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

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Forum topic by TropicalWW posted 08-26-2011 02:14 PM 4629 views 0 times favorited 20 replies Add to Favorites Watch
View TropicalWW's profile


39 posts in 2679 days

08-26-2011 02:14 PM

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?

20 replies so far

View Stuey's profile


43 posts in 2983 days

#1 posted 08-26-2011 03:17 PM

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.


View Bertha's profile


13529 posts in 2720 days

#2 posted 08-26-2011 03:31 PM

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.

-- My dad and I built a 65 chev pick up.I killed trannys in that thing for some reason-Hog

View wingate_52's profile


224 posts in 2596 days

#3 posted 08-26-2011 03:58 PM

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.

View WayneC's profile


13754 posts in 4124 days

#4 posted 08-26-2011 03:59 PM

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…

-- We must guard our enthusiasm as we would our life - James Krenov

View RobFisher's profile


16 posts in 3223 days

#5 posted 08-26-2011 11:05 PM

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.

-- Lancaster, PA

View mvflaim's profile


189 posts in 3118 days

#6 posted 08-26-2011 11:59 PM

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.

View TropicalWW's profile


39 posts in 2679 days

#7 posted 08-27-2011 11:16 PM

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! :-)

View PurpLev's profile


8536 posts in 3675 days

#8 posted 08-27-2011 11:26 PM

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.

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View rance's profile


4258 posts in 3187 days

#9 posted 08-27-2011 11:59 PM

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.

-- Backer boards, stop blocks, build oversized, and never buy a hand plane--

View RobFisher's profile


16 posts in 3223 days

#10 posted 08-28-2011 12:19 AM

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.

-- Lancaster, PA

View rance's profile


4258 posts in 3187 days

#11 posted 08-28-2011 06:10 PM

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.

-- Backer boards, stop blocks, build oversized, and never buy a hand plane--

View Cosmicsniper's profile


2202 posts in 3185 days

#12 posted 08-28-2011 06:33 PM

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.

-- jay,

View RobFisher's profile


16 posts in 3223 days

#13 posted 08-28-2011 09:41 PM

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.

-- Lancaster, PA

View rance's profile


4258 posts in 3187 days

#14 posted 08-28-2011 11:15 PM

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. :)

-- Backer boards, stop blocks, build oversized, and never buy a hand plane--

View TropicalWW's profile


39 posts in 2679 days

#15 posted 08-29-2011 02:21 AM

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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