Internal Keyway Cutter — Custom Tooling for Cutting Keyways Inside a Bore
Search for a “keyway cutter” and you’ll find shelves of off-the-shelf tools — Woodruff cutters, carbide keyseat cutters, slotting end mills. Nearly all of them cut external keyseats: the slot on the outside of a shaft. Very few address the harder problem, which is cutting a keyway on the inside of a bore — inside a gear, a coupling hub, a pump impeller, a sprocket.
That’s what an NMT internal keyway cutter does. It’s not a commodity cutter you pull off a rack. It’s a custom-built keyseat miller, manufactured to your specific bore diameter and keyway dimensions, that mills a full-depth internal keyway in a single pass — including in blind bores where nothing else will reach.
Internal vs. External — Why the Distinction Matters
The terminology around keyway cutting tools is genuinely confusing, so it’s worth being precise:
External keyseat cutters — Woodruff cutters, side-and-face cutters, and slotting end mills that cut a keyseat into the outside surface of a shaft. The tool has open access to the work. These are standard, off-the-shelf, inexpensive, and available from any tooling supplier. If you need to cut a keyseat on a shaft, that’s the tool — and you don’t need NMT for it.
Internal keyway cutters — tooling that cuts a keyway on the inside of a bore. The tool has to enter the bore and operate within its geometry, with constraints on size, chip evacuation, and access. This is a fundamentally harder machining problem, and it’s the one NMT solves. Off-the-shelf internal keyway cutters in the right size for your application usually don’t exist — especially for non-standard bores or blind-bore geometry.
If you’re cutting a slot inside a bore, you need an internal keyway cutter — and for most real applications, that means custom tooling built for the job.
How an NMT Internal Keyway Cutter Works
The NMT internal keyway cutter — our keyseat miller — is custom-manufactured to your bore diameter and keyway dimensions. It mounts in a standard drill chuck, collet, or CNC tool holder and feeds axially into the bore. The bore wall itself guides the cutter, keeping it on centerline, while the rotating cutter mills the keyway to full width and depth in one continuous pass.
Because the bore wall guides the tool, there’s no need for an external fixture, guide bushing, or relief feature. The cutter completes the keyway exactly where it should terminate — even in a blind bore — then retracts cleanly.
The key advantages over any off-the-shelf approach:
- Cuts internal keyways in blind bores — no relief hole, undercut, or cross-hole required. This is the capability that no standard cutter and no broach can match without modifying the part.
- Built to your exact dimensions — bore diameter, keyway width, depth, and length are all custom. Not limited to standard cutter sizes.
- One-pass operation — the full keyway is cut in a single axial feed, not progressive passes.
- Runs on equipment you already have — manual drill press, knee mill, or CNC machining center. No specialized equipment.
- Works in hard materials — cutter geometry specified for your alloy, from mild steel to 4140, stainless, titanium, and Inconel.
What an Internal Keyway Cutter Is Used For
Internal keyway cutters are needed wherever a keyway must be cut inside a bored component:
- Gear bores — locking gears to drive shafts
- Coupling hubs — connecting shafts to motors, pumps, and gearboxes (usually blind bores)
- Pump impeller hubs — securing impellers to pump shafts, frequently in blind, thin-walled bores
- Sprockets and pulleys — chain drive sprockets and belt pulleys keyed to shafts
- Sleeves and adapters — shaft sleeves, bushings, and adapter components
- Sheaves and drums — conveyor pulleys, hoist drums, and material handling components
In nearly all of these, the bore is the constraint — limited access, often blind, frequently non-standard in size. That’s precisely where a custom internal keyway cutter is the right tool and an off-the-shelf cutter isn’t an option.
Why Not Just Buy an Off-the-Shelf Cutter?
For external shaft keyseats — go ahead, an off-the-shelf cutter is the right call. For internal keyways, the off-the-shelf route runs into walls quickly:
Standard internal cutters don’t exist in most sizes. External keyseat cutters are stocked in standard sizes because shaft keyseats are standardized and accessible. Internal keyway applications vary enormously in bore diameter and keyway dimension, and stock internal tooling for your specific combination usually isn’t available.
Blind bores eliminate most options entirely. An off-the-shelf cutter or broach needs a through path or a relief feature. A blind bore has neither. A custom internal keyway cutter built to terminate cleanly inside the bore is the solution.
Non-standard and hard-material applications need purpose-built tooling. When the keyway dimension is non-standard, the material is hard, or the bore geometry is complex (stepped, threaded, shouldered), generic tooling can’t accommodate it. Custom does.
Capabilities
Every NMT internal keyway cutter is custom-manufactured to your application:
- Bore diameters: Under ½” to over 6″ (metric and imperial)
- Keyslot widths: 1/16″ to 2½”
- Keyslot length: Up to 12″ in most materials
- Tolerances: Within ±0.0002″ on width
- Keyway types: Full depth, blind bore (no relief hole), through bore, parallel/opposite, two-step, radius arc, straight splines
- Materials: Steel, stainless, aluminum, bronze, cast iron, titanium, Inconel, and most engineering alloys
- Machine compatibility: Manual drill press, manual mill, horizontal CNC, vertical CNC machining center
- Delivery: Stock tooling ships in 2–3 days; custom tooling in 2–3 weeks
Request a Quote
Need to cut an internal keyway? Tell us your bore diameter, keyway width and depth, material, and whether the bore is blind or through. We’ll confirm feasibility and quote a custom internal keyway cutter built for your application.
Call: 513-541-6682 Email: nationalmachinetoolco@gmail.com
National Machine Tool Co. — Cincinnati, OH — Over 100 years building custom internal keyway cutters and keyseat millers.