Plane Irons and Chipbreakers Explained

The iron is the plane’s cutting blade. It lies on the frog or bed, projects through the mouth by the thickness of the shaving, and is clamped by the lever cap. Everything else in the plane exists to hold it at a fixed angle and depth. How the bed angle and the direction of the bevel set the cutting angle is explained in the hand plane guide; this page is about the iron itself, and the chipbreaker that rides on it in a bevel-down plane.

• Width matches the plane. Replacement irons for Stanley-pattern bench planes run from 1-3/4 in for a No. 3 to 2-5/8 in for a No. 8.
• Thickness varies more than any other dimension. A thicker iron is stiffer and less prone to chatter, which is why makers of replacements for older planes make them thicker than the originals.
• Length sets how much iron is left to sharpen away. Veritas and Hock both make their bench-plane replacements 7 in long.
• The slot is how a Bailey-pattern plane moves the iron. The chipbreaker screw passes through a long slot in the iron, the depth adjuster engages the chipbreaker, and the lateral lever engages a smaller slot near the top of the iron.
• The bevel faces down in a bevel-down plane, where it only needs to clear the wood, and up in a bevel-up plane, where it sets the cutting angle. Grinding and honing it are covered in the sharpening guide.

A chipbreaker — also called a cap iron — is a curved plate screwed to the flat face of a bevel-down iron, with its leading edge set a short distance behind the cutting edge (see the plate above). It has two jobs. It stiffens the iron near the edge, and it bends the shaving over as soon as the shaving reaches it. A shaving that is bent sharply cannot act as a lever: it breaks at the edge instead of prising up the wood ahead of the edge and splitting it along the grain, which is where tear-out comes from.

Lie-Nielsen’s setting is a useful starting point: about 1/16 in from the edge for general work, and closer for the finest smoothing. The chipbreaker only works if its leading edge seats tight against the iron. Lie-Nielsen back-bevels the underside of that edge by 1° to make sure it does; a gap lets shavings jam between the two.

What the research shows

The chipbreaker has been studied under laboratory conditions in Japan. Yamashita, at Shimane University, tested the cap iron in against-the-grain planing of air-dried hinoki at a 10° grain angle, and Chutaro Kato and Yasunori Kawai of Yamagata University studied it as part of a series on the wear of hand-plane blades; both were published in the peer-reviewed journal of the Japan Wood Research Society. Kawai and Kato also filmed, at high magnification, a sharp plane blade with a finely adjustable chipbreaker cutting against the grain.

• The chipbreaker bends the shaving. The film shows the face of the chipbreaker bending the shaving over as the blade advances.
• The closer, the stronger the effect. Wilbur Pan, who subtitled the film in English, reports that the effect was greater the closer the chipbreaker was to the edge, at distances of the order of 0.1–0.3 mm — roughly 0.004–0.012 in.
• The face angle matters. The angle of the chipbreaker’s face changed how close it needed to be.
• Very thin shavings may not need it. A sufficiently thin shaving could be taken against the grain without the chipbreaker.

A replacement iron is the most common upgrade to an older plane, and the most common source of fitting problems. A thicker iron than the original moves the cutting edge forward in the mouth, because the edge sits on the iron’s upper face, which is now further from the frog. That narrows the mouth: on a plane whose frog can be moved back, it can be reopened; on one whose frog cannot, check that the thicker iron still leaves room for the shaving. Makers also design around their own planes: Lie-Nielsen says its replacement blades are made to fit its tools, not other makers’.

Makers state the steel and, usually, the hardness. The figures describe their own blades, and edge-life claims are the makers’ own; they are not independent comparisons.

A bevel-up iron has no chipbreaker and is usually much thicker — 3/16 in in Veritas’s bevel-up planes — because nothing else stiffens it. Its bevel sets the cutting angle directly, so the iron is the place to change the angle. Veritas makes blades at 25°, 38° and 50° for its 12° bed, and suggests a separate blade for each angle you use, because honing a steeper micro-bevel is quick but going back to a low one means removing a lot of steel.

Setting the iron and chipbreaker in practice — depth, lateral adjustment and the mouth — is part of how to use a hand plane. Keeping the edge sharp is the sharpening guide.