As a consultant, I frequently get asked, “My die sections are cracking—what type of tool steel should I use?” or “My punches chip frequently—how do I prevent this?”
Tool steel selection is a crucial part of the die design process; however, using higher-quality tool steel might not always be the answer. Instead, the key often is the machining processes used on the tool steel.
A great deal of tool steel damage can result from improper machining procedures. Metal removal methods such as electrical discharge machining (EDM), drilling, milling, broaching, turning, and grinding all must be performed properly, or tool steel damage could result.
The damage that occurs often is difficult to see or to measure without the use of sophisticated metallurgical equipment. A poor grinding job might look good to the human eye, but unseen stress can cause premature failure.
It’s a real pet peeve of mine to walk into a die shop or stamping plant and find a tool and die technician grinding on a large block of tool steel and leaving little black marks on the surface with each pass of the grinding wheel. Typically when I confront the person and inquire about those little black marks, I am told that this particular grinding pass was only a rough pass and the technician will redress the wheel and make a much lighter finish pass to eliminate the black marks (see Figure 1).
In reality, the technician basically is hiding any highly visible evidence of a poor tool grinding procedure. Think of a grinding wheel as millions of single-point cutting tools (sandlike particles) bonded together into a circular shape with glue. For the grinding wheel to cut the metal rather than burn it, each single-point cutting tool must stay sharp and resist loading. If the particle does load up, it must fall off the wheel to expose a new, single-point cutting tool.
This behavior is a characteristic of soft-bond, coarse-grit grinding wheels. These wheel types are preferred for grinding premium grades of tool steel, such as those that are air-hardened. In contrast, a hard-bond, fine-grit wheel removes a sizable amount of metal with each pass and can cause burning and heat checking of the die section. Heat checks—also known as microcracks—can result in premature cracking and failure.