Understanding the Not-So-Simple Drill Bit

They may be familiar to some, but not every gunsmith knows the ins and outs of drill bits. Here are the basics.


Drilling is one of the simplest forms of machining, yet it is also one of the least understood of the machine arts. Perhaps it is so visually familiar to us, that we take drilling for granted and don’t look into it further. My gunsmithing students made me aware of how much I have neglected this in their education: I had assumed that what had become second nature to me was obvious to others without the same experience. Therefore, I’ll share with you some of the things I’ve picked up about this handiest of the machine shop tools in 36 years of working in and owning machine shops.

When the average person hears the words “drill bit,” the most common image brought to mind is that of the “twist” drill. The name is no mystery, as it looks every bit (pun intended) like it had been gripped by a giant and twisted. In fact, substituting a machine for the giant, and adding a slight pull while twisting, is exactly how drill bits are given their spiraled configuration. Drill bits are first machined with two grooves running longitudinally, and are then twisted under heat. The tempering process is applied after the twisting.

Common twist drills are nothing more than two sharp edges which shave the surface of the workpiece, and lift off the shaved material with inclined planes (flutes). Estimating the torque of a drilling action requires the use of engineering equations for the friction of a wedge (the cutting edge) and the lifting action of an inclined plane. You have probably seen an auger used for digging post holes—somewhat like a curly French fry on a pencil. It also has broken more arms than a Model T crank. This little reminder from the past still lurks in the handles of the common auger and even a little bit in a good half-inch drill.

This danger can be lessened by using a lower angle of the inclined plane, or what is commonly referred to as a “fast-spiral bit.” In this way, torque is traded off for more mechanical advantage in the form of lifting and separating the material being drilled. This is most evident in the post-hole digging auger; they employ a “fast” spiral to make the auger manageable. Lifting has a direct relation to energy (heat), and so it follows that a fast-spiraled drill bit creates less heat since it is not lifting the material shaved off quite so violently as a bit with a slow spiral (or fast lift).

Fast-spiral bits have flutes that make more turns around the shaft of the bit per unit of shaft length. The fast spiral is also known as a “high” spiral, although I’ve never heard of a “low” spiral bit. Slow-spiral bits have a pitch which circles around the shaft less than it tends to progress along the shaft. Most drill bits fall somewhere in between the two extremes, with cheaper bits leaning more toward the slow spiral. Conventional bits have two flutes, as these are stronger than bits with three or more flutes. Three- and four-flute bits are usually finishing bits used to cut less and give a smoother finish.

The optimum drill-point angle is mainly dependent on the type and hardness of the material being drilled. Woods and plastics require a more pointed tip—with its sharper angles—than do harder materials such as steel. The type of steel can often call for a more shallow point angle to reduce tip heat and premature edge failure. A general rule of thumb when buying bits is to look at the finish of the metal, the tip angle, and the spiraling of the flutes; a sharply-pointed tip with a fairly rough finish usually indicates a cheaply made bit aimed at the woodworker. Conversely, a fast spiral and shallow tip angle usually means the bit is made to handle the rigors of drilling metals. I steer clear of bits with an exaggerated rib along the flutes, as this is usually a defense against a poor material’s heating and often indicates a weaker bit.

Drilling speeds vary with bit diameter and the material being drilled, with smaller bits generally requiring more speed for an optimum surface. In my experience, most machinists use much slower speeds than they should when using the smaller sizes of drill bits. Drilling-speed formulas are dependent upon feed rates, which means little when feeding by hand. Automatic feeding, used almost entirely for production work, is rarely encountered in a gunsmith’s shop.

The formula for drill speed is used for relative speeds only; it is difficult to estimate the feed rate of a spindle lowered by hand. Softer materials such as wood require higher rotational speeds. Plastics, on the other hand, tend to heat and smear with speed. Hard steels require slower speeds and feeds to minimize heat generation. The most-used formula for approximating the proper drill speed is taken from Machinery’s Handbook, the bible of machine shops:

Drill Speed = C x 12/3.1416 x D

In this formula, C is the cutting speed in feet per minute (typically 50 to 100 fpm for steel), and D is the diameter of the drill. This formula yields optimum speeds from 14,706 to 29,412 rpm for a #80 drill bit, and 764 to 1528 rpm for a 1/4-inch drill in steel.

Speeds also vary with the style of the bit, the shape of its cutting edges, and the material of which the bit is made. Excessive heat from over-speed can cause a drill bit to drill a hole smaller than its diameter by heating up the metal being drilled. After cooling back down, the hole is often left smaller than it was when the bit passed through it in its expanded condition. The opposite occurs when a dull drill bit is expanded from heat and cuts oversize. It is difficult to predict which will happen when the drill becomes dull, but more often than not, the bit will drill an oversized hole. Drilling holes deeper than three times the diameter of the bit can cause a buildup of chips between the flutes of the bit, resulting in elevated temperatures and, hence, oversized and rough-walled holes.

