Which Features of a Die-Cast Part Should Be CNC Machined?

01A question that is often asked too late

One of the most expensive questions in a die-casting project is often asked too late: which features should be produced by the die, and which should be finished by CNC machining? The easy answer is to machine every critical feature. That approach feels safe, but it can create unnecessary cycle time, fixtures, tools, chips, and scrap. The opposite approach — casting everything possible — can create poor fits, unstable datums, excess draft, flash, difficult inspection, and unreliable threads. The right answer is feature-specific. A sealing face, bearing bore, precision locating pad, or hole with a tight positional tolerance may justify CNC machining. A rib, external contour, logo, clearance opening, or non-functional boss may be better produced as-cast. The decision depends on the functional tolerance, the die-opening direction, alloy, production volume, porosity risk, machine access, and the cost of adding core slides or other tooling. This article explains how an experienced manufacturing engineer evaluates that choice with a buyer or design engineer before the die is built.

02Die casting is a near-net-shape process

In the die-casting industry, die casting is a net or near-net-shape process that can minimize or eliminate finish machining. When machining is required, the decision should consider four factors: the precision capability of the die caster, the alloy, the location and geometry of the feature, and production economics. According to NADCA, these factors should be evaluated together rather than treated as separate design decisions. That means a drawing tolerance alone does not determine the answer. A feature that is difficult to cast on one tool may be practical on another tool with a core slide, better die control, or a different parting-line strategy.

03Features that commonly justify CNC machining

In most programs, CNC machining is appropriate when a feature directly controls assembly, sealing, motion, or measurement. Typical examples include: · bearing and bushing bores requiring controlled size, roundness, or surface finish; · sealing lands and gasket faces where flatness and surface integrity affect leakage; · precision locating datums used to position the part in a downstream assembly; · holes with tight positional tolerances or a specified reamed, bored, or threaded condition; · tapped holes that need a controlled minor diameter, thread depth, or thread quality; · mating faces that must maintain a defined parallelism, perpendicularity, or runout; · surfaces that must meet a finish unavailable from the as-cast die surface. Die castings can be drilled, tapped, reamed, punched, or otherwise machined when the part and die are planned for those operations.

04Features that often should remain as-cast

Features normally better left to the die include non-functional outside contours, cosmetic lettering, ribs, webs, draft surfaces, many clearance holes, and bosses whose location and size do not control a critical interface. Casting these features avoids secondary operations and takes advantage of die casting's ability to combine geometry in one process. Some holes can be cast directly when their diameter, draft, depth, and position are compatible with the die-opening direction. Side holes may be formed with slides, punched after casting, or machined. The lowest-cost choice is not universal; it depends on volume, tool life, flash removal, and the required tolerance.

05The dense skin changes the decision

The outer skin of a die casting is generally denser and less porous than the center. According to NADCA, a typical skin thickness is approximately 0.015–0.020 in (0.38–0.50 mm). Removing too much material can expose subsurface porosity, especially on pressure-tight or cosmetic surfaces. For this reason, a sealing surface is not automatically safer because it is machined. If the machined depth is excessive or the die places porosity beneath the surface, CNC machining can create the leak path it was intended to prevent. Communicate critical machined zones to the die caster before tooling design so that gates, overflows, vents, cooling, and process parameters can be planned around them.

06Every machined feature adds a process chain

A feature selected for CNC machining requires more than a toolpath. It needs stock allowance, a datum scheme, a fixture, tool access, cutting parameters, inspection, chip removal, and a plan for tool wear. On a high-volume part, a few seconds per feature can become a major annual cost. That is why the correct comparison is not “cast versus machine” in isolation. It is the total cost of a conforming part. A core slide may increase die cost and cycle time but eliminate two CNC operations. Conversely, machining a simple hole may be cheaper than building and maintaining a slide, particularly at low volume. In our experience, the cost of CNC work is driven less by how many millimetres are removed than by the number of setups and the cycle time: for the same amount of cutting, a part that needs three setups can cost more than twice as much to machine as one that needs a single setup.

07Tolerance accumulation is often the hidden failure

If a machined face is located from one die half while the fixture references a surface from the other die half, parting-line variation can stack into the final dimension. Where possible, place locating pads on the same die half as the surface to be machined. The 2024 NADCA guide also recommends that datums exist throughout the process rather than being located on machined surfaces. This reduces accumulated tolerance across the parting line and avoids changing the reference scheme between casting and machining. The practical implication for buyers is simple: ask to see the datum and fixture concept, not only the final CNC drawing.

08Threads and bosses need a separate decision

Internal threads can sometimes be cast using spin-out or unscrewing cores, but draft, cycle time, and thread quality often make tapping a separate operation more practical. According to NADCA, tapped holes should be cored to minimize porosity in the thread area and should provide sufficient depth for tool clearance. In our manufacturing experience, a cast boss should not be judged only by whether a tap can enter it. The boss must have enough diameter, wall support, thread depth, and porosity control to survive installation torque and service loads.

