Effect of sharpening and skew on a Bailey bench plane - musings
As I understand the cutting mechanics of a standard Bailey-type iron, there are two main considerations - bevel angle and pitch.
The bevel angle of the iron (including any back bevel) represents a trade-off between edge durability and sharpness. An acute angle is sharper; an obtuse angle lasts longer. There's an obvious bottom limit below which the blade will chip or fold, and an upper limit at which the blade is no longer sharp enough for a decent finish because you need excessive pressure to make it cut. So a conservative range is about 25-40 degrees in my opinion. Furthermore, a sharper blade leads to less tearout, because it will be more likely to shear off fibres rather than "grab" them and lever them out of the wood. So the sharper the blade, the better the result.
25 degrees (less tearout) <-----------------BEVEL ANGLE----------------------> 40 degrees (edge retention, difficulty in pushing)
The pitch of the blade is the angle at which it meets the wood. On a standard Bailey type plane, that angle is usually the bed angle of 45 degrees (common pitch), though you can get a Lie-Nielsen smoother in York pitch (50 degrees) or an HNT Gordon A55 in middle pitch (55 degrees), for example. Pitch is a compromise between ability to avoid tearout on the one hand, and difficulty of pushing, edge retention and surface quality on the other. This is because sheared fibres look better than scraped or abraded fibres, which is why we are using a smoothing plane (apart from the fact that it makes a whoosh noise). Pitch can be increased using a back-bevel, but by no more than 15 degrees (because otherwise we would violate our rule above that total bevel angle should not exceed 40 degrees, and assuming the face bevel to be sharpened at a standard 25 degrees).
Consequently the maximum sensible pitch for a Bailey type plane at common pitch is 55 degrees, the same as that on the HNT Gordon A55 (Terry's other smoother is 60 degrees - I'm not even sure there's a word for that).
45 degrees (smoother cut, edge retention) <--------------PITCH---------------------------> 55 degrees (less tearout)
The instructions for the Veritas honing guide suggest back bevels of up to 20 degrees, but I don't think that's a good idea (and nor does Terry Gordon: HNT Gordon - BLADE ANGLES).
Now on to our last variable, which is skew angle. Despite creating a lower cutting angle, it's pretty clear that a skewed angle helps a lot with wild grain - I suspect that this is because a scything motion leaves a cleaner cut, creating the same anti-tearout benefits as increasing the sharpness of the blade. There is less force upon any part of the blade at any particular point (without going all mathematical, it's a "longer ramp" - more of the blade works on each fiber). So in use this variable can be tuned as follows:
0 degrees (no change) <------------------------SKEW ANGLE------------------------> 45 degrees (less tearout, but lower cutting angle)
I admit this last bit is counter-intuitive, but oh well.
Soooooo....
In theory the best way to attack difficult grain with a Bailey bench plane is to sharpen the iron to 10,000 grit or greater, sharpen a primary bevel of 25 degrees and a back bevel of 15 degrees, and skew the plane at 45 degrees in use.
I tried this to overcome some recalcitrant Jarrah and it worked a treat.
All of which proves that you can do a great deal of thinking, only in order to come a blindingly obvious conclusion that you've been told before (on this forum), i.e. to counter difficult grain whack on a back bevel and skew the blade!
This does make me wonder, however, what the benefits of other types of planes might be, as far as edge geometry goes...
How does all that babbling sound to you guys? What do you do to counter difficult grain (other than buy a plane with a higher bed angle...)?
