## 1 Introduction
Although foam production methods generally yield a near-net shape product, certain design features such as through holes must be machined [17; 18]. Metal foams are hard to machine by conventional machining methods due to high cutting forces exerted onto the foam, which can lead to pore closure and a decrease in porosity. The low cutting force associated with unconventional machining methods, such as wire EDM, minimizes pore closure [19; 20]. The main disadvantage associated with wire EDM machining is its low production rate, which leads to higher production costs compared to conventional machining methods [21]. Pore closure, also known as smearing, can be reduced by introducing molten filler material into the pores before conventional machining. Once in the pores, the filler, most commonly machinable wax, solidifies and acts as a supporting structure for the cell walls. Once machining is completed, the filler material is melted and evacuated out of the pores, leaving no residue [22]. Another use for wax filler was investigated by [PERSON] et al. [23]. In their paper, the influence of cutting speed and feed rate on thrust force and cutting torque was evaluated. A combination of low cutting speed and high feed rate was found to be highly detrimental to the drilling process, as it caused flute clogging, which led to a sharp increase in both thrust force and torque. To alleviate flute clogging, wax was added to the foam, which melted due to the relatively high cutting temperatures. As the wax melted, it acted as a lubricant, which eased chip evacuation and reduced flute clogging. Difficulty in cutting porous materials also comes from the fact that they have reduced thermal conductivity and a high amount of porosity that leads to interrupted cutting, both of which have a significant impact on tool wear [24; 25]. [PERSON] [17] used TiN-coated high-speed steel (HSS) and carbide drills to evaluate the cutting forces and tool wear during the drilling of Distaloy AB steel foam with varied amounts of porosity. Drilling of higher porosity foam produced lower thrust forces due to a lower volume of removed material. Due to the higher wear resistance of the carbide tool, thrust forces remained lower when using a TiN-coated carbide drill. Drilling high-porosity foams caused more wear on the cutting edge, whereas drilling lower-porosity foams caused high flank face wear and workpiece adhesion. Drilling porous materials makes measuring tool flank wear difficult due to high amounts of built-up edge formation. A dependable way of measuring flank wear was proposed by [PERSON] et al. (2018), which involves taking a micrograph of the tool before and after drilling. Overlaying the two micrographs allows for easier identification of built-up edge formation and flank wear. The influence of tool diameter on cutting forces and hole circularity was examined by [PERSON] et al. (2018). Drilling was carried out on closed cell AlSi7 foam using uncoated HSS drills. Their findings showed that smaller tools produced a higher thrust force due to chip build-up in the flutes of the drill. An increase in workpiece material adhesion was also noted with increased cutting speed, which caused an increase in cutting forces. Higher cutting forces caused by increasing the feed rate were attributed to higher chip loading. An increase in circularity errors was also noted using higher-diameter tools. [PERSON] (2018) aimed to determine the influence of tip angle on hole quality, torque and thrust forces during the drilling of steel Distaloy DAB. Testing was carried out with two types of drills, one with a tip angle of 119\({}^{\circ}\) and the other with a tip angle of 135\({}^{\circ}\). Both drills produced lower torque but higher thrust forces with increased cutting speed. A lower drill tip angle of 119\({}^{\circ}\) was less effective at reducing torque but produced lower thrust forces compared to a drill with a tip angle of 135\({}^{\circ}\). Increases in feed rates led to an increase in torque and thrust force regardless of tool tip angle.