Bevel up planes and camber limitations (long and has math)

[Randy Klein]

I've been using LV's trio of BU planes for a few weeks now and I have some comments on limitations of the BU design.

I prescribe to Chris Schwarz's recommendation of after using powered jointer and planer to mill lumber, use a jointer plane to flatten/remove machine marks followed by a smoother for final surface prep. He also recommends cambered irons for the jointer and smoother plane, with the jointer having more camber (and the ability to take a thicker shaving) and the smoother having less camber (and obviously a finer shaving). But the BU design has a significant limitation to cambering. It has to do with the bed angle.

Using a cambered blade, you project the iron just far enough so the corners just disappear into the sole. This will eliminate planer tracks. So how far you can project your iron depends on the camber. And the projection determines the thickness of shaving.

So doing a little trig: On a standard angle plane with bed angle 45 degrees, if you want a 5-6 thou shaving (Chris' recommendation for a jointer plane), you need to have a camber of about 7-8.5 thou. This is coincidentally the amount Chris mentions on his video (that's a good sanity check on the math). However, on a BU plane, the bed angle is 12 degrees, which works out to be a 24-29 thou camber! That's huge.

It's not so bad on a smoother (max 1.5 thou shaving) though. The camber needed for a standard angle is just over 2 thou (again the same number Chris mentions). And for a BU, the camber is just over 7.

So what conclusions can I draw from this?

If you want to follow Chris's method and camber your jointer, a BD jointer would be better (my opinion) than BU.

The smoothers I'm undecided on. On one hand you have BU's ability to easily change angles with different blades (but of course you can use different blades with back bevels for the standard angle), you can close the mouth tighter since there's no chip breaker, and you can use a low angle iron for end grain work.

But how often do you use a smoother for end grain, especially if you have a block plane? And a sharp iron on a standard angle can easily handle end grain anyways.

Or if you like the BU design jointer, you can always not camber your iron and use your smoother to get rid of the plane tracks.

Final thoughts: In the BU vs BD debate, it always comes down to the universal answer: "It depends".

[Derek Cohen]

If you want to follow Chris's method and camber your jointer, a BD jointer would be better (my opinion) than BU.

Randy

It is all an illusion, done with mirrors ...

First off, you are correct - BU plane irons require greater camber than those of a BD plane.

So what?

Just do it. The result is the same.

In the picture below I have chosen to use a LV LA Jack - but I could just as easily used the LV BU Jointer. I am planing across the grain of a Cherry table top (for a sofa table) with a iron bevelled at 25 degrees. This gives me a low cutting angle - quite a bit lower than possible with a BD plane.

It is possible to start with a heavy cut ...



... and then follow it with a finer cut - look at the feathering of the shavings ...



I tend to keep a few blades for different purposes. One is a 25 degree straight bevel for a shooting board. Another is a cambered bevel for across grain planing, and I can switch it between the LAJ and BUJ.

... how often do you use a smoother for end grain, especially if you have a block plane?

With regard the LV BUS, again I agree with you, but I think that you are missing the point. The advantage of the BU design lies in the other direction, that is, a flexibility with high angles of attack. There is no reason to use this type of plane with a low cutting angle. I only use a 60 or 62 degree cutting angle.

I use both BD and BU planes about equally. Generally I prefer the low centre of gravity "feel" imparted by the BU configuration.

On the other hand, here is something that I have not as yet attempted with a BU plane - using a #7 (BD) jointer as a scraping plane. The iron is given a 45 degree bevel, with a slight camber, and used in a BU configuration.

Jarrah shavings with- or against the grain ...



Regards from Perth

Derek

[Randy Klein]

Derek,

I see what you mean by illusion as I can't see any of your pictures but it's probably just the server at work...

[Wilbur Pan]

First off, you are correct - BU plane irons require greater camber than those of a BD plane.

So what?

