Guide
Should you make separate casting and machining drawings?
Separate casting and machining drawings are a two-document way of defining a cast part: one drawing describes the raw casting exactly as it leaves the foundry or die caster, and a second drawing describes only what the machine shop does to that casting to turn it into the finished part.
By Saola Manufacturing engineering team · Published · Updated
The alternative is a single combined drawing that marks which surfaces stay as cast. This guide sets out what belongs on each drawing, when one drawing is enough, when two pay for themselves, and how to keep a two-drawing set consistent. It closes with what the choice means when a cast-and-machined part moves to a new supplier, for example from a Chinese die caster to one in Vietnam. The conventions referred to here come from ASME Y14.8 for castings, forgings and molded parts, ASME Y14.24 for drawing types, and the ISO 8062 series for casting tolerances and machining allowance.
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What goes on a casting drawing?
A casting drawing defines the part in its as-cast state: every surface, including the ones that will later be machined, shown at its cast size with machining stock added. It also carries the process-specific requirements that only the caster can act on, such as parting line, draft, alloy and casting tolerances.
ASME Y14.8-2022 exists precisely because these requirements differ from a machined part’s; ASME describes it as defining drawing requirements for castings, forgings and molded parts. In practice a complete casting drawing tends to include:
- Geometry with stock. Surfaces to be machined drawn at their cast position, with the machining allowance either modeled in or noted, so the caster knows where extra metal is wanted.
- Casting datum targets. Points, lines or small areas on raw surfaces that the caster inspects from and that the first machining operation will locate on.
- Parting line and draft. Where the die halves meet and which way draft runs, or a note that the caster may choose within stated limits.
- Material and process. Alloy designation to a named standard, casting process, and any heat treatment or impregnation requirement.
- Casting tolerances. A general tolerance note that points at a casting standard, such as the ISO 8062-3 grades or the NADCA tolerance tables, plus specific tolerances where function needs them.
- Surface and integrity requirements. Where gates, overflows and ejector pin marks may and may not go, permitted flash, and any porosity or radiographic acceptance level.
Leave finished-part detail such as threads, bore tolerances and sealing-face finishes off this drawing; the caster cannot control them.
What goes on a machining drawing?
A machining drawing starts from the casting as its raw material and defines only the machined features, the datum reference frame they are measured from, and the link back to the casting’s own datums. Everything left as cast is covered by reference to the casting drawing rather than dimensioned again.
- Raw material callout. The casting part number and revision in the material field or parts list, the same way a bar-stock part names its bar.
- Machined features only. Faces, bores, threads, grooves and holes, each toleranced, usually with geometric controls to the machined datums.
- Datum handoff. The machined datum features, and the tolerances that relate them to the casting datum targets so that stock is removed evenly and as-cast walls stay where they should.
- Machined-surface requirements. Roughness, porosity acceptance on sealing faces, cleanliness, deburring and any plating or anodizing that follows machining.
- A clear general note. Something to the effect that unmachined surfaces conform to the casting drawing, so an inspector knows which document governs a surface nobody cut.
How should requirements be split between the two drawings?
Put each requirement on the drawing of the party that can control it: the caster owns everything decided in the die and the melt, and the machinist owns everything decided on the machine. The table shows a common split and how the same content sits on a single combined drawing.
| Requirement | Casting drawing | Machining drawing | Single combined drawing |
|---|---|---|---|
| Alloy and casting process | Yes, full callout | By reference to casting number | Material block |
| Machining stock | Modeled or noted per surface | Not shown | Note or phantom lines, easy to miss |
| Parting line, draft, gate and ejector zones | Yes | No | Yes, crowds the views |
| Casting datum targets | Yes, primary reference | Referenced for the first operation | Yes, beside the machined datums |
| General casting tolerance | Yes, to a named standard | Applies to unmachined surfaces by reference | Needs a second general note |
| Machined features and GD&T | No | Yes | Yes |
| Porosity and leak acceptance | Internal and radiographic level | Exposed on machined faces, leak test | Both, must say which state |
| Finish, plating, marking | Only if applied before machining | Yes | Yes |
The rightmost column is the argument for splitting. A combined drawing has to state, for almost every requirement, which state of the part it applies to. That can be done well, but each sentence of that kind is a chance for the caster and the machinist to read the same line differently.
When is one combined drawing enough?
One drawing is usually enough when the same supplier casts and machines the part, the machining is light, and the casting is unlikely to be used for more than one finished part. In that case the extra document mostly adds revision work without removing much ambiguity.
A combined drawing works best when it follows a few habits. Show the machined surfaces with a finish symbol or a machining note, and give the as-cast outline of those surfaces as phantom lines with the stock dimensioned. Use two general tolerance notes, one for as-cast features that points at a casting standard, and one for machined features. State plainly that the first operation locates on the casting datum targets. ISO 10135:2007, which sets drawing indications for molded parts, including castings, gives symbols for several of these indications if your drawings follow ISO conventions.
Small zinc parts are a common fit for a single drawing. The International Zinc Association points out that zinc die castings often need little or no machining, and where machining is unavoidable it suggests an allowance of 0.25 mm while warning that removing more metal tends to expose porosity (IZA, design rules). With stock that light and only a few machined features, one sheet can stay readable.
When do two separate drawings pay off?
Two drawings pay off when casting and machining happen at different companies, when one casting feeds several machined variants, or when the casting is complex enough that its own requirements fill a sheet. They also help whenever you expect to move either process to a new supplier.
