High Performance Test Drills

A description of many high performance drills along with tips for different methods of utilization.

High Performance Test Drills
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Of all the metal-cutting processes, drilling represents the most ubiquitous. It can also be the most complex, with myriad forces to consider when putting together a cutting strategy. "The tool is always under a constant force," says Joe Kueter, an engineer at M.A. Ford Co. (www.maford.com), Davenport, Iowa. "You always have your teeth (or lips of the drill) in constant contact with the material."
Managing cutting, thrust, torque and other forces is of supreme importance. After initial contact, thrust forces climb until the entire width of the drill is engaged in the material. Features within the drill—the way the grind lines are on the drill point, the quality of the coating, the quality finish and so on—can affect how much material is "smeared" on the point, lip relief and the face of the flute, hole after hole. And during it all, chip evacuation stands always in the footlights.
For years, cutting-tool suppliers have offered drills for specific material types—tweaking the flute form, tool coating, point styles and angles, chisel edges at the tip of the point, and so on—to give maximum performance. Yet with this comes additional tool change-outs and tool-inventory management that can become a burden.
So, the industry has been developing drills that agree with a wider swath of materials. "We continue to manage those trade-offs as best we can," says Kueter.
Physics, chemistry and metallurgy limitations will keep industry away from the "one-tool-for-everything" ideal, of course. Yet driven by the demands of low batch sizes and lean manufacturing, engineers have dissected the drill for optimum performance under those conditions. This month, Fabricating & Metalworking talks with Kueter about some basic characteristics to look for in a high-performance carbide twist drill, to ensure long tool life and compatibility with a wide range of materials.
THE CHISEL EDGE
When looking at that chisel edge head on, analyze the flute gash intersecting that chisel edge, on one side of the chisel edge versus the other. On a finished tool, those angle faces will appear about on the centerline of the two lands. Yet with edge protection—a negative rake on the cutting edge of the drill point, adding strength—"you're moving the face of the gash just slightly, and you end up reducing your chisel edge as well," he says. And leaving a very small chisel edge at the drill point means that tool will cut more freely, changing the effect of forces produced when drilling starts.
When entering the workpiece, the chisel edge makes contact, and the way that edge is positioned can affect hole-location capability. A large chisel edge often will create "walking," hindering precise location as the tool enters the workpiece. "If you're cutting a deep hole, this is really going to cause you headaches," Kueter says.
The edge protection on the chisel edge does increase the required cutting forces; but without it, the cutting edge is weaker and tool life is reduced. Also consider that if coating is placed on a tool without edge protection, "all it takes is the smallest chip of the cutting edge, and you've lost your coating. Soon, you'll start seeing a built-up edge on the drill," he says.
FLUTE FORM
The flute form should be designed to ease the evacuation of chips. High-performance drills offer a curved "saber" or hook design to the lip, giving a radius dimension to the traditionally straight flute face.
Slowing the drilling operation down explains why this can be important. As the lip of the "saber" cuts the metal, that material, due to the curvature of the flute face, is encouraged to bend over on itself (how easily it does this depends on the microstructure of the metal being cut). For evacuation, this compact dimension has major advantages over elongated chips.
Also consider the web diameter that forms the back of that flute form. On a typical drill, the web at the tip of the drill is thinner than up the tool's length, thus creating a gradually shallower flute volume moving up the flute length. On a high-performance drill, however, this web thickness may be constant throughout. This helps with chip evacuation issue, where a "traffic jam" of chips gathers at the tip as they try to move their way up the flute form and evacuate the hole.
"When we do our thrust measurements, we look for one that doesn't have an appreciable spike toward the end of the hole," Kueter says. "Once your tool is engaged, the thrust force should ideally stay constant until the drill exits the hole." The consistent web helps avoid that spike.
BACKTAPER
A drill with backtaper has a larger diameter near the tip than at the top of the flute length and can help significantly reduce friction in operations where deflection concerns arise. If, say, a tube-sheet application requires a drill to cut at aggressive feeds and speeds, the weight of the plate actually may become lighter the more holes are drilled. The reduced weight added to those feeds and speeds cause significant deflection during the drilling operation; as the drill breaks through the bottom of the plate, the plate flexes back toward the drill.
