Thread: Bevel Up Planes With Back Bevel

It is the secondary bevel, which is easy to grind since it is so small. On my bevel up planes I only grind on bevel FWIW.

Mark,I think you may have missed the point. I'm not refering to the micro bevel, I'm refering to back bevel as in the opposite side of the blade to what you normally sharpen.

Oops. Sorry.

Back bevel? Hmmm, I would be confused as well! I can only guess that they meant to write the back [U]of the[U] bevel.

Now I am curious. I have to call Lee Valley this week anyway, I will have to ask them what they mean by back bevel.

putting a back bevel on a low angle bevel up blade

Hi Termite

There are actually TWO reasons why you might use a backbevel on a bevel up blade.

The first is, as the manual states, "to increase the included bevel angle without affecting the cutting angle. The purpose of this is to increase the durability of the edge, particularly when working endgrain". The point here is that the bevel angle used is low angle, that is, the edge is thin and relatively fragile. So the back bevel (of, say, 10 degrees) is used to "thicken up" the bevel edge, thereby strengthening it. Because this angle (10 degrees) is less that the bedding angle (of 12 degrees) it does not effect the cutting angle. (If you did this on a bevel down plane, you would increase the cutting angle by this amount, that is 10 degrees).

The second reason to use a backbevel - and one that I do - is to increase the sharpness of the edge by making absolutely certain that you have two sides coming together at a single point (which is what defines a sharp edge). This is affectionately known as the "Ruler Trick", made popular by the writings (and invention) of David Charlesworth. It involves using a small metal ruler (perhaps 1 mm thick x 1 cm wide) to create a (approzimately) 1 degree backbevel after all sharpening has taken place. It takes a few seconds at most with the 6000/8000 waterstone. Create slurry on stone...place ruler down the one side ... use this to raise the blade 1mm off the stone at the rear of the blade ... rub back of bevel over a 10mm depth of stone until you see the merest glimmer of shine. Done = flat back of blade.

This really works well.

Keep in mind that, to work, the size of a backbevel is unimportant.

Regards from Perth

Derek

Originally Posted by Termite

...putting a back bevel on a low angle bevel up blade...

Not sure what the question is.

Putting a back bevel on the blade of a bevel-up device leads to the geometry in the attached pic. The advantage of a back bevel is speed of sharpening, if the back is not flat. The only risk in putting one on the blade in this situation is that if you make it too big, there is a possibility that the blade will be unsupported by the frog. Whether this happens depends on the depth of cut and the size of the back bevel. It becomes more likely for small depths of cut and for large back bevels. If you draw a diagram with the right angle and the smallest depth of cut, you'll be able to work out the biggest back bevel that still has the blade supported all the way to the end of the frog.

However: this is probably a very small effect, because (1) the frog is very thin at the extremity, and hence has limited rigidity there, (2) the blade probably projects further into the work than the amount of unsupported blade above the frog, and (3) the back bevel ought to be microscopic, so that keeps any blade flex within microscopic limits.

This all assumes that the angle of the back bevel is much less than the angle of the blade support (frog). If the back bevel angle is bigger than this, the plane will cease to function. Most instructions for forming back bevels involve thin shims at the end of the blade to raise it up a poofteenth, leading to a back bevel angle of a poofteenth of a degree, so this shouldn't be a problem.

Enough of this nonsense.

In simple terms, look at the bevel of the bevel up plane, the plane your looking upon is the bevel, add another bevel change the plane, re bevel the micro bevel, this adds another plane turn it over to another plane, look at the bevel, re bevel the backbevel, don't remove the micro bevel when bevelling, this is the back bevel when seen from the plane mentioned, you can't see the main bevel, or macro bevel from this plane of sight, ensure the micro bevel is in tact from the right plane flatten the bevel reinsert the plane blade bevel up, remember its a bevel up plane, not a bevel down plane, bevelling a bevel down plane is a whole new plane field, adjust the plane then plane.


