Drilling vs Broaching: Key Differences, Tolerances & When to Use Each

Drilling involves using a rotating bit to create new holes in solid materials. It is a fast, flexible and affordable process. Broaching uses a multi-tooth linear tool to cut precise internal profiles, such as keyways, splines and polygonal bores, in a single stroke. While drilling is the starting point for most hole operations, broaching delivers superior accuracy and surface finish for high-volume, profile-specific production runs.

Understanding the Two Processes at a Glance

Before we delve into the technical details, it is helpful to compare the two processes. The table below highlights the key differences that engineers and buyers need to consider when choosing between drilling and broaching.

What Is Drilling? Process, Types & Capabilities

Drilling is the fundamental process of creating holes in manufacturing. It is usually the first step taken when a hole needs to be made in a solid workpiece. A rotating cutting tool, known as a drill bit, advances axially into the material, displacing chips upwards through flutes and out of the hole.

Modern CNC drilling centres automate the feed rate, spindle speed, coolant delivery and depth control. This makes drilling highly repeatable, even at a production scale. However, since drill bits are essentially rough-cutting tools, drilling alone rarely meets tight tolerance requirements and is often followed by reaming, boring, or other finishing operations.

Common Drilling Operations

Drilling is not a single operation — it encompasses a family of related processes:

• Standard Drilling — Creates a straight cylindrical through-hole or blind hole. Most common starting point for all hole operations.
• Countersinking — Cuts a conical recess so that a flathead screw sits flush with the part surface. Essential for clean aesthetics in plastic enclosures, panels, and housings.
• Counterboring — Creates a flat-bottomed cylindrical recess for the head of a socket cap screw or bolt. Common in structural assemblies where fastener heads must be recessed.
• Spot Drilling (Center Drilling) — Produces a shallow, precise indentation to accurately locate the starting point for subsequent drilling. Critical for high-precision medical or electronic components.
• Reaming — Uses a multi-flute reamer after drilling to enlarge the hole to an exact diameter with a smooth finish. Achieves IT9–IT7 accuracy — far superior to drilling alone.
• Tapping — Cuts internal threads into a pre-drilled hole to accept a bolt or screw. Sometimes performed in a single combined drill-and-tap cycle.
• Step Drilling — Drills a hole with two or more different diameters along its length in one pass. Used for stepped fittings and modular assemblies.

Drilling Accuracy & Tolerance Data

Understanding the precision limits of drilling is essential for process planning:

2.3 Advantages of Drilling

• Low tooling cost — standard drill bits are inexpensive and universally available
• Maximum material flexibility — drills steel, aluminum, titanium, plastics, composites, and wood
• Suitable for any production volume from one-off prototypes to mass production
• Fast and simple setup — no custom tooling required
• Pairs seamlessly with reaming, boring, or tapping for precision finishing

2.4 Limitations of Drilling

• Limited to creating round, cylindrical holes only
• Poor surface finish and accuracy as a standalone operation (Ra 12.5–50 µm; IT13–IT11)
• Drill bit deflection can cause hole position error, especially in deep holes
• Chip evacuation challenges in deep-hole drilling (L/D > 5) risk tool breakage and heat build-up
• Cannot produce internal profiles such as keyways, splines, or non-circular bores without additional operations

What Is Broaching? Process, Types & Capabilities

Broaching is a highly specialised, precision machining process that involves removing material using a long, toothed tool called a broach. Each successive tooth is slightly larger than the previous one, removing a small, incremental chip with every pass. By the time the final finishing tooth clears the workpiece, the entire cut has been completed in a single stroke.

This single-pass capability is the defining strength of broaching. Unlike drilling, which may require multiple passes, peck cycles or follow-on operations, a broaching operation is performed in one precise, repeatable motion. The result is exceptional dimensional accuracy and surface finish, achieved in just 5–30 seconds per part.

