How to Vet Custom Sheet Metal Fabrication Factories

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  Key takeaways
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  What to verify before you send an RFQ
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  Tolerance stacking: where sheet metal parts actually fail
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  Tooling route: when laser and punch beat a hard die
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  Finishing, hardware, and the secondary operations nobody quotes
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  Certifications, inspection, and confidentiality
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  What a full-process factory adds to a sheet metal program
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  Step by step: how to qualify a sheet metal supplier

A sheet metal drawing usually shows a finished-part tolerance, but the part is made in stages: cut flat, bend, weld, finish. Each stage adds error. If the supplier only controls the flat pattern to ±0.2 mm and then bends on a press brake with worn tooling, the finished hole-to-edge distance can drift well past the drawing callout even though every single step 'passed'.

Ask for three numbers instead of one. The flat-pattern tolerance covers the cut profile. The bend tolerance covers angle and flange length. The hole-position tolerance covers where features land after forming. A supplier who can quote those three separately is measuring the process, not just inspecting the output.

Bend radius matters too. On 1.5 mm 5052 aluminium, an inside radius below the material thickness will crack at the outside fiber. On 304 stainless, springback is larger, so the press brake needs over-bend and sometimes a second hit. Factories that run aluminium and stainless on the same die setup without changing parameters will ship parts that do not fit the fixture.

For parts that locate on holes, plan for a fixture or a post-form drilling step. Punching holes before bending is cheaper, but the hole moves with the bend. Drilling after forming costs more per part and holds position better. Decide which one your assembly needs before you ask for a price.

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  Separate the tolerances
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  Match radius to material
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  Plan hole location

Custom sheet metal fabrication factories usually offer two routes. The first is cut-and-form: fiber laser or turret punch for the profile, press brake for bends. The second is hard tooling: a progressive or transfer die that blanks, forms, and pierces in one pass. The first has near-zero tooling cost and a higher unit price. The second has a large upfront cost and a low unit price.

The break-even sits around a few thousand parts per year for a simple bracket, and higher for a part with several forming stations. Below that, hard tooling is a sunk cost you will not recover. Above it, the die pays for itself and then some. Ask the supplier to state the volume where they would switch routes, and check whether that number matches your forecast.

There is a middle path worth knowing. A factory can build a soft tool or a simple blanking die for the outline, then finish the forms on a press brake. This cuts laser time on high-volume runs without the full die cost. It suits parts between roughly 500 and 5,000 pieces where the profile is stable but the bend count is low.

Watch for tooling ownership. If the supplier builds a die for your part, confirm in the purchase order who owns it and whether it can be transferred. Factories that keep the die as their own asset can hold your program hostage on the next price negotiation.

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  Cut-and-form
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  Hard die
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  Soft tool plus brake

The cut and bend are the visible work. The cost often sits in what happens after. Deburring, tapping, PEM hardware insertion, welding, and surface finishing each add a step and a handling risk. A quote that lists only 'sheet metal fabrication' and one lump price is telling you the secondary steps are estimated, not planned.

Hardware insertion is a good test question. Press-fit standoffs and clinch nuts need a controlled squeeze and the right hole size; too large and the nut spins, too small and the sheet deforms. Ask how the factory verifies pull-out and torque, and whether they use a press or a hand tool. Hand-set hardware fails in the field.

Finishing changes dimensions slightly. Anodizing builds roughly 5 to 15 μm per surface depending on the process; powder coat can add 60 to 100 μm. Threads, dowel holes, and electrical contact areas usually need masking. Get the masking plan in writing, because a masked area that gets coated anyway is a rework cycle, not a cosmetic issue.

Welding distorts. A long seam on thin 304 will pull the part out of flat. Factories that weld before the final bend or skip stress relief will fight the tolerance at assembly. For welded frames, ask for the weld sequence and whether the part is checked on a fixture after cooling.

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  List every secondary step
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  Test hardware insertion
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  Plan masking

Certificates are a scope statement, not a quality score. ISO 9001:2015 covers a documented quality management system. IATF 16949:2016 adds automotive-specific requirements such as traceability and PPAP discipline. ISO 13485:2016 covers medical device manufacturing. If your part is a bracket for a robot cell, ISO 9001 is enough. If it is a battery tray for an EV program, ask for IATF 16949.

Inspection depth is the next question. Sampling inspection is fine for a mature, stable part. For a first article or a tight-tolerance assembly, ask for 100% inspection before shipment with raw material check, in-process monitoring, and final inspection. Dimensional reports, material certificates, and finish thickness readings should be available on request.

Confidentiality matters when your drawing carries a patent or a customer logo. A factory that accepts an NDA before reviewing files, and that keeps uploads secure, is easier to work with than one that asks for the drawing first and the paperwork later. Confirm the NDA is signed before you release the CAD.

Response speed tells you about the engineering bench. A quotation and a DFM analysis inside 12 hours usually means the factory has process engineers who can read a drawing, not just a sales desk that forwards it. That same bench is who you call when a bend radius will not work.

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  Match certificate to industry
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  Ask inspection level
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  Sign NDA first

Sheet metal rarely ships alone. A bracket meets a machined boss, a chassis meets a die-cast housing, a panel meets a 3D-printed duct. When the sheet metal factory also runs CNC machining, die casting, and 3D printing under one roof, the interfaces get checked together instead of blamed across suppliers.

GreatLight Metal Technology runs 3 wholly-owned plants covering a 7,600 m² footprint with 150 technicians, and the equipment list crosses processes: 127 high-precision CNC machines, 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. Maximum processing size reaches 4,000 mm, with a Ø400 mm rotary table for round work.

That mix matters at the assembly level. A sheet metal enclosure with a machined insert can be toleranced as one stack instead of two. A die-cast housing with a sheet metal cover can be checked for flatness and hole alignment in the same inspection report. Fewer handoffs means fewer places for a drawing revision to be missed.

Material coverage is broad: 6061, 5052, 5083, 6063, 6082, and 7075 aluminium; 303, 304, 316, 316L, and 17-4PH stainless; 1018, 1045, 4130, and 4140 steel; plus copper, brass, titanium, Inconel, and magnesium alloys. Since 2011 the company has built parts from one prototype to 10,000+ part runs with no minimum order quantity.

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  One roof, fewer handoffs
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  Capacity for large parts
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  No MOQ