Tuesday, March 6, 2007

More on Sharpening Turning Tools

I had a few questions from a reader that coincided with some info that I wanted to focus on soon anyway, so here goes. Sharpening lathe tools is a different game than most other tools. A few years ago when I taught a turning class with Dave Hout at John C. Campbell, I was shocked to see Dave walk in with his bag of turning tools, all banging around and then dump them out on the table. Dave is a turner, a real turner and I came to learn that turners go straight from the grinder to the lathe, so why bother babying your tools if you're going to grind them anyway!
I should say that I am a turner by necessity and have worked mainly to meet my needs at the lathe as a chairmaker. There are certainly those who know a great deal about turning, like Dave Hout and many have examined the issues more deeply than myself. My goals at the lathe are simply to make the most fluid turnings, as fast and perfectly as I can. I also turn without sanding which is what forces me to sharpen somewhat differently than some turners.
As I stated before, lathe tools need to be sharpened often and have the correct geometry. Correct geometry in this case means that there is no rounding over of the edge. At worst, the bevel can be flat (like a chisel), but if it rounds over, it becomes harder to engage the wood. So the hollow ground left by the grinder gives the best results because the actual edge is so easily introduced to the wood, giving control. It isn't that the edge is sharper, it's the geometry. Under magnification, any edge is seen to be made up of tiny teeth. The grit of the abrasive determines the size of the teeth, grinder (large) through to 10,000 grit stone (fine). The large teeth left by the grinder will break off and dull more easily, which is no problem because of the ease of sharpening on the grinder. The smaller teeth left by honing will leave a smoother surface and stay actually sharper but as soon as they do dull, the edge will gently round and make cutting more difficult. If you can accept the slightly coarser surface left by a tool that has only met the grinder, it is a fine way to always have the best geometry. My experience with this is limited to my parting tool (which I grind and then push into a softwood block to knock off the burr)
My solution to the problem is to grind often and hone minimally. I allow the smallest of flat to appear (see the photo of the gouge) and am quick to grind it away and hone again. Yes, you burn through the steel quicker but the turnings and the ease you will have making them will make up for it. I return to my finest stones often. Sometimes, you will find that the actual edge and geometry are correct, but those tiny teeth have bent over in use. A quick touch up on a fine stone will point the teeth in the correct direction and compress the metal making it harder. I return my skew to the stone after every leg and take a few strokes. The extent of my ability as a turner is limited by the condition of my tools, and I have seen the same in my students. The glassy finish and control are worth spending as much time sharpening as turning. The cruel joke of turning is that as you are learning, you will use the tools for much longer to achieve good results, meaning they are dulled more often in the process. I use my skew for only a few moments per leg, and when the leg is done, return it to the stone. But like I said, I'm a chairmaker and hope I haven't made any of you turners out there cringe!


Greg said...

Thanks Peter, that was really helpful.

Can you clarify one point, I think there was a typo. As written you made two statements regarding frequency of sharpening the skew that seem to conflict:

"I return my skew to the stone after every leg and take a few strokes."

"I use my skew for only a few moments per leg and then return it to the stone."

Peter Galbert said...

Sorry for the confusion, When I say "I use my skew for only a few moments per leg and then return it to the stone." I mean that the entire usage of the skew is limited to very little, just a cleanup pass on all convex surfaces. But this is enough to send me back to the stones when each leg is done. I hope this clarifies and will edit the post to reflect,