The right manufacturing process for a metal part follows from four properties: its geometry, the quantity, the material and the tolerances it must hold. Compact prismatic and round parts go to CNC milling or turning, thin-walled flat or bent parts to sheet metal, large frames to welded assemblies, and complex or repeat-volume shapes to casting with machining of the functional surfaces.
How do you choose a manufacturing process in five questions?
Work through the questions in order and stop at the first yes. Then check quantity and tolerance, which can overrule the geometry answer.
- Can the part be cut flat from sheet or plate and bent, with one wall thickness throughout? Use sheet metal.
- Is it rotationally symmetric, such as a shaft, bush, flange or fitting? Use turning.
- Is it a compact body whose features a rotating cutter can reach from a few sides? Use milling.
- Is it a large frame or structure made from plate, tube or profile? Use a welded assembly.
- Does it have internal cavities, ribs or varying walls that machining would mostly cut away, and does the quantity pay for a pattern or die? Use casting.
Which process fits which part geometry?
The table gives the first choice for each geometry and the reason. These are engineering principles, not limits from a standard, and a particular shop draws its own boundaries.
Table 1. First-choice process by part geometry
| Part geometry | First choice | Why | Watch for |
|---|---|---|---|
| Prismatic block with pockets, bores, flat faces | CNC milling | Material is removed from solid, no tooling | Cost grows with material removed and with the number of setups |
| Rotationally symmetric part | CNC turning | The part rotates against a fixed tool, so round features come from one setup | Flats, cross holes and keyways need live tooling or a second operation |
| Thin-walled, flat or bent part | Sheet metal | Starts at the final wall thickness, cut flat and bent | Only shapes that unfold flat; thick sections and 3D curves do not qualify |
| Large frame or structure | Welded assembly | Joins cut plate, tube and profile with a jig, no pattern | Heat distortion; critical faces are machined after welding |
| Internal passages, ribs, varying wall thickness | Casting plus machining | The mould forms the shape, machining finishes the functional faces | Tooling lead time, machining allowance, internal defects that need an agreed inspection |
Source: editorial summary of process principles; no standard sets these boundaries.
Design rules for each process are in the articles on DFM for CNC milling and sheet metal design. Our network covers CNC milling and turning, sheet metal, welding, casting with machining and assembly.
Material form also matters. A cast alloy and a wrought alloy of the same base metal are different materials with different standards and properties, so check the material designation on the drawing before moving a part from machining to casting.
How does quantity change the choice?
Every process except machining from solid needs something before the first part: bending tools and a nesting program for sheet metal, a welding jig, a pattern or a die. That cost is paid once per part number. Machining needs a program and workholding, which is cheaper to start but removes more material from every part.
The break-even quantity is N = T ÷ (cA − cB). T is the extra one-off cost of process B (pattern, die, jig, first article), and cA and cB are the unit costs of the two processes without tooling.
Calculation with assumed inputs, not a quote: T = 12,000 EUR, cA = 30 EUR for a part machined from solid, cB = 22 EUR for a casting including finish machining. N = 12,000 ÷ 8 = 1,500 parts. Below 1,500 parts over the life of the tooling, machining wins in this example, above it casting wins.
Three conditions change the result. The tooling has to survive N parts. A drawing change after the tooling exists costs a tool modification, where a machined part needs a revised program. And the unit costs cA and cB must come from quotes for the same material, tolerance and inspection scope, or the comparison measures the quotes and not the processes. No standard fixes a break-even volume.
Which tolerance standard applies to each process?
Each process family has its own general tolerance standard, and ISO 2768-1 covers only two of them. The choice of process therefore decides which note belongs in the title block.
Table 2. General tolerance and quality standards by process, status as of 2026-10-30
| Process | Standard on the drawing | What it governs | Status |
|---|---|---|---|
| Milling, turning | ISO 2768-1:1989; ISO 286-1:2010 for fits | Linear and angular sizes without individual tolerance; fits | 2768-1 published, revision listed; 286-1 confirmed 2026-06-19 |
| Sheet metal | ISO 2768-1:1989; ISO 9013:2017 for thermal cut edges | Sizes of formed parts; quality classes of laser, plasma and flame cuts | ISO 9013 confirmed 2022, Amd 1:2024 published |
| Welded assemblies | ISO 13920:2023; ISO 5817:2023 | Lengths, angles, shape and position in four classes; weld imperfection levels B, C, D | Both published 2023, replacing the 1996 and 2014 editions |
| Castings | ISO 8062-3:2023 | General tolerances and machining allowance grades | Published 2023-02-15, replaces the 2007 edition |
Source: ISO catalogue records via ISS (national ISO member) and BSI, links in the source list.