Hi Derek,

Well, my thought was that with a bevel down plane, you'll have to grind away 3 times the amount of steel at the edges to get the amount of camber you need compared to a bevel up plane.

Of course, you could argue that once you establish the camber, you only need to maintain it. But with my crappy sharpening skills, I'd probably just flatten it out. It's only when I'm trying to put a flat edge on a blade that I get awesome cambers.

[Bob Opsitos]

Derek, can you take a picture of the camber on your iron? Curious to see what the amount of camber your running look like?

My BU irons are slightly cambered, only what I get from finger pressure during sharpening. I'll have to check with a square to see how much that has gotten me. I bet it is extremely small.

thanks
bob

[Randy Klein]

If you want to follow Chris's method and camber your jointer, a BD jointer would be better (my opinion) than BU.

Randy

It is all an illusion, done with mirrors ...

First off, you are correct - BU plane irons require greater camber than those of a BD plane.

So what?

Just do it. The result is the same.

Derek, I'm not so sure the result will be the same and here's why. For the same iron width (nominally 2-1/4") cambered to produce 5 thou shaving, the more cambered iron will have a narrower profile since the camber forces it "fall" off to the side faster. So the end result will be a surface with more, but thinner, undulations. The less cambered iron, of course, is opposite. It will have a wider profile and will result in undulations of greater width.

Now, I'm not sure if this difference will actually make an aesthetic difference, but the anal retentive side of me thinks the wider undulations will be more pleasing.

And of course, a thinner profile will need to have more passes taken to cover the width of the piece.

[Randy Klein]

Derek (or anyone else for that matter),

Maybe you can help me figure this out. I was planing some hard maple today that had some reversing grain I wasn't really expecting and ended up with some nice tearout teasing me.

I was already using my LV BU planes with a 50 degree cutting angle and I've been keeping it sharp. So I rehoned a steeper micro bevel and ended up with a cutting angle of 64 degrees (I was actually shooting for 62, but I forgot to change Mk II micro bevel knob). When I started planing with the new angle, the tearout disappeared and the expected increase in force was definitely there.

However, and this is where I need help, the surface doesn't look as good. The other pieces that I planed at the lower angle are significantly more glass-like and smoother feeling. I'm at lost as to why, what to do about it, or even if it will matter after the finish is applied.

The only guess I have is that the angle was too steep and induced a Type III chip formation, that I've heard Rob Lee refer to, leaving a fuzzy appearance.

Any thoughts would be greatly appreciated.

[Derek Cohen]

Randy

I can take some photos of the cambers, but it will have to wait until the weekend.

With regard to the surface finish with a high- verses low cutting angle, it really depends on the wood as much as the plane. Softer woods will not plane as smoothly with high cutting angles (think of a scraper on soft woods - tearing rather than shearing the fibres).

Regards from Perth

Derek

[Randy Klein]

I guess I'll hone some angle in between those two and take a few more passes.

[Bob Opsitos]

Becuause I was curious, I drew the shapes up in CATIA (CAD program here at work) and indeed for a .005" shaving on a 12deg bed and 25 deg, 2-3/8" wide iron you need almost .030" of camber to feather the edges. With that said it really doesn't look ridiculously cambered by any means. it appears to be ~23.5" radius.

Bob

[Randy Klein]

Becuause I was curious, I drew the shapes up in CATIA (CAD program here at work) and indeed for a .005" shaving on a 12deg bed and 25 deg, 2-3/8" wide iron you need almost .030" of camber to feather the edges. With that said it really doesn't look ridiculously cambered by any means. it appears to be ~23.5" radius.

Bob

Bob, I was trying to draw the circles to correspond with the camber, but MS powerpoint is not so good. I did make an excel spreadsheet that has iron width, bed angle, thickness shaving as inputs and it spits out the required camber.

I wonder if anyone can plot these circles to scale on a CAD program. It would be nice to have as a reference as I hone to compare against.

