2022 Volume 61 Issue 1 Pages 218-223
Brass that contains lead is called free-cutting brass, and because it boasts good machinability, which allows fine cutting, it has long been used in a variety of applications. However, lead is known to have negative impacts on the environment. Today, regulations such as the EU’s RoHS Directive limit the use of brass containing lead. So, cutting work must be performed on lead-free brass. However, compared to leaded brass, lead-free brass has very poor machinability. Therefore, to perform cutting work on such lead-free brass without causing adverse effects on the environment, it is necessary to improve its machinability. With this in mind, we have developed brass that uses silicon as a substitute for lead. Adding silicon to the β-phase of brass has been reported to lower the stacking fault energy (SFE). This is because a lower SFE causes the chips formed during cutting to break off short, thus decreasing the cutting resistance. However, there have been no reports on the cutting characteristics (including drill lifespan, cutting resistance, and chip condition) during microdrilling of fine holes in silicon brass.
In this study, we compared the cutting characteristics during microdrilling of fine holes in leaded brass and brass with silicon added to either the α-phase or the β-phase of the solid solution. The results revealed that the addition of silicon to the β-phase led to nearly the same drill lifespan as that for leaded brass. This is due to the lower SFE. However, with silicon brass, chips tend to stick to the drill flank and increase the cutting resistance （thrust force）. Moreover, both leaded brass and silicon brass produce folded chips. However, in the case of leaded brass, this folding occurs due to the solid lubrication action of the lead, whereas for silicon brass it is caused by the lower SFE, which makes it easier for shear failure to occur. Thus, similar folding can be caused by different mechanisms.
