MRO Today - Heed those feeds and speeds

Heed those speeds and feeds
Maximize twist drill life by following these guidelines

by Michael Plankey

A twist drill's life is measured by the amount of holes it makes - from the day it's first put in the spindle, through resharpening, until it is no longer useful.

Many factors affect tool life. Workpiece hardness and material variability affect the drill's abrasive resistance (wear). Other factors include coolant flow and the surface condition of the twist drill (surface treatment or coatings).

Machining parameters (speeds and feed), however, are the most important contributors to tool life.

Speeds are determined by surface feet per minute (SFM), defined as the distance in linear feet that the circumference of the drill travels in one minute. The distance is constant, but the speed (revolutions per minute, or RPM) must be adjusted based on the size of the drill to achieve the proper SFM.

A good comparison of this is car tires of different diameters traveling the same distance. The smaller-diameter tire must turn faster (higher RPM) to cover the required distance in a set time (SFM), just as a smaller-diameter drill must. The larger tire can turn slower (lower RPM) and still cover the same linear distance (SFM), just as a larger-diameter drill does. Smaller drills require more revolutions per minute to cover this set distance, and larger drills require fewer revolutions per minute.

We can determine the speed (RPM) the drill should be revolving by applying this formula:
SFM x (3.83 ÷ drill diameter) = drill RPM.

Feed, or inches per revolution (IPR), is the distance that the drill cuts into the workpiece for each revolution of the cutting tool.

The drill manufacturer usually determines this, but a standard formula used in the industry is:
Tool diameter ÷ 64 = IPR.

One of the biggest problems associated with drill speed is heat caused by friction when the drill enters the workpiece. As the drill enters the workpiece, its cutting lips, chisel edge and sometimes the drill's margin rub the workpiece, causing friction. The friction turns to heat, and if it becomes hot enough, the drill approaches the temperature at which it was hardened. When the drill cools down, the tool material is drawn back, which changes the drill's granular structure and results in reduced hardness at the cutting lips.

Reduced hardness causes a drill to wear prematurely and shortens its life. An excellent example of improper drilling parameters is putting a twist drill into a hand-held drill motor and running it as fast as the motor will run. In most cases, the drill bit will burn up on its periphery because it was run too fast for its size, and maybe fed too lightly as well.

Improper feed affects drill life in a similar manner. For example, if the drill manufacturer recommends a feed rate of .002 to .004 IPR, and the drill is fed much lighter, the chip formation becomes thin and curly. This can cause chip congestion in the drill's flutes, which can also result in premature wear, or even drill failure. When the drill is fed too lightly, the heat doesn't transfer to the chips. It stays in the workpiece, resulting in the problems mentioned earlier, and may even work to harden the drilled material.

Drills that are fed heavily tend to produce broken chips, which in most cases is beneficial because broken chips decrease congestion. However, if the drill is fed too heavily, it can overcome the amount of clearance between the drill lip and the material and produce rubbing behind the cutting lips, which can make the tool fail in the cut prematurely or even destroy the tool by splitting it.

Drilling, like any other manufacturing technique, is most efficient when done correctly. Before drilling, read the manufacturer's catalog or technical guide for speed and feed recommendations to get a starting point for machining parameters.

Michael Plankey is a technical support specialist for Kennametal-Greenfield Industrial Products, a manufacturer of cutting tools.

This article appeared in the August/September 2000 issue of MRO Today magazine. Copyright 2000.