Table of CNC Knife Settings: How to Read Speed, Feed and Depth

• 1
  What a table of CNC knife settings really contains
• 2
  Why the same row does not work in aluminum and in steel
• 3
  How flute count and coating change the settings
• 4
  Where the table stops and the machine takes over
• 5
  How to judge a settings table before you run it
• 6
  Table of CNC knife settings: starting ranges by material

A table of CNC knife settings is not a single list of speeds. Each row describes one cutter in one material, and it carries four independent numbers. Spindle speed (rpm) sets surface speed at the cutting edge. Feed per tooth (mm/tooth) sets chip thickness. Axial depth (ap) and radial width (ae) set how much of the flute is buried in the part. Change one and the other three shift with it.

Surface speed is the number that decides tool life. A Ø12 mm carbide end mill running at 4,000 rpm in 6061 sees about 150 m/min at the edge, a comfortable range for uncoated carbide. The same cutter at the same rpm in 4140 steel runs at roughly 150 m/min too, which is far too fast for that alloy. The rpm has to come down to keep the edge cool.

Chip thickness is the number that decides whether the cut works at all. Feed per tooth below about 0.02 mm rubs instead of cuts. The edge work-hardens the material, heat builds up, and the tool fails early. Feed per tooth above roughly 0.15 mm on a small cutter overloads the shank. The window between those two numbers is narrower than most tables suggest.

Depth of cut is the number that decides how many passes you need. A 0.5 mm axial step-down in aluminum with a Ø12 mm cutter is conservative. Radial engagement of 5–10 percent of diameter with full axial depth is faster in most pockets, but it demands a rigid setup and constant coolant. The table on a machine control usually shows only one of these two strategies.

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  Speed (rpm)Sets surface speed at the cutting edge; the main lever on tool life.
• 2
  Feed per toothSets chip thickness; too small rubs, too large breaks the tool.
• 3
  Axial depth (ap)How deep each pass goes; drives pass count and cycle time.
• 4
  Radial width (ae)How much of the cutter is engaged; drives heat and chatter risk.

Aluminum conducts heat away from the edge quickly. That is why 6061 and 7075 tolerate high surface speeds and large depths without burning. A Ø10 mm three-flute cutter in 6061 often runs at 8,000–12,000 rpm with 0.05–0.10 mm per tooth. The chips come off bright and curl away from the cut. If they come off blue or welded to the flute, the feed is too low for that speed.

Steel does the opposite. Heat stays at the edge because the material conducts it poorly. A Ø10 mm cutter in 4140 or 4340 usually runs between 1,200 and 2,000 rpm with 0.03–0.06 mm per tooth. Coated carbide helps, but the real fix is lower surface speed and a steady feed. Stopping mid-cut in steel is worse than running slightly slow, because the edge dwells and work-hardens.

Stainless grades such as 304 and 316 work-harden fast. The first pass must cut under the hardened skin, not rub on it. Feed per tooth below 0.04 mm on a Ø8 mm cutter in 316 will usually glaze the surface and shorten tool life. A heavier feed with a shallower radial engagement keeps the edge in clean material and gives a better finish.

Titanium TC4 (Ti-6Al-4V) sits at the difficult end. Surface speed stays low, often 40–60 m/min, and coolant must reach the edge continuously. The chips are thin and sharp. If the table shows a titanium row with the same feed as the aluminum row, that row was copied, not tested. Titanium rows need their own numbers and their own tool geometry.

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  AluminumHigh speed, high feed, generous depth; chips carry the heat away.
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  SteelModerate speed, steady feed; heat stays at the edge.
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  StainlessCut under the work-hardened skin; avoid light rubbing feeds.
• 4
  TitaniumLow surface speed, constant coolant, thin sharp chips.

Flute count sets the chip load per revolution. A two-flute cutter in aluminum has more room for chips, so it tolerates a higher feed per tooth. A four-flute cutter in the same material removes more material per revolution but leaves less chip clearance. In deep pockets, a three-flute cutter is often the compromise: enough clearance, enough productivity.

