Specify a tolerance grade under a named standard, not a blanket number. ISO 8062-3 defines sixteen linear dimensional casting tolerance grades, designated DCTG 1 to DCTG 16, with geometrical tolerances handled separately as GCTG grades. The practical approach is to ask each foundry which grade it holds routinely on your process and part size, specify that as the general grade, and then call out individually the few features that need something tighter.
What the Standard Actually Says
Three rules inside ISO 8062-3 catch buyers out.
- Sixteen linear grades exist, DCTG 1 through DCTG 16, with tolerance values that widen as the nominal dimension grows.
- Wall thickness gets one grade coarser. For wall thicknesses in grades DCTG 1 to DCTG 15, one grade coarser applies. A wall is not held to the same tolerance as a linear dimension.
- The tolerance is symmetric by default, disposed half positive and half negative about the nominal dimension. A unilateral tolerance has to be stated.
The standard is also explicit that where general tolerances do not fit, whether tighter for function or looser for economy, individual tolerances must be indicated on the drawing in accordance with the relevant GPS standards. That is the mechanism for a tight feature: one callout, not a tightened blanket.
Why a Blanket Tolerance Is Expensive
A single tolerance applied to every dimension tells the shop to hold the tightest number everywhere. That changes the tooling class, the process control and the scrap rate on features that were never functional, and you pay for all of it on every part for the life of the program.
The cheaper structure is a coarse general grade plus a short list of controlled features tied to datums.
Die Casting Uses a Different Reference
High-pressure die castings are normally quoted against the NADCA product specification standards rather than an ISO grade, using either the Standard or the Precision tolerance set across seven categories including linear dimensions, dimensions across the parting line, and dimensions formed by moving die components.
How Buyer24 Helps
Buyer24 keeps the tolerance requirement attached to the request itself, so every foundry quotes the same specification and differences in the responses are real rather than a matter of who assumed what. Get started →
FAQ
Is "CT" the same as "DCTG"?
They refer to the same family of casting tolerance grades, but DCTG is the designation used in ISO 8062-3 for linear dimensional grades, with GCTG covering geometrical grades. A lot of published material still uses the older CT shorthand, which is worth knowing when comparing sources.
Which grade should I ask for?
Ask the foundry before you write it. Every shop has a grade it holds routinely for a given process and part size, and specifying that grade costs you nothing while specifying a tighter one costs you on every part.
Do tolerances apply before or after machining?
The casting tolerance applies to the as-cast feature. Machined features are governed by the machining print instead, which is why the machining scope has to be explicit in the request.
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