Proper drilling lubricants, formulated to reduce heat, are beneficial on all metals—and necessary with some like copper and stainless steel. There are lubricants made especially for drilling and tapping aluminum, as some of the best lubricants for steel cause aluminum to actually burn with a chemical reaction that is difficult to extinguish. I have caused aluminum to burn—producing an acrid, brown smoke—impervious to the use of water or the fire extinguisher I keep in my shop. Heed the warning, “not for aluminum” found on quite a few machining and tapping fluids.

Boeing, the company that dabbles in airplanes, developed a synthetic sperm whale oil which is many times better than the genuine thing (my apologies to the 747s of the deep). It actually makes “dull” bits cut again, and prolongs the useful life of a cutting edge by several times or more, a point which helps lessen the sting of a drill bit’s high price. To its credit, just a hint of Boeing’s oil on a drill will last for an hour or more. Look for it by the name “Boelube”; even rival aircraft manufacturers use it for their drilling operations.

There are so many specialized types of drill bits, that it would take an entire volume to describe and illustrate them. A few of the ones you’re likely to encounter are shown in an accompanying illustration. Here are brief descriptions of some of the types of bits, points, and terms you most often find when dealing with drills:

Brad Point:  Much like a sheet-metal bit with small, vertical, cutting tips added to the outside end of the cutting edges.

Carbide:  Refers to tungsten-carbide. This is an ultra-hard material of which the hardest bits are made. It is, however, a very brittle alloy and cannot be used for drilling regular steels (it will break).

Cobalt:  Drill made of a very tough and hard cobalt steel.

Counterboring Bit:  A sheet-metal type without the pointed tip, but with a guide; cuts flat-bottomed holes for flush screw heads.

Deming Drill:  Also called a Silvering drill, this is a short, rigid bit which is good for most general work.

High Speed: A description not referring to a bit’s rotational capabilities, but to the type of steel of which it is made.

Pilot Hole:  A hole drilled to guide and provide clearance for the central web of a larger size bit.

Reduced-Shank Bit: A bit in which a portion of the shank’s diameter has been reduced to facilitate use in a smaller-capacity drill chuck.

Sheet-Metal Point:  Zero-degree cutting edges with point in center. The point is just large enough to hold the bit in the same place; the cut is perfectly flat-bottomed.

Split-Point Bit:  One in which the center portion of the cutting edge’s trailing edge is notched to enable a bit with a thicker center web to cut into material without a pilot hole.

Titanium Nitride/Titanium Carbide: Very hard coatings for steel surfaces which act like case-hardening, in that the surface is extremely hard but the metal under it remains softer and less brittle. This coating is approximately .0002 to .0004 inch thick, so it doesn’t add appreciable dimension to the bit’s diameter.

Other Tips

One thing I’ve seen many experienced machinists do wrong is to cool a hot drill in water when they are sharpening the bit on a grindstone. I used to do this also, but learned from Darex—the company that makes fine drill- and cutter-sharpening fixtures and machines—that to do so makes the edge brittle, breaks it off easily, and quickly dulls the tool again. Cooling even carbide-tipped bits in water isn’t wise. Although the carbide isn’t affected by the sudden cooling, the tips are silversoldered onto the bit and the quenching can crystallize the silversolder and cause the tip to come off. Let the bit cool at room temperature.

There is no way that even the most experienced machinist can hand-sharpen a drill bit as evenly as a good fixture can. There are jigs which move the bit across the grindstone with a swinging motion, leaving the relief-angle surface flat. This isn’t the optimum, as it just doesn’t support the cutting edge as well as a generated point. A generated point is one in which the bit is rotated and cammed longitudinally to curve the relieved surface in a radius. This offers the maximum support to the cutting edge without dragging on the material being drilled.

Over the years, I have had many different brands and types of drill-sharpening fixtures and machines, and have formed opinions for several different situations. I have run production and been a tinkerer, and a machine-shop owner—three different scenarios calling for different drilling needs. I have found as a gun tinkerer that Sears, General, or Black and Decker drill-sharpening fixtures will suffice. As a serious gunsmith and small machine-shop owner, I need the superior sharpening obtained with a point-generating fixture, such as that produced by the Darex fixture. This fixture works with a regular bench grinder and has no limitations. For production work, there is no choice but to use a good, self-contained machine from which you can get generated points quickly. One must weigh the cost of replacing drill bits against getting them sharpened by a commercial concern or buying a sharpening fixture.

I started my children out using tools when they were knee-high, as there are always little jobs around the house that require the use of hand tools. My daughter has her own tools, now that she’s off at college, and she takes delight in being able to fix things herself. Of course, drilling is one of the tool skills upon which she depends the most. I wouldn’t have it any other way.

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