Cutting tool sharpening
Video
When you purchase a new lathe tool bit, it might have an angle on the end, but it is not properly sharpened for turning. Grinding
lathe tool bits is a bit of an art. It takes some practice to get good at it. You need to create a cutting edge that is sharp, extends out
so that the cutting edge and not the side of the tool contacts the work, but that still has enough support to maintain sufficient strength to cut metal.
 
How to Grind Tool Bits
Use a bench grinder to sharpen your tool bits. Even an inexpensive bench grinder can do a good job grinding lathe tool bits. In some cases, you might want to purchase a higher quality fine grit wheel.

Keep a small cup of water near your grinder. Grinding generates heat, which can cause two problems. The tool bit will become too hot to hold. Overheating can also affect the heat treatment of the tool bit, leaving the cutting edge soft.

Use a protractor to measure the angles. They are not super-critical, but you should try to stay within one degree of the recommendations.
 
Grind the Front Relief
The first step in creating a tool bit is to grind the front relief. For most work, a relief angle of 10° works well.

While you are grinding the front relief, you are also creating the front cutting edge angle. Make this angle about 10° also, so that the corner formed by the front cutting edge and the side cutting edge is less than 90°.

 
Grind the Left Side Relief.
Form the left side relief next. Again, create about a 10° angle. You don’t need to form a side cutting angle. The side cutting edge can be parallel to the side of the tool blank
 
Grind the Top Rake
The top of the tool bit is ground at an angle that combines the back rake and the side rake. The side rake is most important, because the side cutting edge does most of the work. For cutting steel and aluminium, the side rake should be about 12° and the back rake should be about 8°. For cutting brass, the rake angles should be much less, or even 0°.
 
Round the Nose
A small nose radius allows you to turn into tight corners. A large nose radius produces better surface finishes. Create a nose radius that is appropriate for the tool bit you are creating.