Casting Tolerance Grades: What DCTG 12 Actually Allows

Casting tolerance grades are why two foundries can quote the same drawing and ship parts nine millimetres apart — both of them conforming. The grade was printed on the drawing. Nobody read it the same way.

Here is the chart the argument is actually about, then the four places the reading goes wrong.

Casting tolerance grades at a glance: the ISO 8062-3 chart

ISO 8062-3 defines sixteen linear dimensional casting tolerance grades, written DCTG 1 (finest) to DCTG 16 (coarsest). The number in the table is the total tolerance width in millimetres for that dimension band — not a plus-or-minus figure. By default the standard splits that width evenly either side of the nominal dimension.

Five representative grades, in millimetres, from Table 2 of ISO 8062-3:2007:

Read one cell and the whole quarrel becomes obvious. A 300 mm feature called out at DCTG 12 is allowed 9 mm of spread. Symmetrically disposed, that is nominal ±4.5 mm. A part sitting at either extreme is a good part, and an inspector who rejects it is wrong.

The grades are not evenly spaced in cost either. Every step down roughly halves the width: on that same 300 mm feature, DCTG 12 gives you 9 mm, DCTG 10 gives 4.4 mm, DCTG 8 gives 2.2 mm. Three grades is a factor of four in permitted variation, and usually a different moulding method.

What a casting tolerance grade does not include

This is where quotations get repriced after the first shipment.

Draft is extra, and it is bigger than you think. ISO 8062-3:2023 added tables of draft angles for hand, machine, permanent-mould, pressure-die and investment moulding. For a 160 mm tall feature in hand moulding, Grade A external draft is listed as 1.0°, which the same table converts to 2.2 mm of material at the top of the feature. The general tolerance for that dimension band at DCTG 8 is 1.8 mm. The taper is larger than the tolerance, and it is not an error — it is design. If the drawing does not say where along a drafted face the dimension applies, the disagreement will always exceed the tolerance you argued about.

General form and orientation tolerances do not apply to drafted features at all. ISO 8062-3 states it plainly: general tolerances on straightness, flatness, roundness, angularity, parallelism and perpendicularity do not apply to features with draft. Those need individually indicated tolerances. A buyer who assumes the general grade covers flatness on a tapered wall is assuming something the standard explicitly withholds.

Wall thickness gets one grade coarser. For DCTG 1 to DCTG 15, wall thickness is allowed one grade wider than the rest of the part. A 10 mm wall on a DCTG 10 casting is inspected at DCTG 11.

Geometry is a separate scale. Casting geometry uses seven geometrical casting tolerance grades, GCTG 2 to GCTG 8 — GCTG 1 is deliberately left empty for future finer values. Flatness on a 600 mm face at GCTG 5 is 2 mm; at GCTG 8 it is 7 mm. Nothing about the DCTG number tells you that. If you have wrestled with tolerance stack-up on machined assemblies, castings add a second, independent stack on top.

Required machining allowance: the second letter on the callout

If any face gets machined, the casting has to be born oversize. That stock is the required machining allowance, RMA, and it is graded by letter.

Values published by the German Foundry Industry Association for sand casting, per DIN EN ISO 8062-3:

Two traps live in that table.

The first is the row you pick. RMA is selected from the largest overall dimension of the whole casting, not from the size of the feature being machined. A 40 mm boss on a 900 mm housing takes the 1000 mm row, not the 40 mm row. Machinists who size their stock off the feature run out of material and blame the foundry.

The second is that the casting tolerance still applies on top of the allowance. RMA is not a tolerance and it is not a buffer against one. Add them: an RMA G face on a 900 mm casting carries 5 mm of stock, and the DCTG grade decides how far that 5 mm itself may wander.

The fine RMA grades exist but are not free. Grades at the tight end are applied only in special cases — series production where the pattern equipment, the casting procedure and the machining procedure have all been agreed in advance between customer and foundry. Asking for one on a first order is asking for a tooling conversation, not a tolerance.

Which grade your process can actually hold

A grade you cannot hold is a rejection schedule, not a specification. Naming a tight grade to look precise is the most expensive habit in casting procurement.

For aluminium and magnesium high-pressure die castings, the achievable dimensional grades tabulated from ISO 8062-3:2007 run DCTG 6 up to 50 mm, DCTG 7 from 50 to 180 mm, DCTG 8 from 180 to 500 mm, and DCTG 9 above 500 mm — the achievable grade gets coarser as the part gets bigger, on the same machine, with the same die.

North American die casters work from a parallel system. The NADCA specification splits linear tolerances into "normal" and "precision" classes rather than numbered grades. As tabulated by Engineers Edge, the normal class for aluminium works out to ±0.25 mm across the first 25 mm plus ±0.025 mm for every further 25 mm, with the precision class cutting the base figure to ±0.05 mm. Precision costs more because the die has to be more rigid and thermally stable across the cycle, which means thicker plates and more tooling money — not more care on the shop floor.

