Sunday, June 17, 2012

Sharpening: Extra Coarse

I learned this trick from Mark Duginske a couple of years ago: drywall mesh is great for flattening plane soles. It's also great for flattening water stones. So I wondered to myself: would it also be good as the very coarsest sandpaper in the “Scary Sharp” method? Turns out the answer is “kinda, sorta.”

I'll backtrack a little in case any of this is new to you. Here's a picture of drywall mesh:

You can find it in the big box home stores in the same area as the drywall tools, tape, and joint compound. It is coarse, and the mesh structure lets sanding dust (or iron filings) fall through so the mesh can keep cutting. In drywalling, it's used after you've taped and mudded, to quickly get the surface ready for paint. As you can see from the photo, it's die cut into a tabbed shape to fit a special holder with a handle.

Scary Sharp” is a phrase used to describe a method of sharpening, using sandpaper as the abrasive instead of oil stones or water stones. I think I may have been a witness to the origin of the phrase, which as far as I can tell was on the rec.woodworking group on usenet in the early 90's.

Call it Scary Sharp, or just call it sharpening with sandpaper, it's the cheapest way to start sharpening, and the most expensive way to continue. I recommend it to beginners, and I have coached hundreds of woodworkers to get their first sharp edge with it in my sharpening classes at Highland Woodworking. All you need is a flat surface to glue the abrasives to and you're ready. As I wrote this post, I looked around the web for a good link I could send you to for an introduction. I don't like any I found, so I'll be writing an introduction to Scary Sharp Jim's Way later today, after my bike ride.

Back to today's story. I tried this out on a Harbor Freight chisel that was used for the job it's best at, opening up some paint cans and cutting through some nails. As you can see from this photo, the edge has become roughly serrated:

First thing to do is flatten the back. Here I am flattening the back,

and here's what the back looks like after no more than 64 strokes (I weigh 185 lbs, so I put a good amount of pressure on it):

I like this photo, because you can see how the iron filings fall into the spaces between strands in the mesh.

Next, I put the chisel in a $10 honing jig from Highland Woodworking and started working on the bevel. Here's what 32 strokes along the length of the drywall mesh did to it:

Notice all the iron filings sticking to the edges like a ferric halo! Can someone explain to me why the chisel is acting mildly magnetic? But also notice, the serrations are already almost gone from the edge.

And furthermore, notice the lighter-colored areas on the abrasive surface:

These areas no longer cut, nor do they feel rough. So I assume the abrasive is absent from these areas, and I assume further that putting high pressure on a small area of this abrasive strips it off either mechanically or because of the high temperature - - this thing got HOT as I worked it, far hotter than drywall would ever get.

By the time I stripped all the abrasive off all the mesh, though, the bevel was ready for finer grits. At least I thought it was. The box of 10 mesh pads cost about $8, so I used 80 cents' worth of abrasives to prepare the chisel. NOTE: 3M also makes an 80-grit silicon carbide sandpaper the same size as the mesh, for the same price. It withstands the bevel-grinding process FAR better than the mesh does.

I quickly ran the chisel through the grits of silicon carbide wet/dry sandpaper using water as a lubricant: 180, 220, 400, 600, 800, 1200, 1500, and 2000 grit. It sounds like a lot, but it took less than 5 minutes. Just 32 strokes at each grit, and I was done! (Look for a separate post on this blog later today - -I'll show you how.)

As a test, I tried cutting the end grain of some white birch I had lying around:

Ooops! I thought I had eliminated the last little serration from the edge, but I guess I missed it. And now you all know about it. If you've ever read Franklin's autobiography, you're probably thinking of a speckled axe right now. Anyway, as you can see, the surface left behind is 99% fantastic. I wish you could touch it: it's silky smooth. One of my criteria for a sharp edge is that it should be able to make intact shavings in end grain, and I've certainly met that criterion tonight.

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