The secret to cambering Bevel Up plane blades

[bugbear]

[promhandicam]

I agree with BB that the way that Derek has produced the camber isn't dependant on the form of the wooden template. The only way of using a template like that to reproduce the same form on the blade would be to have a set up similar to that used in key cutting - i.e. a stylus running against the template and using a thin grinding disk - it wouldn't be possible to do it using a belt sander setup.
If you look closely at the photos of the cambered blade, you can see that the striations are not parallel but that they radiate out from the centre indicating that the blade was pivoted around a central point which as BB says could just as easily be a small radius. What is more critical in producing the desired camber is the distance from the pivot point to the grinding surface (to give the desired curve) and the height of the pivot point above the grinding surface to give the angle of the camber.

Maybe a better jig for those who aren't as skilled grinding / using a belt-sander would be something similar to this:


Steve
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'Personally I'm always ready to learn, although I don't always like being taught' - Winston Churchill

[sparky]

Well I'd like to ask a quick question

If someone sharpened just by hand (I do it that way with waterstones) isn't the higher bevel angle easier to work with since it is closer to the final angle.
It seems to me that this whole discussion presumes that the blades are initially ground mechanically. Which is fine, but I was curious about BU blades done by hand.
Also when I say by hand I mean freehand, (without a honing guide) which is a goal of mine to learn.
If anyone has any ideas, I appreciate it.

Sparky
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[David C]

Sparky,

For pure hand honing I think you might like the Krenov method.

The grinding is done at the same angle you want to hone at.

The hollow grind then sits nice and stable on an oilstone.

Honing polishes both the heel and the tip of the bevel.

However once much of the bevel is honed and flat, it takes a long time to remove enough metal to resharpen....

So JK returns to his hand cranked grinder to start the cycle all over again.

David

[David C]

Steve,

I show a concocted jig which works something like your diagram on Page 50 of my second book.

I wanted to convert wooden Jack planes to hollowing planes (a GREAT idea from the Alan Peters book, a scandal that it is no longer in print) and designed the jig to give a fixed radius without any skill or feel involved. i.e. almost totally repeatable.

BB has summed things up rather nicely, as usual, the Derek jig involves some skill and feel, which he clarly has, judging by the nice looking results.

David

[sparky]

Thanks Mr. C and all

So here is my next question. Since I am completely "cordless" where could I find a hand-powered grinder?

sparky
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[Derek Cohen (Perth, Oz)]

[woodbloke]

[ivan]

Yes, it's true that if you didn't move the blade, you'd get a straight(ish) edge. That's also true for a standard straight jig. ??! you say? The point is you have to slide even a straight jig from side to side whilst grinding, either to even out wear in the stone, or because the blade's wider than the wheel. Despite the helpful jig, it's still a 'hand' process needing a bit of skill.

If the camber was produced by the template acting just as a point pivot, the camber would have a rather extreme radius of about 2". From Derek's description and photos, he rolls the template back and forth on the rest. If the edge follows the template, he must do this for about the same number of revolutions at each point. (a bit like DC's camber honing) In practice a steady roll back and forth should do the trick.

I'm assuming that any required precision (straightness, skew, match edge of cap iron etc) can be obtained at the first honing?

[Derek Cohen (Perth, Oz)]

[bugbear]

[Wiley Horne]

I believe Bugbear and Ivan are both correct. Bugbear is correct in that it is operator input that is making the jig work properly. But Ivan has explained that rolling the template along the control bar results in a radius on the blade edge which would require a considerably longer (more distant) point radius. Indeed, one could start with the sagitta = 1/16" and the blade width, and calculate what the equivalent point radius would be, measured along the slant of the primary bevel.

Derek's contribution, as Ivan explains, is that his jig (applied with requisite skill as BB explains!), can produce smaller sagittas without having to use point-pivot jigs with impractically long radii.

Wiley

[MikeW]

The camber produced is slightly larger than that on the jiggy as long as there are no user corrections made, no matter how subtle. That's the physics of it. At least if this were a single-pivot point jig. Here's how I think a single-pivot jiggy would work...

The closer the edge is to the jiggy, the less this offset or difference is. I suspect that without user input the difference on a 2" blade, a 2.5" desired radius, and having a 1.5" projection is about 1.5" difference (a blade ends up with a 4" radius). I believe that same difference in radius holds true using the same projection no matter the start/end radii. At least that's how it seems to my mind.

But, Derek's jig isn't pivoting on a single point. I think if the point on the jiggy and the edge of the blade are kept relatively square to each other at the point of the blade's contact to the sander/grinder wheel, the blade will end up with the same radii as the jigs used. If I was better at using CAD, I would try to draw it. If I had more time, I would make one to try on a piece of scrap saw steel. If nothing else, it is removing some of the radius "growth" inherent in a single-pivot point jiggy.

The skill comes in by not lingering in one spot longer than another--which is helped by drawing the arc onto the non-beveled side of the blade to be cambered. The jiggy keeps the projection relatively even through the arc (unlike a single-pivot). It still takes operator error to not remove say more in the center than the corners and vice versa.

In one sense, it doesn't matter. The main benefit is that it is reproducable.

Take care, Mike
who only cares that his pattern maker's blades and rounds are accurate--they need to match the soles...

[BrentB]

A very interesting discussion, in which I have learnt a number of things about cambered blades.

The similarity between the template and the blade shape leads one to the conclusion that one can only lead to the other. BugBear has noticed and explained well the weakness of the linkage. People trying this at home could easily get a very different curve. Which means of course, a variable bevel angle across the blade. An arc scribed on the back would certainly help.

Assuming a rod based tool rest (rather than a flat tool rest at the desired angle), then I think Derek's jig is better than a point jig. If you use Derek's jig and grind to an arc scribed on the back, then the resulting bevel would have a constant included angle. If you use a point in the middle and grind to an arc on the back, you get a variable included angle (smaller away from the middle).

The standard method, assuming you have a belt sander or grinder and an appropriate tool rest,

1 scribe the desired curve on the back,
2 grind at 90 degrees to achieve that edge shape,
3 grind the bevel at the desired angle just up to the edge,

produces a surer arc faster. Even with a tool rest (which helps with the bevel angle), it would be possible to get a different edge shape without the arc. Grinding to an inscribed arc, then grinding the bevel to that arc, together help to get the desired shape. Trying to do both in one step might lead to a blued edge.

Finally, a question.

An earlier post showed an alternative jig design that was claimed to be better than Derek's design. I cannot see why it is an improvement. Can someone help me on this?
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Brent
--
Victoria, BC, Canada

[Philly]

Welcome to the forum, Brent.
Good to have you aboard,
Philly
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