If your buyer's drawing quotes NADCA and your quality plan quotes ISO, say so in the quotation before the first shipment. The two systems do not convert; they have to be reconciled dimension by dimension.

What changed in ISO 8062-3:2023

The 2023 edition is the second edition and it cancels and replaces ISO 8062-3:2007, including Technical Corrigendum Cor 1:2009. Worth knowing if your drawing template still says 2007:

• Draft angle definitions and five tables of draft angles by moulding method were added as new clauses, so everything after them was renumbered — the linear tolerance table is now Table 7, not Table 2.
• Grade DCTG 16 was renamed DCTG 15wt, to make explicit that it exists only for wall thickness.
• The geometrical tolerance tables became Tables 8 to 11, and the nominal dimension you feed into them changed.
• The title and scope now state that the document covers tolerances "using ± tolerances for indicated dimensions."

A drawing that cites "ISO 8062-3" with no year is now ambiguous about at least the table numbers. Cite the year.

Writing the callout so the inspector accepts it

The callout goes in or near the title block, and it needs to answer four questions without a phone call.

Then do the thing most spec packs skip: put the same numbers on the buyer-facing image. A purchasing engineer in another time zone looks at a picture before they open a PDF. A picture with the machined faces highlighted, the general grade written on it and the RMA marked where it applies removes a whole email round-trip.

That only works if the figures on the image are measured ones. Snap each dimension to the actual edge of the part, then export at the size the buyer's system displays — a number typed over a photo by eye, or produced by an image tool that invents a plausible-looking figure, reads exactly like a real one and fails exactly when it matters. Treat surface callouts the same way, as with Ra vs Rz surface roughness, and the same discipline applies to product dimension tolerance on machined and fabricated parts. For a worked layout of the finished sheet, see the industrial spec diagram format.

Pre-quotation checklist

• Standard cited with its year, not just "ISO 8062-3"
• Dimensional grade (DCTG) named, and confirmed achievable by the intended moulding method and part size
• Geometrical grade (GCTG) named separately — it is not implied by the DCTG
• Machining allowance grade (RMAG) named, and the row chosen from the largest overall dimension of the casting
• Drafted faces carry individual tolerances, since general form and orientation tolerances do not apply to them
• The dimension origin on each drafted face is stated (top of draft, bottom of draft, or parting line)
• Wall thickness treated one grade coarser, as the standard allows
• Datum system declared if general orientation tolerances are used
• Any critical dimension that must beat the general grade is individually toleranced, not left to the grade
• The buyer-facing spec image carries the same grades as the drawing title block

FAQ

What are casting tolerance grades?

Casting tolerance grades are a numbered scale defined in ISO 8062-3 that sets how much a cast dimension is allowed to vary from nominal. Sixteen linear grades run from DCTG 1 (tightest) to DCTG 16, and the table value is the total permitted width in millimetres, split evenly above and below nominal unless the drawing says otherwise.

Is DCTG 12 the same as ±12 mm?

No. The grade number is an index, not a millimetre value, and the table figure is a total width rather than a plus-or-minus. On a 300 mm feature, DCTG 12 permits 9 mm total, which is nominal ±4.5 mm.

What tolerance can sand casting actually hold?

It depends on part size and moulding method, and the honest answer comes from your foundry rather than from a chart. ISO 8062-3 provides informative guidance on achievable grades by process, and for aluminium and magnesium high-pressure die casting it puts achievable grades at DCTG 6 below 50 mm sliding to DCTG 9 above 500 mm. Larger parts get coarser grades from the same equipment.

What is the difference between casting tolerance and machining allowance?

Casting tolerance is how much the as-cast dimension may vary. Machining allowance is extra material deliberately left on faces that will be machined afterwards. They stack: the allowance is sized from the largest overall dimension of the casting, and the casting tolerance then applies to the allowance itself.

Is ISO 8062:1994 still valid?

No. ISO 8062:1994 was replaced by the ISO 8062 series, and ISO 8062-3:2007 was in turn cancelled and replaced by ISO 8062-3:2023. The older standard used a single CT grade; the current one separates dimensional (DCTG), geometrical (GCTG) and machining allowance (RMAG) grades.

Sources & References

• ISO 8062-3:2007 — General dimensional and geometrical tolerances and machining allowances for castings (preview, includes Table 2)
• ISO/FDIS 8062-3:2022 — draft of the second edition, listing the changes from 2007 and the draft angle tables
• Machining Allowances (RMAG E–H) per DIN EN ISO 8062-3, from BDG technical guidelines
• Engineers Edge — Casting Tolerances per NADCA and ISO