The Art and Science of Holemaking
Advances in drill materials and geometries drive drilling productivity
By Chet Parzick
Senior Product Manager
A survey recently conducted in the German metalworking industry shows drilling to be the biggest time-consumer in the machine shop. In fact, 36% of all machine hours are spent performing holemaking processes, as opposed to 25% for turning and 26% for milling. We believe that using high-performance solid-carbide drills, instead of high-speed steel and conventional carbide drills, could significantly reduce the time required for drilling operations, and thus reduce holemaking costs.
Process parameters—specifically, cutting speed—have been improving over the past few years, especially the cutting speed of high-performance solid-carbide drills. Twenty years ago, solid-carbide drills were typically running at speeds of 60-80 m/min. These days, a cutting speed of 200 m/min in steel is common, provided the machine has enough power, stability, and coolant-delivery capability. Still, compared with the cutting speeds common in turning or milling, there are untapped productivity gains to be achieved in holemaking.
Solid-carbide drills require substrates with very high toughness. Wear is acceptable as long as it is controlled and consistent. Therefore, typical drilling grades contain more cobalt than turning or milling tools.
Drills are usually made from micrograin carbide to provide edge strength and ensure consistent wear without chipping. Carbide drills normally run with flood coolant, so temperature at the cutting edge is not extremely high, but thermal- shock resistance is a requirement. The best grades for this work are typically straight tungsten carbide materials without large additions of tantalum or titanium carbides.
For solid-carbide drills, coatings play a much larger role than just increasing surface hardness and wear resistance. They must also provide thermal insulation between the tool and the work material, and remain chemically inert. Adhesion between work material and coating must be minimized to reduce friction, and the coating surface has to be as smooth as possible.
Coatings for twist drills also must resist crack propagation. The dynamic nature of the drilling process can cause microcracks, which must be stopped to maintain tool life. Coating materials can be put into compression by selecting the correct coating process and creating the proper coating microstructure, greatly improving tool life.
Good results are being achieved with multilayer coatings, which have the ability to stop microcracks between their layers. Even when single layers are damaged and flake off, there will still be some left to protect the carbide substrate. There is great potential in nanolayer coatings and coatings tailored exactly to the drilling process.
A new TiAlN nanolayer coating with a TiN top layer, for example, has solved many of the problems encountered when drilling stainless steel. The smooth TiN top layer reduces adhesion and friction, while the TiAlN nanolayer underneath provides hardness and wear resistance. The coating has very good resistance to crack propagation and thermal shock, enabling cutting speeds of 70-80 m/min in stainless steel—nearly twice as fast as conventional grades.
Drill geometry and topography must be optimized to fully utilize the power of modern carbides and coatings. Points, point angles, margin geometry and topography, edge preparation, flute profile, and the number of flutes and margins must be properly adjusted to the application.
High-performance drills typically use one of four point geometries. The four-facet point with web is easy to grind while controlling grinding tolerances; however, its relatively low center clearance reduces feed rates, because the drill flank will touch the hole bottom if feed becomes too high.
A cone point provides more clearance in the center than the four-facet point, and therefore works with a lower thrust. But it's a complex geometry that is hard to produce and manage in a consistent way.
An alternative to these point styles is a screw-point design, which is available in two different styles. The traditional screw point, with a gash, improves chip evacuation out of the center. An advanced screw point has the gash and flank ground simultaneously, eliminating a step and further improving chip flow. Both designs have very high feed capability, because center clearance is higher than other geometries. The advanced screw point also has highspeed capability, and works with lower thrust. The only disadvantage of that point geometry is the relatively complex grinding process required to produce it.
Besides tool life and processing speed, the other major factor in drill selection is required hole quality. Burr reduction has received particular attention in recent years. Deburring is typically a manual process, which is costly and, if not done properly, can cause tremendous problems.
Solid-carbide drills run at high speeds and feeds that generate high pressure on the workpiece material; conventional drill designs or point angles would generate large burrs at the exit of through holes. The easiest way to overcome this is typically to increase drill point angle to 135-145°. With a point angle in this range, the drill generates a disk at the end of the hole and keeps the work material under tensile stress, making it easier to cut instead of just pushing it out of the workpiece. Edge preparation, corner chamfers, and other geometry factors also play a major role in reducing burrs.
Machining gray and ductile cast irons creates different problems. These materials are brittle, and chipping on exit is much more common than burr formation. Chipping is not only bad for workpiece quality; it can also cause drill breakage. A corner chamfer developed specifically for cast-iron machining can help avoid both issues by allowing the drill to break out of the workpiece in a very smooth way, and cut up to the last revolution.
Drill points always need to be adjusted to flute geometry. The number of cutting edges, web thickness, width of flutes, and width of margin lands all play roles, and are very material-specific.