ISO 2768-1 states that it applies to workpieces produced by metal removal or formed from sheet metal. For a machined part the class m values are ±0.2 mm for sizes over 6 up to 30 mm and ±0.3 mm over 30 up to 120 mm. The CNC machining tolerances handbook explains how those values combine with fits and GD&T.
ISO 9013 classifies thermal cuts in material from 0.5 to 32 mm thick for laser, 0.5 to 150 mm for plasma and 3 to 300 mm for flame cutting. These are the ranges the standard covers, not the limits of any machine.
ISO 13920:2023 replaced the 1996 edition. A drawing that cites "ISO 13920" without a year should be checked against the edition the buyer intends. The weld quality side is covered in welded assemblies: ISO 5817, ISO 3834 and EN 1090.
Which parts need two processes?
A cast or welded part with functional faces ends in a machine tool. ISO 8062-3 reflects this: it defines machining allowance grades, the stock a casting carries on surfaces that will be machined.
- Casting plus machining: the cast blank forms the shape, and bores, seal faces, threads and datums are finished on a machining centre. The drawing then needs two sets of requirements: cast tolerances on free surfaces and machined tolerances on the finished ones. See cast parts with machining.
- Welding plus machining: welding heat distorts a structure, so a frame with bearing seats or mounting faces is welded first and machined afterwards in one setup.
- Sheet metal plus welding: enclosures and covers are cut and bent, then welded where a bend cannot close the joint.
A single drawing can therefore carry ISO 2768-m on machined features, ISO 13920 on the welded frame and ISO 8062-3 on a cast part. Each note should say which features it governs.
What should an enquiry contain so the supplier can choose?
A supplier can propose the best process only if the enquiry states what the part must do. Send:
- Quantity per release, per year and over the expected product life, because tooling is judged against the last number.
- The material by standard designation, and whether a cast or wrought alloy is acceptable.
- The general tolerance standard and class, with individual tolerances only on functional features.
- Which surfaces are functional: fits, seals, threads, datums.
- Whether the supplier may offer an alternative process, and whether the part is released for first article inspection before series.
Our audited network in Bursa works to ISO 2768-m as standard and to tighter tolerances where the drawing specifies them. If the enquiry leaves the process open, the quote should name the process it assumes, so quotes from different suppliers compare like with like.
Frequently asked questions
- Is CNC machining cheaper than casting?
- Below a break-even quantity, yes, because machining needs no pattern or die. The break-even is the extra one-off tooling cost divided by the saving per part. No standard fixes it, so it has to be calculated from quotes for the specific part.
- Which process suits a thin-walled enclosure or bracket?
- Sheet metal, when the shape can be cut flat and bent. Milling the same shape from solid removes most of the material, while a sheet part starts at the final wall thickness. Corner joints and frames can be welded.
- Does ISO 2768 apply to castings and welded parts?
- No. ISO 2768-1 is written for workpieces produced by metal removal or formed from sheet metal. Castings use ISO 8062-3:2023 and welded constructions use ISO 13920:2023 as their general tolerance standards.
- Can a casting be used without machining?
- Yes, on surfaces whose function works within the ISO 8062-3 general tolerances. Faces with a fit, a seal, a thread or a datum role are normally machined, so the casting drawing needs a machining allowance there.
- Can a supplier change the process after the drawing is released?
- Only with your approval. A different process can change the material (cast versus wrought alloy), the achievable tolerances and the surface, so a proposed change should come with a revised drawing and a first article.
Sources
- ISO 2768-1:1989, scope (metal removal or formed from sheet metal), stage 90.92, revision listed (ISS, national ISO member)
- ISO 2768-1 linear tolerances, classes f, m, c, v, over 6 to 120 mm (AmesWeb table)
- ISO 286-1:2010, stage 90.93 confirmed 2026-06-19 (ISS, national ISO member)
- ISO 8062-3:2023, castings: general tolerances and machining allowances, replaces ISO 8062-3:2007 (ISS, national ISO member)
- ISO 13920:2023, welded constructions: general tolerances, four classes, replaces ISO 13920:1996 (ISS, national ISO member)
- ISO 5817:2023, fusion-welded joints: quality levels B, C, D, replaces ISO 5817:2014 (ISS, national ISO member)
- ISO 9013:2017, thermal cuts: flame, plasma and laser thickness ranges, confirmed 2022, Amd 1:2024 (ISS, national ISO member)
- BS EN ISO 9013:2017+A1:2024, status current, published 2024-10-31 (BSI)
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