Effect of Different Feed Rates on Chip Evacuation in Drilling of Lead-Free Brass with a Small-Diameter Drill

Free-cutting brass is a material whose machinability is improved by the addition of lead. It is used in a wide range of applications because of its high electrical conductivity and antibacterial properties. However, the use of lead is restricted by laws and regulations to reduce environmental damage. Therefore, the lead in free-cutting brass is being replaced by bismuth or silicon. The machinability of lead-free brass, improved by these additives, has been confirmed to be sufficient for milling and turning. However, when drilling deep small-diameter holes in leadless brass, the problem of premature drill breakage occurs due to chips clumping and clogging in the drill flutes. In a previous study, drilling of holes with an aspect ratio of nine was performed using a 1.0 mm diameter cemented carbide drill, and it was confirmed that the addition of trace elements improved the chip breaking and chip evacuation. However, the waveform of thrust force during machining demonstrated a decreasing trend, and the original machining conditions were not attained. In this study, the effect of feed rate on chip evacuation was investigated using a 0.5 mm diameter twist drill, which is subject to severe chip evacuation at high cutting speeds. As a result of examining chip evacuation by varying the feed rate, the chip evacuation exhibited good performance; where bellows-like chips were evacuated under a slow feed rate, and long-pitch chips were evacuated under a high feed rate. In the drilling process using these feed rates with good chip evacuation, 10,000 holes could be drilled, but the shape of the top of the hole was distorted and the straightness was poor at a slow feed rate. In contrast, hole accuracy was good at a high feed rate, further indicating that a high feed rate with long-pitch-shaped chip evacuation is effective for high efficiency when drilling deep holes of small diameters.