Essential Tips for Troubleshooting Indexable Milling

Normal flank wear lands have a fine, smooth wear pattern, while a land formed by chipping has a saw-toothed, uneven surface. If not detected soon enough, it may be perceived as depth-of-cut notching.

Chipping can also be caused by recutting chips. A good example of this would be a slotting operation where chip clearance or chip gullet space does not allow the chips to evacuate cleanly. In this instance, packing of the chips also occurs.

In most cases, by changing to a stronger grade and/or to a different edge preparation such as a larger hone or T-land, or from a 90˚ cutter geometry to a lead angle cutter geometry, will resolve the problem.

This appears when chipping or wearing at the depth-of-cut line on the rake face and flank of the insert.

Notching is primarily caused by the condition of the workpiece material and can include:

• An abrasive workpiece skin of scale
• Abrasive properties of high-temperature alloys like Inconel
• A work-hardened outer layer resulting from a previous machining operation
• Heat-treated material above 55 HRC

These cracks run perpendicular to the insert’s cutting edge and are caused by the extreme temperature variations involved in milling. In one revolution of a milling cutter, the insert starts to cut and the temperature quickly rises as the insert enters the cut. The varying chip thickness also changes the temperature throughout the cut. When the insert comes out of the cut, air or coolant flow rapidly cools the insert before it reenters the cut.

The proper coolant application is crucial in milling operations to manage temperature and reduce thermal cracking.

This condition involves the adhesion of layers of workpiece material to the top surface of the insert. Hardened pieces of the adhered material periodically break free, leaving an irregularly shaped depression along the cutting edge. This causes damage to the part and insert. Cutting forces also will be increased due to built-up edge.

A relatively smooth, regular depression is produced on the insert’s rake face. Crater wear occurs in two ways:

1. Material adhering to the insert’s top surface is dislodged, carrying away minute fragments of the top surface of the insert.
2. Frictional heat builds up from the flow of chips over the top surface of the insert. Eventually, this heat buildup softens the insert behind the cutting edge and removes minute particles of the insert until a crater forms. Crater wear is rarely encountered in milling but can appear when machining certain steel and cast iron alloys. If crater wear becomes severe, there is a risk that the cutting edge will break, destroying the insert.

Uniform flank wear is the preferred method of insert failure because it can be predicted. Excessive flank wear increases cutting forces and contributes to poor surface finish. When wear occurs at an unacceptable rate or becomes unpredictable, the key elements that must be investigated are speed, feed, grade, and insert/cutter geometry.

NOTE: Inserts should be indexed when roughing (0,38 to 0,50 mm flank wear is reached) and finishing (0,25 to 0,38 mm flank wear or sooner).

When wear, chipping, thermal cracking, and breakage occur at once, the machine operator must look beyond the normal feed, speed, and depth-of-cut adjustments to find the root cause of the problem. Speed, feed and depth-of-cut parameters should be re-examined for accuracy, but the system’s rigidity should also be closely inspected for loose or worn parts as well.