A cap iron — also called a chip breaker (Dutch: keerbeitel) — is the upper blade of a double plane iron, clamped firmly but with slight spring onto the lower part (the actual cutting iron). The cap iron breaks the wood shaving in the plane’s mouth and so helps clear the chips.
How it works
When planing, a shaving rises out of the plane mouth. Without a cap iron this shaving can tear ahead of the cutting edge and damage the surface. The cap iron prevents this:
- Breaking the shaving — the shaving bends against the cap iron and does not tear further.
- Chip clearance — the broken shaving is guided cleanly out through the mouth.
- Spring clamping — the underside of the cap iron is slightly curved, creating the “spring” that keeps it tight against the cutting iron.
The distance between the cutting edge and the edge of the cap iron sets how fine the work is: the closer the cap iron sits to the edge, the less the shaving can tear — important on cross-grain or interlocked timber.
Application
- Hand planes — smoothing planes, jack planes and jointer planes use a double iron with a cap iron.
- Fine finishing — on difficult, “against the grain” wood the cap iron is set close to the edge.
Points to watch
- The edge of the cap iron must sit exactly flat on the cutting iron; a gap lets shavings jam.
- Adjust the cap iron to the timber: further back for coarse removal, closer for fine finishing.
Related terms
- Chisel
- Hand plane
- Wood joints
- Timber species
- Woodworking
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