Productive Trade-Offs in Rough Turning
Maximizing productivity in rough turning operations requires a balance of trade-offs between the properties of the cutting tool substrate and the characteristics of its chipbreaker 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 or chip. Instability in the machine tool, fixturing or workpiece can also precipitate failure.
However, 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 high speed operations. The key is to balance tool properties in light of the workpiece being machined.
Strength and Speed
Some tool grades are engineered for maximum hardness and wear resistance, trading off some impact resistance or toughness for fast speed capabilities. These tools are typically better suited for high-speed roughing on workpieces without interruptions and on stable machining setups. However, some grades are designed to provide long, predictable tool life in unstable conditions. To gain toughness, the grades trade off some heat resistance and high speed capability.
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.
Just as with tool materials, trade-offs 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.
Applying the Technology
The differences in roughing geometries can be seen in the M5, M6 and MR7 designs from Seco Tools Inc. 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.
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-degree positive T-land followed by a 20-degree transition area to the insert’s rake face. The rest of the cutting edge has a 1-degree negative chamfer preceding a 0.31-mm-wide, 5-degree positive T-land before an 18-degree 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 is suited for 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-degree T-land at both the nose and the cutting edge with a shallower 17-degree 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 with that of single-sided inserts for heavy roughing operations.
A combination of geometric features places the performance of the M6 geometry between, and overlapping, that of the M5 and MR7 tools. The 4-degree positive T-land at the nose is 0.25 mm wide (not as wide as the M5 or MR7 tools), and there is a 19-degree transition to the rake face. The 0.30-mm-wide, 0-degree T-land on the cutting edge is followed by a 21-degree transition to the tool’s rake face. This 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 trade-offs and combinations of geometric features can positively affect the final results of roughing operations when applied appropriately.
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