Troubleshooting Eccentricity in Internal Broaching

One of the most common quality problems in internal broaching is eccentricity—the broached feature is not concentric with the pilot hole. This occurs because the broach, supported only by the pulling mechanism during cutting, can be pushed off-center by unbalanced cutting forces.

The symptoms show up in several ways: unprocessed material remaining when the outer diameter is subsequently turned, a go-gauge that won't pass, improperly formed tooth profiles, excessive variation in between-pin measurements, or partial uncut surfaces on the minor diameter.

Diagnosing the root cause requires a systematic approach. A straightforward first step is to mark both the workpiece and the broach so their angular positions are tracked, then broach several parts while rotating the broach mounting position in 90-degree increments. If the direction of maximum runout follows the workpiece orientation, the machine is the likely culprit. If it tracks with the broach position, the broach itself is suspect. If the eccentric direction varies randomly, the workpiece is the most probable source.

Machine-related causes include an out-of-level faceplate (on vertical machines), misalignment between the puller and the travel axis, worn or damaged slides, excessive clearance, or uneven coolant flushing that creates asymmetric forces on the broach.

Broach-related causes include uneven wear or chipping around the circumference of the tooth tips, inconsistencies in chip pocket geometry (often introduced during resharpening), or a bent tool. Runout can be checked by measuring the tooth tip diameter at several circumferential positions along the length of the tool using a dial indicator referenced to the center holes.

Workpiece-related causes include poor roundness, cylindricity, or perpendicularity of the pilot hole, excessive clearance between the broach's front guide and the pilot bore (ideally within 0.03 mm), or uneven material hardness across the cross-section of the part.