I have a brushmower that has a 30" long blade. Only the outer 4.5" is shapened on both side. Is there a jig I should use to get the generally advised 30 degree bevel? Or freehand sharpen it.
Freehand is best for lawnmower blades.
I use the knife sharpening jig (SVM-45). I set the angle to 22.5 degrees. I use the same technique as I'd use to sharpen a wood chisel, with some modifications. The first thing I do is clean the blade using a wire brush. Just like a wood chisel, there's a side with a bevel and a flat side. Keeping the flat side flat is essential. I take sandpaper and clean the portion of the flat side that's near the cutting edge. If necessary, I use a file on the flat side to make sure it's flat, removing any damage that may have been caused by the mower blade hitting something. Then, using the SVM-45 jig I sharpen the bevel side using the same technique that I'd use to sharpen a chisel, with the following modification. I take the blade over to my bench grider and remove some of the steel on the bevel, but I'm careful to stay away from the region of the bevel that's close to the edge. In other words, I remove some of the steel from the heel of the bevel. This saves wear on the Tormek grinding wheel. I move back and forth from the bench grinder to the Tormek machine, inspecting the edge each time. Finally, I finish it off on the Tormek machine.
Now comes the tricky part. It's hard to remove the burr with the honing wheel, so I developed a new technique. I use a small sharpening stone, like the type you might use to sharpen a pocket knife. I clamp the mower blade in my bench vise, and with a little oil as a lubricant, I gently polish the bevel, and the flat side near the bevel. This works quite well.
And now, be careful. You've got a very dangerous instrument in your hand, capable of doing some real damage to your skin. I got a nice slice in my finger to remind me that I need to be careful around sharp mower blades.
Thanks. This is a tremendous help.