How to choose a cutting drill bit for metalworking

What a cutting drill bit must do in metalworking

A cutting drill bit for metalworking has to perform three tasks at once: shear material at the cutting lips, keep the hole on size and on location, and clear chips before they damage the tool or the workpiece. The right choice depends on the work material, hole depth, tolerance, machine rigidity, coolant delivery, and production volume. On a maintenance bench, a high-speed steel or cobalt twist drill may be the most practical option. In repeat CNC production, a solid carbide, modular, or indexable drill may justify its higher cost through shorter cycle time, better consistency, and longer tool life.

A common purchasing mistake is to choose by coating or price alone. Coating can help, but it cannot compensate for the wrong point angle, poor chip evacuation, inadequate feed, excessive runout, or a setup that is not rigid enough for the selected drill style. Public technical references from drill manufacturers such as Michigan Drill and Guhring, along with machining guidance from major cutting-tool suppliers, consistently treat drill geometry and application conditions as one system.

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Start with the work material, not the coating

The workpiece material determines the edge strength, rake behavior, heat resistance, chip shape, and lubrication need. A drill that performs well in free-machining steel can fail quickly in stainless steel or nickel alloy because the chip is tougher, heat is higher, and work hardening can punish rubbing or dwell. Aluminum may need more flute capacity and geometry that resists built-up edge. Cast iron often produces short chips, but it also creates abrasive dust and may call for a different lubrication strategy than gummy materials.

A practical first step is to group the job by material behavior, not by trade name alone:

• Low-carbon and alloy steels: usually tolerate general-purpose or application-specific drills, but hardness and tensile strength strongly affect speed and feed.
• Stainless steels: need sharp, stable cutting action and enough feed to avoid rubbing, with coolant or lubrication planned carefully.
• Aluminum alloys: often benefit from polished flutes, high helix or aluminum-specific geometry, and coatings that reduce adhesion rather than simply increase hot hardness.
• Cast iron: typically creates shorter chips, but abrasiveness can wear margins and cutting edges.
• Titanium and heat-resistant alloys: require conservative process control, strong coolant delivery, and geometry that manages heat and edge loading.

Supplier cutting data is a starting point, not a universal answer. The same nominal material can behave differently depending on hardness, casting skin, forging scale, heat treatment, interrupted surfaces, and fixturing. In production, the correct cutting drill bit is the one that produces stable chips, predictable tool wear, and holes that remain within the required tolerance over a defined tool-life window.

Understand drill geometry before comparing brands

Twist drill geometry includes the point angle, cutting lips, chisel edge, web, flutes, helix angle, lands, margins, and relief angles. These features determine how much thrust the drill needs, how the chip forms, how accurately the tool starts, and how much friction occurs against the hole wall. NASA drill nomenclature references and manufacturer training materials identify these same elements because they are central to how a drill cuts.

Point angle and split point

A general-purpose twist drill is commonly supplied with a 118-degree included point angle. Michigan Drill technical data describes 118 degrees, 9 to 15 degrees of clearance, and a chisel-edge angle range as common factory geometry for general use. Wider point angles, such as 130 or 135 degrees, are frequently used for harder or tougher metals because they reduce the chisel-edge effect and can strengthen the cutting lips, especially when paired with a split point. A wider point, however, is not automatically better. It changes centering behavior, thrust, and the way the tool enters the work.

A split point or web-thinned point reduces the amount of material pushed by the chisel edge near the drill center. This can lower thrust and improve starting behavior, particularly in metal. In manual work, it can reduce walking. In CNC work, it can support more consistent entry, although spot drilling, pilot drilling, or guide features may still be needed depending on hole size, surface angle, and positional tolerance.

Helix, flute space, and chip evacuation

The helix angle and flute shape control chip curling and chip flow. Long, continuous chips are a risk to tool life because they can pack in the flutes, scratch the hole wall, raise torque, and break the drill. Aluminum and other ductile materials may need generous flute space and low-friction surfaces. Cast iron and some brasses create shorter chips, but the drill still needs stable margins and suitable relief.

