First, a fabricator reads your sheet metal drawing in about thirty seconds, and half of that reading is tolerance. So where you wrote nothing, the shop prices a default. By contrast, where you wrote ±0.01, the shop prices aerospace — on a bracket. So this guide hands you the sheet metal tolerance method that keeps both sides honest. Next, it walks the three-tier structure you can apply before the drawing locks. In addition, it shows the typical values by operation, and the DFM rules that protect the price.

The Three-Tier Method in One Paragraph

First, here is the whole pattern. Tier 1 — general class: name ISO 2768-mK (or ±0.2 mm) as the drawing default, and make it cover most dimensions. Also, Tier 2 — functional: call out ±0.1 mm or ±0.5° on the dimensions that control fit and assembly. Finally, Tier 3 — critical: reserve ±0.05 mm or ±0.02 mm with CMM verification. So use that tier for safety or interference dimensions only, and keep it to a minority of the drawing. As a result, the three tiers show at a glance. Then, the shop can price the part correctly the first time. Also, you can compare quotes against a like-for-like specification.

Why Tight Tolerances Double the Quote

First, consider what happens when one dimension tightens. So a hole at ±0.2 mm needs no extra inspection. Next, the same hole at ±0.05 mm needs machined, or drilled-and-verified, control. Then, at ±0.02 mm, it needs CMM measurement plus a process that serves precision. Also, each step up adds measurement cost, process cost, and scrap risk. As a result, the price jumps in step — often doubling or tripling, with no functional gain. So engineers call this “gold plating.” In practice, it shows up as drawings where every dimension carries the same tight callout, out of habit.

So the fix is structural. First, state a general default tolerance for the whole drawing. Then, call out only the dimensions that drive fit, function, or safety. Also, the drawing should name its default explicitly — ISO 2768-mK (medium grade), or an ASME Y14.5 general tolerance table. In other words, both sides price the same part.

Typical Sheet Metal Tolerance Values by Operation

First, the values below are typical industry values for mild steel in the common 2–6 mm sheet range. So treat the table as a planning baseline, not a capability statement. In practice, actual capability depends on material, thickness, part size, and equipment condition.

Operation	Parameter	Typical	Tight	Notes
Laser cutting	Linear dimensions (3–10 mm sheet)	±0.2 mm (±0.008 in)	±0.1 mm (±0.004 in)	Tolerance grows with thickness; kerf ~0.1–0.3 mm
Laser cutting	Hole position	±0.2 mm	±0.1 mm	Nested parts share nest reference datums
Press brake bending	Bend angle	±1°	±0.5°	Springback compensation tracks material batch
Press brake bending	Flange dimension	±0.5 mm (±0.020 in)	±0.3 mm (±0.012 in)	Stable tooling and consistent material thickness
Welded assembly	Overall dimension	±1–2 mm	±0.5 mm with fixturing	Distortion control and weld sequence drive results
CNC machining (from plate)	Linear dimensions	±0.1 mm (±0.004 in)	±0.05 mm; ±0.02 mm with CMM	Precision tier adds measurement and process cost
Material (mill)	Sheet thickness	±5–10% of nominal	Tolerance-rolled, e.g., ±0.05 mm	Affects flange size and fit-up gap
Holes	Diameter / tapped	±0.1–0.2 mm	±0.05 mm; threads to class 6H–7H	Call out thread standard explicitly

Also, check the table with the three tiers in mind. So the “Typical” column is your default tier, and the “Tight” column is your functional or critical tier.

Ten DFM Rules That Protect the Price

First, the ten rules below all do one job: they keep the drawing inside the process’s natural range. So work through them in four groups.

Edges and Corners — Protect the Bend and the Tool

1. First, round internal corners. So keep the internal radius at least 0.5–1× thickness — sharp corners crack during bending, and they wear out tooling.
2. Next, give holes breathing room. So keep hole centers at least 1× thickness from the edge — 2× for tapped holes. Then, that spacing prevents edge distortion.

Features — Protect Assembly and Service Life

1. First, avoid very short flanges. So flanges shorter than ~1.5× thickness distort during bending. Then, if the flange has no function, delete it.
2. Next, choose threads deliberately. First, stamped threads trade off thickness against position. Also, machined taps need edge clearance. Finally, thread inserts cost more, but they survive rework and higher loads.

Bend Layout — Protect the Schedule and the Sheet

1. First, space parallel bends. So keep parallel bends at least 3–5× thickness apart — tighter spacing needs special tooling.
2. Next, prefer standard bend lengths. In other words, regular geometry nests efficiently. By contrast, exotic cutouts reduce sheet utilization and raise material cost.
3. Also, group parts by material and thickness. So parts in the same material/thickness family nest and fixture together. In addition, mixed stacks multiply changeovers.

Access and Surface — Protect Weld Quality and the Finish

1. First, design for weld access. So closed boxes trap welds. Next, provide access flanges or windows, or split the box into sub-assemblies. In addition, weld procedure qualification backs this rule — AWS publications carry the reference.
2. Also, design for coating. So trapped air in thin-walled boxes causes powder-coat defects. Then, add drainage or relief where the finish spec demands it.
3. Finally, tier your tolerances. So use the general class for most dimensions, and the functional tier for fit dimensions. Then, reserve the CMM tier for the dimensions that need verification — per the table above.

