Types of Drill Bits

Walk into any hardware store and the drill bit aisle stretches like a metallic canyon. Hundreds of bits, each with its own particular shape, each designed for a specific kind of hole in a specific kind of material. The variety isn't arbitrary. Every bit geometry represents a different solution to the fundamental problem of removing material in a controlled way.

Twist Bits

The workhorse. The bit everyone pictures when someone says "drill bit." Two helical flutes spiraling up a cylindrical shaft, ending in a pointed tip. This design emerged in the 1860s when Steven Morse standardized the geometry we still use today.

The spiral flutes serve two purposes. They channel chips out of the hole as the bit cuts, and they provide a cutting edge along their length. The point angle at the tip, typically 118 degrees for general purpose bits or 135 degrees for harder materials, determines how the bit first contacts the workpiece.

Twist bits come in an absurd range of sizes. Micro bits smaller than a human hair for circuit board work. Fractional inch sizes following the numbered and lettered drill bit sizing system. Metric sizes in half-millimeter increments. Jobber length, which is standard. Mechanics length, which is shorter and stiffer. Long series for deep holes.

The shank design varies too. Straight shanks for drill chucks. Hex shanks that won't slip in quick-change holders. Reduced shanks that let you use large bits in small chucks. Three-flat shanks for industrial applications.

Manufacturing tolerances matter here. A quality twist bit holds size within 0.001 inches. The flutes are ground precisely to maintain consistent cutting geometry. Lower quality bits might look similar but the geometry drifts, especially in smaller sizes where precision becomes critical.

Spade Bits

Flat paddle of metal with a sharp point and two cutting edges. Sometimes called paddle bits or speed bits. They appeared in the 1960s as a faster, cheaper alternative to twist bits for rough carpentry work.

The geometry is almost primitive compared to a twist bit. A center point to locate the hole. Two flat cutting edges that scrape away material. No flutes at all, just a flat blade that pushes chips forward and sideways.

This creates specific characteristics. Spade bits cut fast through wood because they're removing large amounts of material with each rotation. They need high torque and low speed. The holes they leave are rough, with tear-out on the back side unless you stop before breakthrough and finish from the other side.

Size range runs from 1/4 inch up to 1-1/2 inches typically. They're designed for through-holes in framing lumber. Running electrical wire. Plumbing penetrations. Anywhere speed matters more than hole quality.

The sharpening characteristics are straightforward. Two flat edges to maintain. But the geometry is so simple that many people just replace them when they dull.

Forstner Bits

Named after Benjamin Forstner who invented them in the 1870s, though the modern version looks different from his original design. These are the precision bits for woodworking.

A Forstner bit cuts with a circular rim and internal cutting edges. The rim scribes a clean circle before the center cutters remove the waste. This means they can drill flat-bottomed holes, which twist bits and spade bits cannot do. They can drill overlapping holes. They can drill on angled surfaces without walking.

The geometry involves several cutting surfaces working together. The outer rim does most of the work, guided by the waste material inside the hole rather than a center point. Internal chippers remove the bulk of material. A center point locates the bit initially but isn't load-bearing during the cut.

Size range typically runs from 1/4 inch to 2-1/4 inches. Larger sizes exist for specialty work. They require steady, perpendicular pressure and moderate speed. Push too hard and they burn. Run too fast and they chatter.

Quality variations are extreme in Forstner bits. Premium versions have thick rims and precision-ground cutters. Budget versions have thin rims that flex and rough-ground cutters that tear grain. The difference shows immediately in the quality of the hole.

Auger Bits

The oldest drill bit design still in common use. Archaeological evidence shows similar bits from Roman times. The modern version was standardized in the 19th century for use with hand braces.

An auger bit pulls itself into the wood. It has a threaded screw point that draws the bit forward, a pair of spurs that scribe the hole diameter, and cutting edges that lift chips up the spiral body. The whole system is self-feeding, which is why traditional auger bits work so well in a hand brace where you can't apply much downward pressure.

The spiral body can be a single twist or double twist. Single twist augers clear chips better. Double twist augers cut faster and straighter. The geometry is entirely different from a twist bit even though both have spiral forms.

Modern power auger bits often eliminate or reduce the feed screw because power drills provide enough force without it. Ship auger bits have tighter spirals for deep holes. Electrician's augers have flexible shafts for drilling through multiple studs.

Sizes range from 1/4 inch to over 1 inch in diameter. Lengths vary from standard 7-9 inches to extended versions over 18 inches for drilling through multiple framing members.

The cutting action differs fundamentally from other bits. An auger bit's spurs cut the perimeter first, then the lifting edges remove the center material. This creates exceptionally clean holes with minimal tear-out.

Brad Point Bits

A twist bit modified for woodworking. The key difference is the tip geometry. Instead of a 118-degree point, a brad point bit has a sharp center point and two spurs at the outer diameter.

The center point locates precisely where you want the hole. The spurs scribe the hole perimeter before the cutting edges engage. This prevents tear-out and drift. The bit cuts cleaner than a standard twist bit in wood, though not as clean as a Forstner.

