In the context of VDG P690 and ISO 8062-3, understanding how to interpret Dimensional Casting Tolerance Grades (DCTG) and Geometrical Casting Tolerance Grades (GCTG) together is crucial for ensuring comprehensive control over both the size and shape of cast components.
These two systems work in tandem to address different aspects of dimensional variation in castings and are essential for accurate, fit-for-purpose design and quality assurance.
Tolerance Structure in VDG P690
VDG P690 organizes casting tolerances into two complementary systems:
- Dimensional Casting Tolerance Grades (DCTG 1–16) for linear features, and
- Geometrical Casting Tolerance Grades (GCTG 1–8) for form, orientation, and position.
Together, these grades let you specify exactly how close to nominal your as-cast part must be, without resorting to arbitrary machining allowances.
Dimensional Casting Tolerance Grades (DCTG 1–16)
DCTG defines the maximum permissible deviation for a given nominal dimension.
Specifies the linear dimensional tolerances for features such as lengths, diameters, widths, or thicknesses on cast parts.
It aligns closely with the CT (Casting Tolerance) grade system, ranging from CT1 (tightest tolerance) to CT16 (most lenient).
Table 1. Linear Tolerances by DCTG Grade
| DCTG | Nominal ≤ 10 mm | Nominal 10–30 mm | Nominal 30–100 mm |
|---|---|---|---|
| 1 | 0.02 mm | 0.03 mm | 0.07 mm |
| 2 | 0.04 mm | 0.06 mm | 0.13 mm |
| 3 | 0.06 mm | 0.10 mm | 0.20 mm |
| 4 | 0.08 mm | 0.14 mm | 0.27 mm |
| 5 | 0.10 mm | 0.17 mm | 0.33 mm |
| 6 | 0.13 mm | 0.21 mm | 0.41 mm |
| 7 | 0.16 mm | 0.25 mm | 0.49 mm |
| 8 | 0.20 mm | 0.30 mm | 0.57 mm |
| 9 | 0.25 mm | 0.36 mm | 0.67 mm |
| 10 | 0.30 mm | 0.42 mm | 0.75 mm |
| 11 | 0.36 mm | 0.50 mm | 0.89 mm |
| 12 | 0.42 mm | 0.59 mm | 1.03 mm |
| 13 | 0.48 mm | 0.67 mm | 1.17 mm |
| 14 | 0.55 mm | 0.76 mm | 1.33 mm |
| 15 | 0.62 mm | 0.86 mm | 1.51 mm |
| 16 | 0.70 mm | 0.97 mm | 1.70 mm |
- Tightest Grades (DCTG 1–4) — DCTG 1 and 2 support high-precision processes (e.g., die casting, precision investment casting), holding as-cast features to within 0.02–0.06 mm on small parts.
- Mid-Range Grades (DCTG 5–9) — DCTG 5–9 suit shell-mold and standard investment castings, with deviations of 0.10–0.67 mm on features up to 100 mm.
- Looser Grades (DCTG 10–16) — DCTG 10–16 apply to sand and lost-foam methods, permitting 0.30–1.70 mm on larger dimensions.
Geometrical Casting Tolerance Grades (GCTG 1–8)
GCTG (Geometrical Casting Tolerance Grade) defines the allowable deviations in geometry, including straightness, flatness, roundness, cylindricity, and perpendicularity.
It ensures cast features maintain their intended form and orientation, especially critical for parts with functional interfaces.
| GCTG | Flatness (per 100 mm) | Perpendicularity (per 100 mm) | True Position (per 100 mm) |
|---|---|---|---|
| 1 | 0.02 mm | 0.03 mm | 0.03 mm |
| 2 | 0.05 mm | 0.07 mm | 0.07 mm |
| 3 | 0.10 mm | 0.14 mm | 0.14 mm |
| 4 | 0.20 mm | 0.27 mm | 0.27 mm |
| 5 | 0.30 mm | 0.40 mm | 0.40 mm |
| 6 | 0.50 mm | 0.67 mm | 0.67 mm |
| 7 | 0.75 mm | 1.00 mm | 1.00 mm |
| 8 | 1.00 mm | 1.33 mm | 1.33 mm |
- GCTG 1–2 — Reserve for critical mating surfaces or precision bores.
- GCTG 3–5 — Ideal for general assembly faces, where slight form variation still allows reliable fits.
- GCTG 6–8 — Acceptable on non-critical or large, flat surfaces, like structural castings.
Conclusion
Together, DCTG & GCTG provide a robust framework for defining the dimensional and geometrical quality of cast components.
Designers and quality engineers should always specify both when high performance, repeatability, and fit are critical.
By leveraging these standards in unison, manufacturers can minimize costly rework, improve casting yield, and ensure compliance with international tolerancing practices.
