Can I Broach Odd-Sized or Non-Standard Keyways with CNC Tools?

How custom inserts and step-over programming make non-standard keyways possible

Not every keyway follows a standard size. Some parts require unusual widths, deeper grooves, or dimensions that do not match commonly available tooling. For machine shops, these applications can create challenges when selecting a broaching method.

Many non-standard keyways can be produced using CNC broaching tools without investing in dedicated broaching equipment. Depending on the application, shops may be able to use a standard insert with step-over programming or a custom-ground insert to achieve the required dimensions.

The important step is understanding which approach works for the feature being machined.

What Makes a Keyway Non-Standard?

Standard keyways are generally designed around established dimensions for shaft and hub connections. However, custom components, replacement parts, and specialized equipment may require dimensions outside those standards.

A non-standard keyway might involve an unusual width, a specific radial depth, or a combination of dimensions that cannot be produced with a standard broaching insert.

When evaluating these applications, machinists should consider the keyway width, axial length, bore diameter, and Height of Groove. Each dimension affects tooling selection and whether the feature can be produced with the available equipment.

Can You Broach a Wider Keyway with a Standard Insert?

One advantage of CNC broaching is the ability to use step-over programming to produce certain keyways that are wider than the insert itself.

Rather than purchasing a custom-width insert, a machinist may be able to use a narrower standard insert and program additional cutting passes with small offsets to either side of the keyway centerline.

For example, CNC Broach Tools describes an application using a 0.318-inch-wide insert to produce a 0.377-inch-wide keyway. By offsetting the toolpath 0.0295 inch in each direction, the insert can machine the additional width required.

This approach can be used on CNC mills and CNC lathes equipped with Y-axis capability, provided the tooling and part geometry support the operation.

Step-over programming is not suitable for every application. The insert must still be capable of reaching the required radial depth, and the machine must provide the necessary axis movement and control.

When Is a Custom-Ground Insert Needed?

When step-over programming is not practical, a custom-ground insert may provide another option.

CNC Broach Tools offers custom insert grinding for applications requiring widths or tolerances outside its standard insert offerings. This can be useful when the keyway must meet a specific dimension that cannot be achieved with an available standard insert.

More specialized insert geometries may require additional evaluation to determine whether a custom tooling solution is feasible.

Before ordering custom tooling, shops should review the complete keyway dimensions rather than focusing only on width. A custom insert still needs to work within the available bore clearance, toolholder geometry, and cutting depth requirements.

Height of Groove Still Determines Tooling Capability

One of the most important considerations when evaluating non-standard keyways is the Height of Groove, or H.O.G.

CNC Broach Tools uses this term to describe the radial depth of the keyway. Each insert and toolholder combination has limitations on how much radial depth it can produce.

This becomes particularly important when using step-over programming. Although a narrower insert may be able to produce a wider keyway through additional passes, the required Height of Groove must remain within the insert’s capability.

Bore diameter also matters. Smaller bores may limit the size of the toolholder that can fit inside the part while still providing the necessary insert projection.

These dimensions should be verified before deciding whether a standard insert, custom-ground insert, or different tooling configuration is appropriate.

Setup and Programming Considerations

Non-standard keyways require the same attention to rigidity, alignment, and programming as standard keyways.

The broach should be securely mounted with minimal overhang and accurately indicated so the cutting edge remains properly oriented throughout the stroke. Cutting loads should be controlled through appropriate incremental pass depths and consistent machine movement.

Entry and exit clearance must also be considered.

For blind keyways, a relief groove or cross-drilled hole is necessary to allow the insert to fully clear the feature before retracting. Without sufficient relief, the insert can become trapped in the material and damage the tooling.

When using step-over programming, each offset cutting pass must also maintain proper clearance and remain within the tooling’s cutting limitations.

More Options for Non-Standard Keyways

An unusual keyway dimension does not automatically mean the operation must be outsourced or require a dedicated broaching machine.

In some applications, step-over programming allows shops to produce wider keyways using standard inserts. In others, a custom-ground insert or specialized tooling configuration may be the better solution.

CNC Broach Tools can help evaluate non-standard applications based on the required keyway width, Height of Groove, bore diameter, and length of cut.

Understanding these dimensions early helps shops select the right approach, avoid unnecessary tooling costs, and expand the range of keyways they can produce on existing CNC equipment.