Taiyuan, Shanxi, China-September 11,2026
Technical contributor: Jing Chun Wang, General Manager, SIMIS | Edited by: SIMIS Export Team | August 19, 2026
Casting datum design starts by connecting the finished GD&T scheme with the first rough-casting setup.
Editor’s note: This article continues our discussion of machining allowance. Here we move one step deeper: the finished drawing may define Datum A, B and C very clearly, but the manufacturer still has to decide how a rough casting will be located so those datums can actually be created and repeated in production.
A datum symbol on a finished drawing can look very clear. The difficulty starts one step earlier, when the part is still a rough casting and none of the finished datum surfaces exist yet.
This is where the foundry drawing, machining plan and fixture concept have to meet. The finished part may eventually use a machined mounting face as Datum A, a bore as Datum B and a side face as Datum C. But the first CNC operation still has to locate a rough casting that has normal casting variation, draft, surface texture, parting-line effects and sometimes heat-treatment movement.
So when we review a casting + machining RFQ, we do not only ask, “What is Datum A?” We also ask:
How will we create Datum A from the rough casting?
That question sounds simple, but it often decides whether machining is repeatable or whether every casting has to be adjusted by hand in the fixture.
1. The drawing datum and the rough locating point are not the same thing
In GD&T, a datum is a theoretically exact reference used to orient or locate tolerance zones. The physical surface, axis or other feature on the part that establishes that reference is the datum feature. ISO 5459:2024 and ASME Y14.5-2018 provide the formal language and rules for datum systems and geometric tolerancing. [1][2]
Manufacturing adds another practical layer. Before a datum feature has been machined, the shop still needs physical places to support and locate the rough casting. These may be rough pads, bosses, a cast flange, a pilot diameter or another purposely controlled area. They are manufacturing locating features; they should not be confused with the theoretical datum itself.
This distinction matters because a finished drawing may be perfectly valid while the first machining setup is still poorly defined. The drawing tells us what the finished part must reference. The manufacturing plan must decide how to reach that reference from the casting we actually receive from the mold.
A datum, its physical datum feature and temporary rough locating points serve different roles in manufacturing.
2. We start with function, not with the easiest surface to clamp
A common temptation in machining is to choose the largest or easiest rough surface as the first reference. Sometimes that is exactly the right answer. Sometimes it creates a stable setup but the wrong relationship to the features that matter in service.
For example, consider a pump housing where a machined mounting face controls the position of two bearing bores. If those bores must stay aligned to that face, the machining route should preserve that functional relationship. It is not enough to hold the housing wherever it happens to sit most comfortably in the first fixture.
This is why we normally read the drawing in this order: which surfaces assemble to another component, which bores or sealing faces are critical, which geometric tolerances reference A/B/C, and only then how to create those references efficiently in machining.
A good datum scheme is not merely an inspection convention. It should reflect how the part functions in the final assembly. The production route then has to reproduce that functional scheme without fighting the casting.
3. Primary, secondary and tertiary references must become a real setup
On many machined castings the primary datum controls the main seating orientation, the secondary datum controls the next important direction and the tertiary datum closes the remaining location. The exact formal interpretation belongs to the drawing standard and the designer’s GD&T scheme; our job as the manufacturer is not to rewrite it. Our job is to make sure the rough-to-finished process can establish it repeatably.
The first setup is usually the hardest because the casting still contains the most variation. Once a stable primary machined reference has been created, later operations become much easier to control. This is one reason we like to identify the first datum operation before finalizing casting stock and fixture points.
If the primary datum is a large mounting face, we ask whether the rough casting can be supported without rocking. If the secondary datum is a bore, we ask whether there is enough material and positional margin to machine that bore from the newly established primary reference. If the tertiary reference is a side face or hole pattern, we check whether the sequence leaves enough access and clamping clearance.
The logic should move from rough location to stable machined reference, then from that reference to the critical features.
The finished datum scheme should follow function, while the manufacturing route must be able to establish it repeatably.
4. A practical housing example
Take a ductile-iron housing with one mounting face, two bearing bores and several external ribs. The finished drawing uses the mounting face as Datum A. The bore system is controlled relative to A, and a side face or hole establishes the remaining orientation.
A practical manufacturing route may begin by supporting the rough housing on three designated cast pads and using two additional contacts to prevent side movement. We then machine the mounting face to create Datum A. After that, the housing is re-fixtured from Datum A and the two bearing bores are finished in their required relationship to it.
The important point is that the first rough pads are not automatically the final datum. They are a temporary manufacturing bridge to create it.
This is also where casting design can help machining. If the rough locating pads are too small, sit on a parting line, distort during cooling or are removed during cleaning, the machine shop loses its reliable first reference. A few intentional locating areas on the casting can be worth much more than complicated fixture adjustment later.
The rough locating features are a temporary manufacturing bridge used to create the stable finished datum.
5. Datum selection and machining allowance are connected
This article follows directly from our previous discussion of casting machining allowance because the two subjects cannot really be separated.
Imagine a long housing with a finished base face that becomes Datum A. If the rough casting can bow during cooling or heat treatment, the machining stock on that base face must be sufficient for the whole datum surface to clean up. The fixture must also support the part in a way that does not force a distorted casting flat and then allow it to spring back after unclamping.
The same applies to bores. If a rough bore is shifted slightly by core movement or casting variation, there must be enough radial stock to machine the final bore centerline relative to the established datum structure.
