Turning with Sharp Tools - Parting Tools
Last month, we continued our series of tutorials which are covering the proper sharpening of lathe tools. In the introduction tutorial in that newsletter, we covered the proper ways of identifying dull lathe tools. This month, we're going to explain the proper method for sharpening parting tools. Next month, we will cover roughing gouges.
Different Types of Parting Tools -
There are a variety of different parting tools on the market today: flat parting tools, diamond parting tools, tapered parting tools, fluted parting tools, and beading tools. All can be sharpened in a similar manner, as described below.
When to Sharpen -
Parting tools need more maintenance than many other tools, but usually only require a quick touch up with a sharpening stone or diamond hone, rather than a full regrinding of the tool. Sharpening should be performed when excessive pressure is required when parting a block (which can cause the wood to flex or break while being cut).
Correct Sharpening Angle -
The length of the bevel should be one and a half times the width of the parting tool, as shown in the photo at right below. For example: if your parting tool is 1" wide, then the length of your bevel should be approximately 1.5" long.
Grinding Methods for Parting Tools -
Parting tools can be ground in a number of ways. Let's look at the most common sharpening methods, and how each method can affect the tool's performance.
- Sharpening with a Grinding Wheel
Grinding wheels will produce a slightly concave (hollow ground) surface on each side of a parting tool, due to the rounded shape of the grinding stone. This concave surface causes two potential problems: the turner may have a more difficult time maintaining contact between the bevel of the tool and the wood being turned, and the cutting point can become dulled more quickly. It is highly recommended that a parting tool that is sharpened on the grinding wheel be honed afterwards to produce a more flattened surface along the bevel. This can be accomplished with a belt sander, or by using a hand held file, as detailed below.
- Sharpening with a Belt Sander
A belt sander is an excellent alternative to using a grinding wheel for sharpening the parting tool. A belt sander produces a flat ground surface on each side of the skew, which is ideal when using this tool. The flattened bevel allows the user to more easily keep the tool's bevel in contact with the wood while turning, and will allow the tool to maintain a sharper edge for a longer period of time between sharpenings. Be sure to maintain contact with the cutting edge and heel of the bevel against the sanding belt at all times, and keep the front edge of the parting tool parallel to the direction of travel of the belt to maintain the correct angle at the front cutting edge of the tool.
- Hand Sharpening
Most turners prefer to hand sharpen their parting tools unless they are incorrectly sharpened, or have become damaged (nicked or blunted from using the tool for scraping cuts). Incorrectly sharpened or damaged tools will require more aggressive reshaping, and should be done with a grinder or belt sander. A small diamond file or sharpening stone can be used for this process. Simply maintain contact with the point and heel of the bevel against the stone, and make several light passes on each side. This is typically all that is required to maintain a very sharp edge on a parting tool.
Specialty Parting Tool Grinds -
There are some alternative methods for grinding parting tools, which can manipulate its performance under certain conditions. These modifications are done to to the shape of the cutting edge that is in contact with the wood while parting.
- Notched Tip
A v-shaped notch can be cut into the cutting edge of a flat pointed tool to create a fluted parting tool. This notch creates two cutting points which may reduce pressure needed for parting and/or create a more cleanly cut surface. This notch can be produced by grinding the tip of the parting tool against the corner of your grinding wheel (be sure to use a freshly surfaced wheel to produce a clean, sharply cornered grind).
- Rounded Tip
The corners of the parting tool's bevel can be rounded over slightly (only a very light grind or honing is required here) to reduce the likelihood of catching when doing deeper parting cuts.