Core Pin Tolerance Grades Explained: Standard vs High-Precision Fits
Key Takeaway: Use h7 shaft tolerance for 90% of standard molds — it provides reliable fit and adequate concentricity. Upgrade to h6 only for thin-wall parts (wall < 0.8 mm), medical devices requiring ±0.005 mm positioning, or multi-cavity molds where cavity-to-cavity consistency is critical.
Understanding ISO Tolerance Grades
Core pin shaft tolerances follow the ISO 286 system of limits and fits. The lowercase letter (h) means the shaft is manufactured at or below nominal — never oversize. The number (6 or 7) defines the tolerance band width:
| Shaft ø | h7 Tolerance | h6 Tolerance | H7 Bore Tolerance |
|---|---|---|---|
| ø3 mm | 0 to −0.010 mm | 0 to −0.006 mm | 0 to +0.010 mm |
| ø5 mm | 0 to −0.012 mm | 0 to −0.008 mm | 0 to +0.012 mm |
| ø8 mm | 0 to −0.015 mm | 0 to −0.009 mm | 0 to +0.015 mm |
| ø10 mm | 0 to −0.015 mm | 0 to −0.009 mm | 0 to +0.015 mm |
Fit Types for Core Pins
- H7/h7 (Clearance Fit) — Standard for taperless and straight core pins. The pin slides into the bore by hand. Maximum clearance = H7 max + h7 max = up to 0.024 mm on ø5. Adequate for parts with wall thickness ≥ 1.0 mm.
- H7/h6 (Close Clearance Fit) — Tighter fit with less lateral play. The pin may need a light push to insert. Maximum clearance reduced to 0.020 mm on ø5. Required for thin-wall parts or high-precision medical devices.
- Taper Fit (JIS/DIN) — Straight core pins with taper use a 1:100 or 1:50 taper instead of shaft tolerance for retention. The taper eliminates clearance entirely but requires precision taper reaming of the plate bore.
When to Upgrade from h7 to h6
| Scenario | h7 Adequate? | h6 Recommended? |
|---|---|---|
| Standard parts, wall ≥ 1.0 mm | ✅ Yes | Not needed |
| Thin-wall parts, wall 0.5–0.8 mm | ⚠️ Marginal | ✅ Recommended |
| Medical/optical precision parts | ❌ Insufficient | ✅ Required |
| Multi-cavity (16+ cavities) | ⚠️ Check consistency | ✅ Recommended |
| Long pins (L/D > 8) | ⚠️ Deflection concern | ✅ + core sleeve |
For long pins (length-to-diameter ratio exceeding 8:1), even h6 tolerance may not prevent deflection under injection pressure. In these cases, add core sleeves to provide lateral support along the pin's length.
Measuring Core Pin Tolerance
- Incoming inspection — Measure shaft diameter at three points (near head, mid-shaft, near tip) using a calibrated micrometer. All three readings must fall within the specified tolerance band.
- Bore verification — After reaming, measure the plate bore with a bore gauge. The bore must be within H7 tolerance for the core pin to fit correctly.
- Assembly check — Insert the pin and measure runout (TIR) at the tip using a dial indicator. TIR should be ≤ 0.01 mm for h7 and ≤ 0.005 mm for h6.
According to JIS B 6131, core pin shaft diameter must be inspected per lot with a minimum sampling rate of 5% or 3 pieces (whichever is greater).
Frequently Asked Questions
What tolerance grade should I specify for standard core pins?+
h7 is the standard grade for most applications. It provides reliable slip fit in an H7 bore with concentricity within ±0.005 mm. Specify h6 only when tighter positioning is required.
What is the difference between h6 and h7 tolerance on core pins?+
h6 has a tighter band — for ø5 mm: h6 = 0 to −0.008 mm vs h7 = 0 to −0.012 mm. Tighter tolerance means less clearance in the bore, better concentricity, but requires more precise plate machining.
Can loose core pin tolerance cause part defects?+
Yes. Excessive clearance allows lateral pin shift under injection pressure, producing eccentric wall thickness. For thin-wall parts (< 0.8 mm), even 0.01 mm shift causes visible defects.
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