Back bevel works well (and two questions)

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.....about why back bevels work (on bevel down planes)....

tearout happens in stringy woods when the wood fiber is strong enough to pull up ahead of the cutting edge. think of sliding a putty knife under the lifted end if a splinter. it levers the splinter out of the board, ahead of the edge of the knife. now imagine lifting the handle of the putty knife up until the splinter breaks. you've increased the angle beyond the bending ability of the splinter.
this is what the back bevel does. it forces the shaving to curl to the point that the long fibers can no longer apply upward action ahead of the edge. it also increases the total included grind angle (makes the edge thicker) which tends to make it stay sharp a bit longer between sharpenings.
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On Wed, 05 Oct 2005 11:30:18 -0700, bridgerfafc wrote:

That doesn't make sense to me--unless: The back bevel (we're still on bevel-up planes, right?) is going to change the relief angle. Are the long fibers you're talking about the ones under the blade, after the edge has passed? If so, it makes sense; back bevel in the bevel-up situation will lower the relief angle. The back-bevel could put a teensy bit of pressure on the fibers just below the fibers which are now riding up the blade. These still-in-the-wood fibers can't join the growing 'splinter' ahead of the blade, because we're pinching them back against the wood. I think...
Whatever, I still think OP just had a sharper blade.
--
"Keep your ass behind you"
vladimir a t mad scientist com
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On Wed, 05 Oct 2005 23:14:48 -0500, Australopithecus scobis

the quoted was about bevel down planes. sorry about the confusion...

nah if the heel of that bevel is deeper than the edge the plane will climb right out of the cut.

probably.
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Two thoughts.
First, can you really tell which is the bevel and which the flat surface when you're talking the engaged (<1/8") end of the blade? Answer is no, of course. It's a rake angle, sharpness angle, clearance angle clockwise sum to 90 degrees. Makes no difference which side the bevel is on when considering the sharpness angle..
Second, have you looked at Hoadley, specifically chapter eight? Pretty good stuff.
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Pictures & text here (scroll down):
<http://www.leevalley.com/shopping/Instructions.aspx?pQ880>
djb
--
Life. Nature's way of keeping meat fresh. -- Dr. Who

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Dave Balderstone wrote:

That link has an excellent explanation of back bevels for both bevel up and down blades.
Dave
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BillyBob wrote:

I can't find anything to cite that says a back bevel would reduce tear out in a bevel up plane, but it works on the tear out prone oak I have. Normally, a back bevel is used to increase the effective cutting angle on a bevel down plane. It might be serendipity at work. I'll leave the bevel on that blade unless I have issues with planing other boards.
Dave
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:> :> I cannot argue with your good results. What I don't understand is why this :> worked as well as it did. As I understand it, you added a bevel on the :> opposite side of the normal bevel. This is the definition of a back bevel :> for standard bevel-down configuration. It increases the cutting angle. :> However, on a bevel-up plane such as the LV low angle smoother, it has no :> affect on the cutting angle. To increase the angle, you would shift the :> angle from the factory ground 35 degree angle to something like 40 degrees :> or add a 5 degree microbevel. :> :> Did I misunderstand something? :> :> Bob :> :> : I can't find anything to cite that says a back bevel would reduce tear : out in a bevel up plane, but it works on the tear out prone oak I have. : Normally, a back bevel is used to increase the effective cutting angle : on a bevel down plane. It might be serendipity at work. I'll leave the : bevel on that blade unless I have issues with planing other boards.
I'm confused as well. If you're putting a secondary bevel opposite the main one, won't this prevent the edge from contacting the wood entirely?
    -- Andy Barss
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On Sun, 02 Oct 2005 19:44:25 +0000, Andrew Barss wrote:

Uh, one might move the blade down a little, perhaps? Don't hurt yourself when you slap your head. :)
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wrote:

I tried it and it works. Robin Lee and Rob Cosman both recommend it. As I understand it, the slight back bevel makes the cutting edge absolutely straight and incredibly sharp
--
Lowell Holmes



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You tried it on a bevel up plane?
Bob
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On Sun, 02 Oct 2005 08:58:57 +0000, BillyBob wrote:

Recall Leonard Lee's admonition to strive for the smallest bevel angle consistent with edge retention. The effect of a back bevel on a bevel-up blade might affect the edge's strength, so the OP planed for longer periods with a truly sharp blade.
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Ah, thanks for that insight. I'm going to go do a back bevel on one of my planes. I need to exercise my new Lee Valley honing jig. :-)
Bob
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