Back bevels - True, false, or doesn't matter?

Re: Back bevels - True, false, or doesn't matter?

mikew

>""A bevel down blade gets its edge weakened by a back bevel, while a bevel up blade gets stronger".

Seems to me that a bevel down blade loses steel at the cutting edge, therefore weakening it, whereas a bevel up blade gains more support behind the edge, therefore making it stronger.

Does this make sense? Does it matter? "

Gosh, I hope I didn't write that quote...because it is incorrect. IMO at least, the differences in edge cross-section is at least insignificant. Edge cross-section aside, a BU plane transfers shear forces differently, but that goes to plane geometry, not blade geometry.

There are other issues, but strength of the edge due to a BB isn't one of them. A BB on a BU plane can have less than desirable results if created to steep or large as well.

Take care, Mike

Re: Back bevels - True, false, or doesn't matter?

ThomD

>Plane blades operate as scrapers to the extent that there is a significant clearance angle. On a standard 45 degree bedding, and 25 degree bevel, there is a significant clearance angle of 20 degrees.

On either blade the edge gets fatter. so to that extent they both get stronger.

On a BU, a BB reduces the clearance angle, which strengthens the edge (more metal behind the edge), it also leads to a marginally sharper blade(probably not discernable, but it's a fact). There is no increase in the cutting angle, which means the backforce on the edge is not increased. So net, the edge is stronger in terms of sharpness, meat behind the edge, and no increase in backforce.

On the BD, the BB is on the front: steeper planing angle, means more backforce on the edge, there isn't any additional meat. The sharpness I am not so sure about. The front of the edge is more upright, which is negative, but the overall angle is greater. I'd call it a wash to slightly positive. Overall the BD is weakened a little. However, any problems in practice could be dealt with by simply fatening up the main bevel taking it from say 25-30 degrees.

Re: Back bevels - True, false, or doesn't matter?

Rob Millard

>As long as you alter the grind angle on the main bevel, to compensate for the back bevel, I don�t see any significant change to the durability of the cutting edge. I have a back bevel on my No.9 miter plane allowing me to lower the cutting angle, making it more effective on endgrain. I am also a strong advocate of back bevels on standard bench planes. I see a noticeable improvement in the surface left by a plane bedded at 45� when I use a back bevel. It doesn�t perform as well as a dedicated York pitch plane, but noticeably better than without the back bevel. The back bevels I hone are so small, that they can�t be seen with the naked eye.

Rob Millard

Re: Back bevels - True, false, or doesn't matter?

Arw01

>Purpleheart with a 45 degree stanley. major trouble.

Purpleheart with a 45 degree stanley and a back bevel. Possible.

Alan

Re: Back bevels - True, false, or doesn't matter?

mikew

>"Plane blades operate as scrapers to the extent that there is a significant clearance angle. On a standard 45 degree bedding, and 25 degree bevel, there is a significant clearance angle of 20 degrees."

Well, it depends on chip formation as to whether a blade levers wood from the surface or causes is to break at or below the surface as to whether a plane is acting as a scraper--maybe semantics, maybe not. But clearance angle is not the determining factor.

"On either blade the edge gets fatter. so to that extent they both get stronger."

Yep, that was a point.

"On a BU, a BB reduces the clearance angle, which strengthens the edge (more metal behind the edge), it also leads to a marginally sharper blade(probably not discernable, but it's a fact)."

A BB on a BU plane is no different than a higher bevel angle on a BD plane in that both reduce the clearance angle. It's just when one is dealing with a bedding angle of 12 degrees on a BU plane, it is very easy to remove too much clearance angle.

Also, with a plane blade is the effect of the wear bevel. On a LA BU plane, it is a more significant issue. I believe BBs on a BU blade decreases effective edge retention [really the clearance angle] to the point one needs to hone more frequently. That is my observation anyway.

Apparent sharpness obtained by altering the bevels is not a scientific fact. What feels sharper may not really be sharper.

"There is no increase in the cutting angle, which means the backforce on the edge is not increased."

As pressure against the blade is a result of blade depth and effective cutting angle, I agree.

"So net, the edge is stronger in terms of sharpness, meat behind the edge, and no increase in backforce."

I think a BB as a means of increasing edge strength is a moot point as wear on the edge will cause failure faster than the edge breaking down. I think this is a greater issue as regards a BD plane.

"On the BD, the BB is on the front: steeper planing angle, means more backforce on the edge, there isn't any additional meat."

A BB on a BD down plane increases the cross-sectional dimension--which means it has "more meat." But yep, requires more force and therefore increases what you call backforce against the blade itself.

"The sharpness I am not so sure about. The front of the edge is more upright, which is negative, but the overall angle is greater. I'd call it a wash to slightly positive. Overall the BD is weakened a little. However, any problems in practice could be dealt with by simply fatening up the main bevel taking it from say 25-30 degrees."

