Part Preparation Guide for Rotary Broaching

Pre-Drill Pilot Hole Hole Diameter

Internal rotary broaching requires a properly sized pre-drilled pilot hole to ensure optimal tool performance, dimensional accuracy, and broach life. For best results, the pilot hole diameter should be greater than the minor diameter of the finished form. An undersized pilot hole increases cutting forces, accelerates tool wear, and may adversely affect part quality.

The recommended pilot hole diameters for hexagonal, square, and hexalobular (Torx) forms are calculated using the formulas provided in the charts below.

This additional clearance reduces material displacement, lowers cutting loads, and promotes improved form accuracy and surface finish during the rotary broaching process.

For profiles with multiple lobes or complex geometries—such as serrations and splines, —it is recommended that the pilot hole diameter be as large as your specifications allow of the finished profile. This additional clearance reduces material displacement, lowers cutting loads, and promotes improved form accuracy and surface finish during the rotary broaching process.

Common Pilot Hole Calculations:

• The recommended pilot hole diameter for internal hexagonal forms is 1.03 × the actual across-flats (A/F) dimension.
• The recommended pilot hole diameter for internal square forms is 1.10 × the actual across-flats (A/F) dimension.

Recommended pilot hole percentages are intended as general guidelines and may be adjusted based on workpiece material characteristics. For free-machining materials, the recommended percentage may be reduced to increase form engagement. For materials with lower machinability, higher strength, or greater toughness, the pilot hole diameter should be increased to reduce cutting forces, minimize tool loading, improve chip evacuation, and extend broach service life.

Pre-Drill Pilot Hole Depth

When rotary broaching a blind hole, the pre-drilled pilot hole should be machined to the maximum practical depth. The pilot hole depth must exceed the required broaching depth to provide sufficient chip accumulation and prevent chip compaction at the bottom of the hole. Inadequate chip clearance can significantly increase cutting forces, accelerate tool wear, degrade form accuracy, and potentially result in broach failure.

Polygon Solutions recommends a minimum pilot hole depth of 1.3 to 1.75 times the required broach form depth, depending on the workpiece material and chip characteristics. Whenever possible, an undercut or relief groove immediately beyond the broached form is recommended to provide a chip relief area, allowing chips to fracture and evacuate more efficiently while reducing axial loading on the broach.

Where an undercut cannot be incorporated into the part design, excess chips should be removed by a secondary drilling or boring operation prior to completion of the broaching cycle to maintain optimal cutting conditions and ensure dimensional integrity of the finished form.

Please Note:
When the pilot hole depth is limited by part geometry or design constraints, Polygon Solutions’ Alignment Brake Attachment can be used to perform the broaching operation in multiple incremental passes. Between passes, accumulated chips can be removed from the pilot hole, reducing chip packing, lowering cutting forces, and minimizing tool loading. This staged broaching process improves form accuracy, extends broach life, and enables the successful production of deeper blind forms where chip evacuation would otherwise be restricted.

Lead-In Chamfer

A lead-in chamfer is required for all Polygon Solutions’ internal rotary broaching applications to ensure proper tool entry, alignment, and progressive engagement with the workpiece. Polygon Solutions recommends a 45° chamfer with a major diameter slightly larger than the maximum (major) diameter of the rotary broach. The chamfer provides the necessary clearance for smooth tool entry, reduces initial cutting loads, minimizes edge chipping, and promotes accurate form generation.

For workpiece materials with lower machinability, higher strength, or greater toughness, a 30° lead-in chamfer may provide improved cutting performance by reducing the instantaneous material engagement at tool entry. This can lower peak cutting forces, improve tool stability, and extend broach service life.

Please Note:
Failure to provide a properly sized lead-in chamfer can result in excessive impact loading as the rotary broach enters the workpiece. This may cause cutting edge chipping, premature tool wear, degradation of form accuracy, and, in severe cases, catastrophic broach failure. A properly machined lead-in chamfer ensures progressive tool engagement, reduces entry forces, and promotes consistent broaching performance.