Chip evacuation becomes more difficult as hole depth increases. A short 2×D hole may be forgiving, while a 6×D or 12×D hole can require through-coolant, pecking, specialized flute design, or a dedicated deep-hole method. Sandvik Coromant application materials emphasize coolant security and chip evacuation in demanding drilling, particularly as hole depth increases or when drilling difficult alloys.

Margins and hole guidance

Margins are the narrow full-diameter contact areas that guide the drill in the hole. They support roundness and location, but they also create friction. Too little guidance can reduce hole quality; too much rubbing can generate heat and wear. Four-margin carbide drills are often used where hole straightness and stability matter, while simpler two-margin designs may be adequate for less demanding work. The right choice depends on tolerance, depth, material, and whether the hole will be reamed, tapped, bored, or used as drilled.

Match drill material and coating to the job

Cutting drill bits are commonly made from high-speed steel, cobalt high-speed steel, solid carbide, or brazed carbide. They may also use replaceable carbide tips or inserts. Each type offers a different balance of toughness, heat resistance, cost, and required machine stability.

Drill type	Best fit	Main limitation
High-speed steel	General shop work, hand drilling, low-volume maintenance, less rigid setups	Lower hot hardness and shorter life in demanding production
Cobalt HSS	Stainless steels, tougher alloys, repair work, moderate production	Still slower than carbide in rigid CNC applications
Solid carbide	High-speed CNC drilling, tight process control, abrasive or high-volume jobs	Brittle under poor rigidity, runout, or interrupted entry
Modular exchangeable-tip drills	Production holes where repeatability and fast tip changes matter	Requires matching body, tip, coolant, and application window
Indexable drills	Larger diameters and productive rough holemaking	May not deliver final hole quality without a finishing operation

Coatings should be selected for the heat and adhesion conditions of the cut. Titanium nitride can reduce friction in general applications. Aluminum titanium nitride and related high-performance coatings are commonly used where higher heat resistance is needed, often in steels and difficult alloys. Diamond-like or diamond coatings may be used for abrasive nonferrous materials and composites, but they are not universal solutions for ferrous metals.

Coating selection should follow substrate and geometry selection. A coated drill with unsuitable geometry can still make poor chips, rub at the margin, or fail at the corners. Conversely, an uncoated or simply coated drill with the correct point, flute, and feed may outperform a premium coating used outside its intended range.

Plan for hole depth, tolerance, and what happens after drilling

Hole depth changes the decision more than many buyers expect. As depth increases, chip evacuation, coolant access, tool deflection, and straightness become more important. Many catalogs describe drilling capacity as a multiple of diameter, such as 3×D, 5×D, 8×D, or deeper. This ratio is useful because a 6 mm hole that is 30 mm deep and a 20 mm hole that is 100 mm deep both share the same 5×D depth relationship, even though the machine forces are very different. See also: CNC Machining.

For shallow holes, a standard jobber drill may be acceptable. For deeper holes, a stub or screw-machine-length drill may be used first for rigidity, followed by a longer drill if needed. In CNC work, through-coolant carbide drills can reduce the need for aggressive peck cycles, but only if coolant pressure and flow are sufficient for the drill diameter and depth. If chips remain in the hole, the process is not under control even if the drill itself is expensive.

Tolerance also matters. If the drilled hole will be reamed, bored, or thread-milled, the drill mainly prepares a stable starting hole with appropriate stock allowance. If the hole is used as drilled, the drill must meet size, roundness, location, and surface expectations directly. For a tapped hole, the drill size affects thread percentage, torque, tool life, and gage results. For a dowel hole, drilling alone may not be enough, and a reaming or boring step may be required.

Use speed, feed, coolant, and rigidity as part of tool selection

A cutting drill bit does not work independently from the machine. Spindle condition, holder runout, fixture rigidity, coolant delivery, and programming all affect the result. Solid carbide drills can run at high speeds, but they are less forgiving of runout and vibration than HSS. A flexible drill press, worn chuck, long overhang, or poorly clamped workpiece can make a premium drill fail like a low-cost one.

