Countersinks (AKA countersink drill bits) are tools that chamfer a bit wider at the top of the drilled hole to ensure that the heads of fasteners, when inserted, remain flush with the surface of the workpiece. They also prevent substrate material from swelling as the fasteners are tightened down. A countersunk fastener that sits flush with the surface looks better and won’t snag other materials that move across it. Countersinks vary by the countersink angle they cut, number of flutes, material composition and other important considerations. For more on countersinks and similar tools, see our single-ended countersinks and reamers.