25 degree bevel on a BD plane creates too weak an edge anyway. Especially if one works in something other than knot-free pine or Poplar.

But a BB does not make a BD blade weaker. It does increase the effect of wear bevel, due I think to the shear zone being lowered, thereby both forcing a type II chip formation [or if a steep enough BB is created, a type III chip], which in turn creates more force against the blade's edge and the resultant friction et al.

A negative rake ["The front of the edge is more upright, which is negative,"] obviously doesn't occur until a 90 degree effective cutting angle is crossed. I would say a BB decreases the rake angle of the effective cutting angle. It would take that hair past a 45 degree BB on a 45 degree bedded BD plane to obtain a negative rake.

Take care, Mike

Re: Back bevels - True, false, or doesn't matter?

Pam Niedermayer - Austin, TX

>An edge is formed by the conjunction of two planes; therefore, a back bevel can not make an edge fatter, unless it's not sharp.

Pam

Re: Back bevels - True, false, or doesn't matter?

Jerry Nicholson

>It appears to me that a back bevel on a BU blade would reduce the blades contact area with the body of the plane and partially negate the superior blade support that the BU design promises. It would also reduce the clearance angle. A back bevel of 12 degrees on an LV BU smoother would probably help make the cutting edge really strong but it sure wouldn't cut worth a darn! I don't plan to put any back bevels on my BU smoother's blade.

Jerry

Re: Back bevels - True, false, or doesn't matter?

Derek Cohen (in Perth Australia)

>I believe that the minimum clearance angle for a BB on a BU blade is 7 degrees. Theoretically, therefore, a BB on a BU plane should not exceed 5 degrees, since the bed is only 12 degrees.

In practice, I do use a backbevel on my BU blades, but this is only about 1 degree. In this regard we really should describe it as a micro-backbevel, since it is the type that David Charlesworth recommends in his "Ruler Trick", is part of the sharpening process, and not aimed at modifying the cutting angle.

Backbevels in their true sense, that is those that are aimed at modifying the cutting angle, tend to fall into the 10-15 degree range on BD planes. As I indicated above, these cannot be used on a BU plane.

So, comparing BB on BU and BD planes is really like comparing apples with oranges.

Regards from Perth

Derek

Re: Back bevels - True, false, or doesn't matter?

Neal (San Jose)

>There is a good use for back bevels, and that is adjusting the mouth opening on "non-adjustable" planes, like the ubiquitous 220. If you put a large (wide) back bevel, it moves the cutting edge UP, relative to the back of the blade and away from the wood being planed,. To compensate for this, you have to move the blade forward, thereby closing the mouth.

A good trick to keep in mind. This will work on any BU plane. I have considered doing this on my LN 60.5R. The mouth opening is currently .008, and I've thought I'd like to get it down to .005 or so.

closing the mouth

ydb1md

>An easier method than using a back bevel is to shim the bed of the plane with some thin brass or aluminum. I've used this technique on my veritas apron plane and it closed the mouth up nicely.

More'n one way

Neal (San Jose)

>to remove the fur from a feline. Lee Valley supplies shims with their excellent spokeshaves to allow you to close the mouth. What I like about the back bevel method is if I am at the sharpening station anyway it only takes a little effort and time.

I can see me trying the shim method. Hmmm... gotta find some shim stock. I know, a beer can should work. I guess I'll have to empty one. Now, where are those doggone tin snips? Think I'll make a little more shim stock. Maybe just one more just to make sure. Rats, now I'm too "over the weather". I'd better quit using tools now before I hurt myself. I'll find the tin snips and make the shims tomorrow!!!

Re: More'n one way

Rob Lee

> I'll find the tin snips and make the shims tomorrow!!!

.... but first, you'll need another beer can....

:)

Cheers -

Rob

Re: Back bevels - True, false, or doesn't matter?

ThomD

>"Well, it depends on chip formation as to whether a blade levers wood from the surface or causes is to break at or below the surface as to whether a plane is acting as a scraper--"

You're talking about the chip management, I'm talking about the edge. The part that cuts, and how well it is supported.

"Apparent sharpness obtained by altering the bevels is not a scientific fact. What feels sharper may not really be sharper."

It's a geometric fact. The grooves that a given grit abrade, run out to the edge, and leave finger like projections. The length of these projections and their fragility is related to the angle at the edge. When that profile is then raked through the wood it is then transferred as micro furoughs to the surface being cut. This affects the surface, and the durability of the edge. It is one of the factors that determines how fine the edge is. This is, by the way, one of the things A2 is bad at.

Complex discussion, value your input.

Re: Back bevels - True, false, or doesn't matter?

Derek

>You are correct that it is not the "edge" which is fatter but the "meat" behind the edge. The included angle of the blade is greater with a back bevel so the mass of metal behind the edge is similar to a higher bevel angle.

As an example, a plane iron with a 25 degree bevel and a 5 degree back bevel has the "cross section" behind the edge of a 30 degree bevel iron. If you analyze the geometry, the angle between the leading and trailing edges are 30 degrees.

