Machine frame fabrication standards, in one page
The numbers a machine frame drawing is quoted against, collected in one place. ISO 2768 is a published standard; the welding-standard comparison, coating and material tables are typical ranges across the partner network, not one shop's spec sheet — the shop that quotes your drawing confirms what it holds on your geometry.
General tolerances — ISO 2768-1
Every dimension without its own tolerance falls under the general class named on the drawing. Say nothing, and a shop quotes medium by default — that assumption is already inside the number that comes back.
| Nominal size (mm) | f — fine | m — medium | c — coarse | v — very coarse |
|---|---|---|---|---|
| 0.5 – 3 | ±0.05 | ±0.1 | ±0.2 | — |
| >3 – 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| >6 – 30 | ±0.1 | ±0.2 | ±0.5 | ±1 |
| >30 – 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| >120 – 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| >400 – 1000 | ±0.3 | ±0.8 | ±2 | ±4 |
| >1000 – 2000 | ±0.5 | ±1.2 | ±3 | ±6 |
| >2000 – 4000 | — | ±2 | ±4 | ±8 |
ISO 2768-1:1989, permissible deviations for linear dimensions in millimetres. Frames over 4,000 mm carry the tolerance as a direct note on the drawing rather than a general class. The cheapest drawing calls a coarse or medium class overall and tightens only the three or four features that actually mate — see tolerance stack-up on large weldments.
Machining tolerance and flatness on finished bases
A weldment is not held to the same numbers as a small turned part. These are typical network maxima on the surfaces that get finish-machined after stress relief — mounting faces, ways and bolt patterns — not the as-welded envelope.
| Feature | Typical range | State on the drawing |
|---|---|---|
| Flatness, critical mounting face | ±0.02 mm over the machined face | Which face, and the datum it is measured from |
| Flatness, general machined surface | ±0.05 – 0.1 mm | Only where a mating part actually needs it |
| Bolt pattern position | ±0.1 mm true position | True position with a datum, not four linear dimensions |
| Parallelism, ways or slide faces | ±0.02 – 0.05 mm / m | Length over which it is measured |
| Surface finish, milled/bored | Ra 1.6 μm | The Ra value and which faces it applies to |
| Surface finish, turned | Ra 3.2 μm | State if a finer pass is needed on a fit |
Machining allowance left on a weldment for finishing is normally 3–6 mm per face — enough to clean up distortion from welding without leaving a wall too thin to hold rigidity.
Welding standards compared
The standard named on the drawing decides which shops can even quote it, and which paperwork you get back. Pick it by where the frame is going, not by which sounds most rigorous.
| Standard | Typical buyer | What it actually certifies |
|---|---|---|
| AWS D1.1 | US structural / machine builders | Structural steel welding code — joint design, procedure qualification (WPS/PQR), welder qualification and visual acceptance criteria. |
| ISO 3834-2 | EU, JP, KR — internationally recognised | Comprehensive quality requirements for the welding shop — personnel, equipment, procedures — not a per-joint code. Usually asked for alongside a joint code, not instead of one. |
| EN 1090-2 | EU — structural components sold into the EU | Execution of steel structures; drives CE marking. Relevant when the frame itself is a structural component, not just a machine base. |
| EN 15085 | Rail industry, EU | Welding of railway vehicles and components. Narrow scope — only ask for it if the frame is rail equipment. |
None of these certify the material, the coating or the machined tolerance — each covers the weld only. Full comparison and how to invoke one on a drawing: welding standards for machine frames.
Surface preparation and coating
| Treatment | Typical spec | Notes |
|---|---|---|
| Blast cleaning | Sa 2.5 per ISO 8501-1 | Near-white metal. The usual baseline before primer on an export frame. |
| Primer | Zinc-rich epoxy, 60–80 μm DFT | Applied within hours of blasting to avoid flash rust in humid shop air. |
| Topcoat | 2K polyurethane, 40–60 μm DFT | UV-stable for export and outdoor installs; state the RAL colour. |
| Hot-dip galvanising | 85 μm+ per ISO 1461 | Alternative to paint for structural steel exposed to weather; changes flatness slightly on thin sections. |
| Machined faces | Masked, "as machined" | Any face that mates or is later machined must be called out as masked before coating. |
Coating thickness is not free — it is a dimensional change on close-fit features and a cost line on every other one. What survives a sea transit and what does not: surface treatment for export.
Material equivalents
Approximate cross-references only — chemistry and mechanical properties differ between standards even where grades are commonly treated as interchangeable. Ask for the mill certificate rather than relying on the equivalence.
| Chinese / local grade | Approx. ASTM equivalent | Approx. EN equivalent |
|---|---|---|
| Q235B | A36 | S235JR |
| Q345B | A572 Gr50 | S355JR |
| 20# steel | 1020 | C22 |
| 45# steel | 1045 | C45 |
| HT250 cast iron | ASTM A48 Class 30 | EN-GJL-250 |
| HT300 cast iron | ASTM A48 Class 35 | EN-GJL-300 |
Full grade selection reasoning, not just the cross-reference: steel grades for machine frames. For the cutting, welding and machining technology behind these grades, see fabrication technology and materials.
NDT methods compared
| Method | Finds | When to specify it |
|---|---|---|
| VT — visual | Surface profile, undercut, spatter, obvious defects | Every weld, always, as the baseline hold point. |
| PT — dye penetrant | Surface-breaking cracks and porosity | Non-magnetic materials, or as a cheaper alternative to MT. |
| MT — magnetic particle | Surface and near-surface flaws | Ferromagnetic steel only; faster than PT on a production run. |
| UT — ultrasonic | Internal, volumetric flaws in thick sections | Full-penetration welds on load-bearing joints over ~10 mm. |
| RT — radiographic | Internal, volumetric flaws, with a film record | Where a permanent record is contractually required; slower and costlier than UT. |
Name a method and a sample percentage on the drawing, not just "NDT as required" — that phrase gets quoted at whatever the shop assumes is cheapest. NDT for machine frame welds.
What a quotable drawing carries
- General tolerance class per ISO 2768, or a stated equivalent
- Tight tolerances only on features that mate, each with its datum
- Welding standard invoked by name, with the joints it applies to
- Material grade and form (plate / bar / casting), with an equivalent noted if a substitute is acceptable
- Stress relief method, if the frame needs one, and when in the sequence
- Surface treatment spec, or "as machined", with masked faces called out
- NDT method and sample percentage per joint class
- Weight and overall envelope, for handling and freight planning
- Inspection documents required to travel with the shipment
- Destination and preferred Incoterm
Full walkthrough with a copy-ready title block: the welded machine base drawing checklist.
Cite or link this page
Free to quote, reference or link with attribution. The calculation method is stated above so it can be checked rather than taken on trust.
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