# How do you know the back of a new chisel is flat and polished enough?



## TropicalWW (Feb 20, 2011)

I've finally gotten around to flattening the back of my chisels, and I have a question.

How do you know when the back of a new chisel is flat enough, or polished enough?

Yeah, I can see myself in the back of the tool, but it's not a perfect mirror….I stopped at 1200 grit…it's all I had. (I'm doing Scary Sharp) I can still see light scratches, and if I color the back of the tool with a marker and then rub across the sandpaper, the marker doesn't wear away all at the same time, but with a few more strokes, all the marker goes away. So, is the back flat? Do I need to polish more? Discuss….hell….just tell me what to do!!!!

Oh yeah….and how much of the back do I need to work? The whole thing?


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## Stuey (Apr 21, 2010)

It depends. Some say all you need is about one inch, others insist on flattening the entire back saying it's only a one-time hardship.

I just started flattening a few new chisels, and progressed through several waterstone grits. The final finish is smooth to the touch and with a dull mirror finish. Working at the coarser grits left sub-surface damage that I don't intend to spend hours polishing out.

What are you using to back the sandpaper? Once flat, marjer should be evenly removed from the chisel back. If not, and you see the same behavior repeatedle, perhaps you are applying uneven pressure to the chisel. Nobody can tell you if the chisel backs are flat from your description alone. I'd suggest sarting with only the bottom 1, 1-1/2 inches, and see how you like its performance. If you're happy, you're done.


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## Bertha (Jan 10, 2011)

I generally go for a mirror, up to 2000 or 2500. However, Stuey's right; if you like the way they cut and resharpen, you're done.


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## wingate_52 (May 14, 2011)

Put some polishing compound on a piece of MDF and rub the back of the tool on that for a mirror shine. I use Autosol, Tormek paste will work.


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## WayneC (Mar 8, 2007)

The back only needs to be flattened across the edge on the back. The purposed of flattening it is to prevent a jagged edge from scratches on the back. It should be mirror like. I would say if you flatten all the way across 1/4 of an inch deep you will meet the need…


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## RobFisher (Aug 25, 2009)

First, a mirror polish does not necessarily equal a flat surface. Curved surfaces can be mirror polished. A mirror polish indicates a smooth surface, which is desirable for the back of a chisel, assuming it is also flat. Flatness can be determined by comparing it to a know straightedge. And assuming whatever you are using to polish the surface is flat, the back of the chisel should be flat as well. Second, a chisel back should be polished over it's entireity, because the entire back can be used as a reference surface while in use. A plane blade only needs a very thin area on the back that is polished. Enough to allow the polished bevel to meet the polished back at a sharp and not jagged edge.


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## mvflaim (Dec 8, 2009)

If you asked this question in the 1800's people would look at you strange. Why do you need your chisel's back to be perfectly flat and mirrored? None of mine are yet they work just fine. In fact none of the old chisels I have ever come across at antique tool auctions and shows had their backs perfectly flat and mirrored.


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## TropicalWW (Feb 20, 2011)

Thanks for all the thoughts everyone! I'm not going to worry about how flat or polished anything is….putting the tools to use and going from there is the best advise. Thanks!


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## PurpLev (May 30, 2008)

just to throw some more spices into the mix - people in the 1800 did not know many of the things we know today. making decisions based on things from 200 years ago - may or may not be a good idea - use your own judgement from what info you can gather today.

now that history is behind us lets move on to physics. an (cutting) edge is the line formed between 2 planes. on a cutting tool this would be the bevel plane, and the back plane. if one of those is not flat all across you'll have a cutting tool that sub-performs. if you want an extreme example think of a just-sharpened plane blade that hit a nail and now has a nick in it. that nick is an exaggerated unflatness of the blade and in order to restore the blade to proper condition that nick has to be ground down. now to put this into perspective - a blade that the back is not flat and not honed means that it has many tiny nicks in it - to some people it doesnt matter as long as the tool performs to a certain degree while others really want the blade to cut easier.

bottom line - how sharp should your tools be boils down to what feels good enough to you.

get a sheet of 2500grit paper, give that a try, maybe or maybe it woulnd't make that big of a difference to you - at least you'll know.


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## rance (Sep 30, 2009)

I'm flabbergasted. And I thought all you hand-tool guys were fanatics.  And now you go and talk some sense. Yeah, I agree, 1/4" to 1/2" is gud-nuff. That's the only part that touches the wood. Actually, that's a good gauge, "only as far as touches the wood". That is all that is needed to be flat.

Oh, and flatness requirements for chisels are a whole lot less than for hand-plane blades. The OP question was about chisels.


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## RobFisher (Aug 25, 2009)

What about paring with a chisel? Like paring the wall of a mortise that is more than 1/2" deep, say 3"? Wouldn't a flat chisel back help with keeping the chisel inline with the intended cut? I guess I see flattening a chisel back as a chore that is done once in the life of the tool. Also why are flatness requirements less for a chisel than a plane blade? They are essentially the same thing, one being held by hand, the other held by a plane body.


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## rance (Sep 30, 2009)

Rob, I base that remark on the fact that the chisel is held entirely in your hand and the accuracy of the angle you hold the chisel is far less than what even Harbor Freight can provide in flatness. A plane blade on the other hand is held in a fixture(the plane) which could slightly twist the blade when it is installed.

Another factor that negates the necessity for absolute flatness in the back of a chisel is that a lot of chiseling is slightly back-cut. Once you start doing that, then it is virtually irrelevant.

In all reality, I doubt that the average hand plane user could tell the difference between two hand planes in front of him if one was out of flat by 3 thousanths and the other perfect. I equate this to the colors found in a lot of fishing lures. The color is there more for the fisherman's delight. I doubt the fish cares much.

