that deflect. A pocket deeper than four times its width is hard to hold tight. If the design needs a deep internal channel, consider splitting the part or switching to die casting or 3D printing. The same goes for very thin floors; they vibrate and chatter.

Sharp internal corners cannot be cut by a round tool. The corner radius equals the cutter radius at minimum. A 6 mm end mill leaves a 3 mm inside radius. Designers who draw a true 90° internal corner force a second operation or a wire EDM step. Adding a small corner radius costs nothing and saves a setup.

Threads and text have limits too. Very fine threads in soft aluminum strip easily. Laser marking is limited to a minimum character height of 1.5 mm. Below that, the mark is unreadable. Engraved text is an option but adds cycle time.

Quantity matters in the opposite direction. CNC is economical from one prototype to 10,000+ parts, but at very high volumes a die casting or stamping process usually wins on unit cost. The break-even point depends on geometry, material, and finish. A good quote will show where that line sits.

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  Deep pocketsKeep depth under 4× the cutter diameter.
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  Inside cornersAdd a radius equal to or larger than the tool radius.
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  Thin wallsBelow 0.8 mm, expect chatter and extra passes.
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  High volumePast roughly 10,000 parts, compare casting or stamping.