Ordering sheet metal parts that come back warped, undersized, or full of burrs is an expensive lesson most buyers only want to learn once. Many of these problems trace back to decisions made long before the metal is ever cut. Knowing the most common sheet metal fabrication mistakes puts you in a stronger position when specifying, quoting, and reviewing parts.
Skipping the Design for Manufacturability Review
Sending a CAD file straight to a fabricator without a DFM (Design for Manufacturability) check is one of the fastest ways to generate scrap and rework charges. Common design-level errors include:
- Bend radii too tight for the material thickness — a good rule of thumb is an inside bend radius equal to at least the material thickness; going tighter can crack mild steel and will definitely crack harder alloys like 5052-H32 aluminum.
- Holes placed too close to a bend — holes within 1.5× the material thickness from a bend line will distort during forming.
- Features that require secondary operations you haven't budgeted for — countersinks, tapped holes, and hardware inserts all add cost and lead time.
- Missing or incorrect tolerances — leaving tolerances as "general" on complex assemblies often means the fabricator defaults to ±0.030 in., which may be too loose for mating parts.
Ask your fabricator for a DFM review before final quoting. Most reputable shops offer this at no charge and will flag problems in hours rather than after the run is complete.
Choosing the Wrong Material Specification
"Steel" or "aluminum" is rarely specific enough. A common customer mistake is leaving material choice entirely to the shop, then being surprised when the price, finish, or performance doesn't match expectations.
For structural brackets, 11-gauge (0.120 in.) hot-rolled A36 steel is a practical starting point. For enclosures that need a clean painted finish, 14-gauge cold-rolled 1018 is easier to work and takes powder coat more evenly. Stainless steel grades matter too — 304 works for most corrosion-resistance needs, while 316 is worth the premium only when chloride exposure is a real concern.
Specify the alloy, temper, and thickness range you can accept. Even a ±0.005 in. thickness tolerance on incoming sheet can affect whether bends hit your angle spec.
Underestimating the Cost of Tight Tolerances
Tighter isn't always better. Specifying ±0.005 in. across the board on a sheet metal enclosure will push the job toward laser cutting and precision bending setups that might triple your unit cost compared with ±0.015 in. on non-critical features.
Reserve tight tolerances for mating surfaces, mounting hole patterns, and features that interface with other components. On cosmetic surfaces or non-functional edges, loosen them deliberately. A tolerance stack-up analysis on an assembly drawing can reveal where you actually need precision — and where you're spending money for no functional gain.
Ignoring Finish and Surface Prep Requirements
Powder coat, anodizing, chromate conversion, and e-coat all require different surface preparation and have different thickness buildups. Ignoring this at the design stage causes problems:
- Powder coat adds roughly 0.002–0.006 in. per side — critical if a coated part needs to slide into a channel or accept a close-tolerance fastener.
- Anodized aluminum Type II adds about 0.0001–0.0003 in.; Type III (hardcoat) can add 0.001 in. or more.
- Welds and heat-affected zones can show through thin finishes if not properly ground and primed.
Specify the finish on the drawing, not in an email thread that might get lost. Include masking requirements for threaded holes or mating surfaces.
Comparing Only on Price
Getting three quotes and automatically choosing the lowest is a reliable way to discover sheet metal fabrication mistakes the hard way. A shop that quotes 20% lower may be using worn tooling, skipping deburring, or working with sheet stock that doesn't meet spec.
When comparing suppliers, ask for:
- Sample parts or previous work in your material and gauge
- Their standard tolerances in writing
- Lead time commitments with penalty terms
- Quality control documentation (first-article inspection reports, material certs)
Mercoly makes it easier to compare and find trusted sheet metal fabrication providers in one place, so you can evaluate shops on more than just the bottom-line number.
Not Defining Acceptance Criteria Upfront
If your purchase order doesn't define what "acceptable" looks like, the fabricator's definition wins. Before any run starts, document:
- Dimensional tolerances by feature
- Allowable burr height (typically ≤0.005 in. for hand-assembled parts)
- Surface finish Ra values where applicable
- Reject criteria for cosmetic defects like scratches or dings
This becomes your inspection checklist and gives both parties a shared standard to measure against.
A little planning at the front end eliminates the rework, delays, and hard conversations that define most sheet metal fabrication mistakes — start by finding the right fabricator for your project today.