DEFECTS IN CUTTING TOOLS

There is a cutting tool for every application. As a critical part of the overall machining system, cutting tools are often targeted when manufacturers look for improvements in overall productivity. Technologies such as high-speed machining, dry machining and continuing development of tough workpiece materials place extreme demands on cutting tools. To keep pace, tool suppliers must continue to develop products that can perform at higher speeds and last longer under increasingly rigorous operating conditions.

High speeds and metal removal rates generate more heat. As a result, suppliers of cutting tools have placed a heavy emphasis on the development of heat-resistant tool materials. The result has been a progression from high-speed steel (HSS) tools to cemented carbides, ceramics and super-hard materials.

Regardless of material, all cutting tools have a defined working life. Aside from breakage, cutting tools wear in many ways, including:

• Edge and flank wear
• Cratering or top wear
• Chipping
• Built-up edge (BUE)
• Deformation
• Thermal cracking

Edge and flank wear are normal, slow types of tool wear. Cratering behind the cutting edge occurs frequently in machining of materials that produce long, stringy chips in many steels. If the crater grows large enough to contact the cutting edge, the tool will fail immediately.

Edge chipping is an unpredictable form of failure that sometimes begins when a high point on an edge breaks away. A stronger tool material or different geometry can minimise chipping.

Built-up edge (BUE) is a deposit of workpiece material adhering to the insert's rake face. The deposit can break off, taking carbide from the tool with it. BUE is a common problem when machining ductile materials such as soft steels, aluminum and copper alloys.

Tool deformation is the result of heat build-up. It can be minimised by using a more heat-resistant tool material and/or reducing the cutting speed.

Tapping is an application area dominated by HSS tools. The material is more forgiving than carbide, and HSS taps are about one-tenth the cost of carbide.

Thermal cracking occurs when inserts go through rapid heating and cooling cycles. Causes include interrupted cutting and poorly thought-out application of cutting fluids.

These failures dictate that desirable tool material properties include toughness, wear resistance and hardness, in particular the hardness of heat.