A maddening defect sometimes caused when countersinking is called chatter. After drilling a pilot hole, the bit can vibrate when forming the countersink, resulting in a conical hole that’s badly chewed up in a somewhat rippled pattern. While the sides of the cone are sometimes hidden by the screw head once driven in, the edges of the countersink still look ragged.
Chatter is caused when a countersink bit sets up a harmonic vibration as it spins, and it happens almost exclusively when a countersink is added to a previously driven pilot hole as part of a two-step procedure. Here are some tips for avoiding chatter:
• Piloted countersinks, like those with a drill bit that forms a pilot hole at the same time as the countersink, rarely chatter. Whenever possible, use a piloted countersink.
• Countersinks with an even number of identically sized flutes chatter more than bits with a single (or other odd number) flute. Newer fluted bits vary the flutes slightly, lowering harmonics. Likewise, cross-hole countersink bits — which essentially have only one cutting edge — chatter less than fluted bits.
• Slow down! High speed, especially when combined with light drilling pressure, can easily form harmonic vibrations. Using less speed and a bit more pressure will help minimize chatter.
• No matter what type of bit you use or how you use it, sharp cutting edges cut more smoothly and reduce chatter.