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To some manufacturers, drilling holes is slow, inefficient, and not the most accurate of operations. Others are finding an alternative for holemaking: annular cutters.
A leading supplier and the developer of annular cutters, Hougen Manufacturing, Inc. (Swartz Creek, MI) notes that common twist-type drills are a tool that can slow otherwise very efficient, high-production operations. It maintains that common drills present users with slow cycle times, frequent stoppages to change worn or broken tooling, substantial amounts of chips and swarf, while draining energy using up substantial horsepower and thrust requirements of machine tools. Additionally, secondary operations such as reaming are frequently required to achieve the desired hole size and finish. Old vs. New For the most part, the problems with hole production start with the tooling. The basic design of the most commonly used hole producing tool — the twist drill — has not changed in centuries. It is not an efficient tool, in theory or in practice. Even newer models which include carbide tipped and insert, spade, and core drills, while helping to alleviate some of the symptoms by offering longer life expectancy and better size or finish, have not been the miracle cure.
It’s referred to as a dead zone because at this point the surface speed of the cutting edges, a factor of revolutions per minute and diameter of the drill, approaches zero as the corresponding diameter nears zero. The slower surface speed reduces cutting efficiency and requires that increased feed pressure be applied in order for the cutting edges to bite into the material. In effect, the center of the drill does not cut, but is, in actuality, pushing its way through the material. The amount of thrust required to overcome this resistance will often cause the stock to be deformed or dimpled around the hole, creating a second problem — burrs or flashing around the breakthrough side of the hole. As the material at the bottom of the hole becomes thinner, if the feed is not eased off, the drill will push through the side, typically leaving two jagged remnants of stock still attached. The problem of excessive pressure and thrust requirements is exacerbated by the fact that twist drills usually have only two cutting edges that have to bear the entire workload. This means a slow feed rate as a factor of RPMs and number of teeth, and less than optimum metal removal rates. In addition, the workload placed on just two edges leads to premature wear, which if not remedied by sharpening will result in holes that are oversize, and drills chipping, binding, and ultimately breaking. There is also the element that a twist drill has to push and cut through material over its entire diameter, i.e., a one inch drill makes a one inch cut. The cutting area of twist drills—converting the total diameter to chips—creates tremendous demand for horsepower and torque. This power drain is more evident as the size of the drill increases. For example, the torque required to drill a 1.0" hole is nearly five times the amount necessary to produce a 0.5" diameter hole, while the horsepower (at the drill point) and thrust needed are doubled. There is one other point to be made about drilling with twist drills that creates problems — starting the drill. The geometry of a drill point, when ground correctly, is designed so that the opposing forces exerted on the two cutting edges keep the drill running straight. To start a drill on solid stock, perpendicular to the surface, the initial feed pressure can be increased slightly so that the drill can penetrate the material quickly and establish equal pressure. Attempting to drill a hole with an angle entry is virtually impossible without the aid of a fixture, and if the drill bit is ground incorrectly, the drill will wander or create oversize, out-of-round holes. ‘Hole-Listic’ Cure
Since the cutter is hollow, the actual surface being cut and area converted to chips is significantly smaller than with twist drills, so higher feed rates and less horsepower are required. Also because of its hollow design and smaller cutting area, annular cutters need less thrust, thus reducing energy consumption, and it allows thru-the-tool coolant flow for better finish, chip flush, and longer tool life. The increased number of teeth on annular cutters also reduces the wear and chipping affects often associated with interrupted cuts when using two-fluted drills. Since the annular cutter cuts around the hole, most of the hole is converted to a slug. With fewer chips made, it is easier to keep machine tools clean to minimize maintenance needs, plus the slug material is easier to clean and typically provides a higher reclamation value than chips. Annular cutters, in part because of the number of teeth and partly because they do not rely on the geometry for running straight, are particularly advantageous for such unique capabilities as oblique entry, entry in concave and convex surfaces. All of these can usually be accomplished without any drill bushing or fixture. In many cases, the circular cutting action of the cutters replaces the slow circular interpolation programs and end mill tooling used to produce grooves of various depths, widths, and excellent finishes. Other applications where annular cutters excel are in making radius notches at the edge of steel plates (drilling half of a hole), or to radius the end of tubing for miter joints, drilling overlapping holes, and “inverse” drilling, meaning the slug is not removed, but left as a post on the part. Faster Holes To illustrate holemaking speed, Hougen engineers conducted a test, comparing a 1-1/2" Rotabroach annular cutter to conventional tooling — spade, twist, and indexable insert drills of like diameters. Using 2" thick 1018 steel all drilling tools were fed at their maximum rates using a 10 HP vertical machining center. The Rotabroach annular cutter completed the hole in 13.8 secs. In 13.8 secs. the indexable insert drill had only penetrated to a depth of 1.05", while the twist and spade drills reached depths of 0.47" and 0.41", respectively. A Step Back Taking into account all of the advantages of annular cutters, it should be explained that a few limitations exist when using this type of tooling. First, unless the goal is to make a groove, annular cutters can only be used for through hole drilling. There is no simple way to remove the slug otherwise. The second problem is most often encountered when cutting holes in mild steels and alloys, when the chips tend to be long strings. Annular cutters do not have an effective method to incorporate a chip breaker. Therefore, it is necessary to use a secondary method, such as a short dwell cycle to interrupt the cut, or an external chip breaker positioned just above the hole entrance to break off the strings as they emerge. As beneficial as the center slug may be, it may pose other problems. It is essential that the slug be removed from the tool after each cutting cycle. Rotabroach annular cutters are designed to automatically eject the slug, but in a few instances a chip may become wedged between the tool I.D. and the material. For attended machines, the operator can manually probe the tool to verify the absence of the slug; with automatic machines a sensor can be added to monitor coolant pressure (a drop in pressure after the cut signals ejection). In other applications, such as turning machines, it may be undesirable to eject the slug due to the chuck rotation that could cause the piece to be thrown outward. One method to avoid slug ejection is to momentarily halt coolant flow which will delay ejection, then restart the flow after the tool is withdrawn from the area of the chuck. Hougen Manufacturing - October 2002 |
Annular cutters can offer an alternative to common drills for operations requiring holes in sheet or plate materials.
One of the first points to be made regarding the annular tool is the fact that it is a cutter, where twist drills are part cutting tool and part chisel. The twist drill has a center point, which is not really a point at all but rather the intersecting line where the two cutting edge angles meet at the web of the drill. It is here, at the so called “dead zone” of a twist drill that the troubles begin, explains a Hougen source.
To eliminate these impediments to fast hole productivity, annular (or circular) cutters can be used. The annular cutting concept differs from other drilling tools in that it is a hollow tool and provides multiple cutting edges rather than the typical two edges found on the majority of conventional drills. The multiple cutting edges on Rotabroach tools feature the patented Hougen-Edge, an exclusive end-tooth geometry with alternately lowered inner and outer surfaces on successive teeth, that reduce the work load per tooth providing better finish and longer wear.