Technical Guide4 min read

How to Control Concentricity When Installing Stepped Ejector Pins

Key Takeaway: Pin-to-bore concentricity must be within TIR 0.01mm for precision molds. The three keys: (1) plate bore alignment ±0.005mm, (2) counterbore perpendicularity ±0.02mm, (3) pin head-to-shaft concentricity ±0.005mm (factory-guaranteed).

Why Concentricity Matters

Stepped ejector pins have two diameters that must be concentric. If the head (D1) is offset from the shaft (D2), the ejection force is applied off-center, causing: (1) uneven ejection marks on the part, (2) accelerated bore wear on one side, and (3) premature pin breakage at the step transition.

Installation Alignment Checklist

Counterbore depth: Must equal head thickness ±0.01mm. Too deep = head rattles; too shallow = head protrudes above plate surface.
Bore-to-counterbore alignment: Both machined in the same CNC setup (do not re-fixture between operations).
Plate surface flatness: Better than 0.02mm across the plate face. Check with a precision straightedge.
Pin inspection: Verify head-to-shaft TIR with a V-block and dial indicator before installation. Factory specification is TIR ≤ 0.01mm.

Frequently Asked Questions

How do I measure pin concentricity on-site?+

Place the pin shaft in a V-block, rest the head on a flat surface, and rotate the pin 360°. A dial indicator touching the shaft surface shows the total indicator runout (TIR). For field inspection, a TIR of 0.015mm or less is acceptable.

Can misalignment be corrected after installation?+

Minor misalignment (TIR < 0.02mm) can be corrected by lapping the counterbore. For larger misalignment, the bore must be re-machined. Prevention is far cheaper than correction.