One of these days I want to buy a set of the cheap $3.49 chisels at HF and replace the handles with Cocobolo, sharpen them up and let someone use them to give me an evaluation. I bet they'll praise them. The only difference they'd be able to tell is the frequency that they'd have to be sharpened.


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## Cosmicsniper (Oct 2, 2009)

Assuming your sharpening stone/surface is flat, then it becomes flat when the entire surface of the back becomes etched with the rougher grit abrasive. To help see it, you can put some ink marks over the chisel…when those have disappeared, it is flat. Then, work up to the grits to give the finish polish you need.


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## RobFisher (Aug 25, 2009)

Rance, I understand your point when doing a lot of chiseling chores. However, as much as I can I rest the chisel on a reference surface. In the mortise example above, while finishing chiseling the bottom 1" of the mortise, the back of the chisel is resting on the upper 2" of the mortise wall. If the chisel back is convex it could rock side to side in the cut, messing up the mortise. To quote you from above, "only as far as touches the wood" means 3" of the chisel needs to be flat in the mortise example. And I know there are times when the full length of the chisel back is referenced while making a cut, meaning the entire back needs to be flat, not necessarily polished but flat. Flatness means all of the high spots are coplanar with the cutting edge. Japanese chisels usually have slightly concave backs, but the cutting edge and typically small landing strips along the sides are coplanar allowing the back to function nearly identical to a completely flat back.

Flatness (and polish) requirements for chisels and plane blades are the same if you want to sharpen them to the same degree of sharpness. If you are only chiseling in unseen areas then of course the chisel does not need to be as sharp as a chisel making a finishing cut through end grain, but the same can be said when comparing the sharpness requirements for a jack plane and a shooting board plane.

The entirety of a chisel back does not need to be as polished as the edge, but it does need to be flat because it is often referenced against another flat surface to guide the cut.


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## rance (Sep 30, 2009)

Rob, I agree with everything you just said. Maybe a better question would be "How flat does the back of a chisel need to be?". Does one measure it with feeler gauges, a dial indicator? I would argue that woodworkers go far beyond what is really needed. I'm of the belief that if you can't see it with the naked eye, then it is flat enough.

Please don't confuse my aparent disregard to flatness with my high expectations for sharpness.


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## TropicalWW (Feb 20, 2011)

This is an interesting conversation. What prompted me to start this topic to begin with was that my chisels LOOK like they are set up and sharp, but they work like CRAP! They look like they should be razor sharp, but they can't even start a cut in paper let alone wood. It's back to the sharpening station for me!  Thanks again, everyone!


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## Alwine (Mar 7, 2011)

Sharp is Sharp. Chisels only have to be so flat. Better to be sharp than flat. My great uncle was a machinist and every tool he had was dialed to thousandths of an inch. We drove each other nuts for years. You have to find that happy medium. I got to give it him, his comeback percentage was close to zero, but this is supposed to be fun.


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## wingate_52 (May 14, 2011)

Itake all my bladed tools through the Diamond abrasive range 250,400,600,1200 then to 8000 ceramic before giving the flat face a shine on Autosol and a choice of Jewelers Rouge or Veritas green stick on leather. Flat, shiny and shaving sharp.


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## tirebob (Aug 2, 2010)

I flatten and polish the first inch of all my chisels and plane blades up to 8000 grit on water stones.. Polishing the entire back is great, but definitely overkill as you are probably doing something seriously wrong if you are eating metal up so quick that the first inch doesn't last you a realllllllly long time, even a lifetime for some hobbyists…

Flat and polished does create a sharper tool… Get yourself a decent sharpening book for a much more detailed explanation. The first one I read, and it helped a whole lot, was the following…

http://www.leevalley.com/en/wood/page.aspx?p=32991&cat=1,43072,43091&ap=1


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## Cosmicsniper (Oct 2, 2009)

Speaking from the perspective of a high school math teacher here…

The intersection of two flat planes is a straight line.

The intersection of a curved plane with a flat plane is a curved line.

But in both scenarios, the quality (thinness) of the line (chisel edge) depends on the quality of your sharpening tools and your ability to use them…but both could produce equally sharp edges.

So, regardless of the flatness of the chisel back, you will still produce a sharp edge if you use consistent sharpeners and techniques.

However, the difference is that with two flat surfaces you will produce a dead-straight cutting edge. If one of the planes is curved (or not flat), like on a japanese chisel, the corners of the edge will protrude slightly relative to the center of the edge; it's still sharp, just curved. In other words, in the event that you did have a not flat chisel back, then a microscope would still show consistent sharpness along the entire edge, but it would be wavy to a certain degree (to reflect the peaks and valleys of the non-flat back).

But the real problem is that if the chisel back is NOT flat, then more "proud" areas of the leading chisel edge would dull the fastest simply because those areas make contact with the wood earlier than the rest and, consequently, would spend more time actually ON the work piece.

But I'd venture to say, in performance, that perception of these things is probably not easily seen since, for all intents and purposes, the chisel back is probably flat enough. However, in terms of keeping a sharp edge, I'd imagine those with FLAT backs will give longer lasting edges. I would imagine that a concave-back japanese chisel would circumvent this problem because of the higher quality and hardness of the steel…but technically, on such a chisel, the areas near the corners would dull first.

Another way to think of this is with a gouge tool. A curved plane and a flat plane meet to form a still sharp edge…though if you mostly used the center of the edge while working on the lathe, then the center areas would be the first to get dull. Same as with a cerrated kitchen knife. The more curved or unflat one of the "planes" is, the harder your steel (or other material) better be.


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