CNC Cutting Parameters: How Speed, Feed and Depth Decide the Cut

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  What the four core numbers actually control
• 2
  Why the same numbers fail on a different alloy
• 3
  Tool geometry and machine limits set the ceiling
• 4
  Reading chatter, wear and finish as feedback
• 5
  Typical starting ranges by material

Four numbers drive almost every cut: cutting speed, which is the surface speed at the tool edge in m/min; feed rate, in mm/min for milling or mm/rev for turning; axial and radial depth of cut; and the geometry of the insert itself. Change one and the other three feel it.

Cutting speed sets the temperature at the edge. Push it and the chip carries heat away faster, but the carbide softens and wears on the flank. Pull it back and heat soaks into the workpiece, which is how thin walls start to move. On aluminium 6061 we often run 300–500 m/min; on 316L stainless, 120–180 m/min is a realistic band.

Feed rate sets the chip thickness. Too light and the edge rubs instead of cutting, which work-hardens stainless and burns the insert. Too heavy and you get chatter marks and a rough Ra. Feed per tooth, not feed per minute, is the number to reason about, because it stays meaningful when the tool diameter changes.

Depth of cut splits the load between the axial and radial directions. A shallow radial pass with a deep axial pass keeps radial forces low, which matters on long tools and thin floors. A heavy radial pass clears material fast but deflects the tool and pushes the part out of tolerance.

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  Speed controls heatHigher surface speed moves heat into the chip, up to the point where the insert gives up.
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  Feed controls chip thicknessFeed per tooth below about 0.05 mm rubs rather than cuts.
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  Depth controls forceRadial depth drives deflection; axial depth drives spindle load.

Hardness and thermal conductivity decide most of the answer. Aluminium conducts heat well and cuts at high surface speed, so the tool stays cool and the chips fly. Titanium Ti-6Al-4V conducts heat poorly, so the heat stays at the edge. That is why titanium runs slow, with generous coolant and a rigid setup.

Stainless 304 and 316L work-harden. If the tool rubs, the surface gets harder under the cut and the next pass is worse. The fix is not more speed. It is a heavier feed per tooth so the edge always bites under the hardened skin.

Tool steel and 4140 respond to heat treatment more than to the recipe. A 30 HRC block machines like a different material than the same block at 50 HRC. Ask for the condition, not just the grade, before you set a speed.

Plastics such as POM and PEEK bring their own rules. Heat builds at the edge and melts the chip instead of shearing it. Sharp tools, high rake, and air blast beat flood coolant here.

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  AluminiumHigh speed, high feed, watch for built-up edge on soft tempers.
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  TitaniumLow speed, steady feed, never let the edge dwell.
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  StainlessFeed hard enough to stay under the work-hardened layer.
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  Hardened steelReduce speed, expect shorter tool life, inspect more often.

The insert grade and coating decide how much speed the edge can take. A TiAlN-coated carbide insert runs hotter and faster than an uncoated one. A four-flute end mill leaves a better floor finish than a two-flute at the same feed, but it clears chips worse in a deep pocket.

Tool overhang is the silent killer. Every extra 10 mm of stick-out adds deflection. A 12 mm end mill at 60 mm overhang will chatter at a depth that is fine at 30 mm overhang. Shorten the holder before you touch the feed.

The machine frame matters as much as the recipe. Our 16 simultaneous 5-axis machining centers and 16 mill-turn centers hold ±0.005 mm across a 4,000 mm envelope, but that tolerance depends on a stable setup and a warm spindle.

Spindle speed range, coolant pressure, and chip evacuation all cap what the numbers can do. A program that runs clean on one machine may need a 20% feed cut on another with a weaker spindle.

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  CoatingCoated inserts tolerate higher surface speed and longer cuts.
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  OverhangShorter tools allow deeper passes without chatter.
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  CoolantThrough-tool pressure clears chips in deep holes and pockets.

The cut talks back. A high-pitched squeal usually means the radial depth is too heavy for the tool stiffness. A low rumble points to spindle speed sitting on a natural frequency of the setup. Change one variable at a time so you know which one moved the result.

Insert wear tells the story of speed. Even flank wear across the nose means the speed is close to right. Cratering on the rake face means too much heat. Chipping at the edge means the feed is too light or the material is hard.

Surface finish maps to feed per tooth. To hit Ra 0.8–1.6 μm you usually need a finer feed and a sharper radius. For Ra 1.6–3.2 μm, a standard pass is fine. Chasing Ra 0.2–0.8 μm means slower feeds and often a finishing tool, not a magic number.

Tolerance follows the same logic. Holding ±0.005 mm on a long thin part is a setup problem before it is a parameter problem. Fix the fixturing, then tune the speed.

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  SquealReduce radial depth or shorten the tool.
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  RumbleShift spindle speed off the resonance.
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  Poor RaLower feed per tooth and check for built-up edge.