1. How is a high-performance drill different from a general-purpose one?

Brian Hamil, Kyocera SGS Precision Tools Inc. kyocera-sgstool.com

Designed for specific materials, unique drill point geometries are ground into the drill to make it self-center without centering or spotting operations. This helps to reduce costs by eliminating tools and reducing cycle times. General-purpose geometries are not material specific, and while their point geometries are robust, they are not designed to take advantage of a given material’s machining characteristics. Along with unique point geometries, high-performance drills feature material-specific coatings and edge treatments that further enhance performance and tool life.

2. Besides point geometry, are other material-specific designs needed?

All features of the drill’s design need to be considered – number of flutes, shape of the flutes, web or core diameter, margin size, or width – all these features influence chip formation and flow in the hole-making operation.

3. How do the number of flutes, lands/margins impact drill performance?

General-purpose drills tend to produce holes with a lobing or triangular tool path, as they do not have high-performance geometry. They produce holes that are out of round and not the desired size. By designing special point geometry, along with flute and margin considerations, a drill produces much more accurate holes. Multiple flutes and margins help stabilize the drill, so it follows the intended tool path more accurately.

4. Why are some drills coolant-through and some not?

Coolant-through is mainly used to get coolant into deep holes and aid chip evacuation. This eliminates the need for peck drill cycles which increase cycle times and risk damaging carbide drills. In many cases, shorter length-to-diameter tools, such as 3xD, do not require coolant-through. In cases where coolant-through is not required, non-coolant-through tools are less expensive and will run just as effectively. In longer or deeper drill applications, coolant-through drills, especially when the machine tool has high-pressure coolant through the spindle, can greatly enhance productivity of the hole-making operation.

5. Why are some drills made with nominal plus tolerance and others are made with nominal minus tolerance?

Drill manufacturers produce drills to an industry standard of their choosing: ISO, ANSI, DIN, or JIS. Diameter tolerances are spelled out differently in each standard, which must be taken into consideration when selecting a drill. Is the drill being used as a pilot drill for a deep-hole application? If so, then the pilot hole should be produced with a drill that has plus tolerance, and the following deep drill should have minus tolerance to avoid interference. Drills do not produce holes as oversized as in the past, due to holders and machine spindles that run with lower total indicated runout (TIR). This impacts the hole size produced, so drills with plus tolerances make up this difference.