Sharpening a Badly Warped Plane Iron

WARNING: If you are a sharpening purist or a flat back Nazi, stop reading this right now, go to your safe place, and watch a few cat videos. If you ignore this advice, understand that what you are about to read is guaranteed to trigger you in ways that you’ve never imagined. Continue at your own risk.

I’ve been addicted to using wooden hand planes for about 20 years now. I think they’re beautiful to look at. I love the way that they feel in use and the feedback that they provide to the user. The “ssshhhiiiccckkk” that they emit with each cut sounds like the very definition of autonomous sensory meridian response (ASMR) to my ear. Plus, I’m a huge fan of their thick, laminated, tapered irons, especially when I’m sharpening without the aid of a honing guide.

While wooden hand planes are much simpler in design than their iron siblings, there are some unique challenges to tuning and using them. As any woodworker knows, wood moves seasonally in ways that iron does not. Wedges can shrink, causing shavings to clog in the mouth. Plane bodies can bow, necessitating occasional re-flattening of the sole to keep the plane working properly. Wood movement is just something that needs to be accepted and occasional maintenance of the plane something to plan for when choosing to use wooden bodied hand planes.

The biggest challenge with old wooden bodied bench planes, however, is also their biggest benefit – the thick, laminated plane iron. Old wooden bodied planes almost universally have irons that were made by a blacksmith who would laminate a piece of hard tool steel to a piece of soft iron, similar to how Japanese kanna are still made today. The thick bevel makes freehand sharpening easier, and the cast tool steel can be honed to a wicked sharp edge.

Quenching during the hardening process, though, will usually cause the blade to warp. If you’ve ever seen and heard a piece of red hot steel being plunged into a barrel of water, you can imagine the kind of stress that such rapid cooling might introduce to the blade. The taper of the blade also does not help. While the taper is useful for preventing the iron from retracting in use, the difference in thickness along the blade’s length means that the thinner areas will cool faster than the thicker areas.

The warping in the blade does have some benefit, though. In most old wooden bodied planes that I’ve seen, the blade warps towards the bevel. In other words, the bevel side of the blade will have a concave bend in it along its length. This is actually a good thing because it forces the blade to contact the bed of the plane right at the bevel. Once the wedge is inserted, the blade is then put under compression, like a spring, and the bevel end of the blade is held very tightly down to the bed of the plane, preventing unwanted chatter during planing.

If the blade were warped in the other direction, the tendency would be for the bevel end of the blade to want to rise up off of the bed of the plane. While the chip breaker and wedge help to prevent this to some extent, the thin soft steel of the chip breaker and thin wooden wedge tips are no match for the tension in the thick blade. Ultimately, the blade will still want to lift up at the bevel end and the plane will be much more likely to chatter and perform poorly. So it’s much better for the plane’s performance when the blade is warped towards the bevel, as noted above.

The warping in the blade does result in some sharpening challenges, however, at least according to our modern sensibilities. If the bevel side of the blade is concave, this means that the back side will be convex. This makes it difficult to hold the back of the blade flat on the stone while chasing the burr.

Based on the hundreds of old blades that I’ve inspected, though, this didn’t matter to our ancestors. What I’ve observed are blades that were slightly dubbed over on their back sides, presumably from stropping the back of the blade at whatever angle was necessary to remove the burr from honing the bevel. I’ve never – I repeat, NEVER – seen a blade from an old wooden plane that was lapped flat on the back side, unless it was done by its new owner.

This usually leaves those of us who wish to use these old planes with two options for sharpening the irons. Either we spend the time to lap the convex back of the iron flat, or we sharpen as I believe our ancestors did by just honing the bevel and stropping the back at whatever angle is necessary to chase the burr. My preference is to do the least amount of work, so I typically will just strop as I believe our ancestors did. However, for folks that aren’t comfortable honing freehand, stropping without the aid of a honing guide might feel a bit unnerving.