OK

Originally Posted by derekcohen

..... Because this angle (10 degrees) is less that the bedding angle (of 12 degrees) it does not effect the cutting angle........
Derek

Hmmm, we had some discussion along these lines a few days ago, Derek. Your example leaves you with a clearance angle of 2 deg. I assume you were just using an exaggerated example for illustrative purposes, because I reckon there ain't much room to play with back bevels with a 12 degree bed! My metal-working book recommends nothing less than 6 deg clearance for hard metals (more for soft metals). Wood is a lot more compressive, and gives much more 'bounce-back', which seems to me would require more clearance behind the cutting edge. My intuitive take is that 12 degrees is probably a good average clearance angle, which might have some bearing on its choice for'low angle' planes. Anothe good reason is that the pointy end of the bed becomes pretty useless at supporting the cutter if you get any more acute.

If you want a more robust edge on a low bed angle tool for harder stuff, you're probably better off opting for a secondary bevel. The higher cutting angle would be a good thing, in most situations, would it not?

FWIW
Cheers,

If you want a more robust edge on a low bed angle tool for harder stuff, you're probably better off opting for a secondary bevel. The higher cutting angle would be a good thing, in most situations, would it not?

Ian

I agree.

Also, the instance of a 10 degree BB was indeed for illustration. I have never used one this large in a bevel up configuration, and I could not imagine doing so even though the LV manual says it is OK. I do use a 1 degree BB frequently.

Regards from Perth

Derek

I would think that if you are taking a heavy cut of something soft, then a large clearance angle is essential, but if you are taking a very thin cut on a hard wood, then the clearance angle could be much less without running into too much trouble.

Maybe appropriate here or not, but I have noticed that quite a few of the planes I have bought/seen here come with bed angles at or below 40 degrees. Lowest yet was 36 or so. Couple that with, say, a 30 degree blade bevel and you get 6 degrees of clearance. Thing works fine in all the woods it can handle, being softwood and softer hardwoods.

I have heard that 12 degrees is the minimum myself. I just wonder who said it, how they arrived at that angle, what kind of plane they were using and how long ago it was?

If it was Joe Blow, just a wild guess with a chisel plane sharpened on bluestone 200 years ago, then maybe this so called 'law' might need some overhauling.

Originally Posted by Schtoo

I have heard that 12 degrees is the minimum myself. I just wonder who said it, how they arrived at that angle, what kind of plane they were using and how long ago it was?

If it was Joe Blow, just a wild guess with a chisel plane sharpened on bluestone 200 years ago, then maybe this so called 'law' might need some overhauling.

Schtoo, - It may have been a wild guess, but it's more likely it's one of those empirical laws, arrived at by long trial and error. Doesn't make it any less applicable, though!
I agree with you that it's easy to appreciate that a heavy cut in a wood with a bit of 'bounce' needs plenty of clearance, but even if it's Jarrah, you are also compressing the material slightly at the leading edge of the blade, so you need the clearance, albeit closer to the edge, maybe.
I can't say I've conducted exhaustive tests, but I was made aware of clearance a few months back, when one of my planes with a very thick blade just wasn't cutting well, despite having been recently sharpened. I knew the bevel had shortened quite a bit, but I'd been in lots of hurries, and hadn't reground it for yonks. So I grabbed a protractor and checked the bevel angle. It was down to well over 35* at the edge. This meant less than 10* clearance on the 45* bed-angle plane. A regrind (making sure to get the angle right) and re-hone and all was well again.
I hesitate to tell this story, because Derek and all the jig-honers will jump on me and say "see, that wouldn't have happened to us." But in the interests of the pursuit of knowledge, I'll bear the brunt......
Cheers,

Originally Posted by Derek

Also, the instance of a 10 degree BB was indeed for illustration.

Ah, so it was an abstract plane...

Originally Posted by silentC

Ah, so it was an abstract plane...

Sharp this morning Darren.

Silent, been meaning to speak to you about that jiggling avatar of yours -

Stop it my boy, before you go blind! :eek:

Cheers,

Jiggling? There's no jiggling, must be your eyes playing up. Did you sit up to watch the cricket last night?