Troubleshooting Indexable Milling

Troubleshooting should be performed in a sequential method to identify and solve your milling problems. Problems can be recognized as premature insert edge failure, part appearance, machine noise or vibration, and the cutter’s appearance. Successful troubleshooting requires that we correctly identify the problem, then take necessary corrective action one step at a time.

The five key areas of concern are:

1. Cutting Tool Material (Grade)
2. Cutter/Adapter
3. Machine
4. Workpiece

This section will discuss possible causes and will recommend corrective actions for each of the five areas listed. Remember, if more than one step is taken concurrently, the real cause of the problem may never be discovered. Always perform one corrective measure at a time. 

Edge Condition Problems and Solutions

Appears like normal flank wear to the untrained eye. Actually, normal flank wear lands have a fine, smooth wear pattern, while a land formed by chipping has a saw-toothed, uneven surface. If chipping is not detected soon enough, it may be perceived as depth-of-cut notching.

Chipping can also be caused by recutting of 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.

Appears when chipping or localized wear at the depth-of-cut line on the rake face and flank of the insert occurs.

Notching is primarily caused by the condition of the workpiece material. Material conditions prone to depth-of-cut notch 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, or 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.

These temperature variations create heat stresses in the insert which can result in thermal cracks. To the untrained eye, advanced thermal cracking could appear as chipping.

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.

This matrix explains the specific areas where advanced cutting tool materials perform differently from uncoated and coated carbide grades during the troubleshooting identification process.