Tools which provide controlled contact on the tool face are used to study the action of free machining steels. The mean normal stress on the tool face σc is found to increase with increased undeformed chip thickness (t = feed) or with a reduction in the controlled length of tool face contact. An increase in σc in turn is found to promote the stability of the built-up edge to higher speeds. The high-speed finish produced with a cut-away tool is thus found to be inferior to that produced with a conventional tool. Manganese sulfide is found to have a similar effect on surface finish, but lead tends to improve the finish obtained at a high cutting speed. The cut-away tool provides improved low-speed finish in all cases as does the addition of either manganese sulfide or lead to the steel. Manganese sulfide is found to become more effective with increased undeformed chip thickness t, while lead behaves in the opposite manner. This observation along with several others is in agreement with the idea that manganese sulfide is a poor solid “lubricant,” while lead is an effective solid lubricant. An optimum chip-tool contact length appears to exist at which the tool life will be a maximum at any combination of cutting speed and feed. From this it follows that an optimum combination of sulfur or lead content, degree of cold work, cooling capacity of cutting fluid, or extent of tool-face limitation exists, since all of these quantities influence the resultant length of contact between chip and tool.

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