Good Points: Revive Old Worn Drills With New Sharpening Techniques.
By Curt Anderson. This article appeared in a slightly abridged form in the September 1993 issue of Cutting Tool Engineering.
"A stone cutter's drill, the tool wherewith he bores little holes in marble." This line, quoted by the Oxford English Dictionary, is possibly the earliest recorded reference to the word drill. The words were written in a publication dated 1611 (no doubt the Cutting Tool and Engineering of its day). The drill of the 1600's had more in common with medieval weaponry than it does with today's complex twist drills. Re-sharpening these ancient and rudimentary drills was an exercise in futility. Ignoring the fundamental principles of today's drill geometry can make drill sharpening equally futile.
The familiar modern twist drills dates back to the Civil War. This drill's characteristic helical flutes and a chisel edge cutting tip were the invention of an enterprising Yankee mechanic, Steven A. Morse. In 1864 he founded the Morse Twist Drill and Machine Company.
At that time, Morse compared his invention to its the crude predecessors: "The common drill scrapes the metal to be drilled while mine cuts the metal and discharges the chips and borings without clogging." Those warm words of self praise describe the Morse twist drill in theory only. Cold practical reality tells a different story.
Mr. Morse's contribution is ingenious. Nonetheless, problems beset his drill's performance. Consider how involved the drilling process is. Two different geometry's form the chip produced by a twist drill. The outer cutting lips and the chisel edge create the two geometry's. The cutting action along the lips is rather like that of single point tools. As it cuts, the chisel edge acts like negative rake tool, which deforms and pushes the material outward into the flutes or until the outer cutting lips cut it. This chisel-edge action accounts for fifty percent of the axial-thrust essential in drilling with standard web and 75 percent (or more) of the thrust requisite to wide-web drills. If that sounds simple consider what can go wrong.
After reading that litany of maladies, a person might yearn for the simpler drills of 1611. Happily, in many instances, these twist drill problems have a handy solution: proper drill sharpening techniques, using a drill sharpening machine. A list of the manifold problems includes the following examples.
- Clogging. While the good Mr. Morse was apparently oblivious to the problems of clogging with his drills, experienced drill operators are not. Chips are the material removed from a work piece by the drill. These curls of contention discharge inconsistently through the flutes of conventional twist drills. The relatively narrow flutes restrict chip room when drilling. So if any chip curls and crams the escape route, the flute quickly becomes packed. The problems of curling and the subsequent packing of flutes are most prevalent when drilling ductile materials and deep hole drilling. Clogging also hinders the flow of coolants to the cutting edges, culminating in problem number two, drill wear.
- Drill wear. Wear causes wear. Trite but true. When a drill becomes dull it generates more heat and wears faster. Drills begin to deteriorate when placed into operation. Maximum drill wear occurs at the drill cutting lip. The web, or chisel point edge begins to deform under the heat generated during drilling. The increase in wear at the corners travels back across the lands resulting in loss of size and tool life. We found the following analogy helpful. Running a drill beyond its practical cutting life without re-sharpening is like driving your car on flat tires without sealing the leaks and adding air. Just as keeping your tires properly inflated extends the tire life, proper sharpening increases drill life. As drill wear progresses, the torque and thrust required increases. With accelerated wear, the excessive torque gives way to problem number three: drill breakage.
- Drill breakage. Increased torque stress eventually breaks the drill. Impeded flow of coolant to the cutting edge eventually causes drill breakage. The result is untimely dulling or drill breakdown due to extreme heat. Sharpening the drill naturally shortens drill length. Short drills have a wider chisel edge. The wide chisel edge leads to problem four: walking. 4. Walking. There is a tendency for the chisel edge to "walk" when it contacts the work. Walking is the alternating pivoting from one outside point of the chisel edge to the other. This effect stops the drill from biting into the material. This is especially true with shorter drills with their wider chisel edges. One solution to the problem is to drill a preliminary pilot hole. However, this additional step is more work, therefore a problem in itself. 5. Coiling. Coiling occurs, particularly if the hole is of considerable depth. The chip tends to "snake" as they separate from the work especially, when drilling ductile material. Although these metallic helixes are dramatic, they are an impediment. Their tendency to coil around machinery and parts is common. It's annoying if not downright dangerous. Unraveling these metal ribbons halts the drilling operation. All to often, these corkscrew shaped extrusions obstruct the flutes and stop the flow of the ensuing chips. This takes us back to problem number one: clogging Expand the problem list to include such nemeses as over-sized holes and material breakthrough. Adroit hand sharpeners are a aging breed and are not being replaced.
The preferred route to drill sharpening is to use a drill sharpening machine. Yes, some machinists can create a sharp drill. But is the drill the optimal point angle? With a machine the operator knows that each drill is sharpened exactly as the drill sharpened before. The hand sharpened drill may or may not match its predecessor. If sharpened by hand is the geometry right? For example, even the best off hand drill sharpener can't create an S point drill. Beyond sharp, specific relief and point angle are very important. All of these shortfalls of hand sharpening are compounded when sharpening small drills. Frankly the adroit hand sharpeners are a aging breed and are not being replaced in the work force. Perhaps your company has a machinist skilled enough to sharpen a drills by hand. Great! But are not those valuable skills better used elsewhere? The correct point style and angle can save you a costly secondary drilling operation. With the proper drill sharpening machine, achieving the right drill point is not just guess work.
