Plane blade clearance angle

[Pat Barry]

I was reading through another thread and found this comment by Steve that prompted me to question what the proper clearance angle is for a bevel down blade.

Not that you asked for any advice, but just FYI, my own attempts at very low angle + skew have not been terribly happy…the low sharpening angle needed makes the edge more fragile, the skew makes it still more fragile, and end grain shooting is one place where the edge can't be fragile. If you make it 35°-38°, you will constantly be fighting to get an edge that won't chip and still provides the requisite clearance angle.

Obviously, the larger the clearance angle, the weaker the edge will be. Conversely, not having a clearance could result in the cutting edge not contacting the material and therefore just sliding or skipping across.

So, how much clearance is needed? How much does this depend on the task at hand, ie: planing end grain, vs planing soft woods, vs planing hardwoods with and against the grain? How much does this depend on depth of cut? ie: does a smoother need less of a clearance because it is making such light cuts? Maybe an aggressive cutting scrub plane needs a lot more clearance? Is it just a few degrees minimum? Do you just play with the angle til you are satisfied?

Lots of questions I know but, that's what i do.

[Jim Koepke]

I was reading through another thread and found this comment by Steve that prompted me to question what the proper clearance angle is for a bevel down blade.



Obviously, the larger the clearance angle, the weaker the edge will be. Conversely, not having a clearance could result in the cutting edge not contacting the material and therefore just sliding or skipping across.

So, how much clearance is needed? How much does this depend on the task at hand, ie: planing end grain, vs planing soft woods, vs planing hardwoods with and against the grain? How much does this depend on depth of cut? ie: does a smoother need less of a clearance because it is making such light cuts? Maybe an aggressive cutting scrub plane needs a lot more clearance? Is it just a few degrees minimum? Do you just play with the angle til you are satisfied?

Lots of questions I know but, that's what i do.

One theory of a clearance angle is the wood flexes as the blade is cutting. As the top is sheared what is left below the blade springs back underneath the plane. If there is no clearance, the spring action of the wood would lift the plane and stop cutting. That is likely a simplistic description of all the action, but it may be good enough for this discussion.

Clearance angle requirements are likely going to be pretty much the same whether the plane is bevel up or bevel down. In looking up the discussion in my memory, it was surprising to find it was way back in 2009. This was about bevel up bedding:

http://www.sawmillcreek.org/showthre...l-Up-Bed-Angle

In the booklet supplied with my power sharpening system there was a paragraph or two about blade sharpening angles for bevel down planes. It stated bevels at less than 25° were prone to chatter and quick wear. Bevels greater than 35° would be prone to skipping over the work.

Even on a metal low angle plane the bed, at ~12°, is prone to cracks if the blade is clamped in overly tight or if the user is taking heavy cuts.

Any back bevel either from wear or purposely created by the user will reduce the clearance angle.

As for manufactured planes, the lowest clearance may be on a side rabbet plane at 8°. This at one time was the published specification for the Lie-Nielsen #98 & #99 side rabbet planes.

jtk

[Pat Barry]

Thanks for the reply Jim. Your comment of 35 degree for a primary bevel would make the clearance angle ~ 10 degrees for a standard 45 deg frog (90-45-35). That's pretty small number and the comments regarding springback of the fibers that someone noted was also interesting although much of that discussion was on bevel up blades and bedding angles which might be similar?? I would have to find that Japanese video and look closely to see if they experienced any springback in their testing.

[Patrick Chase]

Thanks for the reply Jim. Your comment of 35 degree for a primary bevel would make the clearance angle ~ 10 degrees for a standard 45 deg frog (90-45-35). That's pretty small number and the comments regarding springback of the fibers that someone noted was also interesting although much of that discussion was on bevel up blades and bedding angles which might be similar?? I would have to find that Japanese video and look closely to see if they experienced any springback in their testing.

The Kato/Kawai video used 10 degree clearance angle. I don't see any significant springback.

I can think of a couple big factors that might cause variation in springback:

- As always, the wood. Softer woods should spring back more than harder ones.

- Vertical force. You can't have springback if you're not pushing the wood down with the blade to begin with, so planing technique matters. The Kato/Kawai setup did not impose any vertical loads.

I usually run 10-15 deg of net clearance on my BD planes and have never experienced springback as a problem.

[Steve Voigt]

Jim's answer is nice and thorough; I'll just add a couple historically based observations.

Nicholson, writing at the beginning of the 19th C., recommended a 35 degree bevel; at the time 47.5 was the standard common pitch bed in a british plane. That gives a clearance of 12.5. Stanley, presumably after lots of testing, never went below 12 degrees for a BU plane, and all the other big modern makers (LV, LN) have followed suit. So, I think there is a lot of historical evidence for respecting the 12 degree limit. In certain situations, for an experienced user, you can get away with maybe 10 degrees. But for most users wanting to cover a wide range of situations, 12 or higher is good. I like higher. To me the benefits of low angles are vastly oversold and I would much rather raise the bed a little, have lots of clearance, and not have to screw around with fighting clearance angles or dealing with diminished blade life from narrow, sub-30 bevel angles.

[Kees Heiden]

In long grain with a sharp blade you can still get a good cut at zero degrees clearance angle. But wear happens on the bottom of the edge and makes that area concave. It kind of lifts the edge into the air and this wearbevel starts to rub on the wood.

