Milling: diagnosing and fixing tool damage

An even wear band along the edge, the same on every insert in the cutter. Life before the change is clearly shorter than expected.

• Cutting speed too high — the main factor.
• Feed per tooth too low: the edge is rubbing, not cutting.
• Insufficient or unstable coolant supply.
• The clearance angle is too small, or the edge is not reinforced enough for this material.

1. Reduce Vc.
2. Check fz: raise it if the chip comes out as dust.
3. Reduce ap and ae.
4. Check the coolant supply, or move to dry milling if the cooling is unstable.
5. Increase the clearance angle, strengthen the edge by honing.
6. Move to a harder grade.

Breakouts along the edge or cracks across the insert. Often on one or two inserts in the set rather than all of them.

• Runout: one insert is doing the work of all of them.
• An impact entry into the material — the cutter is set on the centreline of the workpiece.
• fz and ap too high.
• The geometry is too sharp, or the nose radius too small.
• The chip is not leaving the zone and is being recut by the teeth.
• A non-rigid workpiece setup, or too much arbor overhang.

1. Measure the insert runout, rework the pockets, check the screws.
2. Reduce fz and ap.
3. Offset the cutter from the workpiece so that it does not enter at full width.
4. Reduce the clearance angle, increase the corner angle and radius, strengthen the edge.
5. Move to a stronger grade.
6. Check the number of inserts: on a non-rigid machine a fine tooth pitch overloads it.
7. Shorten the overhang, clamp the part more securely.

Parallel cracks across the edge, like a comb, with fragments then breaking out between them. In milling this is the most typical form of damage.

The edge heats up in the cut and cools outside it — thousands of times in a single pass. It happens most sharply when the coolant is supplied intermittently or the jet does not reach into the zone.

1. Move to dry milling — here that is often a better answer than a half-measure with coolant.
2. If coolant is essential, provide a copious, uninterrupted supply under pressure.
3. Reduce Vc, then fz and ap.
4. Move to a tougher grade with better resistance to thermal shock.
5. Increase the corner angle, strengthen the edge.

Workpiece material welded to the edge, a torn surface on the part, and small pulled-out patches where the built-up edge tore away.

Speed and feed too low on a ductile material: low-carbon steel, stainless, aluminium.

1. Raise Vc.
2. Raise fz.
3. Check the coolant — concentration and supply.
4. A positive geometry with a larger clearance angle and a polished rake face, less honing.
5. A grade with better resistance to adhesion.

The nose has sagged, the edge is intact. Size and flatness drift, and catastrophic failure follows.

A temperature higher than the grade can hold: Vc, fz and ap too high in combination, particularly on high-hardness materials.

1. Reduce Vc, then fz, then ap.
2. A steady coolant supply, or a move to dry cutting.
3. Increase the corner angle, strengthen the edge.
4. A grade with better heat resistance.

A dull or torn pattern instead of an even face, sometimes a regular wave — that is already vibration.

• A worn edge or a built-up edge.
• Speed too low, feed per tooth too high.
• Insert runout: each one leaves its own mark at a different height.
• No wiper insert fitted, or one set incorrectly.
• Chips getting back under the tooth and scratching the finished surface.

1. Check the runout — in milling this is the most common cause.
2. Raise Vc, reduce fz and ap.
3. Check the wiper insert: whether it is fitted, its position, its clearance angle.
4. A sharper geometry, a larger nose radius.
5. Clear the chips away: compressed air or a directed coolant jet.
6. Shorten the overhang, check the rigidity of the arbor and of the workpiece clamping.

A visible step where passes meet, or regular ridges across the width of cut.

The inserts are running at different heights — through runout, a worn pocket, or inserts that actually differ in size. The second cause is the cutter deflecting under excessive cutting data.

1. Reduce Vc, fz and ap.
2. Check the runout and the condition of the pockets, rework the insert set.
3. Increase the clearance angle, reduce the corner angle.
4. Overlap the passes slightly instead of butting them edge to edge.

Breakouts on the edge of the part where the tooth leaves the material. Typical of cast iron; on ductile materials you get a burr in the same place.

As the tooth exits, the material has nothing behind it. The greater the force on the tooth and the more abrupt the exit, the larger the breakout.

1. Reduce fz and ap.
2. Lead the cutter out beyond the part smoothly instead of ending the pass at the edge.
3. Increase the corner angle and the clearance angle, reduce the radius.
4. Break the corner before the main pass.

Once off the machine the part has a dish or an out-of-parallel error that was not there on the machine.

• The cutter and the part deflecting under load.
• The part distorting from the heat generated in the cut.
• The part is clamped out of square, or distorted by the clamping itself.
• Runout, a non-rigid arbor, too much overhang.

1. Reduce Vc and fz, reduce ap on the finishing pass.
2. Let the part cool before the check measurement.
3. Review the clamping: less force, more support points.
4. Check the runout, shorten the overhang, use a more rigid arbor.
5. Split the stock: a roughing pass takes the bulk, the finishing pass runs at a small depth.

A hum or a rattle, a regular wave on the surface, fine breakouts on all the inserts at once.

• Too much arbor overhang, non-rigid workpiece clamping.
• Too many teeth in the cut at once for this machine.
• Feed per tooth too low: the edge is rubbing.
• Conventional milling where climb milling is needed.

1. Shorten the overhang and check the workpiece clamping — this has the strongest effect.
2. Raise fz. Against vibration the feed per tooth is increased, not reduced.
3. Reduce Vc and ap.
4. Move to climb milling — for helical slots this is recommended specifically.
5. Reduce the number of inserts, or use a cutter with an unequal pitch.
6. A sharper positive geometry, a smaller radius.

Chips fly around the working area, get back under the teeth, scratch the finished surface and clog the slots of the fixture.

The chips have nowhere to go: too low a feed produces fine dust, a badly aimed coolant jet returns them into the zone, and the insert geometry does not form the chip.

1. Raise fz — a larger chip leaves the zone on its own.
2. Reduce ap.
3. Move to compressed air instead of coolant, or aim air to carry the chips away.
4. Move to an insert with a chipbreaker.
5. Increase the clearance angle, reduce the corner angle.