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(877) 248-1631What makes it possible and what to watch out for
Blind holes have always been one of the biggest challenges when it comes to internal keyways. Traditional broaching methods rely on pull-through tools, which require clearance beyond the feature. If the bore stops short, those methods simply do not work.
With CNC broaching, blind hole keyways are not only possible, they are often one of the best applications for the process. The key is understanding how the tool cuts, how depth is controlled, and what limitations still apply.
Why Traditional Broaching Fails in Blind Holes
Conventional broach bars are designed to pass completely through the part. The tool needs room to exit the bore, and the cutting teeth are arranged progressively along the length of the broach. In a blind hole, there is nowhere for the tool to go once it reaches full depth.
Because of this, blind hole keyways historically required secondary operations like sinker EDM, slotting machines, or custom fixturing. These options add cost, increase lead time, and introduce additional setup variables.
How CNC Broaching Solves the Problem
CNC broaching uses a single cutting edge mounted in a rigid holder. Instead of pulling a long broach through the bore, the tool feeds linearly under CNC control. Each stroke removes a small amount of material until the full keyway depth is reached.
Because the stroke length is controlled by the program, the tool can stop at a precise depth without breaking through the bottom of the bore. This makes blind hole keyways not only possible, but repeatable.
The spindle remains off during the cut. All motion is linear, which allows the programmer to manage feed rate, dwell, and retract distance with accuracy.
What Makes a Blind Hole Broaching Setup Successful
A successful blind hole broaching setup starts with proper clearance and relief at the end of the cut. Unlike thru-holes, blind keyways require the insert to pass completely through the material and into open air before retracting. Without a relief feature, such as a turned undercut, drilled cross hole, or milled slot, the chips have nowhere to go and will compact in front of the insert, dramatically increasing the risk of crashes or insert pull-out. CNC Broach Tools recommends a relief width of at least 0.250″ to allow for Z-axis deceleration and proper chip evacuation, ensuring the cutter exits the cut cleanly before retracting
Equally important is rigidity and correct tool mounting. Blind hole broaching is unforgiving of deflection, so how the tool is clamped often determines success or failure. Using a split sleeve in a lathe or a hydraulic holder in a mill provides the best rigidity and full shank contact. A solid backstop behind the tool is mandatory to prevent axial movement during the cut. Single set-screw holders should be avoided, as they create a pivot point that leads to chatter, taper, and inconsistent depth, problems that become even more pronounced in blind applications
Finally, proper programming and motion control tie everything together. The Z-axis must reach full feed before contacting material, which is why the cut should begin at least 0.625″ in front of the part. Likewise, the tool must travel past the end of the keyway, typically 0.125″ or more, to decelerate in air and avoid the “phantom retraction” effect built into many CNC controls. Combined with conservative depth per pass, correct feed rates, and retracting straight out of the keyway (never backward through it), these programming practices ensure smooth cutting action and long tool life in blind hole broaching operations
Material and Depth Considerations
Blind hole keyways can be broached in a wide range of materials, including stainless steels, tool steels, titanium, and Inconel. Harder materials typically require slower feeds and more passes to maintain tool life.
Depth is limited by bore length and holder clearance. As a general rule, the keyway depth should be evaluated alongside the available stroke and insert geometry before committing to the process.
Is Blind Hole Broaching the Right Choice?
If your part requires an internal keyway that does not break through, CNC broaching is often the most efficient solution. It eliminates secondary operations, keeps the part in one setup, and delivers consistent results once dialed in.
Blind hole broaching is not a shortcut. It requires proper tooling, accurate programming, and attention to setup details. When those pieces are in place, it becomes a reliable and production-ready operation.
If you are evaluating a blind hole keyway application and need help determining feasibility, tool selection, or setup approach, CNC Broach Tools can support the process from start to finish. Contact us today for questions on your project.
