The Hole Edge: Drilling Performance
Machining a clean, straight and in-tolerance hole through a part is not the only requirement for many holemaking applications.
Courtesy of EXACT
Countersinking tools from EXACT GmbH & Co. KG Präzisionswerkzeuge, Remscheid, Germany. For more information, contact the company. Telephone: +49 2191.36250-0. E-mail: info@exact.info. Web: www.exact.info.
Countersinking holes to increase part functionality.
Machining a clean, straight and in-tolerance hole through a part is not the only requirement for many holemaking applications. In many cases, hole entrances and exits are also critical to part functionality. There are many reasons that hole edges must be further refined, including hiding screw heads, minimizing air and liquid turbulence, improving compressor efficiency and increasing fatigue strength.
Three main processes are used to improve hole entrances and exits—countersinking, radiusing and squaring off. Tapering and other special-shape processes may also be needed in some applications. This article concentrates mainly on countersinking.
Figure 1 (below) illustrates the most common edge conditions. Burrs, which are allowable in many situations, are almost always found on hole entrances and exits when metal is conventionally drilled or otherwise cut (Figure 1a). Gaskets and other products require straight or tapered holes with sharp edges (Figure 1b and 1e), but most machined products require or desire countersunk (Figure 1c) or radiused (Figure 1d) holes.
Countersinking bevels or tapers the work material around the periphery of a hole to create a conical feature. The surface cut by the conical countersinking tool is concentric with and at an angle of less than 90° to the centerline of the hole.
Radiusing, or corner rounding, produces a smooth, blended or rounded edge as opposed to a cone. (See sidebar below.)
Tapered holes differ from countersunk holes only in that the length of the angle is much steeper in a tapered hole (see sidebar). Tapers serve various purposes, such as controlling fluid flow, assuring leak-proof joints, providing tight—almost press—fits and guiding long pins into tight-fitting holes. Tapered holes are more challenging to produce than countersinks because of their longer length and often tighter tolerances.
Tool Designs
My book, “Countersinking Handbook,” published by Industrial Press, illustrates 147 different cutter designs to finish hole edges. Countersinks come in six standard angles (60°, 80°, 82°, 90°, 100°, 120°) and hundreds of sizes. Countersink tools come in left- and right-hand cut, a variety of flute shapes and piloted, nonpiloted, screw-on and slip-on configurations. Some countersinks come as an integral part of the drill. In short, countersinks are almost as ubiquitous as drills themselves.
There are as many variables when finishing hole edges because applications range from printed circuit boards to titanium skins, from aerospace composites to castings, and from sealing critical surfaces to simple deburring. With the exception of the aerospace industry, there are few comparative studies about countersinking tool effectiveness and economics, and few of these studies are published outside company walls.
U.S. and German standards exist for countersink tool designs, but cover only the outer configurations of the more common tools—not the critical flute configurations, rake and relief angles, coatings and unusual designs.
Courtesy of L. Gillespie
Figure 1. Five common hole entrances: (a) burred or raised metal, (b) sharp edged, (c) countersunk, (d) radiused and (e) tapered.
While most countersunk holes are produced on CNC machines, the aircraft industry still finishes millions of holes using manual or robotic tools. These tools use a pilot to assure the countersink is concentric with the drilled hole. In addition to a pilot, aerospace manufacturers also use a pressure pad device to assure material does not crawl up the tool, or delaminate, and to produce the exact depth.
Flutes play a key role when countersinking. Large flutes enhance chip evacuation. Multiple-flute tools generally provide longer life than 1- or 2-flute tools. An odd number of flutes minimizes chatter, but an even number of flutes can also reduce chatter in some instances. Countersinks with multiple flutes cannot be applied for heavy stock removal because there is not enough open area in the flutes for effective chip removal.
The elliptical hole-style tool, often called a Weldon countersink, provides a slicing action to freely cut most workpiece materials. Unlike a multiflute tool, it produces a continuous chip. A Weldon countersink is particularly effective in softer materials because of its high-shearing cutting angles.
Courtesy of EXACT
Figure 2. Countersinks can have radial relief, axial relief and combinations of both.
Countersinks can have radial relief, axial relief or a combination of both (Figure 2). In addition, an external relief, or clearance, reduces heat from rubbing, and a cam relief allows faster feeds in aircraft materials.
Typical coatings for countersinks include TiN, TiCN, TiAIN, AlTiN, PCD and electroplated diamond. The electroplated diamond coating produces a tool for grinding a countersink into a hole.
Because countersinking cycle time is short, many shops have not taken an in-depth look at potentially more economical tool designs.
Table 2. Time in cut for various depths of cut and feeds (seconds)
Depth of cut (in.)
Feed rate (ipm)
1.00
3.00
5.00
10
20
30
60
100
250
500
1000
2000
0.001
0.06
0.02
0.012
0.006
0.003
0.002
0.001
0.003
0.18
0.06
0.036
0.018
0.009
0.006
0.003
0.002
0.005
0.30
0.10
0.060
0.030
0.015
0.010
0.005
0.003
0.001
0.010
0.60
0.20
0.12
0.060
0.030
0.020
0.010
0.006
0.002
0.020
1.2
0.40
0.24
0.120
0.060
0.040
0.020
0.012
0.004
0.002
0.030
1.8
0.60
0.36
0.180
0.090
0.060
0.030
0.018
0.007
0.004
0.002
0.060
3.6
1.20
0.72
0.360
0.180
0.120
0.060
0.036
0.014
0.007
0.004
0.002
0.100
6.0
2.00
1.20
0.600
0.300
0.200
0.100
0.060
0.024
0.012
0.006
0.003
0.125
7.5
2.50
1.5
0.750
0.375
0.250
0.125
0.080
0.030
0.015
0.008
0.004
0.250
15.0
5.00
3.0
1.50
0.750
0.500
0.250
0.160
0.060
0.030
0.016
0.008
0.375
22.5
7.50
4.50
2.25
1.125
0.675
0.338
0.225
0.090
0.045
0.022
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March 2011
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