When engineers send us a request for quotation, one question comes up in almost every conversation: “What tolerance can you hold?” It is a reasonable question — but it is also one where a little knowledge saves a lot of money. Over-specifying tolerances on a cast part quietly inflates costs through unnecessary machining, added inspection, and scrap. Under-specifying them creates assembly problems downstream. This guide explains what investment casting tolerances actually mean, how the CT grade system works, and how experienced buyers use it to get the precision they need at the lowest total cost.
Why Tolerance Language Matters in Casting
Machining shops speak in terms of IT grades (ISO 286) — tolerances measured in hundredths of a millimeter, achieved by removing material with rigid, programmable tools. Foundries speak a different language. Cast parts are formed by liquid metal solidifying inside a mold, and their dimensional variation is governed by wax pattern accuracy, shell expansion, alloy shrinkage, and part geometry.
The international standard for casting tolerances is ISO 8062, which defines a series of “Casting Tolerance” grades — CT grades. The lower the CT number, the tighter the tolerance. Understanding where your part falls on this scale is the foundation of every casting procurement decision.
The CT Grade System at a Glance
CT grades assign a tolerance band based on the nominal dimension of the feature. The same CT grade allows a wider absolute band on a large dimension than a small one. As a practical reference for steel and iron investment castings:
- CT4–CT5 — the practical best case for small investment castings (typically under ~50 mm) in silica sol process
- CT5–CT6 — the standard commercial range for most silica sol investment castings up to ~200 mm
- CT6–CT8 — typical for water glass process castings and larger, more complex geometries
- CT8–CT12 — the territory of sand casting and die casting with high draft and parting-line variation
For comparison, a 100 mm dimension under different grades gives a rough picture: CT4 allows about ±0.26 mm, CT5 about ±0.36 mm, CT6 about ±0.5 mm, and CT9 — a reasonable sand casting figure — about ±1.5 mm. This is why the process choice alone, before any machining, buys you a two- to four-fold improvement in as-cast precision.
What Actually Drives Tolerance in Investment Casting
Four factors control where your part lands on the CT scale:
1. Wax pattern accuracy. Every dimension of the casting inherits the accuracy of the wax pattern that formed it. Modern wax injection with aluminum dies holds patterns to a few hundredths of a millimeter, but wax shrinkage, die temperature, and injection pressure all vary slightly between shots. This is the first — and largest — contributor to casting variation, and it is why dimensional capability is inseparable from pattern tooling quality.
2. Shell and metal behavior. The ceramic shell expands slightly during firing, and every alloy shrinks on solidification — carbon steel roughly 2%, stainless steel 2.5–3%. Shrinkage is not perfectly uniform: it is influenced by section thickness changes, casting position, and how the foundry engineers feeding. Complex parts with abrupt section changes will not shrink identically in every direction, and skilled foundries compensate this in the pattern die.
3. Part geometry. Long thin parts can distort during shell removal or heat treatment; parts with widely separated features accumulate pattern and shell variation across the span. A feature 400 mm away from the datum will never hold the same band as one 40 mm away — not because the foundry is careless, but because error accumulates over distance.
4. Process route. Water glass shells are cheaper but less dimensionally stable than silica sol shells. If your drawing calls CT5 across the board, you are specifying silica sol — and the price reflects it. If CT7 is genuinely enough for non-critical features, water glass may win the quote.
As-Cast vs Machined: The Decision That Saves the Most Money
The single most valuable exercise in casting procurement is sorting every feature on the drawing into three buckets:
Bucket 1 — As-cast is fine. Non-functional surfaces, clearance features, cosmetic profiles. Specifying CT6 as-cast here is completely sufficient. Machining these features is pure waste.
Bucket 2 — As-cast with a controlled band. Features that mate with soft parts, locate loosely, or carry seals may need better than standard as-cast capability. Foundries can tighten these bands with dedicated pattern control, gauge-based sampling, and selective machining of datum preps — without full machining.
Bucket 3 — Must be machined. Bearing bores, H8/H7 fits, threaded holes, gasket faces, gear-mounting surfaces. No casting process delivers IT7 directly; these features get a machining allowance of 1.5–3 mm and are finished on CNC.
Drawings that blanket-specify ±0.05 mm everywhere force the foundry to quote full machining of every surface. Drawings that bucket features intelligently routinely save 20–40% of the machining content. When we review drawings at Ningbo Ruican, this bucketing exercise is the first value we add — before any metal is poured.
Flatness, Straightness, and Other Geometric Calls
CT grades cover size tolerances, but geometric tolerances — flatness, straightness, positional — follow their own logic. Typical as-cast capability for a 150 mm steel investment casting is around 0.5–1.0 mm flatness. If a sealing face needs 0.05 mm flatness, that is a machining operation, full stop. Positional tolerances on cast hole patterns depend on whether the holes are cast or drilled: cast holes follow tooling accuracy (roughly ±0.3–0.8 mm positional at moderate sizes), while drilled patterns inherit CNC precision (±0.05 mm or better). Specifying tight positional callouts on cast-in holes is one of the most common — and most avoidable — tolerance over-specifications we see.
How to Read a Foundry’s Tolerance Claim
When a supplier says “we hold CT5,” ask three questions:
- At what size? CT5 at 25 mm is very different from CT5 at 300 mm. Ask for the tolerance band on your actual critical dimensions.
- On what process? Silica sol or water glass? The quote price tells you, but the drawing should not have to guess.
- With what capability data? A foundry that tracks dimensional results on CMM and can quote Cpk values on critical features is a different supplier from one quoting nominal capability tables. For safety-critical or serial production parts, request a capability study on first articles.
A Practical Tolerance Checklist for Your Next RFQ
- State the default as-cast CT grade for the part (CT5 or CT6 for silica sol is typical), then override only where needed.
- Mark machined surfaces explicitly and define the datum system — machinists need repeatable cast datums to work from.
- Keep the number of critical dimensions small. Ten truly critical features with Cpk tracking beats forty loosely “important” ones.
- Give the foundry the part’s function. Knowing that a bore carries a bushing lets us propose the right combination of cast control and a light machining pass.
- Ask for first-article dimensional reports with actual measured values, not just pass/fail.
The Bottom Line
Investment casting delivers as-cast precision that eliminates most machining — but only when the drawing lets it. Buyers who understand CT grades, bucket their features between as-cast and machined, and discuss functional requirements openly with the foundry consistently get parts that fit, at prices that beat both fully machined parts and less precise casting processes.
At Ningbo Ruican, we manufacture silica sol and water glass investment castings with in-house CNC finishing, and we review every drawing for tolerance optimization before quoting — a service included with your RFQ. Send us your part drawing and let us show you which features we can deliver as-cast, and what that saves you.
Need a tolerance review on an existing part? Contact our engineering team through http://www.cnsandcasting.com.