A Numerical Investigation of Through-Tool Coolant Wetting Behaviour in Twist-Drilling

Abstract

Supplying coolant through internal coolant channels is a common method of transporting large thermal loads away from the tool in twist-drill machining to increase tool life, aid chip evacuation and avoid catastrophic tool failure. In this work a finite element-based numerical model of the machining process is loosely coupled with a finite volumebased numerical method for predicting the distribution of coolant inside the borehole. These methods are employed to study the effect of channel position on cutting geometry lubrication and uses response surface models to show that all designs do not fully flood the borehole and that not all areas of the tool geometry are lubricated with coolant. Visual analysis of results show that coolant, for all designs, primarily lubricates the area between the cutting edge and the coolant hole exit, however, depending on application requirements coolant channel positioning can be used to modify coolant supply to the axial rake, for chip evacuation or to the cutting edge for heat removal.

Computational Fluid Dynamics-Enabled Modelling and Optimisation of Coolant Flow through Helical Channels

“…where D (m) is the helix diameter or distance between the coolant channels. Since this paper focusses on helical flows relevant to twist-drilling applications, taking typical values of d=1mm, D=6.6mm, and 40 m/s ≤ U ≤ 80 m/s [2], yields 40,000 ≤ Re ≤ 80,000, which is significantly larger than Recrit~12,000, so that the flow in coolant channels can be considered to be turbulent. The Dean number is a modified Reynolds number defined by 𝐷𝑛 = 𝑅𝑒 ( 𝑑 𝐷 ) 0.5 (9) which indicates the significance of centrifugal forces generated by the helical geometry (without rotation).…”

“…Drilling devices range in size and application from deep-water exploration in the oil and gas industry to micro-drills in medical applications [2]. In a manufacturing context, drilling is a ubiquitous feature of metal forming processes mainly used for assembly purposes such a holes for fasteners and rivets [3].…”

“…This paper is motivated by the cooling of fluted twist-drills, which are amongst the most common form of drill used in modern manufacturing, with an estimated market worth around $4 billion per annum [4]. These are commonly used to create cylindrical holes in the manufacture of many important industrial products such as aircraft, cars, ships and trains [5]. Twist-drills typically feature two or more cutting edges and helical grooves around the drill body, where the helix angle varies depending on the application, Figure 1.…”

“…Johns et al [5] used a finite element-based numerical model of the machining process to determine the shape of the work-piece and chip and loosely coupled this with a finite volume-based two-phase CFD analysis of the distribution of coolant inside the bore-hole to study the effect of channel position on cutting geometry lubrication. Their results showed how the coolant channel position can be used to modify coolant supply to the axial rake, to provide more effective chip evacuation, or to the cutting edge for better heat removal.…”

“…The recent findings by Oezkaya et al [9] that the pressure of coolant emerging from the helical coolant channels in twist drills has a key impact on the effectiveness of cooling highlights the importance of understanding how flow in coolant channels is influenced by geometrical and operating parameters. Although little research has appeared for the rotating helical flow parameter range specific to twist drilling [2], there is a large literature on flow in non-rotating helical channels due to its importance in a diverse range of applications including heat exchangers, chemical reactors and engine exhaust systems [16]. A number of studies have investigated the secondary flow structures created by the centrifugal forces which drive fluid radially outwards from the axis towards the outer wall where it bifurcates to create two recirculating zones.…”