Every RFQ for a precision casting eventually lands on the same three-way tension: the drawing calls for a tight tolerance, the buyer wants a low unit price, and the foundry wants a process that does not scrap half the batch. Tolerance grade is the single variable that controls all three. Pick CT4 when CT6 was sufficient and you can pay 20–40% more per part for precision no one will ever measure. Pick CT6 when the assembly needed CT5 and you will pay for it later — in machining time, in rework, or in a rejected shipment sitting at your dock.
This guide breaks down what CT4, CT5 and CT6 actually mean under ISO 8062-3, where the cost really comes from, and how to hit ±0.03 mm on critical features without buying tight tolerances across the entire part.
What the CT Grades Actually Mean
CT stands for Casting Tolerance Grade. The system comes from ISO 8062-3, the international standard that assigns dimensional tolerances to castings based on the manufacturing process and the nominal dimension of the feature.
The logic is simple: tolerance bands are expressed as a function of nominal size, not as a single flat number. A 20 mm dimension and a 200 mm dimension in the same CT grade do not get the same tolerance — the larger the feature, the wider the permitted band, because wax shrinkage, shell expansion and metal contraction all scale with size.
For investment casting (lost wax), the relevant grades run roughly from CT4 to CT6 for production work. Investment casting can reach tighter grades than sand casting or gravity die casting because the ceramic shell is dimensionally stable and the wax pattern is injected into a machined metal die — but that precision is not free.
| Grade | Typical band on a 25 mm feature | Typical band on a 100 mm feature | Where it is used |
|---|---|---|---|
| CT4 | ±0.13 mm | ±0.28 mm | Aerospace brackets, medical, tight hydraulic bodies |
| CT5 | ±0.17 mm | ±0.37 mm | Automotive, valve parts, pump components, industrial OEM |
| CT6 | ±0.26 mm | ±0.56 mm | Commercial hardware, non-critical housings, decorative parts |
(Bands shown are indicative and must be confirmed against the ISO 8062-3 tables for the specific nominal range and the agreed drawing standard.)
CT4 vs CT5 vs CT6: A Practical Comparison
CT4 — Aerospace-Grade Precision
CT4 is the tight end of commercially repeatable investment casting. It demands vacuum or air-melt furnaces with tight chemistry control, dies machined to ±0.02 mm, wax injection at controlled temperature and pressure, and autoclave cycles that are logged and traceable. First-article inspection is mandatory, and ongoing statistical process control (SPC) on wax weight, shell thickness and furnace parameters is normal.
CT4 belongs on parts where the casting is the finished geometry — hydraulic manifolds, medical instruments, aerospace brackets — and where any machining at all would compromise the part or the budget.
CT5 — The Workhorse Grade
CT5 is where the vast majority of industrial investment castings live. It is tight enough to support automotive and valve applications, loose enough to run at a repeatable yield, and it is the grade that pairs best with selective CNC machining.
If you are sourcing valve bodies, pump impellers, food machinery components, or general industrial OEM parts, CT5 is almost always the correct default. It absorbs normal process variation without turning every batch into a scrap event.
CT6 — Commercial Castings
CT6 is the economical grade. It is appropriate for parts where dimensions are non-functional, where a generous machining allowance is already planned, or where the casting is a bracket, cover or weight. Buyers who specify CT6 and then machine everything to final size often get the lowest total cost — provided the machining allowance is sized correctly on the drawing.
Why Tolerance Grade Drives Cost
The price difference between CT4 and CT6 is not a markup. It is the sum of real process costs.
1. Tooling and wax pattern cost
Tighter grade means a tighter die. A CT4 die is machined and inspected to a finer band, made from better steel, and often includes more cooling and venting detail. Wax injection parameters must be held in a narrow window — short shots and sink marks that are invisible on a CT6 part become scrap on a CT4 part.
2. Ceramic shell and process control
Shell thickness variation, slurry viscosity, stucco grading and drying humidity all shift final dimensions. A CT4 process requires documented control of each of these variables; a CT6 process tolerates more drift. Documentation, logging and operator discipline are labour costs.
3. Inspection and scrap
You cannot ship CT4 without measuring it. That means CMM time, gauges, fixtures and inspection reports. And because CT4 tolerances sit closer to the process mean, the natural scrap rate rises. Typical scrap on a tight-grade line can run several times higher than on a loose-grade line — that scrap is priced into the unit cost.
4. Machining allowance—the hidden cost
This is where buyers lose the most money. A wide tolerance does not mean “the part is wrong.” It means the as-cast surface can sit anywhere inside a wider band, so the machining allowance must be larger to guarantee cleanup on every part.
A CT5 to CT6 shift can add 0.3–0.5 mm of stock to every machined face. Across a 100,000-piece automotive program, that is extra metal, extra cycle time, extra tool wear and shorter tool life—often far more than the per-kilogram saving on the casting.
How CNC Machining Changes the Tolerance Equation (CT5 → ±0.03 mm)
The most practical 2026 strategy is hybrid sourcing: cast at CT5, then machine only the features that matter.