For holemaking in steel, two-flute twist drills are often the best solution. They are easy to apply, easy to regrind, and are forgiving enough to minimize runout and tolerate instability in the machine and workpiece.
Drills with more than two flutes work better for holes with higher depth:diameter ratios, or when work materials have internal stresses, for example, in cast steels. Three-flute drills are guided better by the three margins, and three cutting edges also provide better self-centering. These drills cannot take too much torque, and therefore are only recommended to machine gray cast iron and nonferrous materials. An alternative, especially when through-coolant capability is required, is a tool with two cutting edges but four margin lands.
A twist drill with four margins does a better job machining steels and cast irons, because it is much more forgiving and can run with about twice the feed rate compared to a straight-fluted drill. This is also the preferred design for deep-hole drilling to depths up to 30x diam, where such drills run about five times faster than conventional gundrills.
For aluminum machining, straightflute drills provide the best hole accuracy, and allow a relatively easy way to produce complex step profiles. A disadvantage of the straight-flute drill is the very high accuracy required for clamping. These drills cannot tolerate runout, excessively high speeds and feeds, or low coolant pressure.
One of the most important issues in drilling, and especially in deep-hole drilling, is the fact that a drill running out-of-center at the beginning of the process simply has no chance to come back to center later on. The margin lands guide the drill in the out-of-center position down to the bottom of the hole. But because of the helix angle of the drill, the hole will describe the form of a helix as well.
To avoid this problem, the most important thing is to have the right drill point with very good self-centering capability, or to have the proper pre-centering strategy. Increasing the guidance of the drill also helps. When a two-margin drill starts drilling, it will only be 25% supported, so in most directions it can be shifted easily out of center, even by small forces. A four-margin drill, however, is supported in all directions, and will therefore generate a rounder and more-cylindrical hole. Four-margin drills also provide better support when the drill has an uneven breakthrough or must machine through cross holes, which is very common, for example, when machining hydraulic components.
In modern holemaking operations, chip evacuation must be 100% controlled, unlike the old days when an operator would peck any time he felt increased thrust. Beginning with chip formation at the point, it's most important to form and break the chips in a way that causes them to fit easily into the flutes, and be transported out of the flutes without friction.
The kinematics of the drilling process actually help control chips, because cutting speed is zero in the center of the point. Therefore, chips more or less flow around the chisel point, and will be strictly formed in the flutes. With the right flute geometry, it's easy to generate consistently sized chips. A negative web taper to the ends of the flute, and polishing of the flutes, will further help to achieve a free chip flow and a controlled drilling process.
Productivity increases and cost reductions can be achieved when the right drills are applied at the right process parameters. But what about tooling costs?
For starters, these advanced drill geometries are more difficult to manufacture than more conventional geometries, and typically are more costly than conventional drill designs. However, such drills can be reground four to five times. Regrinding makes it possible to reduce tooling costs by more than 50%, even factoring in a tool-life reduction of about 10% with every regrind.
The reduction in tool life that occurs after each regrind, however, can cause problems. To be safe, a drill can only be run with a large safety factor, or users must employ a tracking system that allows early removal of reground drills. The only way to avoid this issue is to work with throwaway products—which is typically not economical with solid-carbide drills.
All this can be avoided by a new modular drill design that uses a removable carbide tip, and provides cutting performance and tool life identical to that of high-performance solid-carbide drills. The connection between the tip and the steel shank uses no screws or other elements that can be difficult to handle in small diameters. The tip doesn't have to be engineered for regrinding, so geometry is optimized; and a positive rake angle is applied in the area of the chisel to reduce cutting forces and improve self-centering. This positive rake angle is important to compensate for the reduced rigidity of the alloy steel body as compared to a solid-carbide drill.
Unlike conventional carbide drills, the flute is not produced with a constant helix angle from the front to the rear, but has a right-hand helix for accelerating the chips in the front, and a slightly negative helix angle at the rear. This negative helix angle especially helps to increase the stability of the drill and to reduce vibration. Again, this is very important to compensate for the reduced rigidity of the steel body.
Another difference, when comparing modular drills to solid-carbide drills, which have coolant-hole exits on the flank of the drill point, is that these drills have the coolant exit in the flutes, directed to the rake of the cutting edge. Why is this important? The rake of the cutting edge is typically the area with the highest temperature, and coolant is especially required between the chip and the cutting material. This design optimizes cooling while generating a thermal shock on the chip, which helps to improve chip control. The chisel is taken out of direct coolant flow, further supporting chip formation in this area of very low cutting speed.
In addition to consistent tool life, the big advantage of modular drills is a reduction in tool inventory. Conventional carbide drills require a large inventory, because a lot of drills are usually rotating in the reconditioning cycle. Throwaway products eliminate this reconditioning cycle; the only inventory is the drill on the machine, and maybe a few spares on the shelf.
This article was first published in the July 2006 edition of Manufacturing Engineering magazine.