Extended Reach Tool Geometry

Legend:

• D - diameter, tip diameter, cut diameter, flute diameter - the maximum width you can cut in a single pass
• LOC - Length Of Cut, Effective Flute Length - the maximum depth that you can cut in a single pass
• NECK - a section of the shank that has been ground down to a diameter less than the flute diameters
• OAL - OverAll Length - the total length of the bit
• REACH - (LOC + NECK) - the maximum depth you can cut using multiple passes
• S - shank diameter - the diameter of the unground portion of the tool. The section that inserts into the chuck or collet

Why bother?

The flutes of a cutting tool are formed by grinding away material from a cylindrical blank to form one or more cutting edges. To make the flutes deeper or longer, you need to remove more material. Deeper flutes accommodate higher chiploads allowing higher feed rates. Longer flutes allow you to cut deeper in a single pass. However, as you grind away more material to increase capacity, or to make the flutes longer, you make the tool weaker and less able to withstand the side loads developed when trying to cut at high chiploads.

As a result, there is always a balancing act between flute-length, flute-capacity and strength. For deep-cutting (depth-to-width ratio > 6), two of the most common techniques are to:

• use a bit with flutes long enough to cut as deep as needed
• use a bit with shorter flutes and a reduced-diameter "neck" to make multiple passes to reach the desired depth

The advantage of the first approach is that you can often make your cut in a single pass. The advantage of the second is that, by working with a much stronger tool, you can make multiple passes, at higher chiploads, that might actually get the job done faster and produce a higher quality cut. Like I said, a balancing act.