What is the ideal shape for plane irons?
The shape of an iron's cutting edge affects the kind of cut the plane makes.
For instance, the sharp corners of a straight cutting edge can leave "tracks" in the wood, whereas a slightly cambered, or curved, cutting edge leaves no marks.
Iron cutting edge shapes and their uses
Not including some specialist planes for cutting decorative mouldings, there are four basic shapes for the cutting edges of irons – or iron profiles:
Which one is used depends on the type of plane and the current task.
Straight blades are used in jointer, rebate and shoulder planes.
Without a perfectly straight cutting edge, it would be impossible to cut perfectly flat edges for joining, or to trim mortises (recesses) and tenons (the tongues that fit into the mortises) to make perfect joints.
This profile is preferable when planing away quite large amounts of rough-sawn timber. The rounded edge can dig in deeper than other blades on each pass. Scrub planes are made specifically for this job.
It is also quite practical to grind this profile on a jack plane iron if it's being used for serious reducing work.
Used mainly in smoothing planes, this profile can also be useful in a jack or fore plane when flattening panels that are wider than the blade. The idea is to conceal overlapping strokes on a wide surface by having the middle portion of the blade project from the sole while the corners are safely out of the way.
The surface produced will have a series of broad, shallow, parallel flutes, but the panel will appear to be flat to all but the most careful observer.
This is probably the best all-purpose profile for smoothing and jack planes because it gives near maximum width of cut, allows overlapping strokes on a wide surface without leaving "tracks", and can still be used to plane the edges of boards for joining together.
It is, however, a challenge to hone well, as all the requirements to sharpen a straight edge must be met, plus a smoothly rounded transition needs to be honed on each corner.