In that case, David Charlesworth’s ruler trick can be used in place of freehand stropping. If you haven’t see this trick in action, it’s a really simple concept. Basically, you lay a thin steel ruler along the long edge of your final polishing stone and set the iron on the ruler to do the final removal of the burr on the back side of the blade after honing the bevel. This creates an extremely shallow back bevel on the iron but ensures that the blade is polished and honed right up to the cutting edge with the least amount of effort. It’s a controversial technique among the internet sharpening “experts”, but it works really well.

For the blade I recently worked on, however, the ruler trick just wasn’t going to work. I spent about 15 minutes lapping the back of this blade on a coarse diamond plate to see what kind of shape it was in before I decided to move on to something a bit more risqué. The warp in the blade was so severe that I could fit a 0.018″ feeler gauge between the cutting edge and the body of my square. The only ruler I had that was thick enough to even come close to working for this kind of gap was the ruler from my Starrett square, and I wasn’t going to use that.

Not wanting to give up entirely though (what kind of blog post would that make), I tried another technique I’ve used in the past with some success. However, this method is not without risk either. The concept is to gently bend some (or all) of the warp out of the blade. This tends to work better with thinner blades, like the ones used in vintage iron bench planes, but I figured I’d give it a shot anyway, if for no other reason than to show it as an option here.

In order to bend the blade in the desired direction, dowels are used to transfer the clamping pressure from the jaws of the bench vise to the blade. Be advised, great care has to be taken when using this technique. Hardened steel can easily crack if too much bending pressure is applied. In the photo above you can see that I have bent the blade past the point of where I wanted it to go, expecting some spring back. Sometimes this works (again, it works better with thinner blades), but in the case of this blade, it did not. The blade just sprung right back to its original state when the vise pressure was removed.

This is the point where I will remind you that my preference is to do the least amount of work to the iron possible. So if an old iron cannot be easily made flat enough for the stones, I typically will just freehand strop the back as I believe our ancestors did (I said that earlier, right?), and not go through all of this rigmarole. However, ever wanting to present my readers and students with as many options as possible and not leave non-freehand sharpeners with no alternative whatsoever, I present one final option that will work 100% of the time – the lowly back bevel.

If you ever want to be branded a heretic among the internet sharpening nerds and put your very life in danger, bring up the back bevel. Nothing is guaranteed to ruffle more feathers in the sharpening world than to suggest that the back of a plane iron (or a chisel for that matter, but that’s a post for another day) does not need to be flat. Sometimes the truth is just too much for some people to bear. So it’s probably best to just keep this little nugget of knowledge to yourself and enjoy the extra time you spend working wood while they fire up their scanning electron microscopes to make sure that the edges of their blades can split atoms.

The fact is, the vast majority of the people who will curse me for suggesting the use of a back bevel have never dealt with an iron like the one I’ve been working on here. If they were to run across an iron like this, they’d likely navigate their way to their favorite woodworking retailer’s web site and order a replacement iron because lapping this iron flat would simply be too much work and take much too long to be bothered with.

However, by putting this iron in a honing guide and adjusting its projection until the cutting edge of the iron just touches the surface of the stone with the honing guide running on the bench top, I was able to take an iron that would have taken days to lap flat, wearing out several stones in the process, and put it into use making a finish ready surface on a board in a matter of 5 minutes. The back bevel in this case is really not a measurable bevel. Rather the honing guide is used just to make it easier to hold the back of the cutting edge in constant contact with the stone to prevent it from rocking on the convex surface and pulling the edge up off of the stone.

The result is similar to what would be produced by using the ruler trick, but using a much thicker ruler than usual. In the case of this iron, the back bevel is much wider than what would normally be seen with the ruler trick as well because I was intentionally making a very shallow back bevel. Just like the primary bevel, this angle can easily be reestablished by using a setup block to set the projection of the iron from the honing guide if you want. I won’t bother as I’ll just strop the back freehand in the future, but again, if you’re not comfortable honing freehand, making a back bevel setup block is a perfectly acceptable alternative.

I’m sure if the sharpening Nazis out there ever read this they will verbally flog me for ruining the iron. However, I don’t see it that way. From my perspective, I took a tool that served no purpose other than to decorate the rafters of a Cracker Barrell and made it useful once again. In my mind, the best treatment that you can give an old tool is to make it useful once again.