Turning: diagnosing and fixing tool damage

The size does not drift evenly in one direction but jumps both ways. Adjusting does not help — after a correction the scatter is exactly the same.

• Inserts differ in actual size: tolerance class M has a wider tolerance on nose position than G.
• The tool and the part deflecting under load.
• A built-up edge that periodically tears away.
• The insert does not seat consistently in the pocket: chips under the shim, a worn pocket, an undertightened screw.

1. Blow out and inspect the pocket before every index of the insert.
2. Check for a built-up edge — the cure for that is raising Vc.
3. Shorten the toolholder overhang, check how the part is clamped.
4. Reduce ap and f — less force, less deflection.
5. Move to a sharper positive geometry, reduce rε, increase κr.
6. Move to tolerance class G inserts instead of M where the size is critical.

The first parts are in tolerance, then the size creeps slowly and evenly in one direction. After N parts a correction is needed, and after another N, again.

This is not a fault but flank wear on the edge: the nose recedes, and the size goes with it. The only question is whether it is happening too fast.

1. Measure how many parts you get before it goes out of tolerance and set the change by that number.
2. Reduce Vc — speed has the strongest effect on the rate of wear.
3. Check that the coolant is actually reaching the cutting zone.
4. Increase rε and κr.
5. Move to a harder grade.

The surface is dull, torn or scratched instead of showing an even feed pattern.

• A worn edge — the tool is no longer cutting but pushing.
• Built-up edge: welded metal tears away and leaves torn patches.
• The chip is being dragged between the edge and the machined surface.
• The feed is too high for the nose radius.
• Vibration — in that case the pattern is regular, in waves.

1. Change the insert and look at the result.
2. Reduce f or increase rε.
3. Raise Vc — a low speed on steel and stainless produces a built-up edge.
4. Move to a finishing chipbreaker with a polished rake face.
5. Deal with the chip: a different chipbreaker, a directed coolant jet.
6. Check the overhang and the rigidity.

A whistle or a rattle, a regular wavy pattern on the surface, fine chipping along the whole contact length of the edge.

• The overhang of the toolholder or boring bar is too long.
• The part is weakly clamped, or it is thin-walled or long with no support.
• The feed is too low for a large radius: the edge is rubbing instead of cutting.
• rε too large or κr too small — the radial component of the force grows.

1. Shorten the overhang. This works better than any change of cutting data.
2. Check how the part is clamped, add a steady rest or support it with the tailstock.
3. Increase the feed. A thicker chip stabilises the cut.
4. Reduce ap and Vc.
5. Reduce rε, increase κr, move to a sharper geometry.
6. For deep boring — a bar with a carbide body or a damped bar.

Where the tool leaves the cut, a bent-over ridge of metal is left behind. Consistently so on ductile materials.

At the end of the cut a layer is left in front of the edge with nothing behind it: the metal is not cut off but bent aside.

1. Change the worn insert: a sharp edge leaves a smaller burr.
2. Increase the feed.
3. Reduce ap on the last pass.
4. Break the corner beforehand, or lead the tool out at an angle.
5. Move to a sharper geometry with less honing, increase κr.

As the tool leaves the cut, part of the edge of the workpiece breaks away. Typical of cast iron and hardened steel.

The same cause as with a burr, with the opposite sign: the material does not bend, it fractures.

1. Reduce f and ap as the tool leaves the cut.
2. Break the corner before the main pass.
3. Change the toolpath: exit at an angle.
4. Increase κr, move to a sharper geometry.
5. Check the rigidity of the workpiece clamping.

An even bright band along the cutting edge on the flank side.

The working form of wear — the abrasive action of the material. It is accelerated by too high a Vc, too low a grade hardness and running without coolant.

1. Reduce Vc.
2. Check the coolant supply: jet direction, pressure, concentration.
3. Increase rε and κr.
4. Strengthen the edge with honing or a chamfer.
5. Move to a harder grade.

A hollow on the rake face a little back from the edge. The edge is intact at first; when the crater reaches it, the edge breaks without warning.

The temperature where the chip contacts the rake face. The main factor is cutting speed.

