Keyway Broaching: How to Broach a Keyway (Complete Guide)

Learning how to broach a keyway is one of the fastest ways to cut a precise, standardized key slot in hubs, gears, pulleys, and sprockets. Keyway broaching uses a multi-tooth broach that passes through the workpiece bore in a single pass, cutting the keyway to width, depth, and tolerance in seconds — no CNC setup, no secondary finishing pass. This complete guide walks through the entire keyway broaching process step by step: how the cut works, how to choose the right broach and machine, cutting speeds by material, how to keep the workpiece stable, and the most common broaching faults you will meet on the shop floor — with practical fixes for each one.

What Is Keyway Broaching?

Keyway broaching is a machining procedure that employs a broach — a long, multi-toothed cutting tool — to cut keyways inside the bore of a workpiece. It is appropriate for mass production and is capable of attaining high-precision, high-efficiency, and low-cost processing.

During the keyway broaching process, the broach moves along the axis of the workpiece bore, and each successive row of teeth removes an additional layer of material until the finished keyway is produced. Broaching can process keyways of various shapes, including flat-bottomed rectangular keyways, semicircular Woodruff key seats, and splined grooves.

Keyway broaching is primarily utilized in the automotive, aerospace, and general machinery industries — particularly where high-precision keyways are demanded and the same part is produced in volume. For a comparison of how this internal method stacks up against milling a slot into a shaft, see broaching vs. keyseating for internal keyways.

The tool itself is what makes this possible. Along the shank of a keyway broach, the roughing teeth take the first, heaviest cuts; the semi-finishing teeth bring the keyway close to size; and the final calibration (finishing) teeth set the exact width, depth, and surface finish. Each successive tooth rises slightly above its predecessor — this rise is the tooth lift — so every tooth removes only a thin chip. That is why a single pass can hold tight tolerances without shocking the workpiece with a heavy cut.

How to Broach a Keyway in 6 Steps

• Step 1: Choose the right broaching machine and broaching tool. Select the appropriate machine and tool according to the size, shape, and requirements of the workpiece. The size and shape of the broach should match the keyway you need to cut — a complete keyway broaching set (broach plus guide bushings) covers a range of standard keyway sizes, while the machine side is covered in the keyway broaching machine guide.
• Step 2: Install the broach. Mount the broach on the broaching machine (or in the press adapter) and make sure it sits in the correct position and orientation — the cutting teeth must face the direction of travel, and the shank must be clamped true so the broach pulls straight through the bore.
• Step 3: Fix the workpiece. Secure the workpiece to the broaching machine or press, with its guide bushing seated against the fixture face. The bushing centers the broach in the bore and defines the keyway position, so the workpiece must not move or rotate during the pass.
• Step 4: Adjust the broaching machine. According to the material and size of the workpiece, set parameters such as the speed, feed amount, and cutting depth of the broaching machine, and apply cutting fluid along the broach before starting.
• Step 5: Start broaching. Start the machine and let the broach cut slowly and steadily through the workpiece — smooth, continuous motion, without hesitation mid-pass. Watch the cutting behavior of the broach and adjust machine parameters in a timely manner to protect broaching quality.
• Step 6: Complete broaching and inspect. The pass ends when the broach has completely passed through the workpiece. Shut down the machine, remove the workpiece, deburr the keyway edges, and verify the result before releasing the part.

Inspection after broaching comes down to three checks: keyway width (verified with a key gauge or go/no-go gauge), keyway depth (measured with a depth micrometer from the bore surface to the keyway floor), and symmetry — the keyway must sit centered relative to the bore axis so the key transmits torque evenly. In a keyway broaching set, hardened shims placed behind the broach let you fine-tune the cutting depth or restore full depth after the broach has been reworked, so keep the shims together with the broach.

What Are the Advantages of Keyway Broaching?