Coating changes the temperature the edge can survive. Uncoated carbide in aluminum is normal; coatings can actually increase friction and build-up edge. In steel and stainless, a TiAlN or AlTiN coating lets the edge run hotter without diffusion wear. The table row should say which coating the numbers assume, because a coated tool and an uncoated tool do not share a speed.

Helix angle affects how the cutter enters the material. A 45° helix handles steel and stainless well because the load transfers gradually. A 30° helix is common for aluminum and for finishing. A high helix in a deep pocket can pull the part upward if the fixture is weak, so the table row is only valid when the workpiece is held rigidly.

Corner radius matters more than most tables admit. A sharp corner on a Ø12 mm cutter in hardened steel chips easily. A 0.8 mm corner radius spreads the load and lasts longer, but it leaves a larger internal corner that may need a second operation. The setting row should state the corner radius, or the numbers mean little.

• 1
  Flute countMore flutes raise removal rate but reduce chip clearance.
• 2
  CoatingLets steel run hotter; can hurt aluminum by adding friction.
• 3
  Helix angle45° for steel, 30° for aluminum; affects fixture load.
• 4
  Corner radius0.8 mm reduces chipping but leaves a larger internal corner.

A table assumes a rigid setup. On a part held in a vise with 30 mm of stock sticking out, the numbers may be fine. On a thin wall 1.5 mm thick, the same numbers will chatter. Chatter is not a tool problem. It is the part vibrating at its natural frequency, and the fix is support, not a slower spindle.

Spindle taper and holder length also move the real limit. A cutter in a short heat-shrink holder behaves differently from the same cutter in a long collet extension. Extra length adds deflection. When the table row was measured with a short holder, a long holder may need the feed reduced by 20–30 percent to keep the same finish.

Coolant strategy changes the numbers again. Through-spindle coolant reaches the edge in a deep pocket where flood coolant cannot. In titanium and stainless, through-coolant often allows a slightly higher speed because the edge stays cooler. In aluminum, mist or air blast can be enough, and flood coolant mainly helps with chip evacuation.

The machine control itself has limits. Maximum spindle speed, rapid rates and look-ahead all shape what the table can deliver. A row that calls for 12,000 rpm is useless on a machine capped at 8,000 rpm. The first check before trusting any row is whether the machine can actually reach the numbers.

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  Part rigidityThin walls chatter; add support instead of slowing the spindle.
• 2
  Holder lengthLong holders deflect; reduce feed 20–30 percent if needed.
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  CoolantThrough-spindle reaches the edge; flood mainly clears chips.
• 4
  Machine limitsCheck max rpm and look-ahead before trusting a row.

Start with the material grade, not the material family. 6061 and 7075 are both aluminum, but 7075 is stronger and usually runs slower. 304 and 316 are both stainless, and both work-harden, but 316 is tougher on the edge. A row that says only "aluminum" or "stainless" is a starting point, not a setting.

Next, confirm the tool geometry. Diameter, flute count, coating, helix and corner radius all belong in the row. If the row lists only diameter, the numbers were probably copied from a catalog and not tested on a real part. Ask which holder was used. The answer tells you how much to derate for your own setup.

Then check the depth numbers. Axial depth and radial width together describe the strategy. A row with 0.5 mm axial and 6 mm radial is a different strategy from 12 mm axial and 1 mm radial, even with the same speed and feed. The first is conservative, the second is high-efficiency milling. Both can be right, but they need different fixtures.

Finally, treat the row as a starting point and prove it on one part. Run a test cut, listen to the sound, and look at the chips. Chips that are thin and even mean the feed is right. Chips that are powdery mean the feed is too low. Chips that are thick and blue mean the speed is too high. Adjust one number at a time.

• 1
  Grade first"Aluminum" is not a setting; 6061 and 7075 differ.
• 2
  Full geometryDiameter, flutes, coating, helix, corner radius all matter.
• 3
  Depth strategyAxial and radial together define the cutting approach.
• 4
  Prove one partTest cut, listen, read the chips, adjust one number.