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Why further flatten the back of a chisel?

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Forum topic by jimintx posted 05-07-2018 06:16 PM 1575 views 0 times favorited 13 replies Add to Favorites Watch
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jimintx

916 posts in 1973 days


05-07-2018 06:16 PM

Topic tags/keywords: chisels flattening tuning chisel

Every one always says you have to flatten the back of most new chisels when you get them. I look at the back of new chisels, like say the Stanley Sweetheart or some Narex offerings, and what I see is essentially a flat surface.

I fully acknowledge that if you run one of these over a flattening, abrasive surface, you can see that there are places that are touched more by the grit than others. So, to be clear, I have an understanding of the situation.

But, I also have a question:
Why do they need to be further flattened, when the tool is flat already for all practical purposes?
What difference that a user can identify does it make for a chisel?
Why the quest to further flatten a new chisel?

I will appreciate further understanding of this topic, so thanks in advance!
.

-- Jim, Houston, TX


13 replies so far

View Loren's profile

Loren

10477 posts in 4037 days


#1 posted 05-07-2018 06:25 PM

You can use the flat back of a chisel as a
sort of “fence” for the edge. This is useful
in paring tenon cheeks and things like that.

To get the chisel sharp however only the
part close to the edge needs to be flat. I have
one French made chisel I bought on ebay
because I wanted a wide one for slicing veneer
and I didn’t want to spend much. It was a
bargain and it sharpens up acceptably but
the back was ridiculously bowed and I think
I must have spend close to an hour working
it on a Makita wet grinder with a 1000 grit
wheel to get it close to flat. If I’d known
how long it would have taken I would have
broken out the coarse wheel I’ve never used.

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Rich

4482 posts in 978 days


#2 posted 05-07-2018 06:30 PM

It’s fine if the center is slightly hollow. Take a look at the back of a Japanese chisel to get the idea.

-- Knowledge is not skill. Knowledge plus ten thousand times is skill. -- Shinichi Suzuki

View CRAIGCLICK's profile

CRAIGCLICK

117 posts in 462 days


#3 posted 05-07-2018 06:44 PM

I spent a couple of hours this weekend honing some chisels I had, just for funsies.

They are all cheapo bench chisels from Harbor freight, home depot, Lowes, etc.

I started working on a 1” Harbor freight chisel and the backside was perfectly flat! I say this because it was late and I was tired so I started rubbing it on 800 grit (using the paper and granite method). When I realized what I did after about 5 strokes, I looked at the back and it was completely uniform…not a high or low spot to be seen!

I did it again, just to see if I wasn’t holding it right and, again, no high or low spots.

I gave it a couple of swipes across the coarse stuff just to see if I just wasn’t noticing it with he fine stuff and, still…perfectly uniform.

That cheapo HF chisel now has of place of honor on my bench. It’s like adopting a Pug and then finding out it can do your taxes.

I guess that sometimes, as they say, even a blind squirrel finds a nut!

-- Somewhere between raising hell and amazing grace.

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splintergroup

2689 posts in 1611 days


#4 posted 05-07-2018 07:14 PM

As Loren points out, near the cutting edge is probably the most important place where the back needs to be flat (perhaps for only 1/4”-1/2” from the edge back).

Think of the chisel back and the cutting edge bevel as geometric planes. The intersection of where they meet (the cutting edge) forms a line. If the back has a bump or dip, the line will not be straight and thus the cutting edge will not be straight.

If the back is basically flat, but with lots of grinding marks, the cutting edge will have serrated edge. A serrated edge generally does not cut as cleanly as a straight edge for chisel work. Having a polished and flat back will produce a truly straight cutting edge.

The ability to cut cleanly and accurately with a chisel depends a lot on the cutting edge being straight and sharp. If you do not need the control and/or clean cutting ability for your task, you can forgo these final prep steps and live with the factory finish on your chisel.

