Mark Hennebury
Title Modern Shop Practice V1
Author Howard Monroe Raymond
Publisher American Technical Society
Year 1916
Copyright 1916, American Technical Society

The cap-iron f, which is not sharp, is not, as is often supposed, used for the purpose of strengthening or stiffening the cutting iron, but as a chip-break to prevent the cutting edge of the plane-iron from chipping, tearing, and breaking the grain of the wood below the surface when the grain turns and twists, or when it is knotty and crooked. In such cases the tendency of the plane-iron is to split and tear out the fibres of the wood in front of the cutting edge. To avoid this, the cap-iron is screwed on with its dull edge quite close to the cutting edge, so as to bend and break off the fibres or the shavings before the split gets fairly started below the surface.

The cutting edge of the plane-iron is said to have lead in proportion to the distance it is placed in advance of the dull edge of the cap-iron. The depth of the splits, or the roughness of the cross-grained surface, will be just equal to the lead of the cutting edge. For soft, straight-grained wood, the lead may be 1/32 inch or even more, but this must be reduced in proportion as the wood is curly, cross-grained, or knotty.