Thread: Chisel sharpening

I am no expert in sharpening, but I've learnt a few things lately that I want to share from a beginner's perspective. If I had have known these few things when I first started out rather than thinking sharpening chisels and plane blades was some kind of dark art, I would have been much better off. I have been sharpening my tools using scary sharp for a couple of years but I have recently converted to a bench grinder with an AlOx wheel, a 1200 grit stone and a 6000 grit stone and I find this far superior.
You have to know what the characteristics of a razor sharp edge are so you can set out purposefully to acheive those, otherwise you'll just be waving an chisel over a stone and hoping that all things conspire to give you a sharp edge(which they may do).
So here it is: Two intersecting, optically flat, reflective planes(flat surfaces) is the all you need to acheive a razor sharp edge, simple as that. One plane is formed by the front bevel of the chisel, the other by the back of the chisel. A 30º angle between the back and the bevel will suffice for a chisel.
Why optically flat? It indicates that you indeed have a flat surface and it is the easiest to replicate time upon time. It eliminates all variables- the two surfaces must come together at a fine razor sharp edge.
Why reflective?This helps the durability of the edge. Where the two surfaces meet, any imperfections on the surfaces (like scratches from lower grits) will project off the edge as small serations which although sharp, are weak and will quickly break off and dull the edge. A reflective surface indicates that these imperfections are very small resulting in a more durable edge.
Because the edge is a product of 2 flat surfaces, you have to work on both surfaces, it is no good going to 6000grit on the bevel and neglecting the back. The back must be flat, must be reflective and should be honed to the same grit as the bevel. I noticed the back of many of my chisels curved upwards ever so gently to the edge which I believe is one of my limitations using the scary sharp method. The paper tends to bunch up so slightly ahead of the edge or the paper sinks into the adhesive bed and very slightly rounds the surface (only noticable if you look at the distortion of relfections in the chisel) I could get the chisels quite sharp still, but it made paring more difficult(the cutting edge is slightly higher than the flat of the back).

Why I need a bench grinder - TIME. The hollow grind at ~25º means you do faster resharpens of the 30º bevel because there is less metal to take off than if you hone a flat 25º bevel (which I have done). Get an ordinary bench grinder with the white AlOx wheel or as I have done a half speed grinder to decrease the possibilty of blueing. You don't need to buy a fancy water wheel grinder - send me the money! Make up a litte jig out of two little pieces of ply held together by a couple of bolts and wing nuts. Sandwich the chisel in the jig and tighten it square and projecting the correct amount to cut a 25º bevel when it slides back and forth on the tool rest. When you have set up the correct projection on a chisel, record that length somehow (another small jig such as a piece of timber fixed the correct distance from an edge allows you to quickly set up the chisel in the jig).

So here's what I'd do to get a new chisel to a working state. Firstly flatten the back on a course flat stone or a diamond stone, then hone with 1200 grit and then with 6000grit stone to a reflective state. Then set it up in the bench grinder jig and grind a bevel at ~25º if you need to. When the sparks fly over the chisel you've gone as far as you need with the grinder. Set the chisel up in a chisel sharpening jig like the Stanley one for a 30º bevel. Some people are against these jigs I know, but it doesn't take long to set it up and in the beginning it guarantees reproducible results and you want to eliminate frustration right? Run the chisel and jig back and forth over the 1200 grit stone say 15-20 times to give you a small bevel. Then move to the 6000 grit stone and give it say 20-30 strokes, but you want to finish up with a mirror finish so you might need a little more, a little less. Look at the bevel, it should be flat, reflective right to the edge of the chisel and there should be a "wire" of bur curled over the back edge that you'll feel. Take the chisel out of the jig and hone the back on the 6000grit to remove this bur. You're done. When you need to resharpen, just fit it to the jig and repeat the honing on the stones (only takes a minute or two for each chisel). Eventually the 30º bevel will "grow" longer to a point where resharpening is slowed because of the amount of extra metal to be taken off. Only at this time do you need to redo the 25º hollow grind on the bench grinder. Keep an eye on the back, only if some imperfections have reached the edge do you need to reflatten the back, otherwise just the touchup with the 6000grit will suffice. Now I can consistently and quickly sharpen chisels to where they take the hair off without any sensation, it might be time to invest in my first decent set of Japanese chisels and perhaps learn more about the intricacies of angles and back bevels and microbevels and... nah. I hope this helps somebody to sharpen with consistent results.

Cheers
Michael