With a sharp blade the shaving pushes down on top of the edge and gives a feeling that the plane is being drawn into the wood. The wear bevel under the edge creates a counter force that is pushing the edge out of the wood. When that counter force increase too much you must press down on your plane very hard and the plane feels to have a dull edge.

So, not enough clearance angle makes the plane feel dull quicker.

In endgrain this is worse because the ends of the wood fibers have a stronger wearing action on the underside of the edge.

Edit: I think the springback is very small, just below the edge and you can't see it in the Kato video.

[Derek Cohen]

I was reading through another thread and found this comment by Steve that prompted me to question what the proper clearance angle is for a bevel down blade. ....

Obviously, the larger the clearance angle, the weaker the edge will be. Conversely, not having a clearance could result in the cutting edge not contacting the material and therefore just sliding or skipping across. ...

Pat, I have provided some information about this in that thread.

In brief, low angle BU planes, such as the LA Jack, have a 12 degree clearance. Similarly, the strike block plane ai built with a bed of 37 degrees, when used with a 25 degree bevek, has a clearance of 12 degrees.

The lower the bed angle, the less stress on the blade's edge ... and the lower the bevel angle can be (within reason). A 25 degree bevel angle works well for most steels in this set up. A higher bevel angle is needed as the bed is increased.

Regards from London

Derek

[Patrick Chase]

Jim's answer is nice and thorough; I'll just add a couple historically based observations.

Nicholson, writing at the beginning of the 19th C., recommended a 35 degree bevel; at the time 47.5 was the standard common pitch bed in a british plane. That gives a clearance of 12.5. Stanley, presumably after lots of testing, never went below 12 degrees for a BU plane, and all the other big modern makers (LV, LN) have followed suit. So, I think there is a lot of historical evidence for respecting the 12 degree limit. In certain situations, for an experienced user, you can get away with maybe 10 degrees. But for most users wanting to cover a wide range of situations, 12 or higher is good. I like higher. To me the benefits of low angles are vastly oversold and I would much rather raise the bed a little, have lots of clearance, and not have to screw around with fighting clearance angles or dealing with diminished blade life from narrow, sub-30 bevel angles.

I don't think you can safely assume that the 12-degree bed angle was limited by clearance. They had a lot of sole-chipping problems in those planes that prevented them from going lower, particularly with the #62.

Also the bottoms of BU blades tend to acquire a bit of a back bevel as they wear, so the true clearance angle is often lower than the bed angle.

[Pat Barry]

It makes sense that as a plane's edge wears down through use, that the resulting wear on the bevel would lead to an increase in contact(behind the edge) with the material and result in poor cutting. A steeper bevel angle would minimize this effect due to the geometry. On the other hand, a steeper bevel would be weaker and thus more prone to wear--- ie: its either 6 of one evil or half dozen of another. So through the school of hard knocks, for a common BD plane, a clearance angle of ~ 12 degrees is the best compromise. I can live with that. Thanks to everyone for their feedback on this - I do appreciate the points of view.

[Pat Barry]

I don't have a router plane but would like to get one. I see the Lee Valley router plane looks nice and easy to adjust but the topic of clearance angle has me confused as it appears their router plane has no accommodation for a clearance under the blade. On the other hand the Lie Nielson router plane appears to have the blade set as an angle such that there is a clearance. Is a clearance important to the router plane function? Does the Lee Valley have this even though it doesn't appear to have it?

[Tom M King]

I expect speed is a factor too in the difference between a bench plane and a router plane. Planing with a bench plane, I'm probably two feet a second, while with a router plane is maybe a half inch a second. My Record router planes have a slight clearance angle, but I've never even thought about how much of one. The router plane is more like a controlled little paring chisel without all the complications of a bench plane.

[Jim Koepke]

I don't have a router plane but would like to get one. I see the Lee Valley router plane looks nice and easy to adjust but the topic of clearance angle has me confused as it appears their router plane has no accommodation for a clearance under the blade. On the other hand the Lie Nielson router plane appears to have the blade set as an angle such that there is a clearance. Is a clearance important to the router plane function? Does the Lee Valley have this even though it doesn't appear to have it?

I use Lee Valley blades in my Sargent router plane. The bottom of the cutter is ~7° angled from the post of the blade. It also appears the post on the plane is angled at ~2°.

With a sharp blade it will take nice shavings with no problems.

jtk

[Pat Barry]

I use Lee Valley blades in my Sargent router plane. The bottom of the cutter is ~7° angled from the post of the blade. It also appears the post on the plane is angled at ~2°.

With a sharp blade it will take nice shavings with no problems.

jtk

Thanks Jim. It makes sense there is a bit of an angle there.

[Pat Barry]

I expect speed is a factor too in the difference between a bench plane and a router plane. Planing with a bench plane, I'm probably two feet a second, while with a router plane is maybe a half inch a second. My Record router planes have a slight clearance angle, but I've never even thought about how much of one. The router plane is more like a controlled little paring chisel without all the complications of a bench plane.

Thanks Tom

[Patrick Chase]

FWIW I have several planes set up with 10 deg clearance (35 deg secondary bevel on common-pitch BD planes) and have never had problems. I haven't tried to go any lower than that, but Paul Sellars has claimed that he uses as little as 3 degrees without problem: https://paulsellers.com/2013/04/myth-and-mystery-surrounding-plane-and-chisel-bevels/.

As an engineer I'm deeply skeptical that springback could ever account for anywhere near 10 deg. I think Sellars has this one right, at least for common woods. Maybe if you plane something really spongy (Balsa?) it could be an issue, though :-).