A sheet metal tolerance guide is only useful if the numbers match what the process can actually hold. This page sets out where variation comes from on a laser-cut and bent part, which callouts are worth putting on the drawing, and what to expect back in the inspection report.
Which Tolerances Are Worth Calling Out
Most sheet metal drawings carry too many tolerances rather than too few. A tolerance placed on every dimension makes a part expensive to inspect and no more accurate to make. The practical rule is to tolerance the features that affect function or fit, and leave general dimensions to a title-block note such as ISO 2768-mK.
Tolerance by Feature Type
| Cut profile and hole position | Laser cutting holds a cut-out and a hole position well inside the general tolerance on sheet from 0.5 to 3.0 mm. What moves a hole is not the cutter but the bend it sits near. |
|---|---|
| Bend angle | Angle variation depends on material grade, thickness, springback and tooling. A press brake holds an angle far better than the general tolerance on a title block suggests, but the value has to be agreed rather than assumed. |
| Bend to bend (flange to flange) | This is where stack accumulates. Each bend contributes its own error and the flat pattern contributes its own, so a dimension across three bends is looser than a single bend dimension. |
| Hole to bend line | The governing rule is clearance: a hole needs roughly 2.5 times the material thickness plus the inside bend radius of run-out from the bend line. Hole positions closer than that deform during forming, and no tolerance can rescue them. |
| Flatness on a welded assembly | Welding introduces local heat and the assembly moves. Flatness is controlled by sequence, restraint and the amount of weld metal, and it is far cheaper to agree a realistic value before production than to chase an impossible one afterwards. |
Where Variation Actually Comes From
Three sources account for most of the arguments that follow a delivery. The first is tolerance stack: individual features each within tolerance can still add up to a dimension outside tolerance when the part is measured as an assembly. The second is the flat pattern: what happens at the bend depends on the K-factor used in the calculation, and a flat pattern developed for a different tooling setup will place the hole slightly differently. The third is finishing: powder coating adds thickness to every coated face, which matters when two coated parts have to mate.
All three are predictable. They are handled in the drawing review before quotation rather than discovered at first article, and the values quoted are the ones the process can hold for that specific part.
How Tolerance Is Verified
The characteristics that matter are agreed before the first part is made, together with the method. Calipers, height gauges and gauges cover most dimensions on a bracket or panel; radius gauges cover form features. The result is recorded per characteristic against the requirement taken from the drawing, so the row can be read months later. See quality inspection for what that record contains, and CNC bending for the process side.
Worked Example: A Bracket Across Three Bends
Take a simple bracket with three bends and a hole in each flange. Each bend carries an angle error, and each flange dimension carries a length error, so the flange-to-flange dimension carries the sum of them. If the drawing applies the same tight tolerance to the overall outside dimension as it does to a single hole, the part will be inspected as failing even though every operation was performed correctly.
The practical answer is to dimension from a functional datum: one bend as the reference, the mating features toleranced tightly against it, and the accumulated dimension given a value the process can hold. Where the assembly genuinely needs a tight overall dimension, the fix belongs in the design — an adjustment slot, a shim or a different joint — rather than in a tighter tolerance on the same drawing.
Questions Buyers Ask About Sheet Metal Fabrication Tolerance
Should I use general tolerances or individual ones?
Use a general tolerance in the title block for everything that does not affect function, and individual callouts only where fit or function depends on the value. That keeps the drawing readable and the inspection cost proportionate to the risk.
What if my design needs a tighter tolerance than standard?
It is worth asking before assuming. Some features can be held tighter by changing the process route, for example by bending before cutting a hole or by machining after forming. Where a value genuinely cannot be held, we say so at quotation stage instead of accepting an order we cannot meet.
Do you provide a dimensional report?
For projects that need one, yes. The characteristics, the method and the sample size are agreed before production, and the report is issued against the drawing revision the parts were made to.
Drawing Files Needed for Sheet Metal Fabrication Tolerance
For a useful quotation, send drawings, quantity, material, finish and assembly requirements. Violet Gao will review your project and reply with technical feedback.
Request a QuoteWhat stands behind this quotation
- Manufacturing since 20162,500 m² Ningbo plant — 3 laser cutting machines (including newer TRUMPF), 4 CNC press brakes and 10 welding stations.
- Shipped to more than 30 export marketsEnclosures, cabinets and formed parts supplied to OEM buyers across Europe, North America and Asia.
- 123 documented product casesMedical device housings, HVAC and heat pump enclosures, agricultural and livestock equipment, industrial hydraulics, material handling trolleys and EV charging cabinets.
- Factory evidence you can checkTUV Rheinland audit reference MIC-ASI243729 and Bureau Veritas reference MIC-ASR263630 available for buyer review under NDA.
Before you send drawings: an NDA is signed on request · first-article inspection is carried out before a batch continues · pricing is staged from prototype quantity to repeat volume · pre-shipment photos and packing records are prepared when the project needs them.