Investment Casting Tolerances: CT4-CT6 Explained

Investment casting is a near-net-shape process. In normal commercial production it holds dimensional tolerances of CT4 to CT6, reaches CT3 on high-precision work, and delivers surface roughness of Ra 0.8 to 3.2 micrometres. That combination is what allows castings to be bought with little or no machining allowance.

A tolerance grade means nothing until it is converted into millimetres. Under GB/T 6414, a 50 mm dimension held to CT5 carries 0.50 mm total tolerance, split symmetrically to plus or minus 0.25 mm. The same 50 mm dimension held to CT6 carries 0.70 mm, or plus or minus 0.35 mm. That 0.2 mm difference decides whether a face is machined or used as cast.

Smaller sections are tighter. Up to 3 mm, CT5 is 0.28 mm. From 6 to 10 mm it is 0.36 mm, from 16 to 25 mm it is 0.42 mm, and from 100 to 160 mm it is 0.62 mm.

Tolerance capability is not the same for every alloy. For batch production the handbook gives cast steel at CT5 to CT7, grey and ductile iron at CT5 to CT7, copper alloys and light alloys at CT4 to CT6, and nickel-base and cobalt-base superalloys at CT5 to CT7. If you are designing a stainless steel or superalloy part, asking for CT4 on every dimension is optimistic. CT5 to CT6 is the working band, with CT4 reserved for the few features the foundry has confirmed.

For inch drawings, the US Investment Casters Institute basis is useful. At 25.4 mm the general tolerance is plus or minus 0.25 mm. Below 254 mm, add plus or minus 0.076 mm for each additional 25.4 mm. Above 254 mm, add plus or minus 0.13 mm for each additional 25.4 mm. Wall thickness is excluded, and the minimum wall tolerance is plus or minus 0.51 mm. Straightening and sizing can tighten these figures, which is why a foundry sometimes accepts a tolerance that the raw table says is impossible.

Surface roughness scales with section size and casting weight. Small, thin, light parts come out smoother. A part under 50 mm with wall under 3 mm and weight under 0.2 kg sits at Ra 1.6 to 3.2 micrometres. At 150 mm with 3 to 5 mm wall it is Ra 3.2 to 6.3. At 250 mm with 5 to 10 mm wall it is Ra 6.3 to 12.5. Above 500 mm and 15 kg it is around Ra 12.5.

This is the most useful table in a quoting conversation. It stops the argument where a buyer expects Ra 1.6 on a 10 kg part.

Cast holes behave differently from external dimensions, because roundness depends on the metal around them. Uneven surrounding wall mass distorts the hole, and the effect grows with hole length. For straight holes under 12.7 mm diameter with uniform surrounding metal, a plug gauge tolerance of plus or minus 0.08 mm is realistic. Curved holes carry twice the straight-hole tolerance.

Minimum wall thickness is 0.5 mm on small castings, and local thin sections can go somewhat below the tabulated minimum. Large parts are limited to roughly 2 mm. Minimum cast hole diameter is around 1 mm.

Because the ceramic shell is rigid and the pattern is expendable, the process also holds straightness, roundness, parallelism, perpendicularity, symmetry and coaxiality - characteristics that sand casting struggles with.

Two capability steps matter for high-integrity parts. Hot isostatic pressing lets internal shrinkage porosity and micro-cracks heal by creep and plastic deformation, and reported fatigue life of nickel-base superalloy, titanium and aluminium castings improves by three to ten times with reduced scatter. Modern investment casting has also reached a maximum envelope of 2.2 m and a weight near 1000 kg while minimum wall stays under 2 mm.

Five specification rules follow from all of this. Do not put one tolerance block on the whole drawing: give a general note of CT5 or CT6, then call out the two or three features that genuinely need CT4. Never tolerance wall thickness tighter than plus or minus 0.51 mm without a trial. Expect hole tolerance to be wider than external tolerance, and machine functional bores. Match surface finish expectation to part mass. And specify datum surfaces on as-cast faces, because that is where near-net-shape pays for itself.

Send a drawing and we will tell you which features can be cast to CT5, which need CT4, and where machining allowance is unavoidable. CireCraft is run by investment casting practitioners from XINYOUYUAN, a precision casting foundry in Dongguan, China. Request a casting quote and DFM review at castinmetal.com.

Reference basis: Investment Casting Handbook (China Foundry Association), Tables 3-12, 3-13, 3-14 and 3-28; Practical Investment Casting Technology (Jiang Buju), Chapter 1; Precision Casting New Technology and Quality Inspection, Chapter 2.

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