CT Casting tolerance grade CT
Casting tolerance grade CT · Fundición (ISO 10135) · ISO 1101
Example in plan
How to interpret it
On the drawing, “ISO 8062-3 · CT9 per nominal size” states a casting tolerance grade ct requirement.
What controls
General dimensional tolerance grade applicable to casting dimensions that have no tolerance of their own on the drawing, per ISO 8062. It works like the general machining tolerance but adapted to the casting process: the assigned tolerance grows with the nominal size of the dimension and with the grade number, so the same grade gives very different margins on a 30 mm dimension and on an 800 mm one. It does not apply to machined dimensions, which follow the tolerance stated on the drawing.
How to verify
Measured with calipers, large-capacity outside micrometers, templates or a portable measuring arm depending on size, and the result is compared with the table for the specified grade at the nominal size of that particular dimension. On parts with many as-cast dimensions, a 3D scan compared against the model is much faster than measuring one by one, and it also shows the trend of the lot. It is advisable to measure several parts: the grade describes process scatter, not the result of a single pour.
Workshop guideline values
- General
- Wide grades, on the order of CT11–CT14, typical of hand sand molding or large castings.
- Accuracy
- Intermediate grades, on the order of CT9–CT11, with machine molding and patterns in good condition.
- High precision
- Tight grades, on the order of CT7–CT9, achievable with permanent mold, investment casting or metal tooling.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Applying the CT grade to machined dimensions: those follow the general machining tolerance or the dimension's own tolerance.
- Reading the grade as a fixed value in millimeters: the tolerance depends on the nominal size of each dimension.
- Specifying a tight grade on the drawing without discussing it with the foundry, and discovering the extra cost once the lot is already in production.
How to get it in the workshop / if it comes out of tolerance
Look for the CT grade in the title block or notes before measuring an as-cast part: without it there are no acceptance criteria for untoleranced dimensions. A tighter grade makes the part more expensive because it demands better tooling and more control at the foundry, so it is not tightened “just in case.” On the shop floor, the CT grade tells you how much part-to-part scatter to expect and, therefore, how much machining allowance is needed for machining to always clean up.
If the lot falls outside the grade systematically in the same direction, it is the pattern or the applied shrinkage allowance that needs correcting, not the part. If the scatter is random and wide, look at the molding: flask closure, compaction and core placement. From the machining side, the practical response to a wide grade is to increase the machining allowance and probe every part instead of programming blind.
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