I believe a chipbreaker can actually break the shaving in a way that reduces tear-out if it is set very close to the cutting edge of the blade. This belief is based on the study by Chutaro Kato cited on the Links & References page. I’ve recently done a test that has convinced me this is true.
One objection to using a closely set chipbreaker is that the throat of the plane will become clogged with shavings. I’ve been able to use chipbreaker distances as small as .004” without having this problem. Very thin shavings tend to collect in the throat, but can be removed by turning the plane upside down.
Clearance depends on the nature of the shaving, which varies considerably depending on the species being planed. Soft pine produces strong, flexible shavings that are easy to remove from the throat. When the same plane is used on cherry, a corrugated shaving is produced. On Bolivian rosewood the shavings break into short pieces because the wood is so brittle. Despite the differences in shaving characteristics all of these species can be planed without clearance problems.
When producing a thick shaving that is broken into regular segments (a Type I chip) the distance between breaks in the shaving is about equal to the width of the plane’s mouth. Since a thick Type I shaving doesn’t give the best surface to the wood, this result has little to do with smooth planing.
When producing the thin Type II shaving typical for a smoothing plane, a narrow mouth helps to control tear-out by forcing the shaving to bend sharply right at the cutting edge of the blade. This helps to prevent a Type I chip from forming and instead causes the wood fibers to slide sideways against each other in a shearing action. This “result” is not an experimental observation of my own, but comes from reading the sources cited on my Links & References page. My own planing experience is consistent with this theory.
The testing required to determine optimum mouth dimensions for different species and conditions remains to be done. For now I’m using a mouth dimension of about .008” and believe a mouth half that size would be about right for fine finish planing on most species.
Most testing so far has been done at a cutting angle of 47½º so I have nothing new to add to the well-known fact that higher cutting angles reduce tear-out on hardwoods and lower cutting angles produce surfaces with a “deeper” look on softwoods.
A quick test using a cutting angle of 59½º degrees on Bolivian rosewood confirmed that the higher angle reduces tear-out significantly, leaving the areas of adverse grain with a dull appearance but no visible tear-out.
Trying to produce a Type I shaving has turned out to be much more difficult than I had expected based on my readings. Using a sharp blade, I found I had to increase the shaving thickness way beyond what is typical for smooth planing before the shaving exhibited the regularly broken quality characteristic of Type I cutting.