A key joins the shaft slot to the hub slot to transmit torque

A key is a separate piece fitted between a shaft and the bore of a hub, the part of a pulley, sprocket or gear that fits around the shaft. Contact between the key and slot faces transfers turning force. Here, keyseat means the recess in the shaft and keyway means the matching slot in the hub. Other references sometimes use keyway for both; always identify which part is being cut.

Trace the turning load through the connection: shaft slot face → key → hub slot face, or the reverse when the hub drives the shaft. At the rim, the pulley, sprocket or gear transfers the load through its belt, chain or mating gear teeth. Not every hub is keyed; check how the actual assembly connects the hub to the shaft.

Identify the key by its cross-section and seating shape

Supporting detail: Addendum 1 — Public reference for key forms.

Practice identifying keys

Check key fit and axial retention as separate requirements

For a parallel key, the side fit controls how the turning load is carried. Excessive side clearance permits movement; an overly tight fit can prevent assembly. Verify the specified top clearance too. A tapered key has a different seating action, so do not apply a parallel-key fit rule to it.

Use the drawing's key designation, dimensions and tolerances. Do not choose a key size from shaft diameter alone or assume that half the key height belongs in each part.

The key does not by itself define how the hub is held against movement along the shaft. Check the assembly drawing for axial retention as well as torque transmission. A hub that turns with the shaft is not necessarily secured against sliding along it.

Bearings support the shaft relative to the housing; the keyed hub transfers torque between the shaft and the attached rotating part. For the bearing arrangement and the shaft's axial location, see Shaft Support and Axial Adjustment.

To make and inspect the recesses, continue with Machining Keyseats and Keyways. That lesson connects shaft milling to broaching a through hub keyway.

A key joins the shaft slot to the hub slot to transmit torque

Before choosing a cutting method, identify which part needs the recess: the keyseat in the shaft or the keyway in the hub. See Keys and Keyways in Shaft-Hub Connections for the joint's terminology, diagram and torque path.

Identify the key by its cross-section and seating shape

Use the specified key form and drawing dimensions to choose the feature you need to make. Review key forms, fit and axial retention before planning the cut.

Mill a shaft keyseat with the cutter that matches its end and bottom shape

An end mill can make a straight, flat-bottom shaft keyseat that ends within the shaft's length. Its diameter leaves rounded ends in plan view; confirm that the key and drawing allow that shape. Use a permitted entry method for the selected cutter.

A Woodruff keyseat cutter is a disk-shaped cutter on a shank. Its diameter forms the curved recess for a Woodruff key and its cutting width establishes the slot width. Match both dimensions to the required recess. Not every tool sold as a keyseat cutter is the correct tool for every key.

1. Read the keyseat's width, depth, length or cutter diameter, and axial location from the drawing.
2. Support and secure the shaft against rotation and displacement. Use Milling Vises and Strap Clamps for the setup.
3. Align the shaft axis with the intended machine travel and locate its centerline using Mill Indicators and Part Zero. Set the cutter's cutting plane or path on that centerline, with the specified axial location and depth reference.
4. Choose the tool and entry method with End Mill Anatomy and Cutter Selection and plan the cut with Milling Operations. Cut to the planned allowance, measure, then finish to the required dimensions.

Review Equipment Operating Procedure and mill hazards in the safety collection before operation.

Supporting detail: Addendum 2 — Why cutter selection includes more than width.

Broach a through hub keyway; consider slotting when the geometry needs it

A conventional keyway broach and guide bushing are useful for a straight keyway through a hub bore. See Broaching for the pass sequence. The same arrangement does not make a closed-end keyseat on the outside of a shaft.

Shaping or slotting uses a reciprocating single-point tool to remove material in successive strokes. A suitable internal slotting setup can make a hub keyway when a through broach is unsuitable. Blind work still needs a planned tool runout and chip space; the method must fit the actual geometry.