At Ningbo Ruican, castings produced to CT5 (+/-0.17 mm on a 25 mm feature) are transferred to in-house 3-, 4- and 5-axis CNC centres and turned/milled to ±0.03 mm on critical dimensions, with surface finishes from Ra 1.6 down to Ra 0.8 µm where required. The result:
- The casting carries the geometry cheaply and repeatably.
- The CNC process carries the precision, but only on the 5–15% of features that are functionally critical.
- Overall part cost is typically 20–35% lower than specifying CT4 across the whole component.
The rule of thumb: cast the shape, machine the function. Features that locate, seal, thread, bear or mate should be machined. Features that merely fill space should be cast.
Alloys and Their Tolerance Behavior
Tolerance grade and alloy interact — shrinkage rate, thermal conductivity and hot strength all differ by material.
- 17-4PH (SS630): Precipitation-hardening stainless. Predictable shrinkage, machines well after H900/H1025 ageing. Excellent for CT5 valve and pump parts needing high strength plus corrosion resistance.
- AISI 420: Martensitic stainless, hardenable to ~50 HRC. Used for valve trim and wear parts. Requires careful post-cast heat treatment to avoid dimensional movement — machine after hardening where possible.
- AISI 316L: Austenitic stainless, the corrosion workhorse for chemical, marine and food-contact castings. Higher thermal expansion means dimensional checks should be run at a controlled temperature.
- Duplex 2205 (UNS S32205): Ferritic-austenitic stainless with roughly double the yield strength of 316L and strong chloride resistance. Pouring windows are narrow and shrinkage is less forgiving, so duplex parts are usually specified at CT5 with generous machined features.
- WCB (ASTM A216): Carbon steel for general valve and pump bodies. Good castability, low alloy cost, ideal for larger CT5/CT6 pressure-containing parts.
- CF8M (ASTM A351): The cast equivalent of 316 stainless. The default material for corrosion-resistant valve bodies against EN 10204 3.1 certification.
How to Specify Tolerances Without Overpaying
- Only tighten what functions. Mark critical dimensions with their real requirement and leave general dimensions at the default grade.
- Reference ISO 8062-3 explicitly on the drawing, including the grade and the datum scheme.
- Separate “as-cast” from “as-machined” tolerances so the foundry and the machine shop each own a clear band.
- Specify a machining allowance table, not a single blanket value — large sections need more stock than small ones.
- Ask for a machining capability study (Cp/Cpk on the critical features) rather than a promise. Data beats a datasheet.
- Do not tighten the alloy for tolerance. Change grade, change process control, or add CNC — changing from 316L to duplex rarely fixes a dimensional problem.
FAQ: Investment Casting Tolerances
Q1. What is the difference between CT4, CT5 and CT6? They are Casting Tolerance Grades from ISO 8062-3. CT4 is the tightest, CT6 the loosest. The band scales with nominal dimension, so the same grade gives different numeric tolerances on a 25 mm and a 100 mm feature.
Q2. Can investment casting achieve ±0.03 mm as-cast? No. No commercial investment casting process holds ±0.03 mm as-cast across a production batch. That precision is achieved by CNC machining selected features after casting. A realistic combination is CT5 as-cast (approximately ±0.17 mm on a 25 mm feature) plus machining to ±0.03 mm.
Q3. Is CT5 more expensive than CT6? Yes — typically 10–25% higher per kilogram, driven by tighter tooling, tighter process control, more inspection and higher scrap. Whether that is worth it depends on your machining allowance and assembly requirements.
Q4. Should I specify tolerances on every dimension? No. Over-specification is the most common cause of unnecessary cost. Specify tight tolerances only on functional, mating, sealing and locating features, and leave cosmetic or non-functional dimensions at the general grade.
Q5. How do I document tolerance compliance for an audit? Request a First Article Inspection Report (FAIR) or Initial Sample Inspection Report (ISIR) with dimensional results, plus material certificates to EN 10204 3.1 and heat-treatment records. For automotive programs, a full PPAP package including capability studies is standard.
Partner With Ningbo Ruican for Cast-and-Machine Parts That Hold Tolerance
Specifying the right tolerance grade is half the job. The other half is finding a supplier who controls it batch after batch, then machines to ±0.03 mm without adding a second vendor to your supply chain.
Ningbo Ruican has supplied precision investment castings and CNC-machined components for over 17 years, under an IATF 16949 quality system, with in-house wax injection, shell building, melting, heat treatment (including 17-4PH solution and ageing), CNC turning and milling, passivation to ASTM A967, and full dimensional and material documentation including EN 10204 3.1, FAIR/ISIR and PPAP. We work as an OEM/ODM partner — from tolerance review on your drawing through to production delivery.
Send your drawing and we will return a tolerance and cost review showing exactly which features to cast, which to machine, and what each tolerance grade costs you.
Contact: Wendy She Email: sales@cnsandcasting.com WhatsApp: +86-13777124360 Website: cnsandcasting.com