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CANADA'S LEADING INFORMATION SOURCE FOR THE METALWORKING INDUSTRY

CANADA'S LEADING INFORMATION SOURCE FOR THE METALWORKING INDUSTRY

When turning tools go bad

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How to troubleshoot tough turning problems

by Ed Robertson

Even the most technologically advanced turning tools will eventually wear and fail, particularly when turning hard-to-machine materials.

Knowing how to recognize when these situations are occurring and when the smart thing to do is (a) increase speeds, (b) decrease speeds, or (c) none of the above, does not have to be a matter of trial and error.

How to troubleshoot is important. Troubleshooting should be performed in a sequential manner to identify and solve your machining problems. These problems can be recognized as premature insert edge failure, part appearance, machine noise or vibration (chatter), and tool appearance. 

Successful troubleshooting requires that we correctly identify the problem, then take the necessary corrective action one step at a time. 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. Here’s how to troubleshoot some of the more common problems involving turning tools.

Depth-of-cut notching: Notching is primarily caused by the condition of the workpiece material. Material conditions prone to depth-of-cut notch include an abrasive workpiece skin or 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. Notching appears when chipping or localized wear at the depth-of-cut line on the rake face and/or flank of the insert occurs.

Notching causes       Solution

Grade  —  Use a more wear-resistant grade of carbide.

Feed  —  Reduce feed.

Speed  —  Reduce speed.

Edge prep  —  Use honed or T-land inserts.

Programming  —  Vary depth of cut on very abrasive materials.

Thermal cracks: Turning creates friction and friction creates heat. In turning steels, much of the heat energy is transferred into the chip, whereas in turning titanium or other high-temperature alloys, much of the heat is transferred to the insert. High temperature variations can create stress cracks that run perpendicular to the insert’s cutting edge. To the untrained eye, advanced thermal cracking could appear as chipping.

Thermal crack causes     Solution

Speed and feed — Reduce speed and possibly the feed.

Coolant — Shut off coolant to reduce temperature variations

Grade —  Investigate using coated grade

Chipping: Chipping often 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 causes                             Solution

Grade  —  Use a tougher grade.

Edge prep  —  Use larger hone or T-land possible.

Built-up edge  —  Increase speed.

Chatter  —  Check system rigidity for proper part clamping.

            —  Correct worn gibs/bearings.

            —   Check for improper tool mounting.

Feed   —  Reduce feed.

Recutting chips  —  Use air blast or coolant flow to remove chips.

Built-up edge: 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.

BUE causes                                     Solution

Speed  —  Increase cutting speed.

Feed  —  Increase feed.

Coolant  —  Use mist or flood coolant to avoid chips sticking to the insert when machining stainless steel and aluminum alloys.

Edge-prep  —  Use sharper edge, positive-rake PVD inserts; use polished inserts for non-ferrous materials.

Crater wear: 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.

Cratering causes                           Solution

Grade  —  Use a more wear-resistant grade.

Speed  — Reduce cutting speed.

Edge-prep  — Use smaller T-land or increase feed to proper ange for T-land.

Flank wear: If there can be such a thing as the preferred mode of failure, uniform flank wear is it, notably because it can be predicted. Excessive flank wear increases cutting forces and contributes to poor surface finish.Inserts should be indexed when roughing (.015″–.020″ flank wear is reached) and finishing (.008″–.012″ flank wear or sooner).

Flank wear causes                        Solution

Speed  — Speed should be reduced without changing feed.

Feed  — Increase feed.

Grade  — Use more wear-resistant grade. Change to a coated grade if you are now using an uncoated grade.

Insert geometry  — Inspect insert to ensure proper style is being used.

Multiple factors: 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. In general:

Reduce feed rate to relieve cutting forces.

  • If possible, use a larger nose radius.
  • Use T-land insert.

Use a tougher grade of carbide.

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