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In my continuing attempts to learn all the things another question stemmed from watching a chisel sharpening video from Paul Sellers. When sharpening his new chisel (he explained that he does this to all his chisels) he removed the hard angled bevel and created a smooth convex bevel on his chisel. You can sort of see from the picture below.

Soft convex bevel

I don't think I missed an explanation as to why he chose to put that bevel beyond personal preference. I understand that chisels and plane irons can have different degrees of hard bevels but I don't see that being compared to a convex bevel.

What are some practical reasons to put a convex bevel over maintaining the original hard angle that, in most cases, is already present? Or perhaps even making a concave bevel?

I feel this question could apply to plane irons as well.

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TL;DR warning. Sharpening is a deep and broad subject, with a lot of opinion and personal preference interspersed with the facts and science so be prepared to have to make your own mind up, i.e. choose your reality and stick with it!

I don't think I missed an explanation as to why he chose to put that bevel beyond personal preference.

Paul Sellers does specify in other videos that he thinks it produces a stronger edge, and also that the convexity allows the chisel — when used freehand in a bevel-down configuration — to glide into and out of a curving cut much more easily than if ground flat, which I can confirm from personal use (my chisel bevels have always tended to look like his, which I used to think was a failing but now embrace as a positive benefit).

The tendency towards forming a slight convexity is a natural consequence of the 'geometry' of human sharpening, which Sellers also explains in a couple of his videos. As your arms extend from closer to your body outwards to the end of the stone the presentation angle naturally gets slightly shallower, unless you're able to compensate for it with an almost robotic precision (usually only achieved with long experience, literally years of practice in many cases) or via the use of a honing guide. Accepting that a curved bevel isn't a bad thing, that it doesn't in any way form a poorer edge (as old books often state outright!) you can just exaggerate the motion somewhat and then you achieve convex bevels in the style of Paul Sellers.

Re. the edge being stronger, although I use a convex bevel myself on any chisel I have re-ground completely I have to say I think this is irrelevant. It is very likely true, but it doesn't actually matter in practice. Here's why: the bevel style that's usually criticised for being the weakest — concave grind with a very narrow honed edge — is almost never known to fail in normal use. I have read of only one or two cases where the edge crumpled and that was when used in tough hardwoods. But thousands upon thousands of woodworkers prepare their chisels this way and obviously don't experience failures, so that should be sufficient evidence that it's strong enough for most users in most applications.

Note: the last point is directly tied to honing angle, which is a separate consideration to the style of the bevel. Regardless of grind style, a steep angle (30° or higher) will always be stronger than a shallower angle (25° or lower) for the same chisel simply because there is a greater thickness of metal behind the cutting edge.

What are some practical reasons to put a convex bevel over maintaining the original hard angle that, in most cases, is already present?

Just to clarify one thing quickly first, the majority of Western woodworkers today do not sharpen and then hone just the one bevel. It's extremely common practice, and has been for many years, to have the honing angle be a little higher, often about 5°, than the sharpening or grinding angle (whether flat or concave if formed on a grinding wheel). This forms a secondary bevel which if very small and as shallow as 1-2° is sometimes referred to as a microbevel; I should stress here that although microbevels should be very very narrow by definition the distinction between the two is often arbitrary, to the point where I think the terms could be considered interchangeable as far as common usage goes.

If we just take it that you have a new chisel with the factory grind on it (usually 25°) and if the metal of the chisel, the sharpening media used and the skill and patience of the user allow you can just sharpen and polish that bevel and the chisel will work well. A well-formed 25° bevel has extremely good cutting performance, but is not suitable for all chisels for all uses. A paring chisel can be sharpened at 20° (sometimes even a little shallower than this) to give superior slicing, bench chisels are commonly done at 25-30° and you can go up as high as 45°, although this is best suited for tough jobs like chopping out deep mortises and general work in very hard woods.

Or perhaps even making a concave bevel?

A concave grind is actually not uncommon, also referred to as hollow ground particularly in knife circles. It's a natural consequence of sharpening a bevel on a rotating grindstone, whether very old-school and cranked by hand, on a bench grinder or on one of the high-end systems such as the Tormek. Whatever the radius of the grinding wheel is this radius will be ground into the chisel bevel.

I feel this question could apply to plane irons as well.

Edge geometry is much less relevant to most plane irons as they present their backs to the work (usually dead flat) and not the bevel, as most planes today are bevel-down planes with a cap iron or chipbreaker clamped to the flat back of the blade to curl or 'break' shavings.

There is some slight relevance to bevel-up planes — single-iron planes, as is common with block planes — but such a tiny portion of the bevel interacts with the wood (the shavings only contact the first 0.5mm / 0.02", if that) that it is really only that portion of the blade that matters and this is almost always flat effectively.

Here are the most common bevel profiles seen on Western chisels:

Chisel bevel profiles

All of these are in use by someone and although there are specific advantages to some of them (even if only in shortening the time it takes to sharpen) they all can produce acceptable cutting performance. It is the honing angle and the smoothness of the metal right at the edge that are the two primary factors in how sharp a cutting edge actually is, not the bevel profile.

