The Charlesworth ruler trick is used to speed the sharpening of plane irons by adding a very tiny back bevel to the sharpened edge. When flattening and polishing the back of the iron, it is not necessary to have the whole back polished, only the very small section right along the cutting edge.
The premise of the ruler trick is that you place a small steel rule flat on the surface of your sharpening stone. You lay this ruler along the edge of the stone and then lay your plane iron on top of that. This serves to raise the back of the iron ever so slightly and ensure that you are only sharpening the very edge. This greatly speeds plane iron sharpening and it really does work.
*** One caveat. DON’T DO THIS TO CHISELS *** (I wish I had known this when I first learned about this trick). You do not want a back bevel on your chisels. The back of a chisel is the reference surface and guides the chisel when pairing or when chopping to a knife line.
For the past two years, I have been using a modification of this idea that I think is a significant improvement. I never liked the idea of rubbing the steel plane iron along a steel rule. It’s rubbing metal on metal and the swarf (fine abrasive slurry) from the stones gets in between the metal pieces and chews them up. This ruins the steel rule and it makes it very difficult to read later. Lastly, leaving the rule on a wet stone invites rust. The rule becomes just one more thing to clean up and wipe down when you finish sharpening.
So what you say, small 6-inch rules are cheap, so why not sacrifice one to this task? Fair enough, but when I came up with this modification idea, I only had one small steel rule and didn’t want to ruin it. I got to thinking about it and decided that High Density Polyethylene (HDPE) would be a far better choice. HDPE is the material that most “plastic” cutting boards are made from and is most commonly found as plastic bottles, bottle caps, and milk jugs. You also see it elsewhere in the woodworking world on tool fences as it is a slippery, low-friction surface.
I wanted a small piece of HDPE that was flat and about the same size as the steel rule that it was being substituted for. A milk jug was the first choice, but the standard American milk jug doesn’t have a flat spot anywhere on it. Then it occurred to me that we occasionally buy milk at Costco and they have the newer style Square Milk Jug. I think that Walmart uses this same kind of jug. This style milk jug has been around for less than ten years, and though it was designed for space saving during shipping, has the added benefit of some flat spots on the sides. It’s perfect for what I need. I washed out an empty jug and got started.
Only two of the four sides have areas that are flat enough for this purpose.
Using a scalpel, I cut the two sides out larger than they need to be.
I then trimmed the strips of HDPE down to the size of the rule.
Here’s how it’s used on the stone.
The HDPE strip is low friction and wont ever rust. If you leave it on your wet stone, so what? I think that this is way better than using the steel rule. It even puts a smaller back bevel on the plane iron (more on that below).
It you just want to use this trick, read no further. It will work and you’ll be happy with it. If however, you want to geek-out with me on the nerdier parts of this trick (that in reality have no bearing on use in the shop), read on.
****** Wood Nerd Alert *******
Ok, lets delve into the math. With the HDPE strip placed in the correct position on the stone, I measured from the inside edge of the strip to the part of the stone where the cutting edge of the iron will be making contact and getting sharpened. This distance was 2-inches. I got out my calipers and found that the steel rule that I used was 0.032-inches thick and that the HDPE strip was 0.020-inches thick.
Time for some geometry and trigonometry. On a side note… I distinctly recall asking my trigonometry professor when I was ever going to need this stuff in real life, and he told me, “Never, but you’ll need it to pass college.” He lied!!!
The diagrams below are not to scale, but illustrate the point.
Using the steel rule on top of your sharpening stone creates a back bevel of 0.917°.
Using the HDPE milk jug strip creates a back bevel of 0.573°.
I told you this was going to get nerdy, but you had to read it…
The HDPE milk jug strip definitively creates a smaller back bevel. People are free to argue over whether a smaller bevel is better or not, though I don’t think that this will ever grow into a pins first vs. tails first type of argument. (
It’s tails first, obviously.)
My personal opinion? I think that the back bevel serves a function of eliminating the need to polish the entire back of the plane iron. The smallest bevel that can accomplish this (and not change the other geometry of the plane) the better. In actual practice, I doubt that losing the approximately ½° is a big enough reason to change from a steel rule to the HDPE strip. However, the low friction surface and the fact that it will never rust definitely are reason to switch.
David Charlesworth can have his ruler trick. I’m naming this one the “White Milk Jug Trick”. Sadly, it just doesn’t have the same ring to it. Darn my unexciting last name and its already obvious connection to milk.
I hope you liked this. Now go butcher some milk jugs.
– Jonathan White