I just thought I would share this tutorial on sharpening which I have put together. I had a full set of chisels to sharpen and thought I would make it a bit more interesting by taking pictures as I go. I'm sure some of the newer people to instrument building will find it useful a long with anyone who perhaps hasn't fully utilised these wonderfully versatile tools in their work. I'm sharpening chisels in the pictures, but the process is pretty much the same for plane and spoke-shave irons. Chisels
As we are working on chisels here I'll go into a bit of detail on them. The set you will see below is a vintage set made by Marples, Sheffield dating to probably around 100 years ago. I would advise anyone who is seriously interested in instrument building to buy vintage rather than new, there was a quality to the steel back then which doesn't seem to be matched these days. I'm sure if you were going to spend a lot of money you could get something of similar quality, but the cheaper brands just don't compare. Another great thing about vintage ones is that they are cheap to purchase and once you have done they will last you your life. Some makers of chisels to look out for are Sorby, Marples and Ward. Marples are in fact still going, but the quality is not the same. Ok, lets begin!
Here are the very few tools you will need: An Oil Stone, a Honing Guide and some Cutting Oil....plus whatever you will be sharpening!
Firstly you will need to make sure your Oil Stone is flat, if it is not as this one pictured here isn't, you will need to lap it flat. This can be done over a coarse grit paper attached to a flat surface, the stone is then pushed over this till dead flat (keep checking with a straight edge).
Lets now take a look at the state of the chisels we are going to be working on, or more specifically the bevel. The bevel is the sloped cutting edge, this can vary from 25 degrees to up to 45. Shallower angles are better for shaving while more steep ones for chopping type jobs. In guitar building the latter is rare, so we are going for a 25 degree bevel. The chisels currently have around a 30 degree bevel, so there is going to be quite a bit more work here than simply honing the cutting edge...we need to create a new bevel (or rather re-shape it). As you can see the blade is in good condition with no chips to the edge, if chips were present we would need to re grind it completely on a bench grinder. Another great thing about chisels...they're pretty much indestructible as you can always grind a new cutting edge!
Now lets look at the honing guide, this is the device that holds the chisel at a fixed angle. You will see the different angles mentioned on the guide with a measurment next to it. This is how far the blade needs to project in front of the guide, for a 25 degree bevel we need 25 mm of blade in front of the guide (as pictured).
Here is the guide in action, the oil stone has been wet with cutting oil and the guide with chisel attached is pushed and pulled backwards and forwards. At first in both direction (back and forth) until we get close to the finish, when it is only pulled along the stone.
You need to be careful to distribute pressure across the guide or you will end up like the picture below (see how the right edge is higher than the left?), this is no big deal, it just means you need to put more pressure on the other corner until it evens out. Don't think you can just put even pressure on it and get it right...you cant! You need to keep checking your progress and putting pressure on different points. You should be able to see what we are aiming for now, we want that silver line close to the edge to come all the way down to the cutting edge.
Here you can see we are nearly there, you can just see a tiny silver line on the right edge. This shiny part has yet to be touched by the stone, all the previous grinding has just been grinding the bevel. So we keep going until it has vanished!
And here we have reached the cutting edge, this is how it should look before progressing onto the finer side of the stone. After a few strokes over the fine side you should be noticing a burr on the back edge (flat side) of the chisel, you want this to be quite pronounced before moving on. Feel for the burr with your thumb nail, you will notice it catch and take a tiny part of your nail with it!
Now we have to remove the burr we have just created, this is done by rubbing the back side along the stone like the picture below. Once you no longer feel a burr on this side, check the other side, it will almost certainly be there having bent over from the flat side, so a couple more passes on this side over the fine stone. Repeat this till there is no burr on either side.
How do you know when you are done and your chisel is sharp?...When it does this! All that hair you see is from my arm, a sharp chisel should be able to shave the hairs off your arm easily. If it does not, repeat the last couple of steps (creating the burr and removing the burr) until it does.
The final step is a process called stropping, this gives the blade a bit of polish and sharpens even further. Its basically rubbing it up and down a piece of smooth leather as you may have seen people do with straight razors on TV.
And here are the chisels with their new bevel all done and ready for the next job.
Thank you for reading this, I hope its clear enough and it helps someone.