Clearance in BD plane

[Patrick Chase]

I finally managed to have an honest-to-goodness clearance issue. I've been playing with low-angle frogs in a couple of my smoothers, and had a 40-deg one in my LV custom 4-1/2 last week.

I swapped a new blade in while working some Red Alder with a medium cut (several mils, I didn't measure), and promptly had cutting issues. Swapping to a different, equally sharp blade with a lower bevel fixed them. When I went and looked at my spreadsheet (I know, laugh away) I discovered that I'd grabbed the wrong spare blade, and the one I'd used was set up with a 35 deg secondary bevel. Just for kicks I tried the problem blade on some Sugar Maple and Yellow Birch scraps (both about 1.5X as hard as the Alder) and didn't see issues.

So here's one empirical result for the endless clearance debate: 5 deg is low enough to start to cause trouble on a fairly soft hardwood (~1.3 mpsi modulus).

The reason I'm posting this is because I'm firmly of the opinion that extra clearance is nothing more than wasted blade life, so it pays to know where the real limits are. I think it's good to have some margin, so I'm going to continue to aim for ~10 deg clearance.

[bridger berdel]

Extra clearance being wasted blade life would be entirely dependent on your sharpening practice, says I. I hog out the primary bevel on a grinder and hone freehand. I aim for minimal clearance for a stronger edge, which may be what you meant....

[Warren Mickley]

There is another issue that a higher cutting angle makes for a more stress on the cutting edge, more shear stress. Scraping with a chisel is harder on the edge than paring at a low angle.

I did extensive tests on clearance forty years ago. Eight degrees is where I started noticing definite problems with clearance, although I would expect some variation for different timbers. At small clearance angles the plane might show
problems from dulling earlier.

[Patrick Chase]

Extra clearance being wasted blade life would be entirely dependent on your sharpening practice, says I. I hog out the primary bevel on a grinder and hone freehand. I aim for minimal clearance for a stronger edge, which may be what you meant....

That's exactly right. When I said "wasted blade life" I was referring to the impact of additional sharpening due to faster-than-necessary edge dulling. IMO that's true no matter what sharpening process you use.

Both clearance and edge life are determined by the bevel immediately behind the cutting edge, which I'll call "tip bevel" from here on [*]. In my experience going from, say, 25 to 35 deg tip bevel in a common-pitch plane (45 deg bed) has no effect on clearance, but a big impact on edge durability and life.

[*] I've seen some people argue that the primary or overall bevel matters for clearance, but that can be disproven by one simple observation: For almost any reasonable blade geometry, the primary bevel will be entirely above the plane of the sole.

[Patrick Chase]

There is another issue that a higher cutting angle makes for a more stress on the cutting edge, more shear stress. Scraping with a chisel is harder on the edge than paring at a low angle.

I did extensive tests on clearance forty years ago. Eight degrees is where I started noticing definite problems with clearance, although I would expect some variation for different timbers. At small clearance angles the plane might show problems from dulling earlier.

I've adopted your view w.r.t. cutting angle - that's one of the reasons I'm playing with lower-angled frogs right now.

My comment about clearance and blade life assumed constant cutting angle and therefore constant stress. IMO we should always try for the minimum practical clearance angle, as that gives the highest edge angle and durability. I'm glad to see that I'm in the right ballpark :-).

[Normand Leblanc]

A blade appear dull when I have to add pressure on the toe. This happens because there is a flat being created under the blade. What we should be interested in is to reduce the surface of that flat as much as we can. Obviously a very large relief angle would be the solution but the edge would be fragile.

To calculate that surface, knowing that the width is constant, the relation would approx. be: 1/tanθ. Theta is the relief angle.
Angle: surface
15, 3.7
12, 4.7
9 , 6.3

This means that a 15 degree relief angle will develop a flat but it's size will be much smaller than with a 9 degree RA. This is an approximation but should be in the ballpark. With a high RA you need to have more wear to get to an equal surface as a low RA.