So machining allowance is not just “extra metal.” It is part of the route by which the functional datum system is recovered from a real casting. ISO 8062-3:2023 provides a standardized framework for casting tolerances and machining allowance grades, but the part-specific stock still has to work with the actual datum and machining strategy. [5]
Related SIMIS article: Casting Machining Allowance — publish Article 02 first, then add a two-way internal link here.
6. Sand casting and investment casting need different locating thinking
The functional datum concept does not change with the casting process, but the rough part presented to the machine shop can be very different.
A sand casting may have larger as-cast variation, rougher surfaces, parting-line effects, core shift risk and heavier sections that move during cooling or heat treatment. For a large railway bracket, pump body or gearbox housing, it is often useful to provide clearly defined rough locating areas rather than expect the fixture to reference any available cast surface.
An investment casting can reproduce smaller features and near-net geometry more closely. That can make locating easier, but it does not eliminate the need for a machining plan. A small stainless valve component may still require a sealing face, precision bore or threaded feature to be controlled from a functional datum. If the wax/tooling geometry does not provide a repeatable first location, the near-net advantage can still be lost in secondary machining.
In both processes, the question is the same: how do we move from the casting’s natural variation to the finished part’s functional reference system with the fewest unstable steps?
Real sand-casting finishing before machining. Surface texture, local cleanup and normal cast variation are practical reasons to define intentional locating areas instead of referencing any available rough surface.
7. Heat treatment can change which setup should come first
Another reason to review datums early is heat treatment. A casting may be stress relieved, normalized, solution treated, quenched and tempered, or otherwise processed before final machining. Depending on material, section thickness and geometry, the part can move between the rough-cast and final-machined condition.
If we create a precision datum too early and the part then moves significantly during heat treatment, that reference may no longer be useful for final geometry. On some parts it is better to rough machine locating surfaces, perform the required heat-treatment step, then re-establish the final datum before finish machining.
Stable, intentional rough locating areas can reduce first-setup variation and manual fixture adjustment.
8. What we want to see on the customer drawing or RFQ
SFSA purchasing guidance recommends that the casting drawing identify dimensional tolerances, surfaces to be machined and datum points for locating. The Investment Casting Institute’s 2024 industry consensus similarly recommends a machined casting drawing when machining is required, with critical areas and preferred machining locating areas identified. [3][4]
For a new casting + CNC project, the most useful package is usually:
Finished 2D drawing with GD&T and the intended datum reference scheme.
STEP/STP or Parasolid model if available.
Material grade and heat-treatment condition.
Casting drawing, if one already exists.
Machined faces, bores, threads, sealing surfaces and bearing fits.
Any preferred rough locating pads or surfaces already used by the current supplier.
Annual volume and normal batch quantity.
Known problems such as setup variation, bore position drift, incomplete cleanup, distortion or long machining cycle time.
An anonymized spline-bushing drawing example showing primary, secondary and tertiary datum relationships. Customer identifiers, drawing numbers and traceability information have been removed.
If the customer only has the finished drawing, that is still enough to begin. We can propose a rough casting model, machining allowance, temporary locating areas and an operation sequence for review before tooling starts.
What we do not want is to discover the locating logic only after the tooling is finished. At that point a small change to a cast pad, boss or machining stock can become a much more expensive change.
9. First article inspection should validate the datum route, not only the final numbers
A first article report can show that every finished dimension is within tolerance and still hide a weak production route if the machine operator had to shim, tap or manually adjust every rough casting into position.
For a new project we therefore look at the setup behavior as well as the final inspection result. Did the rough casting sit consistently in the fixture? Did Datum A clean up across the full functional surface? Was the bore system centered within the available stock? Did re-clamping from the machined datum produce repeatable results? Did heat treatment change the location enough to require another operation?
These observations matter because a first article is not only proof that one part can be made. It should help confirm that the planned route can be repeated in production. If pilot production shows excess stock or overly conservative locating pads, the process can then be optimized.
Production inspection of a machined casting feature. Final verification should follow the same functional reference logic used to establish the machining setup and datum route.
10. Our short answer
How should datums be handled on a machined casting?
Let the finished drawing define the functional datum system. Then design the rough casting, machining stock, locating pads, fixture contacts and operation sequence so the shop can create that datum system in a repeatable way.
At SIMIS, we try to connect the customer’s finished GD&T with the real manufacturing steps: where the rough casting sits first, which surface becomes the first stable machined reference, what stock is needed to recover the critical features, and whether heat treatment or casting variation can disturb that route.
The objective is not to make the drawing simpler for the foundry. The objective is to make the finished part function as designed without creating unnecessary adjustment, machining time or production risk.
Have a casting drawing where the datums look clear, but the machining setup does not?
Send SIMIS the finished drawing, STEP file or existing casting model. Our foundry and machining team can review the datum route, rough locating features, machining stock and first-operation strategy before tooling begins.
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About the technical contributor
Jing Chun Wang (center) during an overseas customer visit — engineering discussions around drawings and manufacturability are part of SIMIS project work.
Technical contributor — Jing Chun Wang, General Manager, SIMIS. Ms. Wang has spent decades working in China’s casting industry, with practical experience in process development, production problem-solving and engineer training. This article was edited into English from SIMIS technical discussions so buyers, machining engineers and design engineers can see how the finished datum scheme is connected to the rough casting, fixture and machining route.
About us
Founded in 2004, SIMIS is a professional metal casting and CNC machining manufacturer dedicated to providing high-quality manufacturing solutions for customers worldwide.
Media ContactCompany Name: Taiyuan Simis Investment Casting Co., Ltd.Contact Person: Media RelationsEmail: Send EmailPhone: +86-15034166863Country: ChinaWebsite: https://www.simis-manufacturer.com/