Shopping for drill sharpeners
A quality sharpener quickly pays for itself. It's hardly news to the person paying the bill that drills are expensive. A drill sharpener will keep drills new. Sharpening service costs become history.
If you are considering buying a drill sharpener, investigate various makes and models. Prices vary greatly. A drill manufacturer that we're familiar with offers drill sharpeners from $298US to $2398US. There are drill sharpeners that cost more than a high-priced luxury car. Others are less than a plastic toy car and worth about as much. A good sharpener will be available with a variety of grinding wheels for sharpening high-speed, cobalt and carbide drills. Wheel types include vitreous, CBN or Borazon and diamond wheels.
When shopping for a sharpener, consider the range of drill sizes and styles you most often use. For example, you may use drills of absolute range of 1/16 inch to two inches. Your typical range of drills may be a much narrower. (E.g., ninety-eight percent of the roughly 250 million twist drills used in the US industrial market are under 3/4 inch.) One drill sharpener handles a range from 1/16" to 3/4" and retails well under a thousand dollars. A certain 1/16 to 2 inches capacity drill sharpener costs seven times more. Are you willing to pay thousands of dollars extra to sharpen 2% of your drills?
All drill sharpeners are not created equal.
There are easily dozens of sharpeners available Before buying, consider these potential pitfalls:
- Overly complicated and unduly intimidating drill sharpeners (These sharpeners have bewildering array of knobs, dials and crank wheels. They try to do too much. It often takes more skill and time to operate these machines than it does to sharpen drills off-hand)
- Imprecise and unreliable drill sharpeners (Some sharpeners use a less than accurate "V" block method or angle-iron method to hold small range drills while sharpening. Other shoddily made nonprofessional sharpeners make no serious effort to recreate proper drill geometry).
- Inconvenient drill sharpeners (For example: some sharpeners use a cumbersome collet system. A misplaced or awkwardly dropped collet stops production)
- Expensive and uneconomical drill sharpeners (Some sharpeners cannot be justified budget wise. A sharpener should pay for it self in a reasonable amount of time)
Not surprisingly, the world's best selling sharpener overcomes the listed problems. The most popular sharpeners are precise because they are simple. With these machines, unskilled workers accurately sharpen drills faster than the most experienced off-hand drill sharpening masters. Controls have self-apparent functions and a logical arrangement. Handy multi-jawed chucks firmly grip drills for controlled drill sharpening. Importantly, they are economical.
With optional attachments, some open wheel drill grinders sharpen an amazing variety of drill styles. For example the one drill sharpener will sharpen standard points, helical points, 90 degree angle points, 118 degree angle points, 135 degree angle points, 60 degree angle points, flat bottom points, chamfered points, split point drills, parabolic flute drills, step drills, sheet metal drills, wood drills as well many other drill point styles.
Literature and demonstration videos provided by one leading manufacturer shows how their drill sharpeners produce the same drill point geometry as $30,000 machines used by drill makers. That drill point geometry is a cam generated point. The drill rotates at the same time as the cam action induces point clearance.
Why is a cam generated point significant?
The sharpener generates clearance in the same radial direction as the drill operates in the hole, producing optimal clearance angles. The danger of drag is gone. Drag is frequent problem with a "flat" or non-cam generated point. Additionally, the cam generated point also has the advantage sweeping across the grinding wheel face, eliminating the need for frequent wheel dressing.
There are two basic types of drill sharpener: The open-wheel drill-sharpener and the closed-wheel drill-sharpener. The open-wheel sharpeners are generally more versatile than closed-wheel sharpeners.
As a class, closed wheel sharpeners are newer on the market. They offer superior accuracy, speed and simplicity. The simplicity comes at the expense of versatility. Principally, operator control (and lack-of-control) is eliminated. One prominent drill sharpener manufacturer says of their enclosed sharpener, "Drill sharpening as easy as pencil sharpening." The manufacturer points to an irony. Nobody throws away a ten cent pencil before it's sharpened a dozen times, while expensive drills routinely get tossed into the trash unsharpened.
Enclosed sharpeners also contain and drill grinding dust and particles. The result is a cleaner, safer work place for workers and precision equipment. The sharpeners are especially popular with high production shops that use many drill sizes but few point styles. In at least one sharpener, the sharpening wheels are nickel plated CBN or diamond wheels, which never need dressing.
Clogging and coiling Proper drill sharpening techniques curb these problems. Conventional drills can be re-sharpened into points that break the drilled material into more easily ejected chips
The split point has good chip breaking characteristics. Also a split point has a larger channel with more clearance to "swallow and digest" the chips.
Excessive relief (clearance angle) also causes coiling. Explained more fully, excessive relief causes the coils to be thicker and less apt to break as the form. Ideally, chips, as they part from the shear plane, are small single-curl cones that easily empty through the flutes.
