Measure while drill (MWD) data from blastholes offer enormous potential for improving the resolution of geological models in open cut mines. However, the practical application of MWD data is limited by high variability, which persists even in well-calibrated drilling systems. Without identifying and correcting these sources of variance, analytical methods such as machine learning cannot deliver reliable lithological interpretation. This study addresses a critical but previously overlooked factor influencing MWD parameters: drill bit wear. Using data from more than 4000 bits across ten Pilbara mine sites, we estimate the within‑bit wear response by applying a fixed‑effects design with a penalised spline over cumulative metres, while controlling for lithology and site differences. Contrary to conventional assumptions, the fleet-level curve indicates that penetration rates increase as bits wear, a trend consistently observed across mine sites. These results challenge established expectations about drilling performance and highlight drill bit wear as a systematic source of variance in MWD data. Recognising, recording and correcting for this effect is important for any geological interpretation using MWD data. While this preliminary study focuses on isolating the within‑bit wear trend rather than providing a complete correction formula, the findings offer a practical basis for more reliable use of MWD in stratigraphic interpretation, blast planning, and geotechnical risk assessment.