Knowledge of the Use of CNC Tools

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  Key takeaways
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  What actually controls accuracy and finish
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  How tool geometry maps to the feature you need
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  Coating and substrate choices by material
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  Setting cutting data without guessing
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  Holder, coolant and wear: the parts people skip
• 7
  Which tool type suits which job

A CNC tool is a cutting edge held at a known position by a spindle, a holder and a set of offsets. Accuracy comes from that whole chain, not from the cutter alone. If the holder runs out 0.02 mm, the machine can still position to ±0.005 mm but the wall will not sit where the program says.

Three variables do most of the damage. Runout moves the effective radius of every flute, so one edge cuts deeper than the others. Tool wear changes the edge radius, which raises cutting force and pushes the part away from the cutter. Thermal growth moves the spindle and the part during long cycles.

Geometry decides how the cut behaves. A positive rake angle shears material cleanly and lowers cutting force, which suits aluminium and soft steels. A negative rake edge is stronger under interrupted cuts and hard material, but it needs more spindle power and pushes the part harder.

The rule of thumb: fix runout first, then wear, then speeds. Engineers who chase feeds and speeds while the holder is still running out are tuning the wrong variable.

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  Measure runout at the flute, not the shankIndicator on the cutting edge gives the number that matters.
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  Log wear by part count, not by shiftEdge life varies with material and coolant far more than with time.

Corner radius is the first thing to check against a drawing. A 0.5 mm corner radius in a pocket floor needs a cutter that physically fits, and the tool must also be stiff enough to reach the depth. Long reach with a small radius is the hardest combination in milling.

Flute count trades chip room against stiffness. A two-flute cutter clears chips in aluminium but flexes under load. A six-flute cutter is rigid and feeds fast in steel, yet it packs chips in deep pockets. Three flutes is the common compromise for mixed work.

Helix angle controls the direction of cutting force. A high helix pulls chips up and out, which helps in deep cavities, but it also pulls the part upward and can lift thin plates. A low helix pushes force down into the fixture, which suits plate work and weak setups.

Drills and taps follow the same logic. Point angle, web thickness and flute length set how straight a hole runs. A stub drill holds position; a long series drill wanders. For tapped holes, the chamfer form on the tap decides how much torque the machine must deliver.

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  Smallest internal radius sets the cutterIf the drawing says R0.8, plan for a 1.6 mm cutter and its reach limit.
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  Depth-to-diameter above 4:1 changes the planAdd a stub or pilot operation instead of one deep pass.

Uncoated carbide is still the right answer for aluminium and most plastics. Coatings that contain aluminium tend to drag on aluminium, and the built-up edge that forms ruins the finish. Polished flutes and high rake angles matter more here than any coating.

For steel, TiAlN and AlTiN coatings let the edge run hotter. The oxide layer that forms on the tool surface slows diffusion wear, which is what kills carbide at higher surface speeds. This is where the coating earns its price, on long runs of 4140 or 4340.

Stainless steel is the awkward case. It work-hardens, it welds to the edge, and it holds heat in the cut. Sharp edges, a light feed per tooth and plenty of coolant do more for tool life than a harder coating. 316L is worse than 304 on every one of those points.

Titanium and Inconel sit at the far end. Low cutting speeds, high coolant pressure and rigid setups are mandatory. A cutter that works well in 6061 will fail in TC4 within minutes, so do not carry a program across materials without changing the tool and the parameters.

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  Aluminium: uncoated, polished, high rakeKeep surface speed high and clear chips fast.
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  Stainless: sharp and lightAvoid dwelling in the cut; never let the edge rub.

Surface speed is the starting number. It comes from the tool substrate, the coating and the workpiece, and it is what the manufacturer's chart lists. From surface speed you get spindle rpm, and from feed per tooth you get the table feed. Two numbers, both published, both testable.

Feed per tooth is where most shops go wrong. Too low and the edge rubs instead of cutting, which work-hardens stainless and burns the tool. Too high and the edge chips or the part moves. The safe zone is usually wide enough to find by listening and by reading the chips.

Chips tell you what is happening. Silver and curled means the cut is healthy. Blue or dark means heat is going into the chip instead of the part, which is often acceptable in steel. Fine dust means the feed is far too low.

Depth of cut and width of cut set the load on the spindle and the fixture. A light radial cut with a deep axial cut distributes wear along the flute and suits high-efficiency milling. A heavy radial cut is simpler to program but loads the setup harder.

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  Start at the chart, then adjust one variableChange feed first, speed second, and note what the chips did.
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  Rubbing is worse than cutting fastLow feed per tooth destroys edges faster than high speed.

The holder is part of the tool. A hydraulic or shrink-fit holder holds a few microns of runout at high speed; a worn collet chuck can hold ten times that. On a 6 mm cutter, 0.02 mm of runout means one flute does most of the cutting and the tool life drops accordingly.

Coolant is not always the answer. Flood coolant removes heat and chips, which is what stainless and titanium need. In some aluminium roughing, a strong air blast is better because it clears chips without thermal shock on the edge. Through-spindle coolant is the only reliable option for deep holes.

Wear has stages. First the flank wears evenly and the part stays in tolerance. Then cratering starts on the rake face and cutting force climbs. Finally the edge chips or breaks, and that is when the part is scrapped. Change tools at the second stage, not the third.

On our own runs, in-process monitoring and a defined tool change count per material keep the process inside ±0.005 mm. Wear is measurable. Treat it as a schedule, not a surprise.

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  Check runout on every new holderTwo minutes at the bench saves a batch later.
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  Set a change count per materialThe same tool lasts a different number of parts in 6061 and 316L.