Without adequate backtaper, "you would see extreme friction against the margin of the drill," Kueter explains. Add heat to the mix, and eventually the carbide cracks on that margin and the tool is ruined prematurely. "But by increasing the backtaper and changing some of the margin characteristics, you can avoid this situation."
COOLANT HOLES
Coolant serves two principal purposes in drilling: to keep the point cool as well as flush the chips. Kueter explains: "Coolant holes can intersect the gash on the point. That can help with the chip evacuation by making it easier to get the coolant into and through the flute." This has a trade-off, however. Since the coolant is directed into the flute, "you can lose some of your coolant that helps protect the outside corner of the cutting edge."
To cool the point and effectively flush the chips at the same time, coolant holes on the drill tip may be positioned so that they are not intersecting the gash on the point, yet still positioned just beyond that gash line, such that coolant can still easily flow up the flute. "It helps keep the coolant better positioned so you are also protecting the outer corner of the drill, where a lot heat builds up," Kueter explains, "yet still have the coolant help flush the chips out of the flute"
COATING
Of all the cutting tool advances, coating has seen the most development in recent years. Beyond the types of coating, proper application can greatly assist chip evacuation. For instance, an AlTiN PVD coating may be further polished after initial application, to provide an even smoother surface that resembles the original, uncoated carbide surface. (Note: AlTiN coating wouldn't suit an aluminum application, of course, since the aluminum in the coating reacts adversely with the aluminum workpiece.)
A PLACE FOR CUSTOMIZATION
For many specialized jobs, specific drill geometries and coatings still give the best efficiency. A tool designed specifically for low-carbon steel might have a little more edge protection at the cutting edge. A drill for titanium, on the other hand, would be designed with lighter edge protection, since such hard materials respond better to an aggressive cut, and the chips do not form as well with a negative rake angle.
For especially deep holes, some tools only have coating from the tip to halfway up the flute length. Since cutting forces aren't as strong farther up the tool, chip evacuation replaces tool life as the primary concern. So to aid that evacuation, some tools transition to the smoother uncoated carbide halfway up, easing chip flow.
On extremely deep holes, a flute form's web may include some beveled edges, adding an extra cut to greatly expand the flute volume by reducing the land width about a third to halfway to the top of the drill—creating a "reverse funnel" effect. The channel for chips to flow up and out of the hole becomes larger near the top of the flute length.
An operation with many interrupted cuts may require flute forms that make for a more rigid tool, including cases where the geometry offers a double-margin width on the land to add more stability as the tool enters the hole.
FEWER TOOLS, SIMPLIFIED SETUP
"You're always going to have the shops that cut one material, day in and day out," Kueter says, and for those jobs, a specialized drill makes economic sense. Yet the low batch sizes, lean manufacturing and diversity that pervade many shop floors have spurred demand for a drill designed to cut a variety, from gray cast iron to 4140 to Inconel.
When choosing a tool, consider the flute form, the chisel edge, backtaper characteristics, and the location of the coolant holes, among other characteristics. Kueter concludes that today, through careful selection of drill geometry and coating, cutting-tool technology allows "these shops to run the same drill from job to job to job, without having to stock three or four different styles. It's about reducing the effect of cutting forces and creating strong enough drill geometry to allow people to drill holes and walk away from the machine."
BASIC TWIST-DRILL TERMINOLOGY: A PARTIAL LIST
Chisel Edge: The cutting edge at the drill point.
Flute: Grooves cut into the body of the drill to create the cutting lips and allow for chip and fluid evacuation.
Gash: A feature on the drill point that reduces the non-cutting chisel edge of the drill to reduce thrust forces.
Land: Looking at the drill tip, this is the portion of the drill body between adjacent flutes.
Lip: The cutting edge on the drill end, extending from the chisel edge to the outside periphery.
Lip Relief: The axial relief on the drill point.
Margin: A portion of the land that is not cut away to provide clearance.
Web: The central portion of the drill body that joins the end of the two lands; on a two-flute drill, the web forms the chisel edge.
Sources: ANSI B94.11M; M.A. Ford Inc.



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