What Determines Speeds And Feeds For CNC Machine

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  Key takeaways
• 2
  What speeds and feeds for cnc machine actually mean
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  Workpiece material drives the starting numbers
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  Tool geometry, substrate and coating set the limits
• 5
  Machine rigidity and the holder decide what survives
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  Coolant, chip evacuation and controller behavior
• 7
  Reading the symptoms on the shop floor
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  Surface speed and chip load by material group

Cutting speed is the surface speed at the tool edge, written as Vc in m/min or SFM. Spindle speed is that value converted into RPM for a given tool diameter. Feed per tooth (fz) is the chip thickness each edge removes per revolution. Feed rate is feed per tooth multiplied by tooth count and RPM.

The two numbers are linked. Increase RPM and the surface speed rises, but the chip load stays the same until you change the feed rate. Change the feed rate alone and you change chip thickness, which is what actually breaks or rubs the material.

Chip load is the number worth watching. Too thin and the edge rubs, work-hardens the surface and wears fast. Too thick and the edge chips, the spindle stalls, or the part moves in the vise.

Every recommendation from a tool supplier is a starting point for a specific combination of material, tool and setup. The job on the floor is to find the window where the chip breaks cleanly, the sound is steady, and the finished surface meets the drawing.

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  VcSurface speed at the cutting edge, m/min or SFM.
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  fzChip thickness per tooth, mm/tooth or in/tooth.
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  ap / aeDepth of cut and radial engagement.

Material is the first input because it sets hardness, ductility and how heat leaves the cut. Aluminium 6061 and 7075 conduct heat well and cut at high surface speeds. Titanium TC4 and Inconel hold heat at the edge, so surface speed drops sharply and coolant becomes mandatory.

Ductile materials are the awkward ones. 304 stainless and copper alloys form a built-up edge that welds to the tool face and then breaks off, taking coating with it. The fix is usually a higher feed per tooth to force a thicker chip, not a lower one.

Hardness matters less than people expect. A 4140 pre-hard at 30 HRC machines comfortably with coated carbide. The same alloy at 45 HRC pushes you toward lower surface speed and a more rigid setup.

Thermal conductivity is the quiet variable. Where heat leaves with the chip, tool life is long. Where it stays in the workpiece, you get distortion on thin walls and a surface that looks worse after the part cools.

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  Aluminium 6061 / 7075High surface speed, light coolant, watch chatter on thin ribs.
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  304 / 316 stainlessHigher chip load, sharp edge, no dwell in the cut.
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  TC4 titaniumLow surface speed, heavy flood or through-tool coolant.
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  InconelLowest surface speed, rigid setup, expect short tool life.

A carbide grade is chosen for the material group, not for the brand. The coating does the thermal work. TiAlN and AlCrN raise hot hardness and reduce friction, which is why they tolerate higher surface speed than an uncoated edge.

Geometry controls chip formation. A positive rake, sharp edge suits aluminium and stainless. A stronger edge with a hone or land suits cast iron, hardened steel and interrupted cuts.

Tool overhang is the hidden brake. A 12 mm end mill hanging 60 mm out of the holder will chatter long before its coating fails. Shorten the gauge length first, then raise the numbers.

Number of flutes changes the feed rate directly. A three-flute cutter in aluminium can run a higher table feed than a two-flute at the same chip load, because there are more teeth engaging per revolution.

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  CoatingTiAlN, AlCrN for steel; DLC and uncoated for aluminium.
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  Edge prepSharp for ductile alloys, honed for interrupted cuts.
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  Flute countMore flutes raise feed rate but reduce chip room.
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  OverhangKeep it under 3 × diameter where the geometry allows.

Theoretical cutting data assumes a rigid setup. A 3-axis machine with a Ø400 mm rotary table behaves differently from a 16-station 5-axis center. Short tool assemblies and a solid fixture let you run closer to the recommended numbers.

Chatter is the first symptom of a setup that is too soft. It shows as a rippled wall, a howling sound, or a surface finish that cannot be polished out. The usual answer is to reduce radial engagement, not to slow the spindle to a crawl.

Spindle taper and holder type matter. A shrink-fit or hydraulic holder gives less runout than a worn collet, and less runout means each tooth cuts the chip thickness you calculated.

Long parts are their own problem. A 4,000 mm maximum processing envelope on a large gantry job needs support, or the part deflects under cutting force and the numbers you set are meaningless.

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  FixtureSupport under the cut, not just at the ends.
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  HolderShrink-fit or hydraulic for low runout on finishing.
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  EngagementReduce ae before reducing RPM when chatter starts.

Coolant does two jobs: it removes heat and it moves chips. Flood coolant is the default for steel and stainless. Through-spindle coolant reaches the cutting edge in deep pockets where flood never arrives.

Air blow and minimum quantity lubrication suit aluminium and graphite, where a wet chip is harder to clear than a dry one. The wrong choice here causes recutting, and recutting doubles the effective chip load.

The controller shapes the real feed rate. Look-ahead and feed-rate override smooth the motion on complex contours, so the programmed feed is not always the feed at the edge. On small arcs the machine may slow down, which raises chip thinning and shortens tool life.

Ramp and trochoidal paths keep radial engagement low and constant. That lets you run a deeper axial cut at a higher feed than a conventional stepover, provided the controller can keep up with the blocks.

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  FloodDefault for steel, stainless and cast iron.
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  Through-toolDeep holes and pockets beyond 3 × diameter.
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  Air / MQLAluminium, graphite, medical plastics.
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  Feed overrideLog actual override before blaming the tool.

Sound and chip shape tell you more than the readout. A steady hum and short, curled chips mean the window is right. A high-pitched squeal with powdery chips means the edge is rubbing and the feed is too low for the RPM.

Colour is a rough guide. Straw-coloured chips in steel are normal. Blue chips mean the surface speed is high for that grade and coating, and tool life is being traded for cycle time.

Surface finish is the final check. If the finish meets Ra 0.8–1.6 μm but the tool fails after 20 minutes, the problem is usually heat, not feed. Add coolant flow or drop surface speed by 15–20% and retest.

Measure runout before changing any cutting data. A holder with 0.02 mm runout will overload one flute and produce a finish that no feed change can fix.

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  RubbingPowdery chips, polished edge, work-hardened surface.
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  ChippingBroken edge, heavy vibration, wrong grade for hardness.
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  ChatterRippled wall, reduce ae and shorten overhang first.