On steel and other ferrous materials, chip color is one of the fastest, cheapest diagnostic tools on the machine — a built-in thermometer for your cut.

The color scale

  • Silver / light gray: cool, well-controlled cut — ideal for HSS and cobalt tooling.
  • Light straw: slightly warm, generally fine, common with carbide at higher speeds.
  • Dark straw / brown: getting hot — worth reducing speed, increasing feed, or adding coolant.
  • Blue / purple: too hot — risk of work-hardening the surface, accelerating tool wear, and even drawing the temper of a tap or end mill running dry.

What to do about blue chips

Blue chips usually mean one of: speed too high for the tool material, feed too light (rubbing instead of cutting), insufficient coolant, or a dull edge. Light feeds are a common, counterintuitive culprit — a tool that isn't taking a real bite spends more time rubbing and generating friction heat than one taking a healthy chip.

Non-ferrous materials

Aluminum, brass, and copper don't reliably show heat through color the way steel does. Watch chip shape instead — long stringy chips wrapping the tool suggest built-up edge; small, well-broken chips suggest a healthy cut.

💡 If you only remember one thing: blue chips on steel mean slow down or cool down — don't push through it.