Here's a tip to keep the back of a chisel or plane blade in good shape.

[Jessica de Boer]

As we all know a perfectly flat stone or diamond plate will never create a back that's perfectly flat. The same thing happens when you flatten the sole of a plane with sandpaper on a known flat surface like the bed of a jointer. It will always be slightly convex. Here's what I did to solve it. I bought 2 Atoma base plates and 2 replacement sheets, one 600 grit and the other 1200 grit. A friend of mine works for a company where they do metal work to incredibly tight tolerances and he made both base plates convex by 0.4mm over the length and by 0.2mm over the width of the plate. It might sound counter intuitive but this is a perfect way to keep the backs of chisels and plane blades flat. I don't use these plates for the bevels though, they are solely for flattening my chisels and plane blades.

[Tom Trees]

You can always wreck a nice stone by flattening the back whilst letting the sharp square edges of the blade scrape off the outer edges of the entire hone...
DAMHIK
I agree that it is helpful if you intend to set your cap iron to have infulence, or for honing a shooting plane iron.

And on the plane part, I only know too well
Flattening on a dead flat plate might seem to work with a small plane, but try doing so on anything larger than a no.4 and the convexity will be real apparent.
I used the same method as Bill Carter does for the last plane
Look up the blunt chisel technique, it is a serious skill to have in your arsenal, and will make you able to remove material from the middle of the plane, without causing
the convexity problem.
Do it with an inch and a half cheap chisel, skew it slightly in use, and you will be able to take metal shavings throughout the length of the plane, not having to stop at the mouth, it just ran over my old
Stanley no.8 which has a tight mouth with no issues.

Then just a few laps afterwards to check for flatness..
I adhere to the same principal as what you mention, and make a pair of strips from regular sandpaper rolls.

Thank you Bill Carter for the effort saved using the Blunt chisel technique ...it is soo easy and works wonderfully
He shows it working wood which I'm excited to try, when the time comes
For cast iron it cuts beautifully.

The only other way I could have done it, was to use self adhesve sandpaper strips lesser than the width and length of the plane...
No more!

Tom

[Jim Koepke]

One problem with abrasive sheets is the swarf tends to load up the paper. With paper wider than the plane there is a tendency for the edges of the plane to get a bit more abrasion than the center.

Frequent, meaning ever other stroke, clearing of swarf from the abrasive can help. Attention to the details and constant checking of the surface being worked is important.

jtk

[Tom Trees]

And to think there's one of the most popular utubers that recommends easing off the long edges on a smoother!
One wouldn't be able to take a cap iron influenced tearout free cut,if they didn't have a perfectly flat sole across the width...
as the iron would only be protruding on each end, and not in the middle.
That video should come with a warning

[Tom M King]

I like that idea. You could also use those convex diamond plates to sharpen clipper blades. I use Atoma sheets on a granite surface plate to flatten stones.

[Kees Heiden]

Interesting, but how are we going to get a convex plate like that, without friends in a precision metal working shop?

[Jessica de Boer]

It doesn't necessarily have to be an aluminium plate of course. You could use a block of wood, shape it and then stabilise it with a wood sealer. A thick piece of acrylic would also be suitable I imagine. It doesn't really matter what you use as long as you make it slightly convex.

[Kees Heiden]

Okay. Do you use it each time when sharpening an iron? To keep it flat rather then making it flat in the first place?

[Jessica de Boer]

I usually use it after 8-10 sharpenings or so. Mind you the problem doesn't really exist with Japanese chisels because of the shorter length. By the way, I asked my friend if he remembers the exact amount of convexity just to be sure. He says he made it drop off by 0.2mm to each side over the width and by 0.3mm to each side over the length. It's really just a tiny bit but I found it makes it much easier to keep my long blades flat.

[Luke Dupont]

One problem with abrasive sheets is the swarf tends to load up the paper. With paper wider than the plane there is a tendency for the edges of the plane to get a bit more abrasion than the center.

Frequent, meaning ever other stroke, clearing of swarf from the abrasive can help. Attention to the details and constant checking of the surface being worked is important.

jtk

I used to use sandpaper for sharpening and flattening the backs of chisels, and I noticed this too.

Sandpaper is still the quickest method I know to flatten the backs of chisels and planes, though,
An india works okay as well, but even Aluminum Oxide oilstones will come out of flat when flattening the backs of hard Japanese steels if a lot of work needs to be done -- and such steel will quickly wear out diamond plates, too.