Types of Broaching Operations

• Internal (Pull) Broaching — The most common form. The broach is pulled through a pre-drilled hole to cut an internal profile — keyways, splines, hex bores, square holes, and custom contours.
• Push Broaching — The broach is pushed through shorter workpieces. Used where pull-through access is impractical. Common in keyway cutting on smaller components.
• Surface (External) Broaching — Machines flat or contoured external surfaces. Either the broach or the workpiece moves. Used for slots, grooves, and surface finishing.
• Rotary Broaching — The broach rotates slightly off-axis while cutting. Allows CNC lathes and machining centers to produce hex, square, or splined profiles without a dedicated broaching machine.
• Continuous Broaching — Multiple workpieces are broached simultaneously on a single stroke, maximizing throughput in high-volume production environments.

Broaching Accuracy & Tolerance Data

Broaching consistently delivers the tightest tolerances of all hole-finishing processes in its class:

Advantages of Broaching

• Outstanding precision — IT7–IT6 accuracy and Ra 0.4–1.6 µm surface finish in one pass
• Extremely fast cycle time — 5 to 30 seconds per part, ideal for high-volume production
• No secondary finishing operations required — the broach is the finish cut
• Highly consistent part-to-part repeatability — critical for mass production quality
• Capable of producing complex internal profiles impossible to achieve with a drill alone
• Minimal operator involvement once set up — reduces labor cost per part

Limitations of Broaching

• High tooling cost — custom broaches range from $500 to over $5,000 depending on profile complexity
• Only economical at high production volumes where tooling cost is amortized across many parts
• Requires a pre-drilled pilot hole — cannot initiate cuts in solid material
• Limited to straight-line cuts — cannot follow curved or angled paths
• Material hardness restriction — generally limited to workpieces below ~40 HRC
• Long lead time for custom broach manufacturing (weeks vs. days for standard drills)

Drilling vs Broaching: Deep Comparison Across 6 Dimensions

Accuracy & Surface Finish

This is where broaching clearly outperforms drilling. A standard drilled hole achieves IT13–IT11 accuracy with Ra values of 12.5–50 µm, which is suitable for clearance holes and rough bores but not for precision fits. In contrast, broaching produces IT7–IT6 accuracy with Ra values of 0.4–1.6 µm in a single stroke, eliminating the need for follow-on finishing.

For context, IT7 represents a tolerance of ±0.010–0.025 mm for a 25 mm hole — a level of precision that ensures smooth shaft engagement, tight gear fits and reliable spline connections. Drilling alone cannot achieve this level of precision without significant additional operations.

Cycle Time & Production Throughput

Drilling is faster for a simple through-hole. However, for any profile-specific feature, such as a keyway, spline or non-circular bore, drilling is only the first step in a multi-step process involving drilling, milling and finishing. Broaching replaces all of these steps with a single 5–30-second operation.

In an automotive transmission plant producing 2,000 gear bores per day, this difference is significant. Broaching a splined bore takes under 20 seconds. Milling the equivalent profile after drilling could take three to five minutes. At scale, broaching can reduce machine time for these operations by 80–90%.

Tooling Cost & Total Cost of Ownership

Drill bits are standard tools — a quality carbide twist drill costs between $10 and $100. In contrast, broaches are custom-engineered to a specific profile and can cost between $500 and $5,000, or more. This makes broaching uneconomical at low volumes.

However, the economics reverse at high volumes. A broach can produce 50,000–200,000 parts before requiring regrinding, and it can be reground multiple times. When the cost of tooling is divided across 100,000 parts, the cost per part becomes negligible, while the time savings per part compound to deliver significant reductions in labour and machine costs.

Geometric Capability

Drilling is fundamentally limited to creating round, cylindrical holes. Any deviation from this — such as a keyway, spline, D-profile, hexagonal bore or square hole — requires an additional process after drilling.

Broaching is specifically designed for these non-circular profiles. It can produce virtually any internal geometry that can be ground into a broach tooth pattern, including involute splines, Woodruff keyways, polygonal bores and custom forms. This is broaching’s unique and irreplaceable capability.

Material Compatibility

Drilling is compatible with virtually every machinable material, including aluminium, steel, stainless steel, titanium, Inconel, cast iron, engineering plastics and composites, as well as wood. Specialised drill geometries and coatings extend its capabilities to include very hard and abrasive materials.