The Bending Math Behind the Flat Pattern

First, bending is where flat-pattern math lives. So the K-factor pins down the neutral axis — the line inside the sheet that neither stretches nor compresses. Next, it sits typically between 0.33 and 0.5 for mild steel, and it shifts with inside radius and thickness. Also, the bend allowance is the arc length of that neutral axis through the bend. By contrast, the bend deduction is what you subtract from the sum of outside flanges. So that subtraction yields the flat pattern. As a result, if you get the K-factor wrong, every bend line drifts by the same amount. Then, shops develop and document K-factor tables per material and thickness combination.

So radius rules matter for cost and durability. First, pick the inside radius from the table below. Then, it shows the practical options for mild steel.

Inside radius	Process	Notes
1× to 1.5× thickness	Air bending	Standard, lowest cost; typical default
0.25× to 0.75× thickness	Air bending with tighter tooling or coin finishing	Higher tooling cost; check minimum per shop capability
0 (sharp)	Coining	Expensive; avoid unless function requires it — sharp internal corners crack

In addition, keep two layout rules in mind. First, place holes and edge cuts at least 15 mm from a bend line. So they avoid marks and distortion. Then, sequence bends from the longest to the shortest. Next, later bends reference the geometry the earlier ones already formed.

What Your Drawing Actually Signals

First, understand the three layers a shop reads on your drawing. So layer one is the datums and critical dimensions: mark them clearly — “CRIT” or a critical-dimension note. Also, use GD&T where orientation drives function — true position for hole patterns, flatness for mating surfaces, perpendicularity for flanges. In addition, the governing references are ASME Y14.5 in US-style drawings. Then, the ISO GPS series (ISO 1101 et al.) covers international drawings. As a result, mixing the two on one drawing creates ambiguity.

Next, layer two is the default tolerance clause — the line that names ISO 2768-mK or a general table. Then, layer three covers everything without a mark, and the shop prices it at the default tier. Finally, if 80% of your dimensions carry “±0.01,” the drawing signals an aerospace part — on a bracket. So call out the 10–20% of dimensions that matter, and allow the rest to ride the general class.

Frequently Asked Questions

What default should I state when my drawing names no tolerance?

First, name ISO 2768-mK (medium grade) or ±0.2 mm as the drawing default. So without a named default, the shop applies its house default — usually around ±0.2 mm for sheet metal. As a result, the assumption stays invisible in both quotes. Then, stating it makes the default comparable across suppliers.

How tight can a laser-cut edge go without changing price tier?

First, for mild steel in the 3–10 mm range, ±0.2 mm is the typical band. Also, ±0.1 mm stays inside that band on well-run equipment, with no tier change. Then, anything tighter moves the dimension into the precision tier, where inspection cost joins the cutting cost. In addition, tolerance widens with thickness, because kerf and heat input increase.

What is the K-factor, and who should own it?

First, the K-factor locates the neutral axis inside the bend. So it sits typically between 0.33 and 0.5 for mild steel, depending on inside radius and thickness. Then, it determines the bend allowance your flat pattern needs. Finally, the shop should own a K-factor table per material and thickness combination. So it should state that table when you request a flat pattern with bend tables.

Do I need GD&T on a standard bracket?

Usually no. So reserve GD&T — true position, flatness, perpendicularity — for the dimensions where orientation drives function. In other words, target the mating face, the hole pattern, or the flange plane. Also, over-specification increases inspection cost without adding functional value. Then, it signals a drawing the shop prices at a higher tier.

Why does sheet thickness tolerance matter for fit?

First, mill tolerance is typically ±5–10% of nominal. So for fit-critical assemblies, thickness variation changes flange dimensions and the gap or step between mating parts. Then, specify tighter rolled tolerance where the fit drives function. Instead, you can add thickness variation to your fit-up calculations.

What is the cost of one “±0.01” on every dimension?

First, that is the gold-plating case in full. So the drawing signals every feature at the precision tier, with CMM verification across the board. Also, the part usually works fine at the general class. Then, move the 10–20% of dimensions that matter to the functional or critical tier. Finally, allow the rest to ride the default, and the quote falls with it.

Sources and References

• ISO 2768-1 — General tolerances for linear and angular dimensions (standard reference for the drawing default clause)
• ASME Y14.5 — Dimensioning and tolerancing (GD&T)
• ISO GPS series (ISO 1101 et al.) — Geometrical product specifications for international drawings
• International Organization for Standardization
• AWS publications — welding procedure qualification reference behind the weld-access rule

Send the Drawing Before It Locks

First, send the drawing early. So we flag the tolerance tiers, bend layout, and DFM items before your spec locks. In other words, the expensive changes never start. Also, a sheet metal tolerance review costs a few hours. By contrast, a tolerance change after tooling starts costs a few weeks. So request a quote, and align terminology with the metal fabrication glossary. In addition, the project cases show parts specified with this method. Then, for the paper trail, see our shipping and delivery guide. Finally, for import cost planning, see our 2026 US steel and aluminum tariffs guide.