The parting tool, which really is a uniquely-shaped chisel, gets its name from the tool's ability to remove finished parts from a spindle turning on the lathe. More commonly, though, it simply is used for cutting grooves to specific depths in a spindle turning. By turning a narrow groove square to the axis of the spindle down to a certain thickness, the woodturner can then use other tools to cut tapers or beads down to the required depth without danger of cutting too far.
Using a Parting Tool:
Using a parting tool is very similar to using any other woodturning tool safely. Your back hand will typically be used to gauge the depth and aggressiveness of the tool, while the front hand will be holding the parting tool onto the tool rest, guiding the tool into the stock. When parting or cutting grooves, the parting tool should remain square to the axis of the spindle.
As with gouges and skew chisels, always present the bevel underneath the cutting edge to the wood first. Then slowly ease the tool backwards while raising the rear hand to introduce the cutting edge to the wood.
Checking Thickness of Your Cut:
While using your parting tool, you may need to check the thickness of the stock remaining in the groove with a vernier (a type of gauge used to determine the width of an item). While you could remove the parting tool to check the thickness with the vernier, you can just as easily hold the parting tool in place with your rear hand, and check thickness from the backside (opposite the turning from the tool rest) with the vernier. Merely ease the parting tool forward onto its beveled edge inside the groove without actually cutting while using your free hand to check the thickness with the vernier.
Do not attempt to perform major cutting or remove the parting tool without first putting the vernier down and returning your front hand to the tool and the tool rest. If you attempt to cut one-handed, the tool will not be supported well, which could result in a dig-in. Similarly, if you attempt to remove the parting tool one-handed, you could accidentally dig into the wood on either side of the groove.
When parting with your parting tool, simply begin as you would cutting a groove. When you get close to the point where the part would be cut off of the remaining spindle, remove your front hand from the tool rest and reach over and to the right side of the spindle and, with your palm facing your body, cup the part being removed. Then, slowly continue cutting with the parting tool. The part should gently fall into your hand.
When parting one-handed, be very slow and methodical with your cuts, since you only have one hand controlling the parting tool. Also, for your safety, never reach under the spindle and tool rest from the front side of the lathe to cradle the part in your hand as it is being removed.
Sharpening your Parting Tool:
Your parting tool should be sharpened just like the rest of your woodturning tools. Use a slow speed wet sharpening station or a bench grinder with a tool rest set at the proper angle. Be sure to cut the entire bevel on each side of the tool evenly to create a precise cutting edge.
If you choose to use a bench grinder to sharpen your parting tool, cut in short bursts and cool the tool off in a cup of water in between cuts. Do not grind for more than a few seconds at a time, or you will possibly cause the steel in the tool to lose its temper. Steel that has lost temper will not hold a cutting edge well.
When beginning a cut with a parting tool, particularly on soft woods, the edges of the groove will tend to fray. This can be offset somewhat by making the initial cut into the wood using a scraping cut. Instead of presenting the bevel first, then easing the cutting edge into the wood (with your rear hand beneath your fore hand), try cutting from the opposite bevel.
In other words, keep your rear hand higher than your fore hand, and introduce the parting tool's upper bevel to the wood. Then, ease the tool backwards and slowly move the rear hand down, so the tool is scraping instead of digging into the wood. Once you've begun the initial cut into the spindle, you can switch to the more aggressive (and normal) bevel cutting technique. Since you've already started the cut, you should not see any further fraying of the edges of the cut.
One safety note: NEVER free-hand with a parting tool. Always hold the tool firmly against the tool rest before presenting it to the wood.