# Do chisel backs really need to be flat?



## Smile_n_Nod (Jan 20, 2011)

Dovetails, mortise, tenons, grooves, dados, etc. are frequently shaped with the help of chisels. But all of the surfaces that are touched by chisels are eventually covered by other pieces of wood.

So why are we told to give chisel backs a mirrorlike finish? I know that scratches on a chisel back can lead to imperfections on the wood surfaces, but unless the scratches are huge, they're not going to affect the joint and no one will ever see the surfaces left by the chisels any way.

Are we overthinking this and making too much work for ourselves?

(By the way, this argument does not apply to plane irons, obviously. The surfaces eft by cutting edges of smoothing planes are indeed visible and the backs of their irons need to be flat and smooth.)


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## Davevand (Feb 10, 2016)

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.


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## AlaskaGuy (Jan 29, 2012)

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


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## AESamuel (Jan 20, 2015)

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.


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## Tony1212 (Aug 26, 2013)

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.


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## bondogaposis (Dec 18, 2011)

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.


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## TheFridge (May 1, 2014)

Ditto


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## Smitty_Cabinetshop (Mar 26, 2011)

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


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## SMP (Aug 29, 2018)

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.


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## OSU55 (Dec 14, 2012)

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.


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## MrRon (Jul 9, 2009)

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.


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## Chris889 (Feb 6, 2019)

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


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## pottz (Sep 15, 2015)

> 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?


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## bondogaposis (Dec 18, 2011)

> Well, chisels are chisels.
> 
> - Chris889


Backs are backs and flat be flat.


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## BlueRidgeDog (Jan 2, 2019)

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.


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## Lemwise (Sep 17, 2016)

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.


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## anneb3 (Feb 23, 2014)

Think of it this way a U or a V. Naturally the V shape wins


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## LesB (Dec 21, 2008)

With all this fussing over a "flat" back in order to be sharp can anyone explain to me how it is that curved blades on carving tools get so super sharp. The flat back is a reference for cutting straight or flat surfaces.


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## Lazyman (Aug 8, 2014)

Carving gouges are a different animal than a straight bench chisel and as such the are sharpened in a different way. Regardless of the shape of the steel the key is that you want 2 sides that meet in sharp V. On a flat chisel the easiest way to achieve that is flatten then the back and then hone the bevel to meet it. As noted with the Japanese style chisels, it just needs to be flat along the edge. Think about it. If you have a ripple on the back along the edge, you would need a matching ripple on the bevel for it to be sharp in that ripple and the tool would make nice rippled cuts (think about trying to chisel or plane with a serrated knife edge). On a carving gouge, you still need a smooth even surface on the inside of the curve so that you can hone the bevel to match. Any inconsistencies on the inside edge will leave a spot that is not as sharp and make the gouge difficult to use.


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## Unknowncraftsman (Jun 23, 2013)

I like my coffee hot my steaks thick and my cutting edges shiny and slippery to the wood they cut.


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## bandit571 (Jan 20, 2011)

One can have both surfaces shined up mirror bright….and yet the chisel will have trouble cutting warm butter….don't think so? Look at your butter knife sometime….

get the edges to meet in as little space as you can….until that fine sharp edge arrives….THEN, maybe, worry about the polish….


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## therealSteveN (Oct 29, 2016)

I was going to say, define "flat" and "back" Already answered is that the entire back need not be flat, just the working portion. Bondo gave a figure of 1/2" or so, that is a good place to start, but in reality I have found it to be less than that. Trying less to start then doing more is a good way to find out.

That said all chisels are not created equally, so that "pay dirt portion" is likely to change a bit. But to the overall, yes you need both sides to be flat at the working tip, a primary bevel topside is a must, and if you come upon a way to put on a secondary/micro bevel you will likely never change back to just a single bevel.


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## AlaskaGuy (Jan 29, 2012)

I had no idea this was such a complicated subject.


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## MrRon (Jul 9, 2009)

Chisel backs need to be flat. The top image shows a nice sharp edge with a flat back. The next image shows when a bevel is unintentionally put on the bottom of the chisel. As you can see, the sharp bevel is lost and cannot be restored unless the back is flattened.


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## Bluenote38 (May 3, 2017)

> Well, chisels are chisels. But I think we need to stop overthinking.
> 
> - Chris889


Love it - thanks for the thought and a ray of sanity. Craftsmen of yore created beautiful furniture, chests, boxes, and much more and they didn't have Cr-Ni Steel cryogenicaly quenched and had never heard of mirror polish, flat back, micro bevel etc. Most of what we have today is for the purpose of selling the next big thing and some few are to help overcome our lack of mastery of the "Mysteries".


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## BlueRidgeDog (Jan 2, 2019)

I would not poo-poo the modern understanding of "sharp" and how to get accurate cuts on joinery too much. Yes my grandfather was a master craftsman and my father was a master wood carver who did large commissions world wide, and could put a razor sharp edge on anything (using natural stone hones and a buffing wheel). That ability took him a lifetime to master. In the end, when working in high production mode he would sharpen at least ten of a given tool, use them until they no longer did perfect cuts then grab the next. On sharpening day he would have a piles of knives and chisels to "work up" as he used to say. He used a variety of bevels that each had a purpose with all of them being ground by eye and never a measurement made.

So all the fluffery of today may seem odd compared to how masters worked in days gone by, but the goal is to look at how they worked and come up with a process where a beginner can get something that is at least functional. So, if you have years of experience, you don't need advice, but if you don't, the starting advice is on joinery tools, make the back flat and the edge square. This will get you out of the gate and making usable cuts. Once you get a feel for the metal and for how different angles sharpen and dull, they you can more or less do your own thing.

My dad was also very hard on his tools. He would put a different grind on them as needed and had no issue with them getting shorter and shorter as he used them. They were not toys or collectables. When they got too short to be useful a big box from the foundry would come with a fresh set of metal. Handles too were bought by the box and were not pretty, finished or special.

He also wore them out simply because he rarely used secondary or "micro" bevel. Each sharpening was "the bevel".

I worked with him for years and now have all his chisels…and for the life of me can't even get close to the level of sharpness he could get just by feel. I am good mind you being taught by the best, but still fall short of what he could so with very minimal effort. His tools could work end grain in Ash like a perfect edge in the long grain side of poplar. But I digress. Suffice to say that some of the "rules" are simply crutches for the beginners and they are good crutches, but yes in the long run you won't need them.


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## rob2 (Nov 27, 2007)

Jointery. Requires a flat back with no "ruler trick".


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## AlaskaGuy (Jan 29, 2012)

> Jointery. Requires a flat back with no "ruler trick".
> 
> - Rob Drown


+1


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