- Different suppliers. Each supplier gets the document it is accountable to. The caster ships castings that conform to the casting drawing, the machinist accepts them against that drawing, and a rejected part has one owner.
- One casting, several finished parts. A housing that is machined differently for a left-hand and right-hand version, or for two port sizes, needs only one die and one casting drawing. Each variant gets its own short machining drawing.
- Separate inventory. If castings are bought, stocked or imported as a distinct item, they need their own part number, and a part number needs a drawing to point at.
- Tooling records. The casting drawing is the document the die was built to. Keeping it separate makes it much easier to hand a die, and the knowledge behind it, to another caster.
The cost is two part numbers and two revision histories that must change together.
How do you keep the two drawings in step through revisions?
Tie the machining drawing to a specific casting revision, review every change for its effect on both documents, and keep the 3D models under the same numbering scheme. Most two-drawing problems are revision problems rather than geometry problems.
- Reference by revision where it matters. If a casting change can affect machining, such as moving a cored hole or changing stock, call out the casting revision on the machining drawing or in the parts list, so an old casting is not machined to a new drawing.
- Check interchangeability. ASME Y14.100-2017 sets the general rules for preparing and revising engineering drawings. The core test is whether the old and new castings are interchangeable once machined. If not, the casting needs a new part number, not just a new revision letter.
- Change both, or say why not. A change record for one drawing should state whether the other was reviewed. That single line prevents most orphaned revisions.
ASME Y14.24-2020 describes the types of engineering drawings and the minimum content each needs, which is a useful checklist for either structure.
How should machining stock and datums link the two drawings?
The casting drawing should define datum targets and stock, and the machining drawing should locate its first operation on those same targets and then switch to machined datums. The tolerances that connect the two frames are the most important numbers in a two-drawing set.
The ISO route makes stock explicit. ISO 8062-3:2023 specifies general dimensional and geometrical tolerances and machining allowance grades for castings, applying to all cast metals and casting processes, with the rules for using them in ISO/TS 8062-2. A casting drawing can therefore state a tolerance grade and a machining allowance grade in the general note and leave the numbers to the standard.
Where the datum targets sit also shapes the tolerance you can expect. NADCA notes that flash increases thickness at the parting line and changes dimensions measured across it, and that critical dimensions should be measured within one die half (NADCA, parting line). Put the casting datum targets in the die half that forms the features you care about, and make the machining drawing’s first-operation tolerances realistic for any feature that crosses the parting line.
At first article, report the casting to its drawing, the finished part to its drawing, and show that each machined face cleaned up fully. The tolerances guide explains how the general notes on each drawing are read.
What changes when casting tooling moves to a new supplier?
When a die moves, for example from a Chinese caster to a Vietnamese one, the casting drawing is the document that lets the new caster reproduce the part without guessing. If only a finished-part drawing exists, much of the casting definition lives in the previous supplier’s head and process notes.
Before a transfer, check what you actually hold. Common gaps are a casting model only the old supplier has, datum targets never drawn because the old fixture simply worked, and stock amounts nobody wrote down. Writing a casting drawing now, from the finished part and measured sample castings, is usually cheaper than finding the gaps at first article.
The International Zinc Association observes that most supply disputes over die castings come from differences in understanding between the vendor and the specifier, and that dimensions critical to the part’s function should be specified and agreed with the caster (IZA, specifying quality requirements). A supplier change is when that advice matters most. Who owns the die, and what records come with it, is a contract question to settle in writing with both suppliers; the drawing set is what makes the answer usable.
Saola sources die castings from partner foundries and machines them on CNC milling at its lead CNC plant, with both steps managed on one order, so a single combined drawing is workable for many parts. A separate casting drawing is still worth having if you might ever split the work or move it again. The RFQ package guide lists the rest of what to send, and you can send your drawing set for a quote with either structure.
Frequently asked
Which drawing carries the finished part number?
Usually the machining drawing, because it defines the part the assembly receives. The casting gets its own number and appears as the raw material or a component on the machining drawing.
Should the machining drawing repeat as-cast dimensions?
Only as reference dimensions, if at all. Repeating them as toleranced dimensions creates two sources for the same requirement, and they drift apart at the next revision.
Is a separate casting model enough without a casting drawing?
A model can define the geometry, but the requirements that are not geometry, such as alloy, tolerances, datum targets, porosity limits and permitted gate locations, still need a drawing or an annotated model that the caster accepts as the governing document.
Does the same approach work for sand castings and forgings?
Yes. ASME Y14.8 covers castings, forgings and molded parts together, and the split between an as-produced drawing and a machining drawing is common for all of them. Rougher processes simply make datum targets and stock more important.
Who should draw the casting drawing, the designer or the caster?
The design owner should hold it, even if the caster drafts it. Parting line and gating proposals from the caster are useful input, but a casting drawing that only the supplier holds cannot be handed to anyone else.
Sources
- ASME Y14.8-2022, Castings, Forgings, and Molded Parts, ASME, 2022
- ASME Y14.24-2020, Types and Applications of Engineering Drawings, ASME, 2020
- ASME Y14.100-2017, Engineering Drawing Practices, ASME, 2017
- ISO 8062-3:2023, general dimensional and geometrical tolerances and machining allowances for castings, ISO, 2023 (publisher abstract)
- ISO 10135:2007, drawing indications for molded parts in technical product documentation, ISO, 2007 (publisher abstract)
- Parting line design guidance, NADCA, accessed September 2026
- Zinc die casting design rules, International Zinc Association, 2019 web edition
- Specifying quality requirements, International Zinc Association, 2019 web edition