Keyway broaching has several significant advantages over alternative methods such as milling, slotting, and planing:

High-Precision Processing

1. High dimensional accuracy

• Keyway broaching enables very precise dimensional control. Broaches are precision-designed and manufactured tools with a high degree of consistency in the size and shape of their teeth. During broaching, each tooth cuts the workpiece in turn, ensuring that keyway parameters such as width, depth, and length remain accurate part after part.
• For applications that demand extremely high dimensional accuracy — aerospace components, precision machinery, and similar fields — keyway broaching can hold strict tolerance requirements and guarantee the assembly accuracy and performance of the finished parts.

2. High shape accuracy

• The tooth profile of the broach matches the shape of the keyway to be machined, producing a regular, smooth keyway surface on the workpiece with every pass.
• Compared with other processing methods, keyway broaching is far less likely to produce defects such as deformation or distortion, ensuring the geometry of the keyway meets design requirements.

3. High position accuracy

• The broach cuts along a predetermined trajectory, which guarantees the position of the keyway on the workpiece. This matters most where multiple keyways must mate with each other or with other parts in a precise assembly.
• Precise locating and clamping devices further improve keyway position accuracy, protecting the overall processing quality of the workpiece.

High-Efficiency Processing

1. One-pass forming

• Keyway broaching typically produces the finished keyway in a single pass, eliminating multiple cuts and auxiliary machining operations. This greatly shortens processing time.
• Compared with traditional milling, planing, and slotting, the processing efficiency of keyway broaching can be several times higher.

2. High cutting speed

• Under the right conditions, keyway broaching can run at relatively high cutting speeds. The cutting edges of the broach are sharp and cutting forces are low, so the process stays stable at speed.
• Higher cutting speed further improves output and reduces cycle time, which is exactly what mass production calls for.

3. High degree of automation

• Keyway broaching integrates easily with automated equipment. CNC broaching machines and other automated systems control every parameter of the broaching process precisely, improving accuracy and consistency while reducing manual handling.

Good Surface Quality

1. Low surface roughness

• Broached keyway surfaces are very smooth, with surface roughness values typically in the range of a few microns or less. The teeth of the broach are finely ground and the cutting action is smooth, leaving even, fine machining marks on the workpiece.
• Low surface roughness reduces friction, improves the fitting accuracy and service life of the parts, and contributes to the overall appearance quality of the product.

2. Increased surface hardness

• During broaching, the combined action of cutting force and cutting heat produces a degree of work hardening on the machined surface. This raises the hardness of the keyway surfaces and enhances their wear resistance.
• For parts that must work in harsh environments, the work-hardened surface after keyway broaching improves service life and reliability.

Wide Scope of Application

1. Broad material coverage

• Keyway broaching applies to a wide range of materials: steels, cast iron, aluminum alloys, and copper alloys, as well as non-metallic materials such as plastics. Different materials only require the appropriate broach and cutting parameters.
• Even difficult-to-machine materials — high-strength alloy steels, high-temperature alloys — can be broached efficiently and accurately with the right tooling.

2. Keyways of many different sizes

• From micro keyways on small precision parts to wide keyways in large workpieces, keyway broaching covers the full size range — you simply select the appropriate broach.
• Broaches can be designed and manufactured to custom keyway sizes and shapes, meeting the needs of virtually any application scenario.

What Are the Disadvantages of Keyway Broaching?

For all its strengths, keyway broaching has three main drawbacks you should weigh before committing to the process:

High Tooling Cost

Complex broach manufacturing: manufacturing a keyway broach requires high-precision equipment and experienced toolmakers. The tooth profile is complex, dimensional accuracy requirements are strict, and production involves many operations — forging, cutting, heat treatment, and grinding. These processes make broaches expensive, and special shapes or sizes cost even more.

Demanding tool material requirements: to guarantee cutting performance and tool life, broaches usually need high-performance tool materials such as high-speed steel or cemented carbide. These materials are relatively expensive, which pushes tool cost higher still. Different workpiece materials and processing requirements may also call for different tool materials, adding cost and management overhead.

High Equipment Requirements

A dedicated machine is usually required: keyway broaching is normally performed on a special broaching machine. Such machines are relatively expensive, occupy a large footprint, and demand a suitable amount of processing space. For small shops or low processing volumes, buying a dedicated broaching machine raises both production and maintenance costs — though smaller jobs can often be handled on a press instead (see the broaching press guide).

