How to Machine an Internal Keyway: Broaching Units vs Six Other Methods

Internal Keyways & Splines: One-Setup Driven Broaching vs. Off-Machine Processing

Internal keyways, internal gear teeth, and external splines all tend to stall production at the same stage: the part is already turned on a lathe or machining center, but must be unclamped and sent to a slotter or wire EDM to cut the profile.

Unclamping and re-clamping destroys concentricity and drastically increases lead time. Driven broaching units solve this by converting rotary drive into a linear reciprocating stroke directly in the machine turret or spindle—enabling complete part machining in a single setup.

Why Internal Forms Are Difficult to Machine

Cutting an external slot is simple with a standard end mill. An internal bore presents three unique constraints: tool reach, single-direction tool force, and chip evacuation.

The greatest bottleneck is the blind hole. Conventional push or pull broaches require a through-bore to pass completely through the component. In blind-hole applications, a driven broaching unit or static bar with proper undercut relief at the bottom is mandatory.

Comparing Six Form-Cutting Methods

Method	Blind Hole Capable?	Single Setup?	Speed & Efficiency	Best Use Case
Driven Broaching Unit	Yes (with relief)	Yes (Turret/Spindle)	Very High (Self-reciprocating)	High-concentricity keyways, splines, and teeth in a single setup
Static Inserted Broach Bar	Yes (with relief)	Yes (Turret mounted)	Low (Relies on CNC Z-axis strokes)	Low-volume or prototype keyways where speed is secondary
Dedicated Broaching Machine	No (Through-holes only)	No (Separate machine)	Highest (Single-pass multi-tooth)	High-volume mass production of through-bore keyways
Slotter / Keyseater	Yes	No (Second setup)	Low–Medium	Large workpieces, deep slots, low-cost general shop work
Angle Head Milling	Yes	Yes (Machining Center)	Low	Internal slots in larger bore diameters where geometry allows
Wire EDM	No (Through-holes only)	No (Separate machine)	Very Low	Hardened materials (> HRC 55) or extremely tight tolerances

How a Driven Broaching Unit Works

Mounted on a lathe turret (VDI/BMT) or machining center spindle, a driven broaching unit converts live rotary drive into a rapid linear reciprocating movement.

Unlike a static broach bar—which forces the machine's entire Z-axis carriage to stroke back and forth for every single pass—a driven unit strokes internally while the machine axis simply feeds in $X$ or $Z$. This minimizes machine wear and drastically reduces cycle time.

Stroke Length and System Limits

The maximum slot depth is defined strictly by the unit's mechanical stroke length. Mongtec driven broaching units are available in 25 mm, 35 mm, 50 mm, and 65 mm stroke capacity.

When Driven Broaching Units Are NOT Suitable:

• Slot depths exceeding maximum stroke: Requires dedicated slotting or long-bed broaching.

• Bores that are too small: Insufficient clearance for the bar body or lack of rigidity causes deflection.

• Workpieces already hardened: Standard form cutting requires unhardened stock; use Wire EDM for hardened parts.

• Ultra-high volume through-keyways: Mass-production dedicated broaching machines offer lower per-part costs at extreme scale.

• Low-rigidity machine tools: Reciprocating impact forces load turret indexing mechanisms; ensure the host machine has adequate damping and mass.

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Internal Teeth, External Teeth, and Custom Forms

Driven units are not restricted to standard rectangular keyways. By pairing the drive mechanism with specialized ground insert geometry, users can machine:

• Internal and external involute splines

• Internal and external spur gear teeth

• Polygons (hexagons, squares, Torx-style profiles)

• Non-standard custom internal geometry

Crucial Note: The driven unit is only the actuator. Final accuracy depends on matching bar overhang, tool bar rigidity, and insert profile geometry as a single unified tooling assembly.

What to Have Ready Before Requesting a Quote

• Part Drawing: Illustrating form geometry, target tolerances, overall bore depth, and relief undercut dimensions for blind holes.

• Machine Interface: Turret spec (BMT45 / BMT55 / BMT65, VDI30 / VDI40) or spindle taper (BT, HSK, PSC).

• Bore & Profile Specs: Minimum internal bore diameter and maximum required slot/tooth depth.

• Workpiece Material: Specific material alloy and hardness (HB/HRC).

• Production Target: Expected batch sizes and target cycle times.

  
  

Mongtec Driven Broaching Solutions

Mongtec offers precision driven broaching units featuring 25 mm to 65 mm stroke options designed to handle keyways, internal/external gear teeth, and complex spline forms directly on CNC turning centers and milling machines.

By designing the unit, holder, boring bar, and custom-ground insert as an integrated package, Mongtec ensures interference-free operation and maximum tooling rigidity.

Stroke lengths, interfaces and part numbers for each broaching unit are listed in the Mongtec broaching tool holder catalogue.