Sheet Metal Fabrication RFQ: Files, Tolerances, and Finish Callouts
Sheet metal fabrication quotes vary widely for one consistent reason: incomplete RFQ packages force every shop to guess, and each shop guesses differently. This guide explains what a complete file package looks like (DXF plus STEP plus a controlled PDF drawing), how K-factor and minimum flange rules shape what can actually be bent, what tolerances laser cutting and press brakes realistically hold, how to call out PEM-style self-clinching hardware correctly, and how to write finish callouts for powder coating, anodizing, and plating that a Chinese fabricator can execute without a single clarification email.
The File Package: DXF, STEP, and a Drawing That Controls
Send three things for every part. First, a STEP file of the formed part, because it is the unambiguous record of geometry and the input for the shop's own unfolding software. Second, a DXF of the flat pattern only if you are confident in your unfold parameters; otherwise let the fabricator generate the flat from the STEP, and say so explicitly, since a wrong flat pattern supplied by the customer becomes the customer's defect. Third, a PDF drawing that states which file controls, the material specification and thickness, bend directions and angles, critical dimensions with tolerances, hardware, and finish. A drawing note such as "STEP file controls geometry; drawing controls tolerances, material, hardware, and finish" removes the most common ambiguity in fabrication RFQs. Specify material by standard, not nickname: SPCC per JIS G3141 or the GB equivalent for cold-rolled steel, SGCC for galvanized, 5052-H32 for formed aluminum, 304 per relevant ASTM or GB grades for stainless. If your drawing says only "aluminum," expect quotes on whatever alloy the shop has on the rack.
Bending Reality: K-Factor, Bend Allowance, and Minimum Flange
Why your flat pattern may not match theirs
When sheet metal bends, the material stretches on the outside of the bend and compresses on the inside; the neutral axis where length is unchanged sits somewhere in between, and the K-factor expresses its position as a fraction of thickness. Typical values fall between 0.33 and 0.5 depending on material, tooling, and bend radius, and every shop calibrates its own. This is exactly why supplying your own flat DXF is risky: your CAD system's default K-factor is almost certainly not the shop's measured one. For parts where formed dimensions matter, dimension the formed part and let the fabricator own the unfold. Reserve customer-supplied flats for parts with no bends or where you have validated the unfold with that specific shop.
Minimum flange and hole-to-bend distance
A press brake needs enough material on each side of the bend to sit on the die. The common rule of thumb is a minimum flange of roughly four times material thickness, driven by the die opening; shorter flanges require special tooling or secondary operations and will either be quoted high or deformed. Holes and slots placed too close to a bend distort into ovals; keeping them at least three times thickness plus the bend radius away from the bend line avoids rework, or move the hole to a post-bend drilling operation and accept the cost. These are design rules, but they belong in the RFQ conversation because a good fabricator will flag violations at quote time and a mediocre one will ship distorted parts.
Tolerances: What Laser Cutting and Bending Actually Hold
Fiber laser cutting holds feature-to-feature accuracy on a flat blank comfortably within a few tenths of a millimeter; a realistic general callout for cut features is plus or minus 0.1 to 0.2 mm depending on thickness and machine condition. Bending is the accuracy bottleneck. Each bend introduces angular variation, typically around plus or minus one degree with standard air bending, and angular error converts to linear error that grows with flange length and stacks across multiple bends. A dimension spanning three bends cannot carry the same tolerance as a laser-cut hole pattern, and drawings that apply a blanket tight tolerance to formed dimensions get either inflated quotes or ignored tolerances. The practical approach is a general tolerance note referencing ISO 2768-m for cut features, a separate general angular tolerance for bends, and explicitly tightened tolerances on the few formed dimensions that genuinely matter, with the understanding that those may require bend-and-check iterations.
