Slotting machine or milling machine for keyways?

Keyways transmit torque between a shaft and a gear, pulley, coupling or hub mounted on it. Manufacturing accuracy affects key fit, load distribution and the service life of the joint. Both the width and depth of the keyway and its position relative to the shaft axis and datum surfaces are important.

Keyways can be produced using slotting or milling machines, among other methods. A slotting machine is particularly useful for internal keyways in bores, whereas a milling machine is usually more productive when cutting external keyways on shafts. The best choice depends on component geometry, batch size and required accuracy.

Contents

• How does a slotting machine work?
• Why is it suited to internal keyways?
• When is a milling machine better?
• Accuracy and keyway quality
• Slotting versus broaching
• Comparison of the processes
• How to choose a machine
• Summary

How does a slotting machine work?

A slotting machine removes material using a cutting tool which travels back and forth in a straight line. During the cutting stroke, the edge removes a thin layer; successive passes gradually deepen the keyway. Feed and tool position are selected to suit the material, slot width and machine configuration.

Its main advantage is access to internal surfaces. A suitable cutting tool can enter the bore of a bush, hub, pulley or gear and cut a straight keyway without splitting the part. CORMAK metal slotting machines are especially useful for producing and refurbishing:

• gears and pulleys,
• bushes and hubs,
• coupling components,
• mechanical transmission parts,
• machine components with internal keyways,
• selected internal splines and profiles, where suitable tooling is available.

These machines are useful in toolrooms, maintenance departments and workshops producing one-off or short-run specialist components.

Why is it suited to internal keyways?

Machining a slot inside a bore is more difficult than machining an accessible external surface. An ordinary milling cutter cannot always reach the bore wall, particularly where the diameter is small or the bore is long. A slotting machine guides the tool along that wall, provided the bore diameter, machine stroke and space for tool movement and overrun are sufficient.

The operator locates the component, selects a tool for the keyway width and increases the depth of cut progressively. Changing the size often requires a different tool and adjustments rather than a dedicated broach. This is valuable when repairing older machines or handling frequently changing orders.

Blind keyways present a limitation: the tool needs clearance at the end of its stroke or a specially adapted process. Not every closed bore can be machined on a standard slotter. Slotting is generally slower than milling accessible outside surfaces, but may reduce tooling costs for short runs.

When is a milling machine better?

Metal milling machines are best suited to external keyways on shafts, axles and other readily accessible surfaces. A rotating milling cutter removes material continuously, which usually makes this process more productive for such work.

End mills and side-and-face or disc cutters may be used, selected according to the slot's width, length, depth, shape and material. Stops, dividing equipment, suitable workholding and CNC control can improve repeatability. Universal milling machines are particularly useful in workshops with varied work.

Milling is suitable for:

• keyways on shafts,
• open and accessible surfaces,
• batch production,
• slots of different lengths,
• T-slots and guide slots,
• components requiring several operations in one setup.

Versatility is a major benefit: with suitable tooling, the same machine can also mill flats, steps and pockets. For combined drilling and milling tasks, machines such as the CORMAK ZX7045 mill-drill provide a relevant example.

Accuracy and keyway quality

Both processes can achieve the specified dimensions when tooling, cutting parameters and workholding are appropriate. Rigidity, tool wear, vibration and the component datum affect the result. Width, depth, length and position must be checked against the engineering drawing.

Accuracy when slotting

The cutting tool must be correctly aligned with the bore axis. Incorrect location can shift the keyway or produce uneven walls. Taking too much material per stroke increases the risk of vibration and poor surface finish.

Accuracy when milling

Cutter run-out, geometry and wear, rigid shaft clamping and alignment are critical. Errors can produce an oversized keyway and poor key fit. Burrs should be removed after machining without excessively rounding functional edges.

Slotting versus broaching

Internal keyways can also be produced by broaching. Successive cutting teeth on the broach remove material incrementally, giving high output and repeatability in sufficiently large batches.

However, broaching requires a suitable broach, guide bush and compatibility with bore and keyway geometry. Tooling can be expensive for small runs. Interchangeable slotting tools provide greater flexibility, making slotting attractive for one-off parts, repairs and variable orders.

Comparison of the processes

How to choose a machine

Choose a slotting machine if you regularly produce internal keyways in bushes, hubs, gears or couplings, especially for repairs and short batches. Choose a milling machine if external shaft keyways predominate and additional machining operations are needed.

Before buying, assess maximum workpiece dimensions, tolerances, materials, cycle times, machine travel and tooling costs. For workshops considering particular mill-drill configurations, the CORMAK ZX7045 B1 is another documented option to compare by specification. Slotting and milling often complement each other in a well-equipped workshop.

Summary

A slotting machine is particularly well suited to internal keyways in hubs and bushes, provided the tool has sufficient access. A milling machine is usually more productive for external keyways on shafts and more versatile for additional work.

The final choice should take account of keyway location and geometry, precision, production volume and setup costs. These criteria make it easier to compare appropriate solutions in the CORMAK range.