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jimintx

916 posts in 1973 days


#5 posted 05-07-2018 08:14 PM

Yep, I definitely get that the cutting edge needs to be smooth and flat.
What I did not & do not understand is the need to get to a super-flat back over the entire chisel.

I have always wondered about this, and figured that folks talking about spending hours, and many grits, to flatten the whole back of a chisel are not actually required to do so for the chisel to work quite well in a wood shop and furniture-making environment.

I do understand that it cannot be grossly out of “flat”. The types of wood working chisels I referenced are not, as received, that out of shape.

-- Jim, Houston, TX

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Smitty_Cabinetshop

16125 posts in 3007 days


#6 posted 05-07-2018 09:08 PM

Those to take the time to flatten the backs of chisels over their entire length are overachieving what is needed for solid woodwork.

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

View ColonelTravis's profile

ColonelTravis

1976 posts in 2283 days


#7 posted 05-07-2018 09:25 PM

The only reason I put the back of a new, modern-day chisel on a stone is to get the grinding marks out. And I only do that near the edge. Otherwise, what looks like a flat blade to the naked eye is actually a micro-toothed blade if you look at through magnification. In the grand scheme of things, it won’t ruin your work if you leave it that way. I still haven’t flattened the back of my newest PM-V11, used it right out of the package.

I’ve never spent more than a minimal amount of time flattening (not really flattening but removing factory marks) the back of a new chisel. Maybe 2-3 minutes max?

View Loren's profile

Loren

10477 posts in 4037 days


#8 posted 05-07-2018 09:28 PM

One thing to note is that some chisels, like
Two Cherries chisels, come buffed to a mirror
polish. This is not because they are flattened
at the factory. The buffing may look great
but it can be used to conceal grinding marks
which show up as little serrations in the honed
edge. The chisel may feel sharp, shave hair
and so forth, but the edge is likely to break
down faster. Even though sharpening to 1000
grit is adequate to cut wood very cleanly, the
reason I go to 8000 grit is because the edge
stays sharp longer.

View JayT's profile (online now)

JayT

6207 posts in 2600 days


#9 posted 05-07-2018 09:32 PM



Yep, I definitely get that the cutting edge needs to be smooth and flat.
What I did not & do not understand is the need to get to a super-flat back over the entire chisel.

- jimintx

I don’t understand it, either. The first 1/2in or so being flat to get a good edge is generally enough. The rest of the chisel should be flat enough to use as a reference face. If not, then the quality of the whole chisel is suspect.

-- https://www.jtplaneworks.com - In theory, there is no difference between theory and practice. In practice, there is.

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splintergroup

2689 posts in 1611 days


#10 posted 05-07-2018 09:38 PM


Yep, I definitely get that the cutting edge needs to be smooth and flat.
What I did not & do not understand is the need to get to a super-flat back over the entire chisel.

- jimintx

I don t understand it, either. The first 1/2in or so being flat to get a good edge is generally enough. The rest of the chisel should be flat enough to use as a reference face. If not, then the quality of the whole chisel is suspect.

- JayT

Exactly!

View Loren's profile

Loren

10477 posts in 4037 days


#11 posted 05-07-2018 09:39 PM

Also, with an old chisel with a pitted back the
back can be partially flattened at an angle
on a stone by taping a beer can shim to the
chisel or the stone.

I have some old socket chisels, Witherbys or
something that I found missing handles
(I remember finding one buried in dirt) that
may need a lot of work if I ever get around
to trying to recondition them.

View NeophyteGrant's profile

NeophyteGrant

102 posts in 898 days


#12 posted 05-07-2018 10:53 PM

If the back is flat enough to use as a reference face (and your joinery etc. looks great) then you’re good to go. The PM-V11s come ridiculously flat on back.

Also, are they magnetic? Because they are attracted to metal for sure.

View Woodknack's profile

Woodknack

12804 posts in 2769 days


#13 posted 05-08-2018 12:39 AM

Chisel back is a reference face, also makes sure your cutting edge is flat.

-- Rick M, http://thewoodknack.blogspot.com/

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