I've since just adopted the philosophy of patience. Get a the first 1/4" or so flat enough to sharpen, and not worry about the rest: perfect flatness will come with time. "perfect" being defined as flat enough for my stones to make contact along the whole edge with just enough of a face to consistently register against the stone; whether that is absolutely perfectly flat or not isn't of consequence so long as I can put it on any flat stone and get work off the burr from the whole edge.

That said, I can see how such a nice, slightly convex diamond plate could make that all much easier!

[Luke Dupont]

You can always wreck a nice stone by flattening the back whilst letting the sharp square edges of the blade scrape off the outer edges of the entire hone...
DAMHIK
I agree that it is helpful if you intend to set your cap iron to have infulence, or for honing a shooting plane iron.

And on the plane part, I only know too well
Flattening on a dead flat plate might seem to work with a small plane, but try doing so on anything larger than a no.4 and the convexity will be real apparent.
I used the same method as Bill Carter does for the last plane
Look up the blunt chisel technique, it is a serious skill to have in your arsenal, and will make you able to remove material from the middle of the plane, without causing
the convexity problem.
Do it with an inch and a half cheap chisel, skew it slightly in use, and you will be able to take metal shavings throughout the length of the plane, not having to stop at the mouth, it just ran over my old
Stanley no.8 which has a tight mouth with no issues.

Then just a few laps afterwards to check for flatness..
I adhere to the same principal as what you mention, and make a pair of strips from regular sandpaper rolls.

Thank you Bill Carter for the effort saved using the Blunt chisel technique ...it is soo easy and works wonderfully
He shows it working wood which I'm excited to try, when the time comes
For cast iron it cuts beautifully.

The only other way I could have done it, was to use self adhesve sandpaper strips lesser than the width and length of the plane...
No more!

Tom

Wow, really? I imagine the chisel needs to be harder than the steel you're shaving, no?
I might have to try this... I've got a warped vintage plane iron that I should probably throw away, but feel like I can salvage.

[Tom Trees]

I meant using the blunt chisel technique for cast iron, not tool steel.
Sorry if I drifted off topic a bit, I was just referring to the first line in this thread, stating that a perfectly flat surface won't achieve perfect flatness
with sandpaper/abrasives, if it is wider or as wide as a metal is.

I must say that a perfectly flat back may be a boon when the cap iron is set for complete tearout elimination, as needed for the most demanding timbers.
If one has not used the cap iron to have full infulence before, they may find quite a bit of dust in between the cutter and the cap, even though there was never any
dust produced before, when the cap iron was just close beforehand.
I lapped my planes on a 1800 very worn fancy diamond plate, giving a mirror polish...
Since then I've been using this stone for the backs everytime, and noticed the corners a bit dull (polish wise not the same mirror reflection throughout the width)
I believe I am using even pressure on the iron for taking the burr off, the dullness was caused by lapping it incorrectly, even though I was really careful not to make a
belly in the blade, by going to mad silly lengths alltogther to avoid the belly.
If I was to do it again it would be done with a stone much less than the width of the iron.
Its so little of a blemish that most would call it perfect, but for me planing iroko and other exotics, I need to have the cap iron set real close.

I still get dust and its a bit annoying to tip a no.8 or any plane upside down so that makes me wonder if this nuisance dust can be reduced.
Since were on the topic of flattening backs, even with a convex stone, this occurrence will still happen.
What it would be good for is getting rid of a slight camber, that makes the cap iron unable to work to its full extent.

Onto your possibly salvageable plane iron, I would flatten it with a hammer first...
The Japanese do so with their plane irons.
Maybe I should do the same, but after the really embarrassing amount of time I took lapping my plane irons I dare not.
I might have to do some experiments with a narrower plane, and see if I can come close to eliminating dust when the cap irons real close.
Tom

[Vincent Tai]

Wow, really? I imagine the chisel needs to be harder than the steel you're shaving, no?
I might have to try this... I've got a warped vintage plane iron that I should probably throw away, but feel like I can salvage.

This is called hand scraping; you can hand scrape parts to a millionth of an inch tolerance. The blunt chisel is very nice for woodworking but it is worthwhile to do a little reading about hand scraping in the machinist world and maybe grab an old file and grind a more suitable tool for scraping and mimic some of the scraping techniques used. I;ll admit sometimes I still grab a blunt chisel to scrap a little burr or something out of cast iron but it only takes a couple minutes at a grinder or sander to turn that old file you have somewhere into a dedicated tool.

https://www.youtube.com/watch?v=QJXqHpSh3SE
https://www.youtube.com/watch?v=t0gg9z4gG3A

The second video is all in Japanese but it shows some folks hand scraping large surfaces. Kind of fun and definitely mind boggling the first time around.

[Alan Jones]

Makes you wonder how the cabinetmakers of old ever managed to turn out such fine work