Die Casting Tolerances: As-Cast and Machined Bands

A die casting arrives with the tolerance the die can hold, and machining buys a tighter band only on the features that earn it. That split frames every tolerance decision on a die-cast part. Typical published guidance in the NADCA product specification standards puts standard-class linear tolerance near ±0.25mm on the first 25mm of a dimension formed within one die half, widening by roughly ±0.025mm for each additional 25mm, and the precision class opens near ±0.05mm on the first 25mm in exchange for extra precision in die construction and special process control. Neither number covers the whole part: a dimension that crosses the parting line or passes a moving core carries an extra, plus-only allowance that grows with the casting’s projected area, and alloy choice, part size, draft, and die wear each move the band further. This page works through those mechanics as the tolerance spoke under the die casting hub.

Start from the decision, not the standard. Ask of each feature whether the die holds its tolerance or a cutter does, because the two answers come from different systems with different costs. The as-cast side is what this page tabulates, and the machined side follows the general-tolerance system for machined metals, covered in machining tolerances. Most die-cast parts use both: dozens of dimensions ride on the die, and two or three critical features get machining stock and a machined band.

What the die holds and what machining buys

The die is a net-shape process. Cavity steel fixes the geometry, and the part’s variation comes from shrink, die separation, and thermal effects rather than from tool motion. Two properties follow. First, as-cast tolerance is repeatable on dimensions formed entirely within one die half, because that steel does not move between shots. Second, the casting’s skin is its densest, finest-grained metal; the standard’s guidance is that optimum properties and density sit at or near the surface, so machining removes the minimum material that still cleans up the face.

Machining buys a band the die cannot hold. Once a feature is cut, published shop capability reaches about ±0.02 to 0.05mm on linear dimensions, ±0.005 to 0.015mm on a bored hole diameter, flatness of 0.01 to 0.05mm milled or 0.005 to 0.01mm ground, and hole position around ±0.05 to 0.10mm. That is several times tighter than the as-cast band at the same size, which is why the economical pattern machines the few features that need it and leaves everything else to the die.

The discipline runs the other way too. Every feature specified tighter than the as-cast band adds a machining operation, a fixture, and inspection time, so relaxing tolerances on non-functional features is one of the strongest cost reductions available on a die-cast part. The standard itself warns that precision-class tolerances should be specified only when and where necessary, because additional cost and additional process control come with them.

The two published tolerance classes

The NADCA product specification standards define two levels. The standard class reflects normal die casting production practice at the most economical level, using good manufacturing practice with normal levels of inspection. The precision class buys greater casting accuracy through extra precision in die construction, special control in production, or both, along with more frequent inspection and shorter die life between refurbishments.

The linear values scale with nominal size. For zinc, aluminum, and magnesium alike, standard-class guidance runs ±0.25mm on the first 25mm of a dimension plus about ±0.025mm for each additional 25mm, and precision-class guidance opens at ±0.05mm on the first 25mm with the same growth. For example, a 127mm dimension formed within one die half carries about ±0.35mm at the standard class and ±0.15mm at the precision class, per the standard’s own worked example. Copper alloys run looser, near ±0.36mm on the first 25mm plus about 0.076mm per additional 25mm at the standard class. The size-band values tabulated on this page follow those formulas, and the linear tolerance applies to radii, diameters, and wall thicknesses as well as edge-to-edge lengths.

One correction to most supplier tables is worth making explicit: the standard’s linear tolerance table is identical for zinc, aluminum, and magnesium. The alloys differ in the parting-line and moving-component allowances and in shop practice, not in the published linear band. When a source quotes a per-alloy linear band, it is quoting practice rather than the standard.

The parting-line penalty is plus-only

Projected area drives the extra allowance. A casting’s projected area is its shadow on the parting plane, and injection pressure acts on that area to push the die halves apart. The standard describes the fully closed die as the bottom of the tolerance range: a closed die produces the smallest part, and any separation only makes the part bigger. Parting-line tolerances are therefore plus-only additions rather than symmetric bands, and they widen as projected area grows.

