What is the ideal shape for plane irons?

 
     
     
 Shop for Woodworking Hand Planes 
     
     
 Different shapes of hand plane iron 

The shape of an iron's cutting edge affects the kind of cut the plane makes.

 
     
 Straight hand plane iron 

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

 
 Four basis hand plane iron profiles 

Not including some specialist planes for cutting decorative mouldings, there are four basic shapes for the cutting edges of irons – or iron profiles:

  • Straight

  • Rounded

  • Crowned

  • Rounded corners

 
     
 Various hand plane irons, woodworking planes 

Which one is used depends on the type of plane and the current task.

 
     
 Straight hand plane iron profile 

Straight

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.

 
     
 Rounded iron used in scrub planes and sometimes jack planes 

Round

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.

 
     
 A crowned iron is mostly used in smoothing planes 

Crowned

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.

 
     
 Woodworking plane iron with rounded corners 

Rounded corners

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.

 
     
 Honing a hand plane iron, sharpening a hand plane blade, woodworking plane 

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. 

 
     
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