High machine accuracy requirements: broaching demands accuracy from the machine itself. The guide rails, transmission system, and other components of the broaching machine need good precision and stability to keep the broach accurate and stable during cutting. If the equipment is not precise enough, the dimensional and shape accuracy of the keyway will suffer — and the broach itself can even be damaged.

Poor Flexibility

One broach, one keyway size: the shape and size of a keyway broach are fixed — it can only cut the keyway it matches. To cut a different keyway size or shape, you must change the broach, which adds cost and changeover time. In contrast, methods such as milling and planing handle different keyway sizes simply by adjusting the tool and cutting parameters, giving them higher flexibility at low volumes.

Limited to simpler workpiece geometry: keyway broaching is mainly suitable for keyways in flat or cylindrical surfaces (bores). For workpieces with complex shapes — curved surfaces, special profiles — broaching may be limited, and other methods such as CNC machining or EDM may be required instead.

How to Ensure the Stability of the Workpiece During Keyway Broaching

During the keyway broaching process, workpiece stability is crucial — it directly affects machining accuracy and the quality of the final product. Here are proven ways to keep the workpiece stable:

1. Use fixtures or otherwise secure the workpiece adequately. During broaching operations, clamps hold the workpiece firmly in place, ensuring stability and accuracy throughout the cutting pass.
2. Choose the right metal cutting tool. Select the correct tool for the specific material and job requirements so the cut stays stable and efficient from entry to exit.
3. Determine the cutting parameters of the tool. Set cutting depth and cutting speed sensibly, and adjust the parameters according to material hardness, the number of cutting edges, and machine stability to reduce vibration and improve processing stability.

Two more stability details are easy to overlook. First, the workpiece should be supported as close to the bore as possible — a long overhang or thin-wall hub will deflect under cutting force and spring back, producing a bell-mouthed or tilted keyway. Second, keep the fixture and bushing faces clean and undamaged: a chip or a burr trapped under the workpiece tilts it a fraction of a degree, and that fraction translates directly into keyway symmetry error.

How to Choose the Cutting Speed During Keyway Broaching

Cutting speed is one of the most influential parameters in keyway broaching. Four factors should drive your choice:

1. Consider the Workpiece Material

• For softer, more plastic materials such as aluminum alloys and copper alloys, choose a higher cutting speed — generally between 10 m/min and 30 m/min. These materials cut easily, and higher speeds improve efficiency without causing excessive tool wear.
• For harder materials such as alloy steels and tool steels, reduce the cutting speed — usually to around 5 m/min to 15 m/min. Hard materials wear tools faster, and excessive cutting speed will drastically shorten tool life.

2. Consider the Broach Material and Coating

• High-quality broach materials — high-speed steel or carbide — with good coatings can withstand higher cutting speeds. In this case the speed can be raised appropriately for the workpiece material and processing requirements.
• If the broach material is ordinary or uncoated, reduce the cutting speed to prevent excessive wear and tool damage.

3. Consider Processing Accuracy Requirements

• If the keyway must meet high machining accuracy, reduce the cutting speed. This lowers cutting force and vibration and improves accuracy and surface quality — for precision work, cutting speeds of around 5 m/min to 10 m/min are typical.
• For jobs with lower precision requirements, a somewhat higher cutting speed can be selected to raise production efficiency.

4. Consider Broaching Machine Performance

• Different models and specifications of broaching machines differ in power and stability. If the machine has ample power and good stability, a higher cutting speed is acceptable.
• For lower-power or less stable broaching machines, reduce the cutting speed to keep the process safe and stable.

Common Keyway Broaching Faults and Solutions

Even a well-specified process hits problems in practice. Understanding these five common faults — and their fixes — is critical to keeping processing efficiency and product quality high.

Fault 1: Burrs on the Workpiece Keyway Edges

Burrs rising along the keyway edges are usually caused by damage to the chamfering teeth of the broach. Burrs not only degrade the surface quality of the workpiece, they also accelerate broach wear.

