Sheet Metal Tolerances Explained: What Is Realistic, What Drives Cost

Every tolerance you add to a drawing has a price. Tighten one dimension and the supplier must slow down, inspect more, and scrap more. The skill is knowing which two or three dimensions genuinely matter - and writing those precisely while leaving the rest alone. This is what is achievable, and what it costs.

Start With a General Tolerance Note

The cheapest and most professional way to control tolerances on a sheet metal drawing is a general note, plus a small number of flagged critical dimensions. A typical general note covers:

• Cut and formed linear dimensions: ISO 2768-m or DIN 6935 style general tolerances
• Angles: ±0.5 degree unless stated
• Unspecified radii: inside radius 1.0 t unless stated
• Edge condition: deburred, sharp edges removed
• Surface: free from scratches on visible faces

Then flag the few dimensions your product genuinely depends on. A typical cabinet has three: the panel opening size, the mounting-hole pattern, and the overall height. Everything else can live with a general note.

Cutting Tolerances

The message: laser cutting is inherently accurate. What adds cost is not tight profile tolerance - it is features outside the process window, such as holes smaller than the material thickness, sharp internal corners, extremely narrow tabs, and tolerances so tight that every part must be measured individually.

Bending and Forming Tolerances

Two practical points:

• Tolerance accumulates. Every bend adds a potential error. A box with six bends cannot hold ±0.1 mm overall, regardless of how good the press brake is.
• Critical dimensions are best taken after forming. Where a dimension must be exact - for example a mating panel opening - design for a trimming or machining operation after bending, rather than asking the bend to produce it.

Welding and Assembly Tolerances

This is where tolerances get expensive, because welding introduces heat and heat moves metal.

If a welded assembly needs better than about ±0.5 mm across a large span, the design should carry adjustment features instead of demanding precision from a welding process - slotted mounting holes, an oversized cover, or a machined datum face added after welding.

How Tolerance Stacks Up

Every dimension in a chain contributes. If four bends each carry ±0.3 mm and three welded joints each carry ±0.5 mm, the worst-case accumulated error across the assembly is:

• Bends: 4 x ±0.3 mm = ±1.2 mm
• Welds: 3 x ±0.5 mm = ±1.5 mm
• Worst case total: ±2.7 mm

In reality errors do not always align in the same direction - statistical stack-up gives a smaller figure, roughly the square root of the sum of squares. But you cannot rely on statistics when one part must fit on a customer's bench. The practical rule: if your product needs ±0.5 mm at the end of a long chain, put the precision at the end of the chain, not at the start. Cut or machine the mating feature after assembly.

A Note on GD&T

Geometric dimensioning and tolerancing is a precise language, and it is genuinely useful for features like hole patterns, flatness and perpendicularity. Two cautions for sheet metal:

• Many GD&T callouts are unachievable after forming. A flatness of 0.1 mm on a 1.5 mm laser-cut and welded panel is not a manufacturing reality; it is a drawing exercise.
• Datum selection matters more than the values. Choose datums on features that are actually created accurately - a laser-cut edge or a machined face - not on a folded flange that carries its own tolerance.

If a design carries heavy GD&T, expect the supplier to raise the specific callouts that cannot be met by sheet metal processes. A supplier that quotes without comment is one that will not check.

Tolerance Checklist for Your Next Drawing

• Is there a general tolerance note covering everything unspecified?
• Are the two or three critical dimensions clearly flagged?
• Is any tolerance tighter than the process can hold without a secondary operation - and is that intentional?
• Are holes at least 1 t from bends and spaced at least 1.5 t from each other?
• Are internal corners rounded?
• For welded assemblies, is there an adjustment feature rather than a precision demand?
• Is the thickness tolerance basis stated? Actual thickness can sit below nominal.
• Is the flatness or straightness requirement stated as a number if it matters?
• Does the drawing state the deburring and edge condition?
• Is there a finish specification with colour, gloss and film thickness?

A drawing that answers these ten questions can be quoted accurately by any competent factory, compared fairly between suppliers, and manufactured without a mid-project argument.

Frequently Asked Questions