They bridge the gap between twist bits and Forstner bits. Faster than a Forstner but cleaner than a twist bit. They maintain better straightness in deep holes than Forstner bits because the flutes support the bit along its length.

Size range mirrors standard twist bits, typically 1/8 inch to 1/2 inch. Longer versions exist for pocket hole joinery and other specialized applications.

The geometry variation comes mostly in spur design. Some have prominent spurs for aggressive cutting in softwoods. Others have minimal spurs for hardwoods where aggressive spurs might cause splitting.

Step Bits

A conical bit with multiple cutting diameters. Each step is a different size, creating a bit that can drill multiple hole sizes without changing bits. They emerged in the sheet metal trades where drilling progressively larger holes is common.

The geometry is simple. Each step has cutting edges at its front face. The conical shape means the bit naturally enlarges the hole as you drill deeper. This works well in thin materials like sheet metal, plastic, and thin plywood.

Step bits exist in two main configurations. Single-flute versions cut aggressively and clear chips well but create more vibration. Double-flute versions cut smoother but need more frequent chip clearing.

Size progressions vary. Some bits have 1/8 inch steps. Others have irregular steps matched to common electrical knockout sizes or plumbing fitting diameters. The largest step determines the maximum hole size, typically ranging from 1/2 inch to 1-3/8 inches.

They work best at high speed and moderate feed rates. The self-centering action of the cone shape means they often don't need a pilot hole. They deburr as they cut because the following steps burnish the edge left by the previous step.

Hole Saws

A cylinder with teeth on the edge. Not technically a bit since it doesn't remove all the material in the hole, but it's used for drilling so it lives in this category.

A hole saw cuts an annular ring rather than drilling solid material. This makes large holes much faster than using a solid bit because you're only cutting the perimeter. A pilot bit in the center locates the hole and provides some stability.

The tooth geometry varies by intended material. Wood hole saws have aggressive teeth like a circular saw blade. Metal hole saws have finer teeth, often bi-metal construction with high-speed steel teeth welded to a tougher body. Diamond-grit hole saws for tile and masonry have no teeth at all, just abrasive particles embedded in the cutting edge.

Sizes start around 3/4 inch and extend past 6 inches. The larger the diameter, the more important it becomes to run at appropriate speeds. Large hole saws need slow speeds and steady feed rates or they bind and kick.

Depth capacity is limited by the saw's height, typically 1-1/2 to 2 inches for standard saws. Deeper versions exist for specific applications. The pilot bit usually extends beyond the saw depth to allow drilling through multiple layers.

Chip ejection can be problematic. Sawdust packs inside the cylinder and must be cleared periodically. Some designs include slots in the wall to help with this. The slug of material left behind needs to be pried out after each hole.

Countersink Bits

Conical bits designed to create a beveled opening at the top of a hole. They allow screw heads to sit flush with or below the surface. The geometry is straightforward: multiple cutting edges arranged around a cone, typically at 82 degrees to match standard flathead screws.

Some countersink bits are just the cone. Others combine a twist bit and countersink in one tool, drilling the pilot hole and countersink in a single operation. Adjustable versions let you set the depth precisely.

The cutting action differs from drilling. A countersink bit scrapes material away in a circular pattern rather than cutting straight down. This means they work across grain direction without issue but can chatter if not held firmly.

Material-specific versions exist. Bits for wood have sharper edges. Bits for metal have more gradual cutting angles. The difference relates to how the material yields under pressure.

Masonry Bits

Bits designed for concrete, brick, stone, and tile. The geometry is fundamentally different from woodworking or metalworking bits because masonry doesn't cut, it fractures.

A masonry bit has a carbide tip brazed onto a steel shank. The carbide has a chisel-like geometry rather than a cutting edge. When rotated and struck (in a hammer drill), it chips away material rather than cutting it.

The flute design accommodates masonry dust, which is much finer and more abrasive than wood or metal chips. Wider flutes help clear this dust. Some designs have specialized geometries to improve dust extraction, especially in drilling concrete walls.

Sizes range from tiny bits for tapcons to large bits for anchor bolts. The shank style varies: straight shanks for standard drills, SDS shanks for rotary hammers, spline shanks for larger hammer drills.

The carbide tip geometry varies by intended material. Bits for brick have sharper tips. Bits for hard concrete have blunter tips that can withstand more impact. Bits for tile have different geometries entirely to prevent cracking.

Specialty Bits

Beyond these common types, specialized industries have developed purpose-built bits. Installer bits have extra-long shafts and reduced-friction coatings for running wire through walls. Self-feeding bits for electricians combine aggressive threads with wide flutes. Bell hanger bits reach 4 feet long for drilling through multiple joists. Tapered bits for pocket hole joinery. Plug cutters for concealing screws. Mortising bits that drill square holes.

Each specialty bit represents someone identifying a specific drilling problem and engineering a geometry to solve it. The variety reflects the reality that drilling is not a single operation but a family of related material removal processes, each with its own optimal approach.

Understanding what these different bits actually are and how their geometries relate to their cutting action matters if you're trying to figure out why a hole looks the way it does or why a particular bit needs sharpening after certain materials. The bit type determines everything about the hole it makes.