A marking knife needs to be sharp in the same way as any other lathe tool, and a good habit is to hone it to fine sharpness after every job.
A chisel ground knife – which is ground for either left-handed or right-handed use – is one of three types of marking knife. Sharpen it by holding the blade, bevel side down, flat on the stone with the cutting edge lying along the middle. Use the first and second fingers of the free hand to apply pressure on the blade, then twist the blade past the grinding bevel (20-25 degrees) to the cutting angle of between 25 degrees and 30 degrees.
Maintain this angle, keeping an even pressure on the upstroke and reducing on the backstroke until a burr or wire is formed on the blade. Then turn the knife over, hold it flat against the stone and stroke off the wire in two or three goes. Repeat the operation on successive stones to obtain a really fine cut.
A hollow ground knife, another type, is sharpened in much the same way but as the blade is ground on two sides, it is easier to have the knife at rightangles to the stone.
A variation is the leaf ground knife, which has a curved blade that requires a very different sharpening technique. Hold the knife at a tilt of 25 degrees to 30 degrees, near the far end of the stone, and repeatedly draw the blade towards you, at the same time raising the handle so that the whole curve of the blade is brought into contact with the stone. As you push the blade away, simply reverse the pulling action to cover the arc you have just covered.
All the time keep a firm and even pressure on the knife using the first and second fingers of the spare hand. To remove the wire, run the bladeâÂÂs reverse edge at rightangles to, and along, the middle of the stone.
