Cost-effective turning with hard-to-turn inserts for hard parts

“The key to achieving real improvements in productivity lies in optimizing the feed rate,” explained turning application specialist Martin Saunders. “Moreover, the good news is that increasing the feed rate doesn't compromise the quality of the surface finish. For instance, a feed of 0.3 mm/r can produce a surface finish below Rz 1 μm, and even at a feed of 0.5 mm/r, the resulting surface finish won't drop below Rz 5. In comparison, the typical surface finish of turned parts is around Rz 6.3, and using conventional tips with rounded corners would necessitate a 60-65% reduction in feed to achieve similar results.”

Blades that have demonstrated such impressive productivity gains are the Xcel-type inserts, and their innovative cutting-edge technology has profoundly influenced hard part turning. Turning application specialist Sacha Kotarac, who has extensive experience assisting numerous engineering companies, often selects the blade type based on surface finish requirements. “For finishing operations, I’ll recommend the Xcel blade whenever the surface finish criteria can be met,” he stated. “For roughing processes, I maximize the feed capacity while ensuring stability. With sufficient clearance along the shoulder, the Xcel blade can handle feeds up to 0.5 mm/r. Additionally, the Wiper blade offers double the feed rate of standard tools. The Xcel and Wiper blades are complementary, ensuring stable processes and predictable outcomes.”

“Hard part turning has become increasingly popular as a method to achieve higher precision more efficiently, especially under hardened or case-hardened conditions,” noted another expert. “One of the earlier challenges was the availability of suitable machine tools. While some lathes excel in stability, the clamping requirements remain stringent. Fortunately, most modern CNC lathes meet these demands admirably.”

Stability remains a crucial factor in all CBN blade applications, but the Xcel blade takes this to another level. To achieve optimal results, the aspect ratio should ideally be kept within 4:1. Although the blade can function beyond this ratio, the best cutting conditions are ensured within this range. Standard round head inserts generate the least cutting force but are constrained by the relationship between tool nose radius and surface finish. Wiper blades with a WH-shaped groove offer a good surface finish at high feed rates with reduced stability requirements, while those with WG grooves provide excellent finishes at even higher feed rates but demand greater stability. The Xcel blade stands out by achieving a high surface finish at high feed rates while requiring the highest stability. Even at lower feed rates, Xcel inserts deliver exceptional surface finishes. When minimizing cutting time is the priority, the relationship between feed rate and surface finish becomes crucial.

With a very small lead angle (10 degrees), the depth of cut is naturally limited, but this limitation applies to turning hard parts in general. Conventional round head inserts produce varying chip thicknesses along the cutting edge, whereas Xcel inserts maintain thin and uniform chip thicknesses, enabling high-feed processing. The primary innovation of the Xcel concept lies in integrating the straight part of the cutting edge with a specially designed Wiper, creating a smooth transition to the longitudinal relief angle of the cutting edge.

The depth of cut is determined by the straight edge, which is usually more than adequate for turning operations. Unlike grinding, turning requires less machining allowance, making the pass procedure a rational way to utilize the blade’s capabilities. The entire cutting edge is engaged by the depth of cut capability, and the highest feed rate permitted by the surface finish is utilized. For semi-finishing operations, stability sometimes becomes a limiting factor.