Back bevel works well (and two questions)

[David]

I own the LV low angle smoother and have an extra blade, the high angle optional blade, ground at the factory to 35 degrees. I've known for some time that putting a back bevel on it is supposed to reduce tear out but being the procrastinator that I am, I hadn't ground the back bevel until this afternoon. I put an 8 degree (give or take) bevel, 1 mm wide on the back and gave it a work out on an oak drawer front that I'd set aside early this year because I couldn't tame the tear out without sanding it forever.

Early this year: A couple runs through the planer resulted in tear out no matter how light a cut or which direction. By the time I'd run it through a few times, I decided it was too thin to be installed along side the other drawers and ended up cutting a new front. If I'd owned a thickness sander, I would have put it through that in the first place, but being in a hurry to remove old stain, I ran a bunch of drawer fronts through the planer after scraping off the lacquer finish. The rest came out flawless.

I found that problematic drawer front today and gave it a go with the modified plane blade. It was like a miracle! Almost like using a scraper plane, but faster. The surface was left silky and all the divots disappeared after a half dozen strokes. Very cool.

Oh, and kudos for Robin and company designing the new honing guide. It paid for itself today.

And I know nearly every one of you here already knows about back beveling a blade. I just wanted to share...

Question: Wouldn't a back bevel on planer blades make them chip and wear out much faster?

Is there a 12 step program for procrastinators?

Dave

[George E. Cawthon]

Nope, just two step. One step forward, two steps back. That's the procrastinator. Just reverse that for the solution.

[BillyBob]

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

[George]

Chip form.

[BillyBob]

I don't have any idea what your two word reply means. Would you like to expand that answer?

Bob

[David]

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

[Australopithecus scobis]

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.

[Andrew Barss]

David wrote: :> :> 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

[Australopithecus scobis]

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

[Lowell Holmes]

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

[George]

The back bevel also causes the shaving to curl faster, breaking it over a relatively shorter radius, depriving it of long fiber strength to tear out.

the exaggerated view.

[BillyBob]

Thanks for posting the link. This picture illustates exactly the point I've been trying to make. That's a picture of a bevel down (conventional) plane. The Lee Valley Low angle smoother described by the OP is a bevel up plane, like a block plane. Back bevel does not affect cutting angle on a bevel up plane, so I don't understand why he got better results unless his plane just wasn't sharp to begin with.

Bob

[BillyBob]

You tried it on a bevel up plane?

Bob

[George]

It affects clearance angle, which can increase the sharpness angle while making the edge more durable. See Hoadly on sharpness.

[David]

Maybe that's part of the explanation for why it helped. For grins I ground off the bevel and put a TINY back bevel that was maybe 1/4 to 1/3 of a MM. It wouldn't even cut. I had the blade sticking way out from the sole of the plane and got no cutting action. I guess I got lucky the first time I put a back bevel on (1 MM wide). The blade was sharp before I did that, and I'm not really sure if I had tried it early this year on that tear out prone drawer front. Maybe it would have worked ok with no back bevel, but I must say it worked very well with a 1 MM bevel, but was useless with a tiny BB. I do realize that BB are primarily for bevel down planes.

Dave

[BillyBob]

What is clearance angle? Who or what is Hoadly?

Bob

[BillyBob]

How does it make the edge more durable? It seems to me that it makes the cutting edge thinner, which results in an edge that will dull quicker. Since its a bevel up plane, the angle of cut remains the same and the back bevel just removes more metal behind the cutting edge.

Bob

[BillyBob]

I'm pretty convinced that you simply produced a better sharpening job, rather than making any shift in the planing geometry. That's why it worked.

I was a little surprised you mentioned you had trouble with tear out on the oak to begin with. I've not had much trouble with oak and the smoothers I own. I did not do anything special to them except sharpen them. Now when they were not so sharp, I had all kinds of trouble with tear out.

Bob

[Juergen Hannappel]

No: / / / / | Exaggerated back bevel on bevel up plane | / | / | / |________/

/ / / / | No back bevel on bevel up plane | / | / | / |/

[bridger]

back beveling a bevel up plane makes the edge thicker.

that is if you define the back as the side of the blade without a bevel. if you define the back as the side away from the wood, then a

*front* bevel would make the edge thicker.

regardless of that bit of obfuscation, any secondary bevel, front back or inside out will make the edge thicker. that's all fine and good until the angle of the bevel against the wood exceeds the angle of attack- or as someone else moer clearly just called it, the clearance angle. when that happens the plane ceases to cut.