Hand Tools Archive
Wiley Horne--So. Calif.
Hi David and all,
Steve Elliott and Brent Beach have done testing of all sorts on plane blades. To answer the question David put, here is a page from Steve Elliott:
He shows the wear pattern on upper and lower blade surfaces as a function of lineal feet planed, including the loss of clearance with wear. He has determined that it is the width of wear on the 'clearance' surface (the lower surface of the blade tip) that causes 'dullness', as opposed to general rounding or bluntness of the tip. Leitz uses this same measure of dullness. Steve's subjective feel is that when the lower wear surface gets to 12 microns, the blade feels dull.
Getting to the original poster's question about whether lesser initial clearance can lead to longer life, one can see from Steve's graphs that as the blade's included angle increases from, say, 30 degrees to 40 degrees, the lower surface is becoming closer and closer to parallel with the wood--you're starting out with much less clearance. Thus, as the included angle goes from 30 to 40 degrees, the blade will stand much less metal loss through wear, before it becomes unusable due to clearance loss. In other words, at 40 degrees, you're approaching limiting clearance at the outset, and can stand little further wear before you're at dull.
Hence, the goal for most is to look for the largest initial clearance--the lowest included angle--that the steel will stand before chipping, as the optimum angle for sharpening. Steve finds this angle to be in the low-30's, perhaps as low as 30 degrees, for the steels and wood and 47.5 degree bedding angle he is using. the bedding angle is important here, as Japanese irons with lower bedding will typically be sharpened to around 25 degrees.
Note that Steve also finds, and Brent Beach argues as well, that because steel is lost from upper and lower blade surfaces, a method of sharpening is required that treats the upper blade surface as well as the lower surface--the bevel--unless enough metal is removed during sharpening to completely remove the upper wear surface. Some strop. Some use Mr. Charlesworth's 1 degree elevation (I refuse to say 'ruler trick' again!).
CAVEAT: As with all discussions of this type, the statements above are provisional, and depend on any number of variables and subjective impressions. Steve Elliott himself is anything but definitive--he always maintains and states scientific skepticism about his findings and methods. In short, your mileage may vary!