Effects of boundary lubrication and spindle speed on the surface roughness of UHMWPE in robotic milling

Achieving high surface quality in robotic milling of Ultra-High Molecular Weight Polyethylene (UHMWPE) is difficult: the polymer conducts heat poorly and behaves viscoelastically, so the cut readily softens and damages the very surface it is forming. This study examines how boundary lubrication and spindle speed affect the surface roughness of UHMWPE machined on a KUKA KR120 R2700 robotic system. Dry cutting, palm oil, and ISO VG 32 light mineral oil were compared, with both oils pre-applied by hand so that only a static boundary film was available at the cut. Spindle speeds of 6500 and 9500 rpm were used at a fixed feed rate of 1620 mm/min and a depth of cut of 0.3 mm, and Ra, Rq, and Rz were measured using a Mitutoyo SJ-401 profilometer. Palm oil gave the lowest Ra at both speeds, with values of 1.275 µm at 6500 rpm and 1.004 µm at 9500 rpm, corresponding to reductions of approximately 45% and 39% compared with dry cutting and 23% and 21% compared with mineral oil, respectively. The higher spindle speed improved the finish under every lubrication condition. Within the window tested, palm oil therefore earns consideration as a boundary lubricant for robotic UHMWPE milling. However, these data cannot establish how it behaves over a longer cut: friction, wear, cutting temperature, and tool life were not measured, and these parameters would have to be quantified before the oil could be recommended for production use.