Monday, May 20, 2013

Skew Chisel Sharpening- The Critical Geometry

Do you love to turn but have a panic attack when you pick up the skew chisel? I think this is the most feared tool for a new turner. They buy a set of tools and start turning. Everything is so so with a few successes and a few mishaps. Then they go to sharpen the tools. Doing this for each tool is a bit confusing for the beginner to say the least. There are a great many sources out there nowadays to get you going in the right direction... except maybe when it comes to the skew.

Why do I say that? To me it is mind boggling that with all the discussion about the different angles to sharpen the edge at in relation to this and that and blah blah blah... The most critical part is always left out. DON'T ROUND OVER THE CUTTING EDGE. Maybe I should say that louder. 

What I mean is that it doesn't matter squat what all that other stuff is if your cutting edge is dubbed over. That is to say that their is not a straight line between the bevel and the cutting edge. It must not be sharpened with a rounded edge like you might see on a carving tool. Why? The skew is a simple tool to cut with on the lathe and requires one key to its successful use and that is you must have the bevel in contact with the turning to support the cutting edge while cutting. If you don't then the tool will catch. 

If your edge is rounded over then when you place the bevel against the turning the cutting edge will not be able to make contact with the turning forcing you to lift off the bevel to engage the cutting edge. When you do this you WILL catch. 

Often a related problem to this is not having a rounded edge from improper sharpening but simply a dull edge. A dull edge is another way of saying that your edge is rounded over. Keep your edge sharp and avoid catches

Also it is important to create a smooth arch to the edge. Not one that has transitions from one area to the next which will make problem prone areas for catching since you will tend to rock from one area of the edge to another while trying to keep the edge cutting. This rocking or shifting that you do while trying to keep the tool cutting lends itself to making you lift off the bevel and catch. 

So here are a few photos of what your edge should look like. Don't focus on the angles of anything but look at how the grooves left by the grinder are smooth and don't transition but create one smooth arch across the bevel. 

Notice that when everything is in one plane the light reflects off the edge consistentaly.

One way to help keep your grind smooth is to "bell" or round the face of your grinding wheel and ride across that high spot. Notice the darker area on the wheel in the background. It is the high shot. I am not trying to sharpen across the whole face of the wheel which would give me way too much area to worry about controlling. This gives me a small area to concentrate on while sharpening. I have no use for a 1" wheel as you might guess.

Now with a nice bevel, it is on to a few passes over my 8000 grit water stone and I will have a gleaming edge. Sounds easy. This is the part where you can round over the edge quickly. It is imperative that you keep the bevel against the stone while honing the edge. Firm (but not grinding) pressure should be applied while evenly and smoothly sharpening from heel to toe and back. 

I admit that is not a great description of how to actually do the honing but fortunately someone (Pete Galbert) made a video a good while back that demonstrates this part quite well. More people should be watching it but unfortunately they are probably learning how to do this from a woodworking writer not a professional maker. (The cynic in me coming out.)

Finally, notice again, that the reflection of light is even across its edge. Once you have removed the grinding wheel marks I stop here and go the lathe. I will return every turning to make a few passes to keep me sharp and catch free.

By the way it is nice to have a bench stone holder like the one pictured to have it up nice and high so you can hone with more control. 

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