R17/R19/R22/R25 Thread 28mm Diameter Roof Bolting Bit Pneumatic Rock Drill Compatible Anchor Drill Bit Our 28R series anchor bits cover R17, R19, R22, and R25 thread options. Taking the widely adopted 28R17 model as the core, this 28 mm anchor drill bit is built for roof bolting works in coal mine reinforcement projects. It delivers stable performance under rigorous underground conditions, with balanced structural strength and abrasion resistance to suit variable hard rock