ISO 8062 Casting Tolerances Explained: CT Grades with Real Examples

Typical CT grades by process & size (capability window)

Indicative windows from production programs (gray/grey & ductile iron). Use them to choose a grade; final capability is geometry-specific.

Notes

• Shell achieves cleaner edges & lower Ra (1.6–3.2 µm); green sand is 6.3–12.5 µm.
• Heavier sections and many cores widen variation → higher CT number (looser tolerance).

How to put ISO 8062 on your drawing (correct notation)

Use a general note for all as-cast features, then detail exceptions:

• General as-cast tolerance: ISO 8062-3 CT8 unless otherwise specified.
• Surface texture: As-cast surfaces per ISO 1302; Ra as shown.
• Machined features: don’t assign CT—use GD&T (e.g., ⌀ datum A/B/C, position 0.2) and basic dimensions.
• Per-feature CT (if different): mark specific faces/bosses with CT7/CT9 balloons or callouts.

Draft & shrinkage belong on the casting model/pattern spec, not as CT grades.

Real CMM examples (what we actually see)

Examples from production CMM layouts; single-point linear dims shown in mm (in). Each program passed FAI; as-cast only.

Example A — Gray iron bracket, 180 × 120 × 60 mm; green sand; target CT8

• 100.0 mm boss-to-boss: +0.5 / −0.4 (0.020 / 0.016 in) → within CT8
• 60.0 mm wall length: ±0.5 (±0.020 in)
• 25.0 mm rib height: ±0.4 (±0.016 in)
• As-cast Ra ≈ 6.3–12.5 µm; machining stock used 1.5–2.0 mm (0.06–0.08 in)

Example B — Ductile iron housing, 420 × 280 × 180 mm; resin sand; target CT9

• 300.0 mm face-to-face: +1.2 / −0.9 (0.047 / 0.035 in)
• 120.0 mm boss spacing: ±0.8 (±0.031 in)
• 40.0 mm web height: ±0.7 (±0.028 in)

Example C — Gray iron cover, 220 × 170 × 35 mm; shell; target CT7

• 150.0 mm length: ±0.5 (±0.020 in)
• 80.0 mm width: ±0.4 (±0.016 in)
• Near-net machined faces used stock 1.0–1.2 mm (0.04–0.05 in)

These are indicative—not normative values of the standard—but reflect what buyers can expect with good gating, sand control and inspection.

CT, GD&T and machining allowance—how they work together

• CT grade = as-cast size variation; use it for non-machined features.
• GD&T = functional controls on machined features (position/profile/flatness/runout).
• Machining allowance = stock to clean up as-cast surfaces; pick by size & process:

Choosing a CT grade—simple decision path

• Pick process & size window → read table in §2.
• Check geometry risks (thin walls, long spans, heavy junctions) → if risky, one CT looser.
• Where machining cleans up → leave as-cast CT looser; tighten via GD&T after machining.
• Pilot & validate → FAI + CMM/3D scan; adjust CT/GD&T before cutting hard tools.

Cost & risk notes (why CT tightens price)

Rule of thumb: every step from CT9 → CT8 → CT7 increases piece price and tooling/time—only tighten where it matters functionally.

YBmetal Quality & verification plan

• Metallurgy & sand: spectrometer per heat; AFS GFN/moisture/LOI control.
• Dimensional: FAI + CMM layouts; 3D scan on complex parts; capability (Cp/Cpk) on critical dims.
• Records & traceability: heat/batch IDs; fixture/datums documented; photo records for special features.

RFQ checklist (tolerance-ready)

• 3D + 2D drawings; mark as-cast vs machined clearly.
• General CT grade target (e.g., ISO 8062-3 CT8).
• GD&T for machined features + datum scheme.
• Process preference (green/resin/shell) or required Ra/min wall.
• Inspection scope (FAI/CMM, sampling level); NDT/leak test if any.
• Volume, batches, PPAP level, coating/packaging, destination.