I bought a few countersink bits from Screwfix and Toolstation to process a number of flat steel bar ( already with holes) and they only last a short while. I'm keeping my hand drill on slow but they just stop cutting after a few holes! I bought cutting paste. I want a heat treated hard cobalt bit grade m2 or anything better that won't die off so quickly. I have repeat work so will always want these. Any advice on good countersinks that don't cost the earth maybe and don't fail real quick greatly appreciated, thank you.