Do chisel backs really need to be flat?

If the back of the chisel is not flat you will not have a sharp edge. You need two straight edges coming together to get a sharp point.

Absolutely yes. Don’t even think about not doing it.

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-- Alaskan's for Global warming!

There are two elements for having a flat back, one is to get the thing sharp, and the other is to have something as a reference surface while using the chisel. Bear in mind, you don’t need the whole thing to be flat, I like the first 1 1/2” to be flat.

The reason for mirror polishing the back is to make sharpening easier. Instead of grinding both sides of the bevel each time with each grit progression, you only have to grind the face of the bevel back far enough to remove any wear from the back side of the bevel. Then the back is only touched with the highest grit.

Well, as others said, it needs to be flat on both sides of the bevel to be sharp. That said, the entire back does not need to be flat and polished to a mirror. How much of the back along the edge needs to be polished tends to be personal preference, but realize that as you sharpen the blade and wear away steel, you will need to flatten the back more often if you only do a small amount.

Scratches along the edge introduce weak points that will break and wear faster, meaning that you’ll have to sharpen sooner. The bigger the scratches, the faster the wear. Technically, even after honing on an expensive, high grit Japanese stone, there are microscopic scratches from the stone.

Now, your argument that since the work done with most chisels is not visible is valid. Feel free to hack away at a mortise with a screwdriver, if you like. A sharp chisel just makes it prettier, faster, easier, and more enjoyable.

It’s somewhat ironic that you mention plane irons. A lot of people will put a small bevel on the back of the plane iron – see the ruler technique – to make it cut better. It is flat along the width, but not along the length of the iron.

-- Tony, SW Chicago Suburbs

Yes, they really do need to be flat. Not the entire chisel back, just a 1/2 inch or so. A lot of times in chisel work you need to create a vertical face from a scribe line. I think that would be nearly impossible without a flat back.

-- Bondo Gaposis

Ditto

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

Backs are not flat to leave unscratched surfaces; they’re a reference face and necessary for sharp.

-- Don't anthropomorphize your handplanes. They hate it when you do that. -- OldTools Archive --

You ever use a dull chisel ona step of a project? And then when you finally sharpen it and realize how much easier your job is, and you think to yourself “why didn’t I sharpen this before I started this step?”.

Also, you make a false assumption that wood is covering ALL surfaces touched by chisels.

Your title and question dont agree – flat backs then mirror finish surface.

They need to be flat since the backs are a refence surface in use. As for mirror surface – relates to edge life, not really sharpness or scratches on the wood surface. 600 grit will get an edge that will cut pretty, it just wont cut as long as a polished edge. In use sharp edges micro fracture. The more coarse they are sharpened, the faster the edge fractures away because there is less edge to carry the force due to the scratches from the more coarse grit, as well as each grit scratch is a microfracture – so polish those edges with finer grit.

I’m assuming a chisel will develop a bevel on the flat side due to poor sharpening and instead of trying to restore a flat side, you think it might be OK to have a bevel on both sides of the chisel, much like a kitchen knife edge. This is always a temptation when sharpening, to avoid the long process of getting the side flat. With a double bevel, it is impossible to get a sharp edge.

Well, chisels are chisels. But I think we need to stop overthinking.

-- Jon, St. Louis, https://www.concretecontractorsstlouis.com

Well, chisels are chisels. But I think we need to stop overthinking.

- Chris889

stop over thinking or knowing how to properly put a razor edge on a blade,ive seen a lot of people screw up the sharpening.so what does chisels are chisels mean?

-- sawdust the bigger the pile the bigger my smile-larry,so cal.

Well, chisels are chisels.

- Chris889

Backs are backs and flat be flat.

-- Bondo Gaposis

You can have a chisel with a bevel on each side, which is how you get a sharp chisel without a flat back. I have some for special carving purposes, The double bevel though will create unpredictable cuts in joinery which by design requires square edges and perfect lines. So for caring, no a flat back is not needed. For joinery, yes you want a single bevel on one side only.

People need to stop obsessing over a mirror polish. I don’t know who started it all but he or she should be banned from using edge tools the rest of their life. A MIRROR POLISH DOES NOT MEAN IT’S SHARP! Natural stones will often not produce a mirror polish but the finest natural stones create an edge that’s finer than any synthetic can produce.

Anyway, this is when you can achieve maximum sharpness. Notice how the polish (not a mirror polish!) on the back of my oirenomi is perfectly even. 75% of the cutting performance is determined by the back.