Thursday, November 20, 2014

Understanding the 5 Physical Properties of Workpiece Materials

By Tim Aydt, Product Manager – Milling

Whether you are machining parts from cast irons, low-alloy steels or nickel-based alloys, all such materials exhibit five basic physical properties in varying levels. Those properties are abrasiveness, hardness, thermal conductivity, tendencies toward adhesion/ductility and strain hardening.

The proportions of the individual properties in a given workpiece material largely determine its machinability. Relatively soft low-alloy steel will exhibit strong tendencies to adhesion that can lead to edge buildup on a cutting tool and diffusion wear. On the other hand, poor thermal conductivity of a tough nickel-base alloy can generate extreme cutting temperatures that will cause a tool to deform and fail.

In theory, a material’s specified mix of alloying elements determines the type of cutting tools and cutting parameters that will, in turn, produce predictable and normal wear patterns and help increase productivity. The reality, however, is that cutting tools and parameters indicated for a certain workpiece material may not produce such desired results, and often the reason is variability in material composition.

To better understand how the five properties affect machinability, Seco partnered with steel suppliers and other metalworking-related companies to develop an analysis system that measures workpiece properties. The system charts data from quantitative measurements of the five material properties on a five-pointed grid or pentagram with low values appearing near the center and high values toward its borders. The area enclosed by the data points provides a graphic image of the particular material’s tendencies. Using the pentagram, machinists can better match tool features and cutting parameters to the actual properties of the workpiece.

Several common guidelines have resulted from Seco’s analysis system.

  • Material adhesion tendencies create a need for tough tool substrates with tough coatings, sharp edge radii and high rake angles as well as cutting conditions aimed toward temperature control. This means speeds high enough to carry heat away in the ductile chip. Adhesion tool wear patterns include micro chipping, built-up edge, flaking and notch wear. 
  • Tools aimed at handling material hardness should have strong substrates (depending on the feedrates employed) as well as cutting edges with small rake angles applied at low feedrates and shallower depths of cut. Typical tool wear includes plastic deformation, chipping and breakage.
  • Machining materials that tend to strain harden require tools with toughness and small nose radii and cutting edge geometries for low cutting speeds, high feedrates and deeper depths of cut. Prominent tool failure modes include plastic deformation, chipping and notching.
  • Materials such as superalloys that exhibit poor thermal conductivity mandate the use of tools with high compressive strength, high rake angles and strong cutting edges. Low cutting speeds and feeds are typical. Tools usually fail via plastic deformation or simply from a higher-than-normal wear rate. 
  • Tools intended for abrasive workpiece materials should be engineered with abrasion-resistant substrates and strong cutting edges. Low feedrates and cutting speeds but high depths of cut are appropriate. Wear mechanisms include flank and crater wear and notching.

For more information on how workpiece material properties influence the machining process, please feel free to contact me.

About the Author
Tim manages Seco’s indexable milling product line for Seco NAFTA. In his spare time, he enjoys playing golf and working out.

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