Dimensional Tolerances and Machining Allowances for Aluminum Castings per ISO 8062-3

Casting tolerances are specified by the DCTG class according to ISO 8062-3, and the machining allowance by the RMA value, selected from the RMAG class and the largest casting dimension. For aluminum alloys, typical classes in series production are DCTG 9–12 for hand-molded sand casting, 7–9 for machine molding, 6–8 for permanent mold casting, and 5–7 for high-pressure die casting. Below we explain how to read these designations, how to calculate the raw casting dimension, and how to specify the requirements on the drawing.

In brief

• DCT is the total tolerance, split equally into plus and minus: DCTG 8 for a dimension of 100–160 mm is 1.8 mm, i.e. ±0.9 mm.
• Typical classes for light alloys: hand-molded sand DCTG 9–12, machine-molded 7–9, permanent mold and low-pressure casting 6–8, high-pressure die casting 5–7; for complex castings and for wall thicknesses, one class coarser.
• The RMA allowance is selected from the RMAG class and the largest casting dimension, e.g. RMAG F for a casting of 160–250 mm gives 2 mm per side.
• Example specification for a permanent mold casting about 200 mm in size: ISO 8062-3 – DCTG 8 – RMA 2 (RMAG F).
• Dimensions that cross the mold parting plane are less accurate – place critical dimensions within one mold half or allow for machining on them.

What ISO 8062-3 specifies

ISO 8062-3 belongs to the GPS (Geometrical Product Specifications) family of standards. It defines general dimensional and geometrical tolerances for castings and machining allowance grades. Instead of tolerancing every dimension individually, the designer specifies a single grade on the drawing. The foundry then knows which deviation applies to every dimension without an individual tolerance.

Three designations you need to know:

• DCTG (dimensional casting tolerance grade) – the dimensional tolerance grade. The lower the number, the tighter the tolerance. The tolerance value, or DCT, is read from the standard's table for the given grade and nominal dimension range.
• GCTG (geometrical casting tolerance grade) – geometrical tolerance grades: straightness, flatness, roundness, parallelism, perpendicularity, symmetry and coaxiality.
• RMAG (required machining allowance grade) – the grade of the required machining allowance, designated by a letter. The RMAG grade and the largest dimension of the casting determine the RMA, i.e. the allowance in millimeters for each machined surface.

The standard does not cover, among other things, positional tolerances other than those listed, angular dimensions, cylindricity, or 3D models used without dimensions on the drawing. Such requirements must be specified separately.

2007 edition (PN-EN ISO 8062-3:2009) or 2023 – which one to cite

The first edition of ISO 8062-3 was published in 2007 and replaced ISO 8062:1994, which used CT grades. In Poland it is adopted as PN-EN ISO 8062-3:2009, and this designation still appears in technical documentation and tenders.

The second edition, ISO 8062-3:2023 (EN ISO 8062-3:2023 in Europe), was published in February 2023. The most important changes:

• draft angle tables (tables 2–6) were added for manual and machine molding, permanent molds, die casting and investment casting,
• the dimensional tolerance table was renumbered from 2 to 7,
• the former grade DCTG 16 is now called DCTG 15wt and applies to wall thickness only,
• according to the foreword of the standard, the DCTG grade values themselves have not changed.

Practical advice: state on the drawing or in the order which edition you refer to. When updating documentation, older drawings with a note such as “ISO 8062 – CT12” should be rewritten to the DCTG system and confirmed with the foundry.

Which Tolerance Classes Are Realistic for Aluminum Castings

Technology	Typical DCTG grade	Typical RMAG grade	Practical tolerance per the handbook “Odlewnictwo”
Sand mold, manual molding	DCTG 9–12	F–H	up to 150 mm: ±0.8 mm; above: ±(0.8 mm + 0.3% of dimension)
Sand mold, machine or shell molding	DCTG 7–9	E–G	as above; molds compacted under high pressure give tolerances 2–3 times tighter than loosely compacted molds
Permanent mold, low-pressure casting	DCTG 6–8	D–F	±(0.4 mm + 0.2% of dimension); across the parting plane an additional ±0.3–0.8 mm
Die casting	DCTG 5–7 (DCTG 5 by agreement)	B–D	±0.2% of dimension; across the parting plane an additional ±0.1–0.15 mm

