Sheet metal tolerances are not one number. What a laser holds on a cut edge, what a press brake holds on a bend angle, and what a welded assembly holds after distortion are three quite different bands, and a drawing that applies one tolerance to all of them will either be unbuildable or unnecessarily expensive.
Cut features are the tight ones
Laser and punch positioning are very repeatable, so hole positions and outline dimensions within a single flat blank are the tightest things on the part. If a dimension has to be close, putting it between two cut features on one face is the cheapest way to get it, because no forming variation enters the chain.
Bends are looser, and they stack
Bend angle varies with material thickness within its mill tolerance, with grain direction, with springback and with how consistently the operator positions the blank. Each bend adds its own variation, so a dimension measured across three bends carries all three. That is why fabricators quote tolerance per number of bends rather than as a single figure.
Welded assemblies are looser again
Heat shrinks the weld as it cools and pulls the assembly, so post weld dimensions depend on fixture, sequence and how much weld was laid. Where a welded assembly must hold a close dimension, the usual answers are to machine the critical feature after welding, or to design the joint so the critical feature sits on a single part rather than across the joint.
Designing so the tolerances are achievable
Put critical dimensions between features on one flat face. Dimension from a single datum rather than chaining. Avoid measuring across multiple bends. Where a hole must locate accurately relative to a formed feature, punch it after forming or machine it. And ask the fabricator for their tolerance table before dimensioning rather than after quoting.
Questions people ask about sheet metal tolerances
What tolerance can sheet metal hold?
It depends on the feature. Cut features within one blank are tight, bend angles are looser and stack with the number of bends, and welded assemblies are looser again.
Why do tolerances stack across bends?
Because each bend carries its own variation from material thickness, springback, grain direction and operator positioning, and a dimension across several bends inherits all of them.
How do I hold a tight dimension on a fabricated part?
Put it between cut features on one face, or machine the feature after forming and welding. Do not chain it across bends or a weld.
Does material thickness tolerance matter?
Yes. Mill tolerance on the sheet feeds directly into bend allowance and therefore into finished dimensions, which is one reason bend tolerances are looser than cut ones.