Engaging a metal fabrication shop: flat patterns, welds and finish

Material grade and finish are half the quote

Sheet is not simply steel or aluminium. Grade, temper, thickness tolerance, surface condition and whether the material arrives pre-finished all affect price, availability and how a part behaves when formed. Stainless in a decorative finish must be protected through the whole process and gets rejected for handling marks that would be invisible on a painted part. Specify the grade and acceptable surface condition, state whether protective film has to remain until delivery, and identify which faces are cosmetic. A shop asked for mild steel buys whatever is in the rack, which is a fair reading of an unspecific drawing.

Bends are where the drawing meets the machine

A folded part comes from a flat pattern, and that pattern depends on material, thickness, bend radius and the tooling available on that shop's press. Internal radii drawn arbitrarily may not exist as tooling; features placed too close to a bend cannot be formed; a relief the designer omitted will tear. Send the folded model and let the shop develop its own pattern rather than supplying one built on different assumptions. State which dimensions are measured after forming, because a tolerance achievable while the part is flat may be unachievable across a bend.

Welded assemblies have tolerances your drawing probably lacks

Heat distorts. Components individually within tolerance become an assembly that is not, and how much depends on weld sequence, fixturing and heat input. Most drawings dimension the pieces carefully and say nothing about the assembly, leaving the shop to guess what matters. Provide assembly-level dimensions with tolerances, identify the datums, and mark features that must hold after welding, which are often those machined afterwards. Specify weld type, size and extent as well, since a fabricator told merely to weld will produce something strong and possibly unsightly, or occasionally the reverse.

Fixtures are a hidden setup cost that repeat orders recover

Any welded assembly of consequence needs jigs holding parts in position, and somebody pays to design and build them. On a single order that cost sits inside the price and makes the job look expensive; across repeat orders it disappears into the background. This is why fabricators quote much better rates for continuing business, and why moving an assembly to a new shop costs more than the piece price suggests. Ask explicitly whether fixturing is included, who owns it, and what a repeat order would cost, because a quotation silently assuming ongoing work gets revisited when the work does not arrive.

Finishing usually leaves the building

Powder coating, plating, anodising, galvanising and passivation are typically subcontracted, adding transport, lead time and a party you never selected. Finish defects are also the commonest reason for rejection, and they appear at the end of the process once the most value has been added. Specify the finish system rather than a colour, state the required film properties and which surfaces must be masked, and agree how the fabricator handles a batch returned from a coater with defects. Their liability for a subcontractor's work should be explicit, since both parties tend to assume the other is carrying it.

Frequently asked questions

Should we send a flat pattern or a folded model?

Send the folded model with the drawing and let the shop unfold it. Flat patterns depend on bend allowances specific to the material, thickness, radius and tooling of whoever does the work, so a pattern built on your assumptions produces parts that are dimensionally wrong after forming. Supply your own pattern only as reference, if at all, and make clear that the folded geometry governs. The dimensions you care about belong on the formed part.

Every part measured correctly but the welded assembly is out of tolerance. What happened?

Almost certainly weld distortion, which is normal and gets designed around rather than eliminated. Heat pulls material, and the outcome depends on sequence, fixturing and heat input. The remedies are practical: dimension and tolerance the assembly rather than only its components, let the fabricator choose a sequence that manages distortion, leave machining stock on critical features to be finished after welding, and agree where measurements are taken.

Who is responsible when a subcontracted coating fails?

Your contract is with the fabricator, so ordinarily they are, but only so far as the agreement makes them responsible for subcontracted work. Many quotations quietly exclude finishing defects or cap liability at the coating charge rather than the value of the assembly. Read that before the first order, specify the finish system properly, and require inspection on return from the coater so faults are caught before parts are packed and shipped.