The DCTG and RMAG grades are typical values for light alloys in series production, as given, among others, by a British aluminum foundry based on ISO 8062-3 and the tables of the earlier ISO 8062:1994 – this is not a quotation from the annex of the 2023 edition. For complex castings and for wall thicknesses, a grade one step coarser is usually assumed. For single castings and short runs, the standard provides separate, usually coarser grades – agree on them with the foundry when requesting a quote.

Two conclusions follow from the table. First, the two scales agree: for a 100 mm dimension, the practical tolerance in a permanent mold is ±(0.4 + 0.2) = ±0.6 mm, i.e. 1.2 mm of total tolerance. That is close to DCTG 7, which is 1.1 mm for dimensions of 63–100 mm. Second, in sand molds, accuracy depends on the molding method and the compaction of the sand, which we discuss in our article on molding sands and their effect on the casting.

The handbook also recalls a principle that protects against unnecessary costs: a process running at the limit of its capability is a worse choice than one with margin to spare. So don't specify DCTG 6 in a permanent mold if DCTG 8 is enough for the function of the part.

Here are the practical tolerances from the table above, calculated for two dimensions (excluding the allowance for the parting plane):

Dimension	Sand mold	Permanent mold	Die casting
100 mm	±0,8 mm	±0,6 mm	±0,2 mm
300 mm	±1,7 mm	±1,0 mm	±0,6 mm

What it means in millimeters – how to read DCT

DCT is a total tolerance and is split symmetrically: half plus, half minus. Two examples from the standard's table:

• DCTG 8 for a dimension of 100–160 mm: DCT = 1.8 mm, i.e. ±0.9 mm,
• DCTG 7 for a dimension of 63–100 mm: DCT = 1.1 mm, i.e. ±0.55 mm.

You can read the remaining values from Table 7 of the 2023 edition (in the 2007 edition and in PN-EN ISO 8062-3:2009 it is Table 2). The row defines the nominal dimension range and the column defines the DCTG class. Tolerance increases with both the dimension and the class number.

Machining allowance: RMAG, RMA and the as-cast dimension

The RMA allowance is read from the table for the given RMAG class and the largest dimension of the entire casting – not for the dimension that is being machined.

Largest casting dimension [mm]	RMAG D	RMAG E	RMAG F	RMAG G	RMAG H
up to 40	0,3	0,4	0,5	0,5	0,7
40–63	0,3	0,4	0,5	0,7	1
63–100	0,5	0,7	1	1,4	2
100–160	0,8	1,1	1,5	2,2	3
160–250	1	1,4	2	2,8	4
250–400	1,3	1,8	2,5	3,5	5
400–630	1,5	2,2	3	4	6
630–1000	1,8	2,5	3,5	5	7

RMA values in mm per machined surface, according to the ISO 8062-3 table; classes A–C (including B–C, typical for die castings) and J–K can be read from the standard's table.

The allowance does not include the casting tolerance. For even a casting at the tolerance limit to have the full allowance, the as-cast dimension must be increased by half of the DCT. The same relationships were already given in ISO 8062:1994 (clause 10.1):

• external dimension machined on both sides: as-cast dimension = finished dimension + 2 × RMA + DCT/2,
• bore machined around its entire circumference: as-cast dimension = finished dimension − 2 × RMA − DCT/2.

Example (nominal values). A permanent mold casting has a largest dimension of approx. 200 mm, RMAG F is assumed, so RMA = 2 mm. A section with a finished width of 120 mm is machined on two sides, class DCTG 8 (for 100–160 mm, DCT = 1.8 mm). The as-cast dimension is 120 + 2 × 2 + 0.9 = 124.9 mm, i.e. 124.9 ±0.9 mm. Even the smallest permissible casting (124.0 mm) has 2 mm of material on each side.