If the included or point angle is too aggressive, material is gobbled up too fast, adding to the probability of troublesome stringy coils. A less pointed drill has a slower feed rate and creates thinner and more fragile, thus desirable chips.
In high tensile materials, the chips tend to break up into pieces rather than coil. The latter is an advantageous circumstance. Standard twist drills, properly sharpened, usually cut harder, tougher materials without a problem.
In drilling operations cutting more ductile materials, to ensure facile chip ejection, it is necessary to break up chips properly. One of the best ways of obtaining that proper chip breaking is to decrease the rake angle of the cutting lips. The resultant chips are the preferred thin fragile ones.
The aim here is to create small chips not powder, which may pack flutes. Drill wear and breakage. Unnecessarily aggressive point angles lead to unnecessarily aggressive feed rates and drill wear. A drill with a point angle of 135 degrees has less overall lip length than does a 118 degree drill. The essentially stubbier 135 degree point meets less stress than an elongated point, which fights greater torque. If the drill is clogged, the lack of lubricants and coolants combine to increase to torque and heat. The combination of strains results in drill work hardening induced fragility.
S points last longer than conventional drills. Precision Twist Drill Company states that helical points have an even torque loading and lower thrust requirements extending tool life.
Concerning drill wear, there's a delicate balance achieved when settling upon the perfect lip relief. (Generally, increase the relief angle on smaller drills, and reduce it for larger drills.) As in the case of any cutting tool, the surface in back of these cutting lips must not rub on the work, but must be relieved to permit the edge to penetrate. Without lip relief the corner of cutting lip and the heel are at approximately the same plane. The incorrectly relieved drill will not penetrate the metal but merely rub the surface.
Further, insufficient lip relief adds to thrust forces, precipitates lip wear, and causes extreme heat. On the other hand, too much relief weakens lips, chipping or breaking the drill. Study conditions and materials and experiment if drill wear or breakage rates is unacceptable. A simple sharpening change of angle in one direction or the other may yield cost-saving dividends. Walking. The remedy for walking is often simply a sharper drill. Sharp drills penetrate more easily and walk less. The split point is noted for its self-centering abilities. Aircraft builders developed the split point. Because of the tremendous amount of hand drilling in airplane construction a self-centering point is a necessity.
Other problems and solutions. The obvious culprit of oversized holes is non- concentric sharpening. However, preliminary tests by Vern Hancock of Darex Corporation's research and development team implicate a second suspect: excessive relief. Hancock theorizes that the drill with too much relief and its accompanying higher load levels operates somewhat out of control. Oversized holes are a problem because the hole is essentially the wrong size and the holes are not uniform. Keeping drills consistently concentric is a challenge. Keeping drills consistently eccentric is impossible. According to the Precision Twist Drill Company, helical or S points cut close to actual diameter and may make reaming unnecessary.
Certain points tend to burr more than others when breaking through material. For example, material breakthrough problems are prevalent with helical points. Material breakthrough is the irregular exiting problem. The drill overpowers the thin final layers of the material as you break through, tearing material around the exit. The resultant burrs need to be cleaned in a secondary deburring operation. Reaming is a typical solution. Depending on the material, various drill point styles are available to solve this problem in one step. In sheet metal and other thin metal applications (tubes) a brad point with its "W"-profile is the best drill to obtain clean holes. In other materials a change in relief and included angle are appropriate. The brad point or sheet metal/wood point with its "W" shaped profile produces burrless holes. The prevailing Just-in-time philosophy of forward thinking manufacturer decrees that impediments to productivity be identified and eliminated. A good drill sharpener provides endless solutions to production obstacles. Drilling removes more metal than any other machining process. Thus, it follows that the humble drill, so frequent the "working end" of machining operations, merits our attention and occasional care. Steven Morse would likely agree.
There are three basic solutions to the difficulties concerning Mr. Morse's chisel edge.
- Splitting the drill point or thinning the web shortens or eliminates the chisel edge. Splitting or thinning the point has long be used to reduce the size of the chisel edge and reduce or eliminate walking. A point splitting operation usually creates two secondary edges. They extend to or almost to the center of the drill and eliminate most or all the chisel edge. Hole size and finish improve. A wide variety of split points, thinned points, as well as four six and other multiple-facet points, have evolved as aspects of this trend.
- Reforming the straight line chisel edge into a crowned "S" shape or helical point. Variations include the Bickford (tm) Point and Racon (r) Point. The second main approach to over come the ill effects the chisel edge has been the development of helical points or S point. In these, the chisel edge is retained, though it takes the form of a crowned S curve. The S shaped chisel is a continuous edge. These points are self centering, produce better- quality holes and reduce thrust requirements.
- Combining parts of solutions one and two this third major direction of drill evolution in recent years combines some of the features of both the split and S point trends into a non chisel geometry's. These are variations of the two proceeding drill types. Brand names include OSG Hosoi, Sandvik, Valenite, Mitsubishi New Point among others. A word of caution: these exotic drills have exotic price tags to match. Often they are re-sharpenable only at specified licensed locations or with an expensive machine. However, there is hope for those who demand performance, convenience and value. Darex reports great success in sharpning many of these points.