  • Thank you SO MUCH for the pictures. I'm in the process of trying to figure out how to get my tools sharpened, and this helps so much. – Charlie Kilian Jul 25 '15 at 23:07
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    @CharlieKilian, no problem! Re. sharpening in general, my view FWIW is summed up in what I tacked onto the end of this Answer (also see the comments below relating to strops). Obviously feel free to ask a Question if you want a broader range of opinions on media & method (freehand v. jig, or a bit of both). – Graphus Jul 26 '15 at 8:08
  • The convex bevel certainly looks prettier when photographed with a macro lens. I'm not sure how much that helps when it's actually being put to work, though. – FreeMan Mar 21 '16 at 15:15
  • One of Sellers' goals is to streamline the workflow so that he can sharpen as quickly and as frequently as possible and return to work the wood with a minimum of interruption. Sharpening by hand, without a jig, and by allowing for a natural convex edge to take shape accomplishes this goal. I doubt this convex shape is appreciably stronger than either a secondary bevel or microbevel, but the first two require either a jig, fixture, or very steady hand... while allowing for a convex shape relieves you of this burden and barrier to sharpening. – aaron Mar 24 '16 at 11:39
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I've actually read Paul Seller's blog posts about sharpening a little while ago, and tried out his method on my chisels. Until now I used a sharpening jig to keep the straight factory bevel, and got the chisels pretty sharp.

With Paul Seller's method of creating a convex bevel it's much easier to sharpen by hand (i.e. without the jig), and the process of sharpening goes much quicker for me (I have a cheap jig, which is quite fiddly to setup), while still production a similarly sharp edge.

From Paul Seller's explanations, the advantage of the convex bevel is in putting more metal behind the cutting edge - due to the convex shape, the actual cutting angle might be 30-35 degrees, while the overall angle of the bevel is still the common 20-25 degrees (which Paul calls the macro-bevel). A micro-bevel give a similar benefit, but Paul claims it gives a weaker edge. One can think of the convex bevel as a large number of micro-bevels, starting from the maximal angle and slowly decreasing until reaching the overall bevel angle.

I don't know if the convex bevel has a real advantage over a micro-bevel (or just a single angle bevel) in edge endurance, but this is what Paul claims.

From my (short) experience with Paul's technique, the greatest advantage is ease of use when hand sharpening, without a jig. If you can already sharpen to a fixed bevel (with or without a micro-bevel) free-hand, then this probably doesn't matter.

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If you are sharpening on a grinder -- including slow and wet-wheel grinders -- the face will tend to follow the curve of the convex wheel and become concave. A larger wheel, such as the full-sized Tormek machines and their equivalents, would produce a larger radius and thus a less-concave surface.

I can't imagine creating that shape any other way... and I'm more than half convinced that he's trying to claim that this is an advantage simply because he can't avoid it. It would mean the area directly behind the cutting edge is thinner, but also less well supported.

Given that every other sharpening method produces a flat there (except for the microbevel at the cutting edge, of course), and that traditional grinding wheels had even larger radii than the tormek -- and given the razor edges folks produce that way -- I don't think this is something you need to emulate. If you're using a smaller grinding wheel it will happen, and enough people have sharpened that way that I'm not sure you need to be paranoid about avoiding it either.

Just an artifact of one particular approach.

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    Note that Paul Sellers is sharpening to a convex bevel, while you are talking about a concave one. He is also using flat stones, that start to have a concave shape over time, helping create the convex bevel on the chisel/iron. – Eli Iser Jul 25 '15 at 8:25
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    Ah. I was being misled by the photo, and got me wires crossed. Others cave addressed convex quite well. I'm still convinced the difference is mostly artifact of sharpening method and folks are fooling themselves when they claim an improvement -- but I would be delighted to have good evidence that I'm wrong! – keshlam Jul 25 '15 at 16:47
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I think his method makes sense. Using a smaller rake angle should make it easier to get a smooth cut and will make the blade less likely to bite.

Professional machine tools for cutting wood often have negative rakes for this reason.

I have not tried the method myself, but my first impression is that it is probably worth doing. If it makes sharpening easier, so much the better.

I should probably make the caveat that sharper blades require thinner angles, but take more skill to use. The sharper your blade, the easier it is to screw up. The advantage of a sharper blade is that it works faster, however, for an amateur woodworker, you are not usually trying to get done as fast as possible. In the old days when professional woodworkers were being paid by the acanthus leaf, it made sense to aim for the sharpest possible blade. But for a modern hobbyist, steady, easy and reliable makes more sense. From this perspective Paul Sellers has the right idea.

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    I don't agree that a sharp chisel is not important for the amateur woodworker. A sharp chisel cuts wood cleanly, making it easier and safer to work, as well as more enjoyable (nothing beats the feeling of paring tissue thin slivers on endgrain to make a joint tight). I would agree that in the area of diminishing returns, the amateur woodworker will probably invest less in sharpening than a professional woodworker. – Eli Iser Mar 22 '16 at 6:48

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