I did a test (I'll post that soon) where a plane is used with different relief angle (same wood, same sharpening method). I begin planing and, after 5 cuts, then 10, etc. I check if the plane engage a cut without any weight added. I'm pushing the plane by the sole behind the handle. When I can't get the plane to engage the test is complete.

Results for a Stanley blade(relief angle; number of cuts):
15,50
12,35
9,12.5

12.5 is the result of two test because I could not believe that I was only getting 15 cuts. Made another one to get only 10.

This kind of result tells me that the edges of our BU planes do not last as long as our BD planes (I should say that the BU planes, because of their relief angle, do not last as long as a BD). Again, I've tested that and there is quite a margin between a BD at 12 degree relief and a BU at 15 degree.

[Luke Dupont]

By clearance, I assume you mean the difference in angle between the angle which the iron is bedded at, and the angle which the bevel is sharpened at?

I had some odd clearance issues with my Japanese plane once - it's bedded a little lower than standard western irons, at around 41 degrees. And I suppose I was sharpening the bevel around 30-35 degrees free hand - I wasn't really measuring. As a result, I had some weird behavior where the plane ceased to cut well, acting as if it were dull after a short period of time. I'll bet the very edge was getting rolled slightly back into the plane after working with hardwoods, resulting in the plane riding the bevel - especially noticed this problem on rough sawn timber or uneven surfaces.

I reshaped the bevel at a lower angle, about 25-30 degrees, and haven't had any issues since.

[Patrick Chase]

A blade appear dull when I have to add pressure on the toe. This happens because there is a flat being created under the blade. What we should be interested in is to reduce the surface of that flat as much as we can. Obviously a very large relief angle would be the solution but the edge would be fragile.

To calculate that surface, knowing that the width is constant, the relation would approx. be: 1/tanθ. Theta is the relief angle.
Angle: surface
15, 3.7
12, 4.7
9 , 6.3

This means that a 15 degree relief angle will develop a flat but it's size will be much smaller than with a 9 degree RA. This is an approximation but should be in the ballpark. With a high RA you need to have more wear to get to an equal surface as a low RA.

I did a test (I'll post that soon) where a plane is used with different relief angle (same wood, same sharpening method). I begin planing and, after 5 cuts, then 10, etc. I check if the plane engage a cut without any weight added. I'm pushing the plane by the sole behind the handle. When I can't get the plane to engage the test is complete.

Results for a Stanley blade(relief angle; number of cuts):
15,50
12,35
9,12.5

12.5 is the result of two test because I could not believe that I was only getting 15 cuts. Made another one to get only 10.

This kind of result tells me that the edges of our BU planes do not last as long as our BD planes (I should say that the BU planes, because of their relief angle, do not last as long as a BD). Again, I've tested that and there is quite a margin between a BD at 12 degree relief and a BU at 15 degree.

All I can say is: I don't get results at all like that. Nor do the many, many, many people successfully using BU planes with 12 deg bed angles. Wear bevels certainly do develop on the blade back near the tip, forcing you to re-flatten the back every so often. That happens over the course of thousands of strokes, though, not 30. There is something extremely unique about your setup, and not in any positive sense of the word.

Is it possible you're dubbing the edge somehow? Are you using non-rigid media (leather or elastomer strop, mdf, etc) at any step in your honing process?

EDIT: I don't suppose you lift the plane when you draw it back?

[bridger berdel]

My first read on your post was that by honing at a lower angle you were removing more metal and that was the source of reduced blade life. Which sort of makes sense, until you examine it. As I was finishing my post I realized that that probably wasn't what you meant, but I was out of time, so I sent it as it was.

That's exactly right. When I said "wasted blade life" I was referring to the impact of additional sharpening due to faster-than-necessary edge dulling. IMO that's true no matter what sharpening process you use.

