Technical Guide4 min read

What Bore Specifications Do Two-Step Ejector Pins Require?

Key Takeaway: Two-step bores require three concentric diameters machined in a single CNC setup. All three bores must be concentric within TIR 0.01mm. Use an H7 reamer for each diameter step.

Bore Profile Specification

Bore Section	Matches Pin	Tolerance	Surface Finish
Counterbore (top)	Head diameter D1	H7	Ra ≤ 1.6μm
Middle bore	Middle shaft D2	H7	Ra ≤ 0.8μm
Tip bore (bottom)	Tip diameter D3	H7	Ra ≤ 0.8μm

Machining Sequence

Critical: all three bore diameters must be machined in a single CNC setup without re-fixturing the plate. This ensures concentricity. Steps: (1) drill pilot hole full depth, (2) ream tip bore (D3 + H7) full depth, (3) ream middle bore (D2 + H7) to intermediate depth, (4) counterbore head pocket (D1 + H7) to head thickness depth. Verify concentricity with a bore gauge before removing the plate from the fixture.

Frequently Asked Questions

Can I use EDM instead of reaming for two-step bores?+

Yes — EDM provides superior concentricity for tight-tolerance bores. However, EDM is 3–5× slower and more expensive than reaming. Use EDM only when bore concentricity requirements exceed what CNC reaming can achieve (TIR < 0.005mm).

What if the middle bore is not concentric with the tip bore?+

Misalignment creates a step at the transition that will score the pin shaft, leading to premature wear. If misalignment exceeds TIR 0.02mm, the bore must be re-machined or a new plate produced.