Most sheet metal features are cut by laser or punch, but some cannot be: a countersink that must be precise, a bore that has to take a bearing, a slot in a face that has already been formed, or an edge that must be machined flat after welding. That work is sheet metal machining, and its recurring difficulty is holding thin material still.
When machining beats cutting
When a hole must be round and to size rather than merely present, because a laser hole carries taper and a heat affected edge. When a countersink or counterbore is required. When a feature must be added after forming or welding, so the flat blank stage has passed. And when a welded assembly has to be machined flat or bored true across a joint.
The workholding problem
Thin material has almost no stiffness against the cutting force and will lift, chatter or deflect. Vacuum tables are the usual answer for flat sheet, along with sacrificial backing boards for routing. Formed parts need purpose made fixtures that support the area being cut. That fixture is the real cost of machining a sheet metal part, and it is why the operation is avoided where possible.
Routing against milling
A CNC router with a vacuum bed is the fast way to cut and profile sheet aluminium and plastics, and it is common in signage and enclosure work. A machining centre gives better accuracy and can hold tighter tolerances on features, and it is what you want for bores, threads and precision countersinks. Shops usually have one or the other, so ask.
Designing to avoid it
Put precision features on a machined component that is fastened to the fabrication rather than machining the fabrication itself. Use self clinching hardware instead of tapped holes in thin sheet. Accept a laser cut hole with clearance instead of a reamed one wherever the fit allows. Each of these removes an operation and a fixture from a part that is otherwise cheap to make.
Questions people ask about sheet metal machining
Why machine sheet metal at all?
For precise round holes, countersinks and bores, for features added after forming or welding, and for machining a welded assembly flat or true.
What makes it difficult?
Thin material has no stiffness against cutting force, so it lifts and chatters. Vacuum tables, backing boards and purpose made fixtures are needed.
Router or machining centre?
A router with a vacuum bed for fast profiling of sheet. A machining centre for accuracy on bores, threads and countersinks.
How can I avoid machining a fabrication?
Put precision features on a separate machined part that fastens to it, use self clinching hardware for threads, and allow clearance on laser cut holes.