Payoffs And Tradeoffs of CBN Inserts

CBN inserts incorporating reinforced, chamfered edges eliminate the edge breakout common when cutting materials harder than 50 RC.

Consider the entire application. Less expensive carbide inserts that can do the job in terms of tolerance and surface finish may be costly when the time spent indexing and replacing inserts is considered. Real productivity results from an understanding of the tradeoffs in throughput, cycle time and insert performance.

In one specialized, low-volume example, a sintered titanium carbide gas turbine blade was milled successfully with coated carbide cutting inserts. At 120 sfm, the carbide cutting edge cut well for just 5 to 10 minutes. Acceptable insert life is typically placed at 15 to 30 minutes in high volume production with difficult materials, but with a low-rate part, the short insert life and frequent tool changes are not major drawbacks. Longer insert life does become important in full production, however, to decrease tool-changing downtime and labor and to increase machine utilization and throughput. Carbide works well for the turbine blade for now, but should the part go to higher volume production, the application may justify harder, more costly inserts made of CBN.

Productivity with advanced material inserts requires adopting the right feeds and speeds. CBN inserts incorporate reinforced, chamfered edges to eliminate the edge breakout common when cutting materials harder than 50 RC. Yet even despite this toughness, CBN inserts demand cutting machine parameters held to tight tolerances. Cutting speeds 10 percent too low or 10 percent too high can dramatically hamper performance.

If faced with the need to machine a difficult material, consider contacting your cutting tool supplier. Suppliers can offer solutions based on how others have approached the same problem. When experimentation is required, careful trial-and-error generally starts with carbide inserts and moves on to harder and more costly cutters. Modern insert geometries, rigid toolholders and refined machining routines often make less costly carbide inserts suitable for tough jobs.