Too little machining allowance risks unmachined (black) spots after machining, especially with mold half or core shift. Too much lengthens machining time, and in die castings it removes the dense surface layer and exposes pores from the interior. We show how allowances affect part cost in our comparison of casting versus milling from solid material.

Dimensions across the parting plane, cores and draft angles

A dimension that crosses the mold parting plane always carries an additional deviation, because the mold halves do not close perfectly. For aluminum this is an additional ±0.3–0.8 mm in a permanent mold and ±0.1–0.15 mm in a die casting mold. That is why one of the design rules is: the shape of the casting should allow its dimensions to be formed within a single mold half.

In practice this means a few decisions at the drawing stage:

• place critical dimensions in one mold half or plan to machine them,
• do not base measurement datums on the parting line or on surfaces formed by cores,
• give dimensions that depend on core position wider tolerances or machine them,
• for casting draft angles, use Tables 2–6 of the 2023 edition or agree on values with the foundry.

We discuss the selection of draft angles, wall thicknesses and parting plane position in more detail in the aluminum casting design guide for engineers.

Surface Roughness of the As-Cast Part

Technology	Ra of an as-cast aluminum casting
Sand mold (bentonite-bonded sand)	4–10 µm
Permanent mold	4–6 µm
Die casting	0.8–1.6 µm

According to the handbook "Odlewnictwo" (Table 3.22) for non-ferrous alloys. Molds for aluminum are made from finer sand than those for cast iron, which is why the surface is smoother. Any surface that requires lower roughness must be machined, and a machining allowance must be provided on it.

How to specify tolerances and allowances on a drawing

1. General class in the notes field: e.g. ISO 8062-3 – DCTG 8 – RMA 2 (RMAG F). The notation format conforms to the standard; the specific values are an example for a permanent mold casting approx. 200 mm in size.
2. Standard edition: state whether PN-EN ISO 8062-3:2009 or EN ISO 8062-3:2023 applies.
3. Critical dimensions: assign them individual tolerances. If they are tighter than the process allows, mark them as machined.
4. Machined surfaces: mark them with a machining symbol and specify the required roughness. Without this, the foundry does not know where to add an allowance.
5. Datums: indicate the datum surfaces or points of the as-cast part, away from the parting line, flash and gate locations.
6. Geometric tolerances: if required, add a GCTG class or individual tolerances; agree on the notation format with the foundry.

We describe what else should be included in the documentation in our guide on how to prepare a request for quotation for a foundry.

Most Common Mistakes on Casting Drawings

• General tolerances for machined parts applied to the as-cast casting. Classes intended for machining are usually too tight for cast surfaces.
• Tight tolerances on all dimensions "just in case." They force a more expensive process, more inspection and a higher scrap rate – see how tolerances and process affect the price of a casting.
• A datum on the parting line or on a core-formed surface. Any mold shift is then carried over into all dimensions.
• No marking of machined surfaces. The foundry has to guess where to add the allowance.
• Mixing CT and DCTG classes and omitting the standard edition. This is a sure path to disputes at acceptance.

Frequently asked questions

We will select the tolerance classes for your casting

Send us a drawing of the finished part and a 3D model. We will propose a DCTG class and allowances suited to the process, and we will produce the functional surfaces on 3- and 5-axis CNC machining centers as part of our aluminum machining. You will find the inspection rules for acceptance in technical acceptance conditions for castings. Request a quote based on your drawing.

Sources and standards

• ISO 8062-3:2023 / EN ISO 8062-3:2023 (2nd edition, 2023) – general dimensional and geometrical tolerances and machining allowances for castings.
• PN-EN ISO 8062-3:2009 – Polish version of the 2007 edition.
• ISO 8062:1994 – earlier CT grade system, found on older drawings.
• ISO 10135:2007 (EN ISO 10135:2009) – indication of molded parts in technical documentation (draft angles, parting plane).
• M. Perzyk (ed.), Casting, WNT, Warsaw – ch. 3.2 (table 3.12, pp. 169–171), 3.3 (tables 3.21 and 3.22, pp. 186–188), 3.4 (process selection, p. 194), 5.2 (high-pressure molded forms, pp. 281–282).