Broaching is effective with steels, aluminium, brass and engineering plastics, but struggles with very hard materials (above ~40 HRC). Hard workpieces can quickly wear down the teeth of the broach, reducing tool life and increasing regrinding costs. For hardened steel components, grinding or hard milling is usually the preferred method.

Setup Flexibility

Drilling offers maximum flexibility. Changing the size, depth or position of a hole requires only a different drill bit and an updated CNC programme, both of which can be achieved in minutes. This makes it ideal for prototype work, custom parts and short production runs involving frequent design changes.

Broaching is inflexible by design. Each profile requires a custom-made broach, and changing the profile necessitates a new tool, which takes weeks to produce and costs hundreds to thousands of dollars. While this rigidity is acceptable for stable, high-volume production, it makes broaching impractical for custom or prototype work.

When to Use Drilling vs Broaching: Decision Guide

Choose Drilling When:

• You need to create a new hole in solid material — drilling is always the starting point
• The required geometry is a round cylindrical hole with standard tolerances
• Production volume is low to medium (prototypes, custom parts, short runs)
• Design changes are anticipated — you need flexible tooling
• The material is very hard (above 40 HRC) or highly abrasive
• Budget is limited — no justification for custom tooling investment
• You need to drill multiple different hole sizes and positions on one part

Choose Broaching When:

• You need an internal profile: keyway, spline, hex bore, square hole, D-profile
• Dimensional accuracy requirements are IT7 or better
• Production volume is high (hundreds to thousands of identical parts per run)
• Cycle time is critical — you need the fastest possible per-part machining time
• Surface finish requirements are stringent (Ra < 1.6 µm) without secondary operations
• Part-to-part consistency is critical for assembly fit and function
• You have a stable, repeating part design that justifies tooling investment

Drilling vs Broaching in Real-World Applications

Automotive Manufacturing

Both processes are used extensively in the automotive industry, often on the same component. The production of a transmission gear begins with drilling: a twist drill creates the initial bore. This is then broached to cut the internal spline that engages the transmission shaft. Drilling takes seconds, whereas broaching adds another 15–20 seconds, but achieves IT6 spline accuracy, which a drill could never match.

At a production rate of 2,000 gears per day, the speed and consistency of broaching are essential. In contrast, manual milling of the same spline profile would take 4–5 minutes per part and introduce unacceptable variation.

Aerospace & Defense

Aerospace components require tight tolerances and high material integrity. Turbine disc fir-tree slots are broached because the complex, interlocking tooth profile required to secure turbine blades can only be achieved with the repeatable precision that broaching delivers. EDM or milling is far slower and more expensive for high production volumes.

Meanwhile, structural airframe holes for fasteners are drilled. These are round holes with moderate tolerances, for which drilling is the clear choice due to its flexibility and low cost.

Plastic & Composite Parts

Both drilling and broaching are used in plastic machining, but the techniques differ significantly. Plastics require sharp tools and controlled feed rates, and often no coolant (or air cooling only), in order to avoid heat distortion. Drilling is the fastest and cheapest method, making it the default choice for fastener holes, wire passages, and clearance holes.

Broaching plastic parts is less common, but it is used for keyway slots in plastic gears and bushings, where dimensional consistency is required. The single-pass nature of broaching also reduces the risk of heat accumulation that multiple drilling passes might cause.

Medical Devices

High-precision holes are often required in materials such as stainless steel and titanium for medical manufacturing. While these materials are challenging, they can be drilled with the correct tooling and parameters. The standard approach for implant and instrument components is spot drilling for precise location, followed by reaming to achieve IT7 accuracy.

Broaching is used in the manufacture of orthopaedic instruments that require non-circular profiles, specifically for the production of intramedullary canal reamers and implant driver interfaces that require splined or keyed connections.

The Drilling-Then-Broaching Workflow: How They Work Together

It is critical to understand that drilling and broaching are not always competing alternatives — in many applications, they are sequential steps in the same workflow. Broaching cannot begin in solid material. Every broaching operation requires a pre-drilled pilot hole through which the broach is threaded and pulled.

