It is common knowledge that skewing a plane reduces the blade’s effective angle of attack in relationship to the wood. If that’s not common knowledge for you, think about it this way, if you walk straight up a ramp, you are walking up its “pitch” angle. If you walk diagonally up the ramp, you are traveling at a shallower angle. It’s farther, walking up diagonally, but easier.
When a plane blade is pushed straight ahead into the wood, the shaving follows the pitch of the blade. When you skew the plane, the shaving follows the longer, but shallower diagonal path up the blade. This fact can come in handy if you are planing, for example, end grain and need to shear the fibers at a lower angle of attack to get the best finish. This lower angle of attack comes with the same bevel and relief angles on the blade. If you honed a low-angle plane to match the skewed angle you may need to grind the blade to a thin, fragile edge.
I recently spent too much time trying to understand the relationship between the skew angle and the resultant effective pitch angle when John Whelan came to my rescue. I found an excerpt from his book: The Wooden Plane: Its History, Form, and Function at Nichael Cramer’s website: http://homepages.sover.net/~nichael/nlc-wood/index.html. The passage is worth reading but the part I needed was this: “…the sine of the effective pitch is the product of the sine of the actual pitch and the cosine of the skew angle.” This sounded to me way more complicated than it needs to be so I cut angles off a couple of rear blocks from our plane kits and started measuring and doing the trig.
Turns out he’s right, of course. To save you some time I did a bit of Excel work and came up with a chart to show the effective angle of a the following pitches and skews:
|Skew angle –>||10°||15°||20°||25°||30°||35°||40°||45°|
A common pitch plane (blade bedded bevel down at 45°, like your trusty #4, etc.), skewed 30° will attack the wood at an effective angle of 37.8°. If you skew the same plane 45°, it will cut the wood as would an edge at 30°.
I find this sort of research interesting and hope you do as well. I’m sure there are aspects of this study that I haven’t thought of and would appreciate your comments and additions.
And I think I showed some restraint in the literary application of the skew/screw pun for the title. I thought of a lot of others…