Cast Parts with Machining: Process and Allowance Guide

A cast part with machining is a casting poured close to its final shape and then finished on CNC machines, so that only the functional surfaces such as bores, seats, sealing faces and threads receive final tolerances. Sand casting, investment casting and die casting differ in alloys, part size, tooling cost and as-cast accuracy, and ISO 8062-3:2023 provides the tolerance grades and machining allowance grades that connect the casting drawing to the finished part.

Why are cast parts machined afterwards?

Casting produces shapes that are slow or wasteful to cut from solid: closed cavities, ribs, bosses and curved outer contours in one piece. It does not produce the surface finish and dimensional accuracy that a bearing seat, a threaded port or a flange face needs. Machining supplies those on the few surfaces where function demands them.

ISO 8062-3 follows the same split. It gives general tolerances for the casting as it leaves the foundry, and it gives machining allowance grades for the surfaces that will be cut afterwards.

The first design decision is therefore which features stay as cast and which are machined. A surface that only has to clear a neighbouring part can carry the general casting tolerance. Mating faces, bores for fits, sealing faces and the datum surfaces for the machining setup get an allowance and a finished tolerance. Our network covers casting with machining as one process, and the process selection guide shows when casting beats milling from solid, sheet metal or welding.

How do sand, investment and die casting compare?

The table ranks the three processes against each other. It states no tolerance values, because the grade a foundry holds depends on alloy, casting size, pattern or die quality and the individual feature.

Table 1. Sand, investment and die casting compared (qualitative)

Source: general foundry practice, ranking within these three processes. Grades and allowances: ISO 8062-3:2023 (values not reproduced).

Which casting process suits which part?

Work through material, quantity and geometry, in that order.

Material decides first. Die casting is limited to alloys with a lower melting range, in practice aluminium, zinc and magnesium. A ductile iron bracket is normally a sand casting. Spheroidal graphite iron is specified in EN 1563:2018 (published in Germany as DIN EN 1563:2019-04, current), and aluminium casting alloys in EN 1706:2020+A1:2021. A small stainless valve body with thin walls and fine detail points to investment casting.

Quantity decides between the remaining routes. A die costs far more than a sand pattern, so die casting needs a series large enough to repay it. We give no break-even quantity here: it depends on part size, alloy and die complexity, and the foundry can price both routes from the same drawing.

Geometry then sets the limits. Heavy sections and large parts favour sand casting. Thin walls, fine detail and tight tolerances on small parts favour investment or die casting. Deep undercuts and internal passages need cores in all three processes, and each core adds a position tolerance to the cast feature.

The mechanical properties in EN 1706 are measured on separately cast test pieces, according to the abstract of the standard. Properties inside a thick section of the real part can differ, so the drawing should say where and how the part is tested.

What is a machining allowance in ISO 8062-3?

The machining allowance is the material left on a cast surface so that machining removes the as-cast skin and brings the surface to its final dimension. ISO 8062-3:2023 specifies machining allowance grades alongside the general tolerances for castings. It applies to all cast metals and alloys.

In practice the allowance has to cover three things: the dimensional deviation of the casting on that surface, the skin and surface defects that have to be cut away, and the position error between the casting datums and the machine setup. A larger casting and a coarser process need a larger allowance. The grade values sit in the paid standard and are not reproduced here. Check which edition a quoted table comes from.

How do you tolerance a cast part that is machined?

Use two sets of tolerances on two states of the part. The casting carries the general tolerance from ISO 8062-3, noted in or near the title block. ISO 8062-3 covers linear dimensions and seven geometrical characteristics: straightness, flatness, roundness, parallelism, perpendicularity, symmetry and coaxiality. Angular dimensions and cylindricity are outside it. The machined features carry individual tolerances or a general class such as ISO 2768-m, see ISO 2768 general tolerances. Our network works to ISO 2768-m as standard on machined features, and tighter where the drawing specifies it.

Define the datums once and use them everywhere. The casting drawing names the surfaces or points on which the foundry measures, the fixture clamps and the first machining operation locates. If the foundry inspects from one datum and the machinist clamps on another, the allowance calculated for the first no longer protects the second.