1. Reduce Vc.
2. Reduce f and ap.
3. Copious coolant, preferably under pressure into the contact zone.
4. A chipbreaker with a larger rake angle.
5. A grade with a thicker thermal barrier coating (CVD).

A local notch exactly where the edge leaves the material. The rest of the edge is in good condition.

The work-hardened layer left by the previous pass, scale on the workpiece and a concentration of stress at one point. Typical of stainless and heat-resistant alloys.

1. Vary ap between passes so that the depth-of-cut line does not stay at one point on the edge.
2. Reduce κr.
3. Reduce Vc.
4. Strengthen the edge, move to a stronger grade.
5. For workpieces with scale — make the first pass deeper than the thickness of the crust.

Small breakouts along the edge. Appears suddenly, often on the first part after an insert change.

• Interrupted cutting, impact on entry.
• The geometry is too sharp for these conditions.
• Vibration or a non-rigid setup.
• A built-up edge tearing away.
• Running with a very thin chip.

1. Reduce the feed.
2. Check overhang and clamping, get rid of the vibration.
3. A geometry with a reinforced edge, a larger rε.
4. A stronger grade: for interrupted cutting toughness matters more than hardness.
5. A smooth entry into the cut instead of an impact against the face.
6. If there is a built-up edge — treat the built-up edge, not the wear.

Isolated cracks along or across the edge, without a thermal comb. They often precede the insert breaking altogether.

Mechanical overload: too much feed and depth, an impact on entry, play in the clamping, a worn pocket or shim.

1. Reduce f, then ap.
2. Check the pocket, the shim and the clamping screw.
3. Shorten the overhang, clamp the part more securely.
4. A thicker or larger insert.
5. A tougher grade, a reinforced edge.
6. Change the entry path.

A row of parallel cracks across the edge, like a comb, at even spacing. Fragments then break out between them.

Not the temperature itself but the swings in it: interrupted cutting, leaving the cut, and most often an intermittent coolant supply.

1. Decide on the coolant: either copious and uninterrupted, or dry cutting. There is no option in between.
2. Reduce Vc, then f.
3. A grade with better resistance to thermal shock.
4. Strengthen the edge, increase rε.
5. Reduce the number of entries and exits in the program.

The nose is neither fractured nor worn but has “flowed” — sagged and spread.

A temperature higher than the grade can hold: Vc, feed and depth too high in combination.

1. Reduce Vc, then the feed, then the depth — in exactly that order.
2. A steady coolant supply.
3. Increase rε and κr, strengthen the edge.
4. A grade with better heat resistance and a thicker coating.

Workpiece material welds itself to the edge, the surface becomes torn and the size jumps. The built-up edge periodically tears away, taking part of the edge with it.

Too low a cutting speed plus a material prone to adhesion: low-carbon steels, stainless, aluminium, copper.

1. Raise Vc — the main action.
2. Increase the feed.
3. A positive geometry with a sharper rake angle and a polished face.
4. Check the coolant.
5. A grade with better resistance to adhesion.

The chip is blue or purple, the part is hot, and the size after cooling does not match what was measured at the machine.

More heat is generated than the chip and the coolant can carry away. It then turns into cratering, deformation or thermal cracks.

1. Reduce Vc, then f and ap.
2. Sort out the coolant: pressure, direction, concentration.
3. A sharper positive geometry — less heat for the same metal removal.
4. Reduce the edge honing.
5. A grade with better heat resistance.

A continuous ribbon that wraps around the part, the boring bar or the turret.

The chipbreaker is not working: feed and depth have fallen outside its range. Most often the feed is too low.

1. Increase the feed.
2. Change ap, redistribute the stock between passes.
3. Choose a chipbreaker to match the cutting data you actually run — this is the main solution.
4. On soft and low-carbon steels with X-type geometries, raise Vc.
5. Reduce the rake angle, change κr.
6. Aim the coolant jet to carry the chip away.

Small hard fragments fly about, clog the slideways and leave scratches on the machined surface.

The chipbreaker is breaking the chip too aggressively for these conditions.

1. Reduce feed and cutting speed.
2. A chipbreaker with gentler chip breaking.
3. Increase rε.
4. Aim the coolant so that the jet leads the chips into the pan.