At the standard class, the published addition for aluminum and magnesium runs from about +0.14mm on parts up to 65 square centimeters of projected area to +0.61mm at 1290 to 1935 square centimeters, with zinc running from about +0.11mm to +0.46mm across the same bands. Precision-class additions run roughly two-thirds of those values, opening near +0.089mm for aluminum and magnesium and +0.076mm for zinc on small parts and reaching about +0.406mm and +0.305mm respectively at the top band. Above 1935 square centimeters the published tables end and the die caster is consulted.

For example, an aluminum housing with 484 square centimeters of projected area and a 127mm dimension crossing the parting line combines the linear ±0.35mm with a parting-line addition of about +0.30mm, for a working band of +0.65 and −0.35mm, again from the standard’s worked example. The design lever becomes obvious once the arithmetic is visible: locate critical dimensions within one die half wherever the part allows.

Moving cores add more

Dimensions formed by moving die components, the core slides that create side holes and undercuts, carry a larger addition still, because each moving element adds its own closure and locking error. Published standard-class allowances run from about +0.20mm for aluminum and magnesium parts of small projected area up to about +1.0mm at the largest tabulated band, with zinc from about +0.15mm to +0.81mm. The standard’s worked aluminum example at 484 square centimeters adds +0.61mm to the linear band for a total of +0.96 and −0.35mm. Precision-class moving-component additions run near two-thirds of the standard values.

Mismatch: parting-line shift

Separately from separation, the two die halves can sit offset from each other. The published parting-line shift tolerance runs about ±0.10mm on castings up to 323 square centimeters of projected area, ±0.15mm at 323 to 645, and ±0.20mm at 645 to 1290. Thermal differences between the die halves, machine wear, and die alignment wear drive it, and interlocks and guide blocks reduce it at the price of closer tooling maintenance.

Alloy behavior: shrink, aging, and draft

Three alloy mechanisms scale the tolerance story. The first is solidification shrink, and the useful published fact is the volumetric ordering: zinc shrinks about 3 percent, magnesium about 4 percent, and aluminum most of all, with published values spanning roughly 3.5 to 6.5 percent. More shrink means more variation the die has to absorb, which is the physical root of aluminum’s looser as-cast reputation and of the machined-critical-feature pattern on aluminum parts.

The die compensates by cutting the cavity oversize, and the exact correction factor is die-specific, set from solidification simulation and refined at first article. Public per-alloy linear shrink factors conflict badly, from roughly 0.3 to 1.3 percent for the same alloy family depending on how the value is defined, so treat any single quoted factor as a starting point and let the grade datasheet and the die caster’s correction govern the steel.

The second mechanism belongs to zinc: room-temperature aging. As-cast zinc’s supersaturated structure decomposes slowly at ambient temperature, contracting parts by a few hundredths of a percent over the first weeks, with published by-grade values over five weeks spanning roughly 0.028 to 0.065 percent and stabilizing over about three to five weeks. The standard notes that artificial aging may be essential for holding critical dimensions in zinc, particularly on parts that will be machined. The published remedy is a stabilization anneal at 70 to 100°C for 3 to 6 hours, which drives the change to completion so parts can be machined immediately without drifting afterward.

Draft is the third mechanism. Recommended draft runs about 1 to 2 degrees generally, aluminum needs more than zinc at roughly 1 to 2 degrees outside and 2 to 3 inside against 0.5 to 1 degree for zinc, and inside surfaces need about twice the draft of outside surfaces because the alloy shrinks onto the steel that forms them. Draft also interlocks with tolerance: a drafted wall is deliberately larger at one end, so the drawing must state which end carries the dimension. Derived from the angle itself, one degree of draft over a 50mm depth moves the wall by about 0.87mm, an offset far larger than any tolerance band discussed here. The per-alloy draft values and the feature-by-feature drafting rules are carried on the design rules page. Alloy behavior across the families is tabulated on this page, and grade-level property values live in materials.

