Investment Casting Tolerances | ISO 8062 CT Grade Guide

Investment Casting Tolerances: ISO 8062 CT Grades, Error Travel and the Machining Boundary

As-cast investment casting tolerances typically fall between CT4 and CT8 on the ISO 8062-3 scale; silica sol processes hold CT4-CT6, water glass processes sit at CT7-CT8. Why the gap? The answer lives in the three-stage error chain of wax pattern, ceramic shell and solidifying metal, not in a marketing claim.

Key takeaways

• As-cast, investment casting tolerances usually map to CT4-CT8, with CT4-CT6 for silica sol
• A CT value is the total tolerance band; divide by two for the ± value on a drawing
• At 50 mm, CT4 gives a 0.36 mm band (±0.18) and CT6 gives 0.70 mm (±0.35)
• Stainless solidification shrinkage runs about 2.0-2.5% by volume, wax compensation about 0.6-1.0%

What investment casting tolerances mean: the ISO 8062 CT system

A casting tolerance states how far a real dimension may drift from nominal. ISO 8062-3 defines grades from CT1 to CT16, tighter as the number drops; China's GB/T 6414 is identical, and German buyers also reference VDG P690, where D3 approximates CT4 and D2 approximates CT5.

Most reading errors around investment casting tolerances come from one point: the CT value is the total tolerance band. A drawing callout of ±0.25 mm corresponds to a 0.50 mm band, which is CT5 in the 40-63 mm range. Reading CT as a ± value doubles the precision you are actually requesting, and both quotes and scrap rates suffer.

For context on where casting sits among the forming routes, see the six precision parts processes at Yujiaxin Tech.

CT1 to CT16: how to read and pick a grade

Three steps: find the nominal size range, pick the CT column, divide by two. For a 50 mm feature, in the 40-63 mm band, the values are:

For investment casting tolerances, pick grades by function: tight on critical dimensions, one or two grades looser on general form. Assigning the tightest grade to every dimension inflates cost and scrap without adding functional value.

How tight the process can go: silica sol and water glass

The same CT table lands differently by shell-building route. Silica sol shells deform little at firing temperature, so as-cast investment casting tolerances hold at CT4-CT6 with a surface roughness of Ra 6.3-12.5 μm. Water glass trades precision for cost and typically delivers CT7-CT8. Yujiaxin Tech runs silica sol as its main route and quotes CT5 as the standard production landing point, with CT4 confirmed after process validation.

How dimensional error travels: wax pattern, ceramic shell, solidification

Listed side by side, the three error sources look equal; in production they relay. First the wax: injected wax cools and shrinks, and the die is cut oversize by roughly 0.6-1.0% for stainless alloys, the most predictable stage. Second the shell: the ceramic expands and contracts through dewaxing and firing, held below ±0.1 mm on most features under tight control.

Third, solidification: austenitic grades such as 304 and 316L contract about 2.0-2.5% by volume, and uneven sections add distortion on top. This relay decides where your final investment casting tolerances land. The workshop sequence behind each stage is covered in the nine-step investment casting process, step by step.

Shrinkage and compensation

Compensation works on two layers. At the wax stage, die cavities are enlarged by a factor regressed from first-article measurements rather than copied from a table. At the metal stage, the alloy sets the baseline: duplex 2205 scatters more than 304 because of its two-phase transformation, and 17-4 PH changes dimension during precipitation hardening, so tolerances are verified in the final heat-treated condition.

For complex parts, solidification simulation moves part of the compensation into the gating design. Alloy composition and solidification behavior are documented in ASM International's materials resources.

Tolerance allocation and machining allowance

Allocate tolerances surface by surface:

The division is simple: CT governs as-cast surfaces, GD&T governs machined ones. Yujiaxin Tech applies this split across its precision metal component product lines. In effect, the table turns tolerance allocation into a per-surface decision rather than one blanket number.

The secondary machining boundary

As-cast precision has a ceiling. When tolerances run tighter than CT4, geometric tolerances demand better than roughly 0.1 mm, or the surface needs Ra below 3.2 μm, CNC machining becomes mandatory. Between those thresholds, general assembly dimensions close at CT5-CT6 as cast.

Every tighter grade raises tooling, process control and inspection cost, so the economic boundary is real. Investment casting tolerance vs machining is, in practice, a question of which errors the process removes and which the cutter removes.

Inspection and verification: CMM, Cpk and acceptance

CMM measurement is the baseline for dimensional acceptance, paired with GD&T evaluation and full-dimension reports. Batch consistency is expressed through Cpk, with 1.33 the common floor on critical dimensions. First articles run through FAI, production follows sampling plans.

Measurement traceability is part of the same discipline, as NIST's weights and measures resources explain. Yujiaxin Tech issues CMM inspection reports on key dimensions; the wider system is described under quality and inspection control at Yujiaxin Tech and in precision part inspection and measurement capability.

Investment casting tolerances: common questions

What investment casting tolerances can you hold as cast? CT4-CT8 in general; silica sol processes hold CT4-CT6. Tighter callouts require secondary machining.

How do I convert a CT value to ±? Divide by two. At 50 mm, CT5 means a 0.50 mm band, or ±0.25 mm.

How is shrinkage compensated? Wax dies are enlarged by about 0.6-1.0%, the metal baseline follows the alloy, and first-article measurements close the loop.

Can as-cast surfaces replace machining? For general assembly dimensions, yes; sealing faces and precision fits keep their machining allowance.

What Cpk counts as capable? 1.33 on critical dimensions is the usual floor, subject to the technical agreement.

Conclusion: tolerance control is the floor

Read the grade, trace the error, calculate the compensation, allocate the tolerances, verify the result. ISO 8062 supplies the framework; the drawing and the technical agreement decide acceptance, and tighter grades always cost more. Yujiaxin Tech evaluates drawings against this framework; send your drawing and tolerance requirements to our engineering team.