Solution: inspect the chamfering teeth of the broach and repair any damaged teeth promptly. Checking and maintaining the chamfered teeth on a regular schedule effectively reduces burr formation and keeps keyway edges clean.

Fault 2: Scratches on the Keyway Side Faces

Scratches along the keyway sides typically come from the sharp trailing corner of the broach dragging across the finished side faces. This happens when the rear guide section of the broach is too wide, or when the key width of the rear guide was not ground to the end at the factory, leaving an inverted taper that scrapes the keyway on the way out.

Solution: measure the width of the rear guide spline of the broach. If it is greater than or equal to the width of the calibration (finishing) teeth spline, grind it down until the rear guide spline is 0.04 mm narrower than the calibration tooth spline. Grind the end corners of the rear guide splines into arc shapes and polish them. Precise measurement and grinding will significantly reduce keyway side scratches and keep the workpiece within dimensional and shape specification.

Fault 3: Scratches on the Keyway Bottom

Scratches on the keyway bottom often appear on broaches that have been sharpened many times: as regrinding progresses, the diameter of the finishing teeth drifts — precision cutting teeth end up larger, calibration teeth smaller — and the transition between them starts to mark the keyway floor.

Solution: do not sharpen all calibration teeth at once. Sharpen only the first calibration tooth initially; when its diameter has been reduced by 0.02–0.03 mm, move on to the second, and progress through subsequent calibration teeth in the same sequence. This disciplined sharpening order extends broach life and prevents bottom scratches.

Fault 4: Scale-Like Tears on the Broached Surface

A scaly, torn appearance on the broached surface is usually linked to improper broaching speed or tooth lift, an unsuitable rake angle, or the wrong cutting fluid. This fault seriously affects surface quality of the keyway.

Solution: run trial passes at a broaching speed below 2 m/min or above 5 m/min (avoiding the problematic band in between), use a smaller tooth lift, increase the rake angle appropriately, change the heat-treatment state of the material, and switch to an extreme-pressure (EP) cutting fluid. Adjusting the broaching parameters combined with the right cutting fluid effectively eliminates the scaly texture and restores surface quality.

Fault 5: The Broach Swings Up and Down While Broaching

If the broach oscillates up and down during the pass, the guide length of the broach guide core (mandrel) is likely insufficient, allowing the tool to wander off its intended path.

Solution: maintain the correct working position of the broach, preserve the perpendicularity between the workpiece datum face and the pre-machined bore, and keep the end faces of both workpiece and fixture clean and flat. Optimizing the broach guide system and keeping the setup clean eliminates broach swing and restores machining accuracy and efficiency.

Understanding and analyzing these common faults — and acting on them early — pays off directly in processing efficiency and product quality. Inspect the chamfered teeth of your keyway broaches regularly, control the trailing-guide geometry that causes side scratches, follow the correct sharpening sequence against bottom scratches, adjust broaching speed and tooth lift, use an appropriate cutting fluid, and keep broach and workpiece clean. These habits are the most reliable way to reduce faults and raise processing quality.

Learn more: Troubleshooting for Broaching Operation and rotary broaching troubleshooting.

How to Choose a Suitable Keyway Broach

Choosing the right broach determines whether the whole process succeeds. Consider these four aspects:

1. Select Based on Workpiece Material

Softer materials:

• For lower-hardness materials such as aluminum alloys and copper alloys, choose high-speed steel (HSS) broaches. HSS broaches offer good cutting performance and toughness, cut under low cutting forces, and are not prone to chipping.
• HSS broaches are also relatively inexpensive, making them economical for larger batches of softer workpieces.

Harder materials:

• For higher-hardness materials such as alloy steels and tool steels, select carbide broaches. Carbide has higher hardness and wear resistance, withstands larger cutting forces, and suits hard materials.
• Carbide broaches are more expensive and more brittle, so guard against edge chipping during use and handling.

2. Select According to Keyway Size

Keyway width:

• The width of the broach must match the keyway. Generally the broach is sized to the nominal keyway width so that the finished slot holds its dimensional accuracy after the pass. Standard keyway widths and depths for a given bore diameter are tabulated in the keyway dimensions and standards reference.
• An oversized broach width causes excessive cutting force and can damage the broach or workpiece; an undersized broach leaves the keyway width out of specification.