Practice choosing a method

A broach divides a shaped cut among a row of teeth

In conventional broaching, a tool moves in a straight line relative to the work. Successive cutting teeth reach farther into the material. Rise per tooth is that increase in cutting height between successive teeth; it is not the tooth spacing along the tool. The spaces between teeth hold chips.

Roughing teeth remove most of the allowance. Semi-finishing teeth approach the final profile with smaller cuts. Finishing teeth establish size and finish; several may have the same height. The exact arrangement depends on the tool.

Supporting detail: Addendum 3 — Why the finishing teeth need not keep getting taller.

Push or pull describes the drive; internal or external describes the surface

A push broach is driven from behind; a pull broach is drawn through by its leading end. An internal broach works inside a prepared opening. An external broach works on an outside surface, such as a flat or an external spline. These are separate distinctions: internal does not mean pull-only.

A suitable hole broach can produce a square, hexagon or internal spline; a keyway broach cuts a slot in an existing bore. Conventional through broaching needs an exit path. Choose the tool from the required finished profile and starting-hole specification, not just a nominal shape name.

Repeated parts can justify dedicated tooling because many cutting edges complete the profile in a short cycle. A small keyway set can also suit repair or short-run work. Compare Material-Removal Process Families and Selecting a Material-Removal Process; Vertical Band Machining covers cutting contours with a continuous band instead.

Supporting detail: Addendum 4 — Production advantages depend on the job.

A bushing guides the keyway broach; shims move its cutting edge outward

The hub is the part surrounding the shaft. A slotted bushing fits its bore and guides the broach. A shim is a thin spacer behind the broach that increases the depth reached on a later pass. Broach, bushing and shims must be a compatible set.

Before operation, use Equipment Operating Procedure and press and mill hazard guidance in the safety collection. Follow the specific press and broach instructions for capacity, alignment, support, stroke, cut length and lubrication.

1. Check the drawing, prepared bore and required keyway. Select the matching broach, bushing and specified shim schedule.
2. Seat the bushing in the hub. Insert the broach in its slot for the first pass, normally without a shim on a multi-pass set.
3. Support the hub squarely on the press bed with clearance below for the entire broach to pass through. Align the ram, tool and guide.
4. Apply the specified lubricant and press the broach through according to its instructions.
5. After the completed pass, remove the broach and clean its teeth. If another pass is specified, insert the prescribed shim behind it, reinsert, lubricate and check alignment again.
6. Repeat with the specified shim stack for each remaining pass. Stop at the prescribed final setup; deburr and inspect as described in Machining Keyseats and Keyways.

Do not add a shim after the final pass. One-pass tools and some small broaches require none. Adding arbitrary shims is not a way to correct an uncertain dimension.

Supporting detail: Addendum 5 — Manufacturer reference for the pass sequence.

Count the first pass separately from the added shim passes

Rise per tooth and depth per pass are different quantities. For a simple planning exercise, let d₁ be the depth reached by the unshimmed pass and s the extra depth from each added shim increment. After N passes, depth is d₁ + (N − 1)s.

For a target depth D greater than d₁, the minimum count to reach at least the target is 1 + ceiling((D − d₁)/s). Ceiling means round upward to a whole number. If the first pass already reaches the target, the count is one. All depths here are measured radially from the original bore surface.

Example: an invented first-pass depth of 1.0 mm and two 0.5 mm increments give 1.0, 1.5 and 2.0 mm: three passes. This is counting practice, not a real broach specification. If rounding upward exceeds the allowed finished depth, that shim schedule cannot make the part to size.

Practice counting passes

Rotary broaching uses an angled end-cutting tool

Rotary broaching can make polygon forms such as a hex socket using a special holder on a suitable lathe or mill. A small angular offset makes successive portions of the tool's end engage as the tool and work move relative to one another. It does not use the long row of progressively taller teeth shown above.

It needs a prepared opening for internal work. Tool form, available thrust, material, depth and chip clearance limit the application; a blind socket needs a planned place for the chips. This is a process overview, not a setup procedure.

Supporting detail: Addendum 6 — How the rotary holder produces the cutting motion.