Form: flatness, concentricity, and angularity as-cast

Size bands do not control shape, and the published form tolerances run wider than the linear ones. Flatness guidance holds about 0.20mm over the first 76mm of a surface, adding roughly 0.08mm per additional 25mm at the standard class; the precision class opens at 0.13mm and grows about 0.05mm per additional 25mm. A 254mm diagonal surface works out to about 0.76mm total at the standard class and 0.48mm at precision. Concentricity within one die section runs about 0.20mm total indicator reading over the first 76mm plus 0.05mm per additional 25mm, and features that sit in opposite die halves are judged through projected area at the parting line. Angularity, which folds in parallelism and perpendicularity, runs about 0.13mm over the first 76mm plus 0.025mm per additional 25mm, and it is added to the other tolerances rather than standing alone.

Design controls form more than inspection does. Uniform walls, symmetric layouts, and freedom from heavy bosses or thick sections beneath flat faces are the levers the standard itself recommends. Where a face must be flatter than the as-cast guidance, it becomes a machined face, and milled flatness of 0.01 to 0.05mm is available after the cut.

Die wear and cavity-to-cavity spread

A tolerance has a life inside the run. The standard notes plainly that dies wear over the course of production, so the number of shots before die repair or replacement runs lower for tighter tolerances and higher for wider ones. Precision-class work assumes rigorous tooling development, disciplined process control, and more frequent inspection, which is a large part of why it costs more. Dimensions that passed first article can drift late in the tool’s life, so re-verification intervals belong in the conversation with the die caster. On multi-cavity dies, wear accumulates in each cavity over its own shot history, so identifying the producing cavity at first article and at later dimensional audits is the practical way that cavity-to-cavity spread is traced.

Machining stock: how much to leave

The published normal minimum machining allowance is 0.25mm. The maximum allowance stacks that minimum with the machining allowance and the casting allowances, and it depends on where the machined feature’s datums sit. In the standard’s worked example, a 127mm precision-class dimension carries maximum stock of about 0.56mm when its datum lies in the same die half and about 0.86mm when the datum sits across the parting line, because the casting tolerance feeding the machined face is wider in the second case. Large castings get separate guidance: machining stock up to about 1.5mm on all faces, general walls of 3.5mm ±0.5mm, a minimum cored hole of 6mm, and cored-hole length-to-diameter limits of about 4 to 1 below 12.5mm diameter and 10 to 1 above.

Two physics reasons keep stock thin. The skin is the densest metal in the casting, and cutting deep also risks opening subsurface porosity, which is why sealing faces are specified deliberately and impregnation exists for pressure-tight parts. The features that justify the stock are consistent across shops: threads, tight or H7-class bores, precision locating features, and sealing faces, in practice anything needing better than roughly ±0.05 to 0.1mm or flatter than about 0.1mm. CNC machining finishes those features while the rest of the part stays as-cast, and the machined bands follow the machined-metal system in the CNC tolerances reference table. Surface finish follows the same split: as-cast surfaces commonly run Ra 1.6 to 3.2µm with some sources extending to 6.3µm, machined faces reach 0.4 to 0.8µm, and the full scale sits in the surface finish Ra reference table.

Frames of reference and drawing practice

Two standards frame the numbers. Internationally, ISO 8062 assigns CT grades to as-cast castings plus separate machining-allowance grades, and its current series covers high pressure die castings, which older editions did not. Die casting sits at the tight end of the casting family, with commonly cited grades falling roughly CT4 to CT8 and zinc toward the tight end, though published mappings spread. Two cautions travel with it: CT grades apply to the as-cast condition only, and they are not the IT-grade system that governs machined parts, so once a feature is machined it is toleranced in the machined system, ISO 2768 and GD&T. For choosing between casting metal and molding plastic at the tolerance level, the process comparison in die casting vs injection molding carries the summary, and the injection molding hub holds the molding-side bands.

On the drawing, declare the general as-cast class once, then write specific bands only on functional features. Mark which dimensions cross the parting line or a moving core and give them the allowances they will actually need rather than the numbers you wish they held. State the measurement state as well: drawings dimension parts at 20°C, aluminum’s thermal expansion makes a warm casting a measurably different part, and castings are dimensionally checked after temperature normalization. First-article inspection validates the process before production, with nominal, tolerance, actual, and measurement method recorded per characteristic; the measurement side of that practice is covered in quality inspection and metrology. Drawing conventions for datums and callouts are in technical drawing requirements, and the vocabulary these conventions share with the rest of the site is collected in the manufacturing glossary.