Effect of MQL on the tool life of small twist drills in deep-hole drilling

Drilling of deep and small boreholes using twist drills has to be considered as one of the most difficult metal cutting operations. There are many reasons for this, one of them being that the cutting fluid has to be supplied externally. This research work investigates in how far the manner of supplying and the type of minimum quantity lubricant have an effect on the tool life of coated and uncoated high-speed steel twist drills of 1.5 mm diameter. Deep-hole drilling is performed as the holes, drilled into plain carbon steel, had a depth of 10 times the diameter. The feasibility of dry machining as an appropriate alternative to MQL in deep-hole drilling has also been investigated. This work shows that, compared to a continuous supply of the minimum quantity lubricant, a discontinuous supply brings about a significant reduction in tool life, especially in the case of heat-sensitive drills. With respect to the type of minimum quantity lubricant, a low-viscous type with a high cooling-capability gave rise to a notably prolonged tool life. It is also shown that dry drilling is associated with strongly accelerated tool wear for most of the twist drills tested, resulting in a significant reduction in tool life.