Feed is especially important. Feeding too lightly can cause rubbing, heat, work hardening, and premature dulling. Feeding too heavily can overload the lips, pack chips, or break the tool. The correct feed produces chips that evacuate cleanly and leaves wear patterns that are gradual rather than catastrophic. Supplier speed-and-feed charts should be treated as starting values, then adjusted based on spindle load, sound, chip color, hole finish, tool wear, and dimensional results.

Coolant has two roles in drilling: temperature control and chip transport. NIOSH describes metalworking fluids as mixtures used to cool, lubricate, and remove chips during machining. That function is critical in drilling because the cutting zone is partly enclosed by the hole itself. Flood coolant may be enough for shallow work, while through-tool coolant can be essential for deeper CNC drilling. Minimum quantity lubrication can work in selected applications, but it must be validated against chip evacuation and heat control, not selected only to reduce fluid use.

Safety should be included in process planning. OSHA machine-guarding guidance identifies hazards such as rotating parts, flying chips, sparks, and splashing cutting fluids. Drilling operations can create all of these. Proper guarding, eye protection, chip control, and safe chip removal practices are part of a reliable drilling process, not optional extras.

A practical selection workflow

The following workflow helps narrow the choice without relying on brand claims alone:

1. Define the hole. Record diameter, depth, tolerance, finish requirement, through or blind condition, and whether the hole is final or semi-finished.
2. Define the material condition. Include alloy, hardness, heat treatment, scale, interrupted surfaces, and whether the material tends to work harden or form built-up edge.
3. Check the machine and holder. Confirm available spindle speed, torque, coolant type, coolant pressure, holder accuracy, and setup rigidity.
4. Select drill style. Choose HSS, cobalt, carbide, modular, or indexable based on rigidity, volume, diameter, and required productivity.
5. Select geometry. Match point angle, split point, helix, flute form, margins, and drill length to the material and hole depth.
6. Choose coating last. Use coating to improve heat resistance, lubricity, or wear behavior after the base tool design is correct.
7. Validate with chips and holes. Inspect chip shape, hole size, location, burrs, surface finish, spindle load, and tool wear before releasing the process.

This workflow helps prevent a common purchasing error: comparing drill bits as isolated products rather than as parts of a holemaking process. A lower-cost cobalt drill may be the right choice for repair work on a manual machine. A through-coolant carbide drill may be the right choice for a stable CNC process. An indexable drill may be productive for large holes that will be finished later. The correct decision is the one that matches the technical and economic requirements of the job.

Frequently asked questions

Is a 135-degree drill bit always better than a 118-degree drill bit?

No. A 135-degree point, often with a split point, can work well in harder or tougher metals, but it is not automatically better for every material or setup. A 118-degree point remains common for general-purpose drilling. The better choice depends on material, centering needs, machine rigidity, feed, and whether a spotting operation is used.

When should a shop choose carbide instead of HSS or cobalt?

Carbide is most attractive when the machine is rigid, runout is controlled, coolant delivery is suitable, and the job volume justifies higher tool cost. HSS and cobalt remain useful for maintenance, manual machines, interrupted conditions, and lower-volume work where toughness and forgiveness are more valuable than maximum speed.

Why do drill bits fail even when the coating looks suitable?

Coating failure is often a symptom rather than the root cause. Common causes include wrong feed, poor chip evacuation, excessive runout, inadequate coolant, too much overhang, unstable fixturing, or geometry that does not match the material. The wear pattern on the lips, corners, margins, and chisel edge usually gives better clues than the coating name.

Does deeper drilling always require peck drilling?

Not always. Peck drilling can help break and clear chips with conventional drills, but it can also increase cycle time and may introduce thermal cycling or rubbing if poorly programmed. Through-coolant carbide drills are often designed to drill deeper holes without traditional pecking, provided coolant flow, pressure, and chip formation are under control.

What is the simplest way to improve hole quality?

Reduce runout, shorten tool overhang, use the shortest drill that reaches the required depth, match feed to the drill maker’s starting data, and verify chip evacuation. If the hole has tight location or finish requirements, consider a spotting, piloting, reaming, or boring step instead of expecting one drill to do every job.