Friday, May 16, 2014

Productive Tradeoffs in Rough Turning

By Chad Miller, Product Manager – Turning and Advanced Materials 

Maximizing productivity in rough turning operations requires a balance of tradeoffs between the properties of the cutting tool substrate as well as the characteristics of its chipbreaker geometry. 

Seco M6 Roughing Geometry
A tool’s cutting edge must be harder than the material it cuts. High hardness, especially at elevated temperatures generated in high speed machining, will prolong tool life. A harder tool, however, is also more brittle. Uneven cutting forces encountered in roughing, especially in interrupted cuts involving scale or varying depths of cut, can cause a hard cutting tool to fracture. Instability in the machine tool, fixturing, or workpiece can also precipitate failure. 
On the other hand, boosting a tool’s toughness by including a higher percentage of cobalt binder, for example, will enable a tool to resist impact. But at the same time, reduced hardness also makes a tool subject to rapid wear and/or deformation in higher-speed operations. The key is to balance tool properties in light of the workpiece being machined. 
For example, Seco’s TP0500 grade is engineered for maximum hardness and wear resistance and trades off some impact resistance or toughness for faster speed capabilities. It is best suited for higher-speed roughing on workpieces without interruptions and on stable machining setups. The TP3500 grade, on the other hand, is designed to provide long, predictable tool life in unstable conditions. To gain toughness, the grade trades off some heat resistance and high-speed capability. 
Incidentally, Seco’s Duratomic coating enhances the performance of both grades. The aluminum and oxygen in the coating’s outer layer are arranged on the atomic level to maximize hardness, toughness, and lubricity, allowing cut material to flow freely minimizing heat buildup.
While the substrate material and coating of a cutting tool provide a foundation for roughing operations, the tool’s chipbreaker geometry enables fine-tuning of tool performance.
Seco TP0500 Grade

Just as with tool materials, tradeoffs are involved in the engineering of tool geometries. A positive cutting geometry and sharp cutting edge reduce cutting forces and maximize chip flow. However, a sharp edge is not as strong as a rounded one. 
Geometric features such as T-lands and chamfers can be manipulated to strengthen the cutting edge. A T-land – a reinforcing area behind the cutting edge – set at a positive angle can provide sufficient strength to handle specific operations and workpiece materials and minimize cutting forces as much as possible. A chamfer squares off the weakest part of a sharp cutting edge, at the price of increased cutting forces. 
“Hard” chip control geometries guide the chips through a relatively acute angle to curl and break them immediately. These geometries can be effective with long-chipping materials but place extra load on the cutting edge. “Soft” chip control geometries put less load on the cutting edge, but generate longer chips. 
Good examples of differences in roughing geometries are the M5, M6, and MR7 designs from Seco. Listed basically in the order of their capability to handle increasing DOC and feed rate in roughing operations, the inserts are negative in overall geometry in that they have perpendicular flank faces and are engineered for two-sided use. 
Seco M5 Roughing Geometry
The M5 geometry combines high edge strength with comparatively low cutting forces. At the insert nose, the tools have a 0.30 mm wide, 5˚ positive T-land followed by a 20˚ transition area to the insert’s rake face. The rest of the cutting edge has a 1˚ negative chamfer preceding a 0.31 mm-wide, 5˚-positive T-land before an 18˚ transition to the rake face. The chamfer boosts edge strength, and an open chip groove facilitates the flow of ductile long-chipping alloys. The M5 geometry suits a variety of workpiece materials including steel, stainless steel, cast iron and superalloys.
The MR7 geometry, in comparison, is engineered to handle heavy interruptions and tough applications such as roughing forgings/castings with skin and oxide scale in steel, cast iron, and stainless steel. To maximize edge strength, the tools have a wider 0.35 mm, 0˚ T-land at both the nose and the cutting edge with a shallower, 17˚ transition to the insert’s rake face. While the wider, flat T-land and shallower transition angle provide increased strength, they also generate higher cutting forces because the tool overall is less sharp. Together, the geometry features provide strength comparable to that of single-sided inserts for heavy roughing operations.
A combination of geometric features places the performance of the new M6 geometry between, and overlapping, that of the M5 and MR7 tools. The 4˚-positive T-land at the nose is 0.25mm wide, not as wide as the M5 or MR7 tools, and there is a 19˚ transition to the rake face. The 0.30 mm-wide, 0˚ T-land on the cutting edge is followed by a 21˚ transition to the tool’s rake face. This chip-control configuration combines strength and a wide chip control groove to expedite flow of the cut material. 
Although the three geometries differ in details and areas of application, they share an engineering strategy aimed at protecting the cutting edge, minimizing cutting forces and maximizing efficiency of chip evacuation. The tools illustrate how tradeoffs and combinations of geometric features can positively affect the final results of roughing operations. 
About the Author
Chad manages Seco's turning and advanced materials product lines, including all CBN and PCD products. When he's not helping customers implement advanced metalcutting solutions, you can find him training for and running 5K, 10K and 1/2 marathon races and triathlons. 

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