Keyway depth:

• The length of the broach is determined by the depth of the keyway: it must be long enough to cut the full depth in one pass while remaining rigid and stable. Note that a through-bore is the standard precondition — cutting a slot that ends inside a blind bore requires a different approach, covered in blind keyway broaching.
• A broach that is too long may lack rigidity and vibrate during cutting, hurting quality; one that is too short may not cut the workpiece completely, leaving the keyway short of depth.

3. Select According to Processing Accuracy Requirements

High precision requirements:

• For high-accuracy keyways, choose a broach with a higher accuracy grade. The dimensional and shape accuracy of the teeth of high-precision broaches is better, which guarantees the finished keyway’s accuracy.
• High-precision broaches also have better surface finish, which reduces friction during cutting and improves both processing efficiency and surface quality.

General accuracy requirements:

• When accuracy demands are moderate, a lower-accuracy-grade broach cuts broach cost while still meeting general processing requirements.

4. Select According to Production Batch Size

Mass production:

• For volume production, choose a broach with high durability and stable cutting performance — carbide broaches or coated broaches. Their superior wear resistance holds processing quality steady over long production runs.
• Also consider replaceable-blade broaches: when the blades wear, you replace the blades instead of the entire broach, lowering production cost.

Small-batch production:

• For small batches, lower-priced high-speed steel broaches are usually the better fit — simpler to make, cheaper, and entirely adequate for short runs.
• A universal (multi-size) broach set is another option for varied work, raising the utilization rate of the tooling investment.

Keyway Broaching FAQ

How do you broach a keyway?

Mount a guide bushing and the workpiece on the press or broaching machine, insert the keyway broach so its first cutting teeth enter the bore, then push or pull the broach through the bore in a single steady pass. Each row of teeth removes a little more material until the last (finishing) teeth bring the keyway to final width and depth. Remove the workpiece, deburr the slot edges, and check width, depth, and symmetry against spec before releasing the part. The detailed six-step workflow is covered earlier in this guide.

What size broach do I need for a keyway?

Match the broach to three dimensions: the keyway width you need (the broach’s finishing teeth set the final slot width), the bore diameter of the workpiece (which determines the guide bushing), and the required keyway depth (which the broach’s tooth stack must reach in one pass). For standard keys, these dimensions are not free choices — they follow recognized keyway standards for each bore size, so start from the standard tables in the keyway dimensions and standards guide and pick the broach (and bushing) that matches your bore.

Can you broach a keyway without a press?

You need some form of controlled axial force — pure hand pressure is not enough for a clean cut. The most economical entry point is an arbor or hydraulic press together with a keyway broaching set (broach, guide bushings, and shims), which is how most small shops cut occasional keyways without buying a dedicated machine. Beyond that, the options are a vertical or horizontal keyway broaching machine or outsourcing the operation. Choosing the right capacity is covered in the broaching press guide.

How deep should a keyway be?

Keyway depth is standardized, not improvised: for a given bore diameter and key type, standards such as DIN 6885 and ANSI B17.1 define the nominal keyway width, depth, and tolerances for both the hub and the shaft. The broach’s finishing teeth are ground to those dimensions, so in practice the keyway comes out correct as long as the broach matches the standard you are working to. If you are sizing a job from scratch, look up the depth for your bore in the keyway dimensions and standards reference.

What is the difference between a keyway and a keyseat?

In everyday shop usage the two terms are used almost interchangeably, but strictly speaking a keyseat is the slot machined into a shaft (usually by milling), while a keyway is the slot machined into the hub bore — which is exactly the operation this guide covers, and why internal broaching is the standard method for hub keyways. The comparison of methods, tolerances, and cost is laid out in broaching vs. keyseating for internal keyways.

Deep Dive: Keep Learning About Keyway Broaching

Want more? Browse the related resources on this site: all keyway broaching articles, the broaching machines category, and the broaching tools category for broach selection, maintenance, and buying guides.