Table 2. Standards for cast parts and their status as of 2026-10-30

Source: ISO and AFNOR catalogue entries, DIN Media, DIN project page for EN 1559-1; see the source list below.

EN 1559-1 sets general technical conditions of delivery for castings made from cast metallic materials. It excludes copper alloy castings and metallic castings for further reprocessing such as forging ingots. The DIN project page shows a revision of the 2011 edition begun on 2026-06-30, and DIN Media still lists the 2011 edition as current.

What goes wrong when cast parts are machined?

Machined castings have three typical failure points.

The first is porosity exposed by the cut. Gas and shrinkage porosity sit mostly below the as-cast skin, and machining through the skin can open pores on a sealing face or a thread. Limit the stock to what the surface needs, and write acceptance criteria for pores on critical faces into the drawing or the order. Where a face has to seal, call for a leak test.

The second is too little or too much allowance. If a core or the mould shifts and a surface does not clean up, the part is scrap, because material cannot be added back. Too much allowance costs cutting time and tool wear, and scale or sand inclusions in the skin wear tools faster.

The third is movement after the first cut. Removing the skin releases stress, and heavy or asymmetric castings can distort. Rough machining, a stress relief where the part needs one, and finish machining in that order is the usual route.

What does a supplier need to quote a cast part?

Send the following with the request:

• The finished part drawing with the general tolerance class and every individual tolerance, as in the technical drawing checklist.
• The alloy by its standard designation, not a trade name.
• The expected annual quantity and lot size, because quantity decides between sand, investment and die casting before any other factor.
• Which surfaces are machined and which stay as cast.
• Test requirements: pressure or leak testing, material certificates, and where properties are verified.
• The edition of ISO 8062-3 that the casting drawing refers to.

With these, a supplier can propose the casting process, the allowance and the machining setup in one offer.

Frequently asked questions

What is the difference between sand, investment and die casting?

Sand casting pours metal into a mould of bonded sand and suits the widest range of alloys and sizes with the lowest tooling cost. Investment casting uses a wax pattern and a ceramic shell and gives the tightest tolerances on complex small parts. Die casting injects metal into a steel die and pays off in large series, mainly in aluminium, zinc and magnesium.

What is a machining allowance on a casting?

It is the extra material left on a surface of the casting so that machining can cut away the as-cast skin and bring the surface to its final dimension. ISO 8062-3:2023 specifies machining allowance grades for this purpose.

Which standard defines casting tolerances?

ISO 8062-3:2023, published in February 2023, replaces the 2007 edition. It gives general dimensional and geometrical tolerances and machining allowance grades for castings in all cast metals and alloys. The German edition is DIN EN ISO 8062-3:2023-05.

Can every alloy be die cast?

No. Die casting is used mainly for alloys with a lower melting range, in practice aluminium, zinc and magnesium. Steel, stainless steel and cast iron parts are made by sand casting or investment casting.

Do the mechanical properties in EN 1706 apply to the finished part?

Not directly. EN 1706 gives mechanical properties of separately cast test pieces. Values in the part, especially in thick sections, can differ, so agree with the foundry where and how the part itself is tested.

Sources

1. ISO 8062-3:2023, castings, general dimensional and geometrical tolerances and machining allowances (AFNOR)
2. ISO 8062-3:2023, scope, status 60.60, replaces ISO 8062-3:2007 (Genorma)
3. EN ISO 8062-3:2023-03, current (DIN Media)
4. DIN EN ISO 8062-3:2008-09, withdrawn, replaced by DIN EN ISO 8062-3:2023-05 (DIN Media)
5. ISO 8062-1:2007, dimensional and geometrical tolerances for moulded parts, vocabulary (AFNOR)
6. DIN EN 1559-1:2011-05, founding, technical conditions of delivery, general (DIN Media)
7. Revision project for EN 1559-1, begun 2026-06-30 (DIN)
8. DIN EN 1706:2021-10, aluminium castings, chemical composition and mechanical properties (DIN Media)
9. NEN-EN 1706:2020, abstract: mechanical properties of separately cast test pieces (Intertek Inform)
10. DIN EN 1563:2019-04, spheroidal graphite cast irons (DIN Media)

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