The Crowning Touch
Call it camber, call it radiusing, call it rounding, call it what you will. The questions is, why crown your plane irons? Well, there are three very good reasons. First, a gentle crown on smoothing and polishing planes will eliminate lap marks or “tracking”. Second, a heavy crown on scrub and fore planes will allow cross grain planing without tear out. Third, a medium crown on jack planes will allow for the removal of thicker shavings while planing with the grain.
The first order of business is to determine the thickness of the heaviest shaving you intend to remove with a particular plane. This dimension will become the “arc height” of the crown you establish on the iron. Now, here’s where the science comes in to play. Because the iron is bedded at an angle (other than 90 degrees to the sole), the actual arc height must be increased. Why? Because as the cutting angle decreases, so does the arc height. The simple chart will demonstrate this geometric conundrum and provide you with the approximate increase in arc height required.
Chart indicating cutting angle/arc height relationship
For example: If you wanted a .001 crown on an iron that would have a cutting angle of 45 degrees, you would divide .001 by .71 (71%). The actual arc height would be .0014 (give or take a “squeak”).
Remember, crowning can be achieved by simply applying alternating pressure to the edges of the iron while you’re honing. It’s just not that tough.