Monday, December 7, 2015

Sharpening Chisels

Hopefully this is a post that I can write with razor sharp wit... okay bad start.

For those (probably few) who are interested in how to prep a chisel from out-of-the-box-new to wood slicing sharpness, read on. For the rest (most) of you, feel free to just ignore this one.

In what I can describe as only "frustrating" and "excitement blunting," (I'm on a roll, but honestly running out of material fast) new chisels are not sharp. Yes, this is annoying and somewhat surprising. You don't buy a table saw blade and then go off to sharpen it before it's ready to use, but alas, chisels require sharpening.

Look, its my fingerprint! But mostly, focus on the visible ridges of steel.
I tried to zoom that picture in, but it might benefit from getting clicked on and embiggened. That is a chisel before any sharpening. If you can, you can see repetitive parallel lines that are the remains of the chisel being forged. Among the factors that make something "sharp" is our (no one's) old friend the pressure formula, which is an inverted relationship between surface area and pressure, i.e. the smaller the surface area that you press on, the bigger the pressure. That relationship is squared too, so, cut the surface area in half and you quadruple the pressure. ("Imagine your foot beneath an angry woman's stiletto heel; hurts more than getting stepped on by a regular shoe right?" Jon's high school physics teacher, Mr. Balzer, said, which is a lesson he weirdly remembers a decade later). Anyway, back to the forging lines, they make for unevenness, and adversely affect that pressure formula.

Not sharp.
Those ridges do something else we don't want too: They make for an uneven surface. The bottom of a chisel rides along the material being cut, the smoother it is, the more control you have and the more precisely *flat* your chiseling. Flat is important. Really important. Especially when it comes to hand cutting mortise and tenons or, God-willing, dovetails. You need all the help you can freakin' get.

To the rescue: um... a rock.
So in steps the sharpening mechanism of your choice. I opt for Japanese waterstones for a variety of reasons including 1. I'm a sucker. 2. I'm weirdly fascinated with Japanese furniture making techniques, and 3. They work really well, albeit slowly.

Don't mind the fingernail. I was polishing shoes before polishing chisels; it was a polishing kind of day.
So basically here's the procedure: 1. Acquire waterstones in a various set of grits, like sandpaper. 2. Soak waterstones. 3. Rub chisels on waterstones from roughest grit to finest until polished.

For more detail:

I used the following grits: 250, 800, 1000, 4000, 6000.

First I took the 250 grit stone and worked the bottom of the chisel, attempting to grind the forging marks away. Doing this is to "lap" the chisel. This took, by far, the longest amount of time. Looking at the picture, basically I worked the chisel across the stone forward and back, left-to-right, right-to-left, and back, rotating the stone periodically, and moving the chisel off the edges of the stone. The goal is to wear the stone away evenly (which helps make your "flat" chisel actually flat instead of wavy, or the technical term of "caddywompus") It left something like:

There are scratches instead of ridges
Once the forging marks are gone, then I progressed through the grits all the way to 6000. The goal is a mirror shine. This is an objective that is much easier to reach than it is to photograph, so, uh, I don't really have a picture to show you.

This'll help, sort of.
This is a mortising chisel that I lapped. The cutting edge is off frame to the right. You can see a line where I stopped polishing. It's the best I've got on the photography front. Cut me some slack, cameras are trickier than power tools.

The more dangerous side/end.
After the chisel is lapped, then it's time to get after it as to the operative side. Basically, the procedure is the same, only you have to hold the chisel at an angle to the stone that matches the angle of the bevel on the chisel. Luckily, there are guides for that. I used the Veritas Mk II Honing Guide, which helps hold the chisel at the appropriate angle. I strongly urge you to use something like this.

The last step is to polish away the burr that forms from the polishing process. Basically, the process of polishing leaves a rough, flat, dull, hook of steel on the side opposite from the side you just polished (i.e. if the last thing you polished was the bevel, then the burr is on the bottom of your chisel). Remember our (no one's) friend the pressure formula. Yep, that'll make your chisel dull, which is, just in case it wasn't obvious, not the objective of "sharpening" your chisels.

To clear off the burr you take out the finest stone you've got and just briefly polish the surface again. So, for me, I took the 6000 grit stone out and just worked the bottom of the chisel back and forth one or two passes and it was good to go.

If you can zoom in you can see there's no burr.
I was taught that a good rule of thumb for sharpness is to place the blade against your fingernail with minimal pressure and a half an inch off the table. If it falls down, then it's not sharp enough. If it stays put, then it's sharp.

Lastly, and this is something I've never really seen anyone else do, but I polish the sides of my chisels. The way I see it is anything that is going to slide along the material you're cutting benefits from being polished. It also could be that I was polishing my chisels watching sci-fi and western movies and drinking beer while my fiance was away, so why not go all out?

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