How a sheet metal assembly is joined decides its cost far more than how its parts are cut. Welding needs fixtures and introduces distortion; pressed hardware needs neither. Choosing the joining method early lets the parts be designed for it, and choosing it late means paying for fixtures nobody budgeted.
The methods and what each costs
Fusion welding is strong, permanent and distorts the parts, and it needs a fixture and a skilled operator. Spot welding is fast, cheap in volume and needs access from both sides. Riveting and clinching are cold, so they do not distort, and both need access for the tool. Self clinching hardware presses a nut or stud into the sheet and is usually the cheapest way to get a thread. Adhesive bonding spreads load and needs surface preparation.
Distortion is the recurring problem
Heat from welding shrinks the weld as it cools and pulls the assembly out of shape, and the thinner the material the worse it is. The answers are staggered tacking, balanced weld sequences, fixtures that restrain the parts, and, above all, using less weld. A design that specifies a continuous seam where a series of stitches would do buys distortion it did not need.
Fixtures are the hidden cost
A welded assembly needs something to hold the parts in position, and that fixture is designed, made and stored. On low volumes it can exceed the cost of the parts. The way out is self locating design: tabs and slots cut into the parts so they hold each other in position, which laser cutting makes essentially free.
Designing for the method you chose
For welding, allow access for the torch and provide tack points. For spot welding, allow access from both sides and keep flange widths adequate. For self clinching hardware, respect the minimum distance from an edge and the correct hole size, because both are specified by the hardware maker and both are commonly ignored. And decide the finish before joining, since welding after plating destroys the coating.
Questions people ask about sheet metal assembly
What is the cheapest way to join sheet metal?
Usually self clinching hardware and mechanical fastening, because they need no fixture, introduce no heat and require no skilled operator.
How do I stop a welded assembly distorting?
Use less weld, stagger it, sequence it to balance the shrinkage, and restrain the parts in a fixture. Stitch welds instead of continuous seams wherever strength allows.
What is self locating design?
Tabs and slots cut into the parts so they hold each other in position during assembly, removing the need for a welding fixture. Laser cutting makes it essentially free.
Should parts be finished before or after assembly?
After, in almost all cases, because welding destroys coatings. Where parts must be finished first, the joining method has to be one that does not damage the finish.