Guided by YB Metal (Suzhou, China) — fillet radii for iron castings: green sand, resin/no-bake & shell molding under one roof, with machining, simulation-led gating/risers, and traceable QA.
Excerpt: Smart fillets, radii, and transitions lower stress concentration, improve fill/feeding, and reduce machining and scrap. Use these 9 proven rules, copy-paste tables, and notes you can drop into your drawings and RFQs today.
Table of contents
- Why fillets & radii matter (stress + flow)
- 9 proven rules (copy & use)
- Radius tables by wall & process (mm/in)
- Transitions & Kt: what to avoid, what to blend
- Flow-friendly features (hot-spot & feeding tips)
- Machining & GD&T notes (ISO 8062 CT, Ra, stock)
- Drawing/RFQ checklist
- FAQs
Why fillets & radii matter
Sharp corners spike stress (higher Kt) and create hot spots that shrink late and trap gas/inclusions. Proper radii stabilize flow, shorten fill paths, raise yield, and cut machining minutes by avoiding heavy stock in corner clean-ups.
9 proven rules (copy & use)
Indicative ranges. Your drawing/spec and foundry capability prevail.
- Blend every internal corner — avoid zero-radius re-entrant edges.
- Radius ratio: start from R ≈ 0.25–0.50 × t (t = thinner wall) for internal fillets; R ≥ 0.2 × t for external.
- Meet process reality: green sand needs a larger R than the shell at the same wall.
- Step-downs: use conical/filleted reducers (3:1 to 5:1 taper) instead of abrupt steps.
- Bosses & pads: tie into ribs with blends; avoid isolated thick bosses.
- Rib roots: R ≥ 3–5 mm (0.12–0.20 in), plus 50–70% rib thickness fillet at base.
- Hole to wall: keep edge distance ≥ 1.5–2.0 × hole Ø or add a pad with blends.
- Machine reliefs: if a sharp outside edge is functional, cast with R and machine a small chamfer later.
- Call out ranges, not absolutes on drawings (e.g., internal fillets R 3–6 mm), letting foundry optimize for flow.
Radius tables by wall & process
Indicative design starts. Your foundry’s simulation and pattern capability prevail.
Table A — Minimum recommended internal fillet radii by wall & process
| Thinner wall t | Green sand | Resin / no-bake | Shell mold |
|---|---|---|---|
| 3–4 mm (0.12–0.16 in) | R 1.5–2.0 mm (0.06–0.08 in) | R 1.2–1.8 mm (0.05–0.07 in) | R 1.0–1.5 mm (0.04–0.06 in) |
| 5–7 mm (0.20–0.28 in) | R 2.0–3.5 mm (0.08–0.14 in) | R 1.8–3.0 mm (0.07–0.12 in) | R 1.5–2.5 mm (0.06–0.10 in) |
| 8–12 mm (0.31–0.47 in) | R 3.0–5.0 mm (0.12–0.20 in) | R 2.5–4.0 mm (0.10–0.16 in) | R 2.0–3.5 mm (0.08–0.14 in) |
| 13–20 mm (0.51–0.79 in) | R 4.0–7.0 mm (0.16–0.28 in) | R 3.5–6.0 mm (0.14–0.24 in) | R 3.0–5.0 mm (0.12–0.20 in) |
Table B — External edge radii（machining to sharp later if needed）
| Edge type | Start from | Notes |
|---|---|---|
| Outside corners | R 0.2 × t (min R 1.0–1.5 mm / 0.04–0.06 in) | Cast safe, machine to sharp or chamfer |
| Pad/boss edge | R 2–4 mm (0.08–0.16 in) | Keeps coating & flow stable |
| Rib tips | R 1.5–2.5 mm (0.06–0.10 in) | Reduces sand breakage & fins |
Transitions & Kt: what to avoid, what to blend
Table C — Transition choices vs stress concentration
| Geometry | Bad | Better | Why |
|---|---|---|---|
| Wall step t₁→t₂ | Abrupt 90° step | 3:1 cone + fillet | Lowers Kt; smoother flow |
| The boss on the wall | Rib into the wall | Boss + pad + blend | Cuts root stress; sand-friendly |
| Avoids hot spots & sink | Sharp root | Large root fillet | Cuts root stress; sand friendly |
| Junction of 3 walls | Sharp tri-corner | Blend with spherical fillet | Avoids last-to-freeze pocket |
Rule of thumb: every time you halve the radius, expect a noticeable Kt rise and worse feeding. Err larger if space allows.
Flow-friendly features（feeding & hot-spot tips）
- Uniformity first: keep sections even; shift mass toward risers; avoid isolated thick pads.
- Chills & sleeves: allow fillets to stay large while controlling freeze order.
- Core prints: add blends where cores land; sharp prints crack sand and leak metal.
- Runner junctions: avoid sharp T-joints; use radius tees and flow-split blends.
- Vents & fillets: generous fillets improve gas escape paths in sand and reduce erosion.
Machining & GD&T notes（ISO 8062 CT, Ra, stock）
- CT grades (typical capability): green sand CT9–CT10; resin/no-bake CT8–CT9; shell CT6–CT7 (ISO 8062-3).
- Surface finish targets: as-cast Ra 6.3–12.5 μm (250–500 μin) green sand; 3.2–6.3 μm (125–250 μin) resin; 1.6–3.2 μm (63–125 μin) shell.
- Machining stock by mass（iron castings）:
- 5–10 kg (11–22 lb): 1.5–2.5 mm green / 1.2–2.0 mm resin
- 10–50 kg (22–110 lb): 2.5–3.5 mm green / 2.0–3.0 mm resin
- GD&T: use datums that sit on blended pads; avoid relying on sharp cast edges.
- Functional sharpness: if sealing edges must be sharp, cast with a radius, then machine the sealing feature.
Drawing / RFQ checklist（paste-ready）
- Process & CT：e.g., No-bake, ISO 8062-3 CT8
- Fillets & radii：Internal fillets per Table A; external edges per Table B
- Transitions： Use conical/filleted reducers; no abrupt steps
- Surface & stock： Machined faces Ra 3.2–6.3 μm; stock per mass range
- Hot-spot control： Allow use of chills/sleeves; simulate before tooling
- Units & spelling：mm/in、kg/lb；include gray/grey once
FAQs
CTA — design it right the first time
YBmetal uses simulation to size fillets, ribs, and transitions before tooling. We quote with CT/Ra assumptions, yield notes, and a pilot run plan so your team can approve fast.