Both clearance and edge life are determined by the bevel immediately behind the cutting edge, which I'll call "tip bevel" from here on [*]. In my experience going from, say, 25 to 35 deg tip bevel in a common-pitch plane (45 deg bed) has no effect on clearance, but a big impact on edge durability and life.

[*] I've seen some people argue that the primary or overall bevel matters for clearance, but that can be disproven by one simple observation: For almost any reasonable blade geometry, the primary bevel will be entirely above the plane of the sole.

[david charlesworth]

Warren's research makes good sense to me.

I have sharpened all my bevel down, 45 degree pitch, bench planes at 35 degrees for donkey's years, and this works very well.

David

[Kees Heiden]

Steve Elliot looked at the formation of the lower wear bevel, that is the rounded bulge just behind the edge at the underside of the blade. The part touching the wood. This lower wear bevel tends to lift the edge out of the wood, ultimately leading to the feeling that you must sharpen again. Populair believe is that the roundness of the very tip of the edge makes the unsharpness, but Steve's test proves that's not true. The tip quickly looses its very sharp tip and after that the radius of the tip doesn't change much until the edge becomes very very blunt, way beyond fine cabinet making standards. http://bladetest.infillplane.com/htm..._profiles.html

So, the wear bevel at the clearance side of the blade is what makes the tool feel dull. It would be reasonable to think there is a correlation between clearance angle and the formation of this lower wear bevel. But I don't know of any study specifically adressing this issue. I did stumble across something though when I did my tests to compare high cutting angles to chipbreakers. The article is here: http://planetuning.infillplane.com/h...pbreakers.html

The second part of this study was about the wear behaviour. I meassured the horizontal force (pushing the plane) and the vertical force (the pressure of the edge against the shaving) Especially the vertical force is interesting in this case because it represents the effect of this wear bevel. When the blade is sharp this force is ideally negative, meaning the edge is pulled into the wood. Wear makes it less negative or even positive, meaning the edge is more and more pushed out of the wood, neccessitating us to press harder on top of the plane (or sharpen the blade).

Distance02.jpg

Now, I first did my tests with a 60 degree plane with only a 15 degree clearance angle. I saw that the vertical force increased faster then my 45 degree planes. Don Willans complained about this, saying that was not enough clearance. I repeated the test with a 30 degree clearance angle, and behold, the wear rate was a lot better! (Still, not quite as good as the chipbreaker plane of course ;-))

My conclusion: High cutting angles and low clearance angle are not an ideal combination. This of course still prooves not much about planes with a standard cutting angle.

[Kees Heiden]

Normand, I think your test is a good effort to find a correlation between clearance angle and wear bevel. The effect you see is exactly what I was measuring with the vertical force sensors, an increase of wear at the clearance side diminishes the ability of the edge to pull itself into the wood. Of course, this is a limited test and therefore still open to debate.

And Patrick, he was looking at how often he could raise a shaving without any additional pressure on the plane, except the weight of the plane itself. In real use you always exert some pressure, increasing that pressure as the blade gets duller and duller. So you can milk out a lot more plane strokes.

[Stewie Simpson]

Warren's research makes good sense to me.

I have sharpened all my bevel down, 45 degree pitch, bench planes at 35 degrees for donkey's years, and this works very well.

David

David; based on a minimum clearance of 10*; I am not understanding why you would have a starting point of 35* on a bevel down plane pitched at 45*.

Stewie;

[Pat Barry]

.... When I went and looked at my spreadsheet (I know, laugh away) I discovered that I'd grabbed the wrong spare blade, and the one I'd used was set up with a 35 deg secondary bevel. .

ROFL... Seriously man, you have far too many tools if you can't keep track of them without a spreadsheet.

[david charlesworth]

Stewie,

When I sharpen at 35 degrees, (this is actually just polish at the tip,) Clearance angle is reduced from 15 degrees to 10 degrees.

And it works fine.

David