Dimensional Compensation In Tooling
Who this helps: casting Design Engineers / Buyers / Tooling Engineers building patterns and core boxes for gray/grey iron, ductile iron, and ADI parts.
What you’ll learn: practical shrinkage ranges, how to pre-comp for predictable distortion, and a fast T0→T1 feedback loop using CMM/3D scan.
Author: YB Metal Solution Engineering Team (hereafter YB Metal)
Table of contents
- Why compensation matters
- Linear shrinkage—starting numbers that actually work
- Warpage modes you can predict
- Models: from simple scale to morph fields
- T0→T1 workflow with CMM/3D scan feedback
- Worked example
- Process notes (green vs resin vs shell)
- Acceptance & documents
- What YB Metal delivers
- FAQs
Why compensation matters
Even perfect CT grades on the drawing won’t hold if linear shrinkage and shape distortion are not pre-compensated in the tooling. Good compensation reduces:
- First-article rework and tool re-cuts
- Machining stock “insurance”
- Iterations between buyer and supplier
Linear shrinkage—starting numbers that actually work
Use these as initial rules. Confirm with T0 data and adjust by section size and gating restraint.
| Material & process | Typical starting allowance |
|---|---|
| Gray iron (green/resin sand) | 0.9–1.2 % (≈ 1/8 in/ft ≈ 1.0 %) |
| Ductile iron (green/resin sand) | 0.7–1.1 % |
| ADI (after austemper) | Base ductile shrinkage, then allow +0.02–0.06 % dimensional change from heat treat (verify with coupons) |
| Shell molding (iron) | Similar scale, but tends to show freer shrink; start at the high end of the range |
| Large, heavy sections (>50 mm / 2 in) | Scale toward lower end (restraint ↑) |
| Thin sections (<8 mm / 0.31 in) | Scale toward higher end (cooling rate ↑) |
Pattern size = CAD_nominal × (1 + shrinkage_fraction)
Example: 200.00 mm × (1 + 0.010) = 202.00 mm
Warpage modes you can predict
Not all error is scale. Common low-order modes worth pre-comping:
- Bowl/Bow on plates/brackets (one-sided cooling, non-uniform modulus)
- Coning on rings/housings (spoke layout, hub restraint)
- Twist on long arms (uneven ribbing, cores not balanced)
- Ovalization on big bores (core weight/print deflection)
- Parting “step” (mismatch from mould deflection or handling)
Map the likely mode to a morph: a smooth, low-order deformation you add to the tool opposite to the expected distortion.
Models: from simple scale to morph fields
Level 1 — Global scale
One factor applied to all axes (most iron castings): Sx = Sy = Sz = 1 + s
Level 2 — Directional scale
If distortion is one-axis dominant (long housings/arms): Sx = 1 + sx ; Sy = 1 + sy ; Sz = 1 + sz
Level 3 — Shape morphs (warpage fields)
Add a smooth displacement field Δ(x) on top of scale: Compensated CAD = (CAD_nominal × S) + Δ(x)
Typical Δ(x): bowl (quadratic), cone (linear radial), twist (helical), oval (2θ harmonic).
Level 4 — Local features
Specific offsets on bores/pads to counter core shift, seating deflection, or gravity sag.
T0→T1 workflow with CMM/3D scan feedback
- Choose base scale from Section 2; add one morph for the dominant mode.
- Soft tool/printed pattern → pour T0 samples.
- Inspect on functional datums (not best-fit): CMM or 3D scan with datum lock.
- Decompose error into: rigid (ignore), uniform scale, low-order shape, local features.
- Update tool: tweak scale ±0.1–0.2 % if needed; refine morph amplitude/phase.
- T1 run → confirm critical (SC/CC) features and ISO 8062 CT bands.
- Freeze compensation; move to hard/production tool.
As-cast tolerance target: call out ISO 8062 CT by size band (e.g., CT8–CT10 typical for these sizes).
Worked example
Part: ductile-iron pump housing Ø300 × 120 mm wall rings.
Assumptions: s = 0.9 % global; expected coning away from hub.
- Global scale:
- Sx = Sy = Sz = 1 + 0.009
- Coning morph (radial, outward at rim):
- Δr = +0.25 mm at OD, tapering to 0 at hub
- Compensation on OD:
- CAD_OD = 300.00 × 1.009 + 0.25 = 303.95 mm
T0 result: OD mean 303.80; coning remaining +0.10 mm → reduce Δr to +0.15 mm and keep scale.
T1: all OD/ID within stock; face flatness improves to within spec.
Process notes (green vs resin vs shell)
- Green sand: more mould compliance → restraint varies with squeeze; control compactability and moisture to stabilize. See Green Sand capability
- Resin sand: stiffer mould/cores → stable shape, slightly lower apparent shrink in heavy sections. Compare .
- Shell: thin cores can sag—support prints, shorten spans, or add temporary ribs in tooling.
Acceptance & documents
- Compensation report: initial scale/morph, inspection plots, revision log.
- Dimensional pack: datum scheme, CMM/scan, best-fit overlay (for info only), CT grade check.
- Control plan: SPC on pattern wear (key dims) once in production; see Pattern Life & Maintenance
What YB Metal delivers
YB Metal combines simulation-backed gating, pattern design, and CMM/3D scanning to close the loop quickly:
- Comp plan: recommended scale + morphs by section, with risk notes.
- Try-out timing: T0/T1 schedule (soft → hard tool).
- Evidence: scan heat-map, datum-locked reports, PPAP submissions on request.
Want a part-specific compensation plan? Upload your drawing and YB Metal will return the proposed scale/morph and a quote.
FAQs
CTA — specify with proof, not guesses
Ready to trade weight, NVH and cost with real data? Upload your drawing to /rfq and YB Metal will send a standards-aligned proposal within your target lead time.