The standard workflow for producing a precision internal spline, for example, proceeds as follows:

1. Drill a round pilot hole to a diameter slightly smaller than the finished profile
2. Thread the broach through the pilot hole and secure it to the broaching machine pull head
3. Execute a single broaching stroke — roughing teeth, semi-finishing teeth, and finishing teeth each remove material incrementally
4. The finished profile is complete — no additional operations required

This complementary relationship means that when evaluating ‘drilling vs broaching,’ the real question is often: ‘Do I need a simple round hole (drilling only), or do I need a precision internal profile (drilling + broaching)?’

Frequently Asked Questions

Q1: Can I use drilling instead of broaching for a keyway?

Not directly. A drill creates a round hole; it cannot produce a flat-sided keyway slot on its own. For prototypes or very small quantities, a keyway can be milled using an end mill after drilling. However, for production quantities, broaching is significantly faster, more accurate, and more cost-effective than milling the keyway after drilling.

Q2: Is broaching more accurate than drilling?

Yes, significantly. Standard drilling achieves an accuracy of IT13–IT11 with a surface finish of Ra 12.5–50 µm. However, broaching delivers IT7–IT6 accuracy with a surface finish of Ra 0.4–1.6 µm, which cannot be achieved through drilling alone. Even a process involving drilling followed by reaming (which achieves IT9–IT7) does not match the accuracy of a precision broach in terms of profile geometry and finish.

Q3: Why is broaching so expensive?

The high cost of broaching stems from the tooling required. Broaches are custom-engineered for a specific profile, manufactured from high-speed steel or carbide, and precision-ground to tight tolerances along their entire length, which can be between 300 and 1,500 mm. Unlike drill bits, which are commodity items, every broach is a bespoke precision tool. However, a single broach can produce 50,000–200,000 parts before requiring regrinding, resulting in a very low per-part cost at high volumes.

Q4: Can broaching be done on a standard CNC machining center?

Traditional broaching requires a dedicated machine. However, rotary broaching is an exception — a rotary broach can be mounted in the spindle of a CNC lathe or machining centre to cut hexagonal, square or splined profiles. This makes rotary broaching accessible to shops without broaching equipment, although it is limited to relatively shallow profile depths.

Q5: Which process is better for plastic parts?

Drilling is the primary hole-making process for most plastic part applications due to its low cost, flexibility, and compatibility with all engineering polymers. However, broaching is used specifically for creating precise internal profiles (such as keyways, splines, or non-circular bores) in plastic gears, bushings, and couplings. In these instances, broaching provides the level of accuracy and surface finish that drilling cannot deliver.

Q6: What materials can be broached?

Broaching is effective on most steels, aluminium alloys, brass, bronze, cast iron and engineering plastics. The main limitation is material hardness: materials with a hardness rating above approximately 40 HRC wear broach teeth rapidly and are generally not suitable. For hardened steel components, grinding, EDM or hard milling are preferable.

Conclusion: Choosing Between Drilling and Broaching

Drilling and broaching serve fundamentally different purposes in the machining workflow. Drilling is the universal starting point for creating holes: it is versatile, affordable and compatible with every material and production volume. Broaching, on the other hand, is the precision finishing specialist, delivering unmatched accuracy, surface quality and cycle speed for internal profiles at high production volumes.

In practice, the two processes complement each other rather than competing with each other. Broaching always begins with a drilled pilot hole. The key engineering decision is not whether to drill or broach, but whether your application requires simple round holes, which drilling delivers, or precision internal profiles, which only broaching can produce at scale.

Use the comparison table and decision guide in this article to match your accuracy, production volume, geometry and budget requirements to the right process — or the right combination of both.

Key takeaway: If you need a round hole, drill it. If you need a precise internal profile, such as a keyway, spline, hex or custom contour, at production scale, use broaching. For the best results, create the pilot hole using drilling, then finish the profile using broaching in a single, precise stroke.