Cabinet specifications often fail in one of two ways: no tolerances are given at all and the parts will not assemble, or every dimension is marked ±0.05 mm and the quotation doubles. Tolerances are not "the tighter the better" — they should be applied where assembly actually depends on them. This guide covers the six tolerance categories that matter for sheet metal cabinets.
1. Overall Dimension Tolerance
The deviation of the cabinet's length, width and height:
- Standard cabinets: ±0.5 mm
- Large cabinets (over 2000 mm high): can be relaxed to ±1.0 mm
- Non-standard shapes: agree case by case according to structure and assembly needs
Note that overall dimensions and diagonal deviation are two different things. A cabinet can measure correctly overall and still be skewed — see section 3.
2. Mounting Hole Tolerance
This is the category that most deserves strict specification, because it decides whether equipment can be installed:
| Hole type | Suggested tolerance | Why it matters |
|---|---|---|
| Profile system holes (modular grid) | ±0.1–0.2 mm | Accessory compatibility and bayed interchangeability |
| Equipment mounting holes | ±0.2 mm | Specify per the actual equipment hole pattern |
| Cable entry openings | ±0.5 mm | Control opening diameter when glands are used |
| Lock and hinge holes | ±0.2 mm | Affects door swing and lock alignment |
3. Diagonal Deviation (Most Commonly Overlooked)
Diagonal deviation measures how square the cabinet body is. The check is simple: measure the two diagonals of the front face and take the difference.
- Standard cabinets: ≤1.5 mm
- Large cabinets (over 2000 mm high): ≤3.0 mm
- Cabinets intended for baying: keep tighter (≤1.5 mm) or gaps appear when joined
Excessive deviation causes doors that will not close properly, seals that do not compress evenly and misalignment in bayed line-ups. These are structural defects that cannot be corrected by later adjustment.
4. Flatness and Straightness
Applies to doors, panels and mounting plates:
- Doors and panels: generally controlled to ≤1 mm per metre
- Mounting plates: also control deflection under load, typically within 1/200 of the span
- Inspection: surface plate with feeler gauge, or straight edge checked against light gaps
5. Bending Angle and Formed Dimension Tolerance
- Bend angle: normally ±1°; ±0.5° for visible cosmetic parts
- Formed edge length after bending: ±0.3 mm (affected by thickness and springback)
- Bend radius should suit the material thickness; too small a radius tends to crack
Note that the actual outer dimensions of a bent part differ from the calculated flat pattern (neutral axis shift). Drawings must state whether a dimension is an outside or inside dimension — this is one of the most common sources of dispute between buyer and fabricator.
6. Welding and Weld Tolerance
- Post-weld squareness of the frame matters more than individual weld length: control diagonal deviation and twist
- Sealing faces need continuous welds; verify airtightness where required
- Dressing: exposed welds must be flush, with no spatter, undercut or burn-through
- Distortion from welding heat must be handled in the welding process itself — never by forcing parts together at assembly
Three Practical Rules for Specifying Tolerances
- Mark only critical dimensions. Apply tight tolerances to features affecting assembly, sealing and interchangeability; leave the rest to general tolerances (e.g. ISO 2768-m)
- State the datum. A hole tolerance means little without saying which face or edge is the datum
- State the measuring method. For example "diagonal deviation ≤1.5 mm (difference between the two front diagonals)" removes ambiguity at inspection
Measuring Tools for Inspection
| Inspection item | Common tools |
|---|---|
| Overall dimensions, thickness | Tape measure, caliper, micrometer |
| Diagonal, squareness | Tape measure (diagonal check), square |
| Flatness, door gap | Straight edge with feeler gauge |
| Precision hole positions | 2D vision measuring machine / CMM |
| Coating thickness | Coating thickness gauge |
FAQ
Q: What tolerances are typical for sheet metal cabinets?
Overall dimensions ±0.5 mm; mounting hole positions ±0.1–0.2 mm; diagonal deviation within 1.5 mm. Values follow assembly requirements — tighter is not always better.
Q: Is a tighter tolerance always better?
No. Tighter tolerances raise machining and inspection cost, and tolerances beyond process capability either get ignored or inflate the quotation. Specify strictly only where assembly depends on it.
Q: Why does diagonal deviation matter so much?
It determines whether the cabinet body is square. Excessive deviation causes doors that will not close, seals that do not compress and misalignment when cabinets are bayed — none of which can be corrected later.
About Xingyuan Electric
Xingyuan Electric (Suzhou) Co., Ltd. is located in Lili Town, Wujiang District, Suzhou, with a complete production line covering laser cutting, CNC bending, welding and powder coating. We manufacture to drawing tolerances, provide dimensional inspection records, and support first-article approval and build-to-print customization.