HSS M6 Counter bore for through holes - Medium Grade
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PUTNAM TOOL Counterbore, Counter Bore, .340" Pilot x .438" +/- Cut, # 1 MT Shank | eBay
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7Pcs/set Counterbore Drill Bit Sets HSS Counterbore End Milling Cutter M3.2-M12.4 Countersink Drill Bits Pilot Slotting Milling Cutter End Mill Cutting Tool Set for Copper Aluminum Cast Iron - Amazon.com