A tip for flattening the backs of chisels and plane irons!

[Luke Dupont]

I wanted to share a quick tip for flattening the backs of chisels and plane irons.

I use a lot of vintage tools, and I've never owned any really high end chisels that come precision ground either.

In both cases, the backs of the chisels can be seriously out of whack, such that it can take many hours of grinding on even coarse diamond stones or sandpaper to bring them to a sharpenable state.

In any case, here's a simple trick that I use which reduces hours of work:

If the back of your chisel or plane iron has a hump, such that you're only contacting the middle of the back and not the edges, which is so often the problem, hollow it out with sandpaper!

I can spend hours on a stone getting nowhere, but as soon as I sand the middle bit down, the process goes extremely quickly.

Be careful as you don't want to cut yourself or damage the edge, but I generally find cutting a small piece of sandpaper off and just rubbing the high parts with my finger works. You can use another object in place of your finger, or some other method besides sandpaper if you have one (I'd love to hear!) but the principle is the same. Just work on the high parts off the stone. You will not think that you are removing much metal, and you won't be, but the next time you take it to your stone and check, you will be able to quickly achieve flatness.

I did this just earlier with a stubborn chisel, and afterwards all I needed to bring the back into flatness was a Washita. It's now dead flat.

My theory is that we can waste a lot of time not correcting this problem even on a coarse stone, because when there's a hump in the middle, the chisel will continue to rock back and forth (even if by immeasurably tiny fractions of a millimeter, which we can't notice) which just retains the "bump" in the middle, rather than working it down. So special attention to that hump in the middle may be necessary, or in any case is very benefitial, to achieve a flat registration as well as reducing the surface area in contact with the stone.

Anyway, I just don't see this talked about much, which is why I mention it.

Most people seem to really skip over the "flattening the back" step, or make it look very easy with high end tools (some folks just very lighly polishing the back of some LN chisel on a relatively fine stone which would take decades to flatten most tools I've touched!), and I think this is a disservice to beginners who can easily find themselves grinding for hours and still unable to get a serviceably flat (ie, sharpenable, such that the edge makes contact with the stone) back. And so they use an unflat tool for weeks or months, struggling to sharpen it the whole time. A very coarse flat stone and some attention to high areas will save a ton of trouble and time!

[Scott Winners]

Good tip. Thanks.

[James Pallas]

I take a flat piece of steel or an old file. Wrap emery around it. Clamp it in a vise protruding about an 1/8 above the jaw. Hold the tool or chisel perpendicular to the steel and go. Depending on the grit and the tool steel this can go very quickly. You can get lots of pressure easily so don’t push down hard enough to bow the tool. Great tip. Only remove what you need to to get the high areas.
Jim

[Jim Koepke]

With a narrow strip of sandpaper and a piece of scrap as a fence to help keep the hump on the abrasive the only thing you have to worry about is cleaning the swarf from the abrasive every few strokes with a brush or a strong magnet wrapped in paper or inside a plastic bag. The paper or bag makes it easy to separate the filings from the magnet.

My days of flattening chisels and planes is mostly in the past. Though one never knows when a tempting set of chisels might show up out in the wild again.

jtk

[Luke Dupont]

With a narrow strip of sandpaper and a piece of scrap as a fence to help keep the hump on the abrasive the only thing you have to worry about is cleaning the swarf from the abrasive every few strokes with a brush or a strong magnet wrapped in paper or inside a plastic bag. The paper or bag makes it easy to separate the filings from the magnet.

My days of flattening chisels and planes is mostly in the past. Though one never knows when a tempting set of chisels might show up out in the wild again.

jtk

That is genius! I will have to try that next time.

[Charles Guest]

I think the first indication of quality of manufacture is that the bevel was ground on the correct side of the cutter, which leaves the back concave and not convex.

But, yes, if there's a hump it's better to try to knock it down locally by spot sanding or scraping.

[Jack Dover]

I just grind any humps down with a rotary tool. Any rotary tool would work - a Dremel, a die grinder, even a flex shaft and a drill. I used valve grinding bits chucked a drill press for this. Hollowed out a whole back once similar to Japanese chisels, just for fun. It works great by the way, just don't go too deep esp. close to a business end. There are many ways of ensuring proper depth, but just a steel rule and some eyeballing would tell when the middle part is sunken enough.

A few points for anyone who might want to try it.

1. The most economic way is valve grinding bits, medium grit, they're often sold in a discount section of borg stores, I have collected like a bucket of them. Medium grit stones actually take very little material, 'cos it's hardened steel. Most probably there's too little material would be ground on the first attempt, but better play it safe.

2. Cylindrical bits work the best, just shape it to a barrel shape with a diamond T-stick. Pick the best abrasive bit, sometimes they have hard inclusions that leave a deep score mark or wear uneven. Wouldn't affect functionality tho.

3. It's totally possible to do it freehand, but with a simple fence\jig results come out perfect. Maybe you would figure a jig for a bench grinder - that would save tons of time. If your tool is stationary and you hold a chisel - please make sure that in case of a grab a chisel would be pulled away from you. On a drill press it's a matter of choosing the right side (which is left actually), starting at the handle and pulling a chisel towards yourself.

4. The hollow is not of a uniform depth. It should be deepest close to a handle and feather out to zero about 1/4" before cutting edge. Just think about how the back wears with every sharpening and you will immediately understand why.

I did this while figuring whether I could grind a hollow at hole or not. Rarely doing this for restoration, since the set up takes time, so it has to be a really wide, really hard chisel that dubbed over from decades of sharpening on a dished stone such that I can't put it even remotely flat on abrasive. Otherwise I won't bother, 48" of 80 grit PSA and a good holder will take care about any back issue in 15-20 mins, be it a chisel or a plane iron.