What Is the Difference Between a Blind Keyway and a Through Keyway?

Why relief features are critical when broaching blind keyways

At first glance, a blind keyway and a through keyway may appear to differ only in how far the slot extends through the part. From a CNC broaching standpoint, however, the difference is much more significant.

A through keyway allows the insert to pass completely through the workpiece before retracting. A blind keyway does not. Because of this, blind keyways require additional design considerations to provide a safe place for the insert to exit the cut.

Understanding this difference is essential when selecting tooling, programming the operation, and evaluating whether a part is suitable for CNC broaching.

What Is a Through Keyway?

A through keyway extends completely through the bore, allowing the broach insert to enter one side of the part and exit the other.

Because the insert fully clears the feature before reversing direction, through keyways are generally the simplest type of internal keyway to broach. The program can maintain a straight cutting stroke through the entire feature without concern for the insert stopping inside the material.

For many applications, this makes through keyways the more straightforward broaching operation.

What Is a Blind Keyway?

A blind keyway stops before reaching the back of the bore. Since the insert cannot simply continue through the part, it must have somewhere to go after the keyway is complete.

For CNC Broach Tools applications, this means incorporating a relief groove or cross-drilled relief hole beyond the end of the keyway. Without one of these relief features, the broach insert has nowhere to escape before retracting, greatly increasing the likelihood of damaging the tool.

The relief feature becomes part of the broaching process, not simply an optional design detail.

Why the Relief Feature Matters

During a broaching cycle, the insert should continue past the end of the keyway and into the relief area, then drop down out of the slot completely, before reversing direction.

CNC Broach Tools commonly recommends programming the insert to travel approximately 0.125 inch past the end of the keyway before retracting. This allows the machine to decelerate outside the finished feature instead of inside it, helping prevent taper or unwanted geometry at the end of the keyway.

Attempting to slow down or retract while the insert is still inside the keyway can negatively affect both tool life and part quality.

Additional Considerations for Blind Keyways

Blind keyways require more than just careful programming.

Toolholder geometry, insert projection, bore depth, and available clearance must all be evaluated before selecting tooling. On vertical machining centers, chip evacuation also deserves additional attention because gravity can cause chips to collect inside the relief area. In these applications, a cross-drilled relief hole can help provide both insert clearance and improved chip evacuation.

Reviewing these details during the quoting stage helps ensure the application is compatible with the selected broaching system.

Choosing the Right Broaching Strategy

Blind and through keyways may perform the same function, but they are approached differently during CNC broaching.

Through keyways allow the insert to exit naturally through the part. Blind keyways require a properly designed relief groove or cross hole so the insert can safely clear the feature before retracting.

Recognizing this difference early helps avoid tooling issues, protects finished parts, and leads to a more reliable broaching process.