When should you replace the carbide tips on your lathe tools?

Built up edge.

Built up edge (BUE) occurs when material starts to adhere to the cutting tip, impeding chip formation, spoiling finish and reducing cutting efficiency while increasing heat generation from friction.

Built up edge can happen for many reasons, but the most common reasons are:

• Chemical reaction between a carbide inserts coating and the cutting material. Coatings containing aluminium like Titanium Aluminium Nitride (TiAlN) especially have a tendency to chemically bond with titanium. This is one of the reasons inserts for cutting aluminium are uncoated
  
• Heat. Cutting metals on the lathe can generate high temperatures, which can cause many metals with lower melting points to plasticize and become easily adhered to other metal. Proper use of air or fluid cooling can both reduce temperatures and clear chips from the cutting face before they have a chance to bond to the cutter tip. Selecting an insert with an appropriate chip breaker is also important as chip formation helps transfer heat away from the workpiece and into the chip
  
• Friction. At high pressures galling can occur between two metals, which is a form of friction welding that can occur even at low temperatures. Ensuring cutting at an appropriate angle, with the carbide insert properly centred on the workpiece is necessary to prevent excess friction which is generated when a cutter is over-centre and ends up rubbing more than cutting

Chipped Edge

A chipped edge on a carbide insert is one of the most obvious signs that replacement is necessary. Chipping typically occurs when the tool experiences excessive force or impact, such as from interrupted cuts, workpiece irregularities, or tool vibration.
When inspecting your carbide tips, look for:

• Small fractures along the cutting edge
• Irregular edge geometry that differs from the original profile
• Sections of the edge that appear broken off or jagged

Even minor chips can significantly affect your work quality. A chipped edge will transfer its irregular pattern to your workpiece, creating poor surface finishes and potentially causing the tool to grab or catch during operation. Additionally, chips tend to propagate—what begins as a small defect can quickly develop into a catastrophic failure.

Don't attempt to "work through" a chipped edge. The increased cutting forces will stress your machine, possibly damage your workpiece, and potentially create dangerous conditions in your shop.

Worn Cutting Radius

Unlike chipping, which can happen suddenly, wear on the cutting radius develops gradually over time. This wear pattern is characterized by the rounding or flattening of what should be a crisp cutting edge.
Signs that the cutting radius has worn beyond acceptable limits include:

• Increased cutting forces required to make the same cuts
• Deteriorating surface finish on your workpieces
  
• Dimensional inconsistencies in your machined parts
  
• Tool generating excessive heat during operation
  
• Audible changes in the cutting sound—often a higher-pitched noise

Many machinists use a simple test: if you can catch your fingernail on the edge of a new insert but can't on your current one, significant wear has occurred. For more precise assessment, a tool microscope or magnifying glass can help you compare a worn insert to a new one.

Remember that different materials cause different wear patterns. Cast iron typically causes abrasive wear, while steel often leads to crater wear on the tool face. Understanding these patterns can help you select the appropriate carbide grade for your specific application.