Mold Guide Post and Bushing Fits — Clearance Tolerances, Oil Grooves, and Ball Cage Preload Standards

The guiding system forms the alignment backbone of stamping dies and injection molds. If the clearance between the guide pillar (post) and bushing is too loose, upper and lower die components will experience misalignment, uneven punch cutting clearances, and accelerated mold wear. Conversely, if the clearance is too tight, frictional heating at high speeds can cause seizure (galling) and machine shutdown.

1. Sliding Guide Fits: H7 / g6 Standard

For standard stamping dies and plastic injection molds operating under moderate cycle speeds, the ISO H7/g6 fit is universally adopted as the primary precision sliding standard:

• Bushing Bore Tolerance (H7): Internal diameter precision held within +0.015 / 0 mm (for Ø25 mm).
• Guide Post OD Tolerance (g6): External diameter precision held within -0.007 / -0.020 mm (for Ø25 mm).
• Initial Working Clearance: Typically 0.007 mm to 0.035 mm, providing adequate room for an unbroken hydrodynamic lubrication film.

2. Clearance Reference Chart by Guide Post Diameter

3. Hardened Steel vs. Self-Lubricating Bronze Bushings

The pairing of materials between post and bushing is critical to preventing identical-metal galling:

• SUJ2 / 20Cr Steel Guide Post + Hardened Alloy Bushing (HRC 60+): Delivers highest rigidity for precision stamping dies, but requires continuous forced oil or grease lubrication through spiral or diamond oil grooves.
• SUJ2 Post + Sintered Graphite Bronze Bushing: Embedded solid graphite plugs provide micro-lubrication without requiring constant external oil. Ideal for cleanroom medical molds and automotive stamping dies where oil drops would contaminate parts.

4. Ball Bearing Cage Guide Sets (Zero-Clearance Preload)

When stamping thin metal foils (≤ 0.2 mm) or high-speed connector terminals at 600+ strokes per minute, sliding clearances can cause punching burrs.