| Feature type | Realistic general tolerance | Notes |
|---|---|---|
| Laser-cut features, flat blank | ±0.1 to ±0.2 mm | Thickness and kerf dependent |
| Bend angle, air bending | ±1° | Material springback varies by lot |
| Formed dimension, one bend | ±0.2 to ±0.5 mm | Grows with flange length |
| Dimension across multiple bends | ±0.5 mm or looser | Errors stack; tighten only where functional |
Hardware: Calling Out PEM-Style Self-Clinching Fasteners
Self-clinching nuts, studs, and standoffs, generically called PEM-style hardware after the best-known brand family, are pressed into punched or laser-cut holes and rely on the sheet being softer than the fastener. Callouts fail in predictable ways. Specify the full part designator including thread, material, and shank or thickness code, because an S-type nut for steel sheet and its stainless or aluminum-sheet variants are not interchangeable. State the installation side, since self-clinching hardware installs from one side only and a mirrored insertion is unusable. Confirm the sheet thickness is within the fastener's specified range, and remember that hardware in stainless sheet needs hardened variants. Two process notes worth adding to the drawing: hardware is installed after deburring but before powder coating unless threads are masked, and installed hardware must meet the manufacturer's published pushout and torque-out values, which your inspection plan can sample. Chinese fabricators routinely stock domestic equivalents of common self-clinching hardware; if brand authenticity matters for your product, say so in the RFQ and expect a price difference, and verify it the same way you would any material claim during incoming inspection.
Finish Callouts That Survive Translation
Powder coating
A complete powder coat callout has five elements: color by RAL number rather than a color name, gloss level, texture, minimum cured film thickness in microns, and masking. Masking is the one most often omitted: threads, ground contact points, and press-fit bores must be called out as masked or the coating will interfere. Add the substrate pretreatment expectation, such as iron phosphate or zirconium conversion for steel, because corrosion performance depends on it more than on the powder itself.
Anodizing
For aluminum parts, call out anodize type and class using MIL-A-8625 language, which Chinese finishers understand: Type II for standard sulfuric anodize, Type III for hardcoat, Class 1 for clear, Class 2 for dyed with a specified color. State a coating thickness range, whether sealing is required, and note that anodize films are thin but do change hole fits on tight tolerances. Alloy matters: 5052 and 6061 anodize predictably, while high-silicon castings and some 2xxx alloys do not take cosmetic anodize well.
Plating
For zinc plating on steel, reference ASTM B633 or the ISO 2081 designation system, specifying thickness class and chromate type, and require trivalent chromate if the product falls under RoHS, since hexavalent finishes are still cheaper and still offered. For bright nickel or chrome on decorative parts, specify the underlying polish level with a sample or surface roughness value, because plating amplifies rather than hides surface defects.
Quoting: What Drives the Price You See
Fabrication pricing is dominated by setup counts, not material: each bend is a press brake setup touch, each hardware insertion a station, each finish a subcontract pass. This is why two visually similar brackets can quote very differently, and why consolidated orders across parts sharing material and finish quote better than the same parts sent separately. When comparing quotes from suppliers found through the fabricated metal products sector or a broader manufacturer search, ask each shop to state its assumed material grade, K-factor ownership, hardware brand, and finish subcontractor. Divergent answers to those four questions explain most quote spreads and predict most quality disputes.
Key Takeaways
- Send STEP plus PDF drawing for every part, state which file controls, and only supply flat DXFs when the unfold has been validated with that shop.
- Let the fabricator own the K-factor and flat pattern; dimension the formed part and respect minimum flange and hole-to-bend rules.
- Tolerance cut features and formed features separately: laser cutting holds tenths of a millimeter, bends hold about a degree, and errors stack across bends.
- Call out self-clinching hardware with full designators, installation side, and sheet thickness compatibility, and sequence installation against coating.
- Write finish callouts with standards (RAL, MIL-A-8625, ASTM B633), thickness values, and masking, and make every quote state its assumptions.
This guide is editorial reference material, not legal or transactional advice. Verify supplier claims and regulatory requirements independently. See our data sources and editorial policy.