And for my case, a 2-1/4" iron, bedded at 12 degrees and a desired shaving of 0.006", the camber needed has a radius of 22". Compare this to a bed angle of 45 degrees and the radius is 75". I haven't seen these as circles yet though, so may that huge disparity in radii is only slight.

[Randy Klein]

Derek, I'm not so sure the result will be the same and here's why. For the same iron width (nominally 2-1/4") cambered to produce 5 thou shaving, the more cambered iron will have a narrower profile since the camber forces it "fall" off to the side faster. So the end result will be a surface with more, but thinner, undulations. The less cambered iron, of course, is opposite. It will have a wider profile and will result in undulations of greater width.

Now, I'm not sure if this difference will actually make an aesthetic difference, but the anal retentive side of me thinks the wider undulations will be more pleasing.

And of course, a thinner profile will need to have more passes taken to cover the width of the piece.

I actually need to retract what I said here in the quote, since I figured out that I was mistaken. The profile will end up being the same, since the bed angle of the LA will "flatten out" the more cambered blade.

However, another issue I thought of: If you are cambering your jointer iron 28 thou, this means you need to project it 28 thou from the sole, in order to get the proper thickness of shaving. As opposed to the a normal plane where you only need to project it about 8 thou.

I was wondering if this larger projection has any affect, since the blade is unsupported. Or maybe the extra thickness of the iron takes care of this already.

[Bob Opsitos]

Very slight, I plotted them out 1:1 and I doubt you'd be able to tell the difference with only a 2.25" chord length (blade width) without a square/straight edge to compare them. The difference is only .517mm in camber. The two lines are blurred together on the screen at that point.

Attached is a 1:1 of the 22deg radius across a 2.25" blade width. You'll likely have to play with the printing scale to ensure it comes out correct.

Bob

[Wilbur Pan]

However, another issue I thought of: If you are cambering your jointer iron 28 thou, this means you need to project it 28 thou from the sole, in order to get the proper thickness of shaving. As opposed to the a normal plane where you only need to project it about 8 thou.

I was wondering if this larger projection has any affect, since the blade is unsupported. Or maybe the extra thickness of the iron takes care of this already.

I think that whether you use a standard angle or a low angle jointer, either one of them should be set up to take a 0.006" shaving. As you say, because of the differing angles, the low angle plane will need a greater degree of camber to achieve this.

However, the edge of that blade should still only project about 0.006" below the surface of the sole of the plane. The blade probably projects further out from the base of the frog, but because of the low angle, the downward projection is less.

[Wiley Horne]

Randy,

I don't believe blade support on a bevel-up plane is going to be an issue, even with the camber. Bear in mind that on a bevel-down plane, the entire primary bevel is cantilevered, though it is also pre-tensioned by the cap iron.

What is going to happen when you camber a 12-degree-bedded, bevel-up iron, is that the blade edge will need to be managed within tighter tolerances over time, than a bevel-down iron. In the situation you describe, each time you resharpen, you have two conditions to meet: maintain the camber of approx. 1/32", and maintain the final honing angle. And in doing this, you will want to remove enough metal with each sharpening to get beyond the wear on the back of the blade. This is manageable, and it's manageable on a hand-held basis, but will require more attention to tolerances than in the bevel-down situation.

In general, the bevel up plane has a blade geometry which is sensitive to changes occurring on the backside (clearance is affected), as well as changes on the bevel side (changes the attack angle). As long as you're aware of this as you resharpen over time, it's manageable.

Similar considerations attend the sharpening of bevel-down irons over time, but the tolerances are usually greater. I say 'usually' because in certain situations, for example with Japanese planes bedded in the low-40's, maintaining the bevel requires great discipline, because clearance is limited and bevel-rounding due to sloppy sharpening will cause clearance problems fairly quickly.

Each different group of planes has blade and bedding geometries that impose discipline on the way we sharpen.

Wiley