09Common mistakes

Machining every tight-looking dimension. Not every close dimension on a 3D model is a functional tolerance. Machining a feature that the die can hold repeatably adds cost without improving the product. First classify dimensions as functional, assembly-related, cosmetic, or non-critical. Adding stock as a substitute for casting control. Large machining stock may appear to protect against dimensional variation, but it can remove the dense skin and expose porosity. According to the 2024 NADCA standard, a normal minimum machining allowance is 0.010 in (0.25 mm), while the final value must account for casting tolerance, flatness, parting-line variation, and the actual operation. Machining stock is added to the existing casting tolerance, so it should not be used to hide unstable casting control. In our experience, precision mating faces are best split into roughing and finishing: roughing removes most of the stock, stress is allowed to relax in between, and only about 0.1 mm is left for the finishing pass; large flat faces need extra stock according to the casting distortion actually measured. Ignoring cutter access and clamping loads. A feature may be theoretically machinable but impossible to reach with the selected tool, or too weak to withstand clamping. Machined features must permit machine-tool access, and casting features receiving machining or clamping loads need adequate strength and rigidity. Moving the machining decision after tool release. Changing an as-cast feature to a CNC feature after the die is built can require new slides, more stock, a different fixture, or a revised inspection plan. Conversely, discovering that a hole could have been cast after buying a dedicated machining station is equally expensive. Treating “as-cast” as “uncontrolled”. As-cast does not mean unmeasured. An as-cast feature still needs a defined tolerance, draft, parting-line condition, and inspection method. If the function allows an as-cast feature, specify its capability instead of leaving it ambiguous.

10Our typical approach

Experienced suppliers make the cast-versus-machine decision at feature level, before final die design. They balance functional tolerance, die complexity, CNC access, porosity risk, inspection, production volume, and total cost. The goal is not to machine as much as possible, but to machine only where it creates measurable product value. We create a feature-by-feature manufacturing plan before final die design. The plan normally divides the model into four categories: cast as-is, cast then trim, cast then machine, and cast with an optional operation depending on volume or capability. For each machined feature, we define the machining datum, stock, access direction, fixture support, tool sequence, inspection method, and acceptable porosity condition. We then review whether a core, stepped core, slide, punch, or revised parting line could produce the feature economically as-cast. Core slides can eliminate machining, although the added tooling and cycle time must be weighed against recurring machining savings. We also protect the surfaces that must remain pressure-tight or cosmetic. We keep gates and overflows away from critical areas where possible, and compare the planned machining depth with the expected dense skin. If a feature will be drilled or tapped, we verify that the core geometry and wall thickness support the operation. We validate the actual sequence, not just an as-cast sample. We run representative castings through trimming, fixturing, roughing, finishing, washing, and inspection. We check hole position, flatness, thread gauges, surface finish, leak performance, and any evidence of exposed porosity. For production, we keep separate controls for the casting and machined condition; more than one gage may be required.

11From our projects: dividing the work on a clutch basket

A motorcycle clutch basket we make is a clear example of dividing the work between cast and machined features. The centre bore, held to ±0.02 mm on diameter, is the datum for assembly and for all machining, and CNC guarantees it; the slot widths, held to ±0.05 mm, are formed directly by the die. As-cast does not mean uncontrolled. The slot cores are long and slender and run hotter than anything else in the die; over thousands of thermal cycles they wear and move, which is the main source of batch-to-batch drift in slot width. We cool the slot cores conformally, log their shot count, measure the trend in slot width every 50,000 shots and replace the cores on schedule at 120,000; every finished part is checked with a go/no-go gauge, and the gauge itself is periodically verified on the CMM. In stable production slot width holds within ±0.05 mm, and sticking complaints from the customer's assembly line have fallen to zero.

12Questions buyers should ask

1. Which features are functionally critical, and which are only cosmetic or nominal? 2. For every proposed CNC feature, what tolerance or performance requirement cannot be achieved reliably as-cast? 3. Could a core slide, stepped core, punch, or revised parting line eliminate the operation? 4. What is the planned machining stock, and how does it relate to the dense skin and porosity risk? 5. Are all machined surfaces accessible with standard tools and supported against cutting and clamping loads? 6. Do the machining datums come from the same die half as the critical machined features? 7. Which holes should be cast, cored, punched, drilled, reamed, or tapped, and why? 8. How will threads be protected from porosity, insufficient depth, draft, and boss dilation? 9. What will be inspected before machining and after machining? 10. What annual volume or tool-life assumption makes the selected cast-versus-machine trade-off economical? The supplier's answers should show a complete manufacturing system, not simply a list of CNC capabilities.

SOURCES AND NOTE

[1]North American Die Casting Association (NADCA). NADCA Product Specification Standards for Die Castings: Aluminum, Aluminum-MMC, Copper, Magnesium, Zinc and ZA Alloys, 12th edition, revised for 2024. Key sections: Secondary Machining Preplanning, pp. 2-14 to 2-15; Cored Holes for Cut, Formed and Pipe Threads, pp. 4A-28 to 4A-33; Machining Stock Allowance S/P-4A-13-24, p. 4A-34; Quality Assurance, pp. 7-12 to 7-16; Tooling and Engineering & Design guidance.
[2]North American Die Casting Association (NADCA). Product Design for Die Casting: In Recyclable Aluminum, Magnesium, Zinc and ZA Alloys, 7th edition, 2015. Key sections: Product Design, pp. 37-45; Fastening and Tapped Holes, pp. 50-51; Machining, pp. 63-65.
[3]North American Die Casting Association. 2021 NADCA Aluminum Die Casting Alloys (2021). Supporting reference for alloy selection and the interaction between alloy characteristics, casting capability, and machining requirements.

This is a technical introduction and design guide, drawing on the sources listed above and on Jinxiong's many years of production practice. It is for reference only and is not a commitment for any specific project. For a specific project, the technical agreement between both parties governs.

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