Also, as you point out, the edge is the intersection of 2 planes so the sharpnes of the edge is no different with or without a back bevel if the same degree of sharpening care is used. How wide the "wedge" at the bevel is does not indicate sharpness. I refer to L. Lee's Sharpening book.

Of course, a keener edge (a smaller included angle) does affect cutting a lot. I'm not arguing that the angle of the wedge does not matter. Of course, the higher the included angle, the higher the minimum attack angle of the leading edge.

And NO, I did NOT angle this post as an attack against anyone or their views. I did not intend to be sharp or cutting towards anyone. :>)

Re: Back bevels - BU bedding at risk?

Norman (Metcalfe, Ont.)

>One of the advantages to the BU setup is that the iron is completely bedded from the top of the blade to the sharp tip. Wouldn't a back bevel remove this advantage, even a miniscule back bevel.

Factors to consider: thickness of iron .. Thicker -> less chatter

If the iron isn't thick enough and we instantly remove support behind the iron at the very bottom, or business end, this might introduce the dreaded chatter and subsequent results of an unsupported blade..

My 0.02 cents,

Norman

Re: Back bevels - True, false, or doesn't matter?

ThomD

>Man you guys must be using some special steel and stones! I've never seen the planar intersection you refer to on a real tool. Real tools have jagged 3d edges, and the reality of how they cut (while probably largely conjecture in no mater whose book) is a little messier, like dragging fingers through sand, the length of the fingers maters. And the back bevel affects that length. Examine a toothing plane for an extreme example of how the bevel or bb geometry affects the size of the rake effect.

Re: 50%

Moses Yoder in White Pigeon, MI

>I always liked the true or false questions on tests the best, because I knew even if I didn't have the slightest idea what I was talking about that there was a 50% chance I would get the question right. Over 90% of the time I had studied the material at least well enough to guess the right answer.

It seems to me that the question would actually have a correct answer that could be calculated if one had the right equipment. I don't see a whole lot of sense in setting around theorizing about it, so my answer to the question is "it really does not matter to me". If I had to know the answer to this question in order to plane effectively, I would be in dire trouble. But perhaps that explains why my projects always come out not looking quite as good as I thought they would.

Lee Valley

ydb1md

>Rob,

I'm ashamed of you?! You didn't plug the fact that you carry some nice brass shim stock -- in a variety pack no less?

:)

PS My wife thinks that you make wonderful planes -- she likes the idea of invention.

knock it off and get back to work

Adam Cherubini

>

God bless Adam; ever the pragmatic voice of reason

ydb1md

>And I mean that in the best possible way . . . .

Dave

I beg your pardon????

Denis Ch�nard, Orl�ans, Ont.

>Are you telling me that my question is worthless?

I'm trying to understand the technology behind planes, and my question seems to be a valid one.

Unless you can elaborate a bit, I consider your response to be rude, to say the least.

DC

BTW, I posted the question at 12:15 AM. I work days...

Re: Back bevels - True, false, or doesn't matter?

mikew

>"It's a geometric fact. The grooves that a given grit abrade, run out to the edge, and leave finger like projections. The length of these projections and their fragility is related to the angle at the edge."

OK, the grit does affect the refinement of the cutting edge. Under a microscope the grooves can be readily seen. But when we are talking about the difference between 30 or 25 or 55 degree angles I would maintain that unless one is using an exceedingly coarse stone, the grooves--serrations--do not go deep enough to affect those parts which are not grooved--the mean surface.

It seems to me that from the photographs I have seen of edges under magnification, the serrations are intermittent and puncuated with mean surface between them at the angles under discussion. One would need to drop well below usable bevel angles to be affected by the serrations--grooves.

"When that profile is then raked through the wood it is then transferred as micro furoughs to the surface being cut. This affects the surface, and the durability of the edge. It is one of the factors that determines how fine the edge is. This is, by the way, one of the things A2 is bad at."

No contention there. Maybe clarification. The serrations actually cause a raised portion upon the surface while the mean edge surface the furrows--a mirror of the edge tool.

"Complex discussion, value your input."

And I yours.

Sincerely, Mike

Re: Back bevels - True, false, or doesn't matter?

mikew

>Yes, yes. Poor wording on my part. Thank you for the clarification.

It is that part behind/beyond the actual intersection of the bezel which increases in cross-sectional dimension with an increase in bevel angle due to either an increase in a back bevel, primary or secondary [or whatever "ary"] bevel.

Take care, Mike

Re: I beg your pardon????

Moses Yoder in White Pigeon, MI

>I certainly wouldn't take it personally, Derek. But if you are really intersted in knowing the right answer, I would find a local lab with the right testing equipment to see whether a back bevel strengthens or weakens steel. The only responses I have read so far are just theorys. If you take a theory and build on it, what have you got? You got nothin.