Jigs, fixtures, soft jaw inserts, location and setting gauges, machine guards, part-handling trays, protective caps, patterns for casting, and replacement components for equipment no longer supported by its maker. The common feature is a low-volume item that is quicker to print overnight than to machine, and where the alternative is a machine tool occupied on work that produces nothing billable.

Because it determines the material range. PLA and PETG print acceptably on an open machine. ABS, ASA, polycarbonate and most nylons contract significantly as they cool, and without a heated chamber holding the part warm through the build they warp off the bed and split between layers. If the intention is functional engineering parts rather than models, an enclosed machine is the requirement rather than an upgrade.

A printed part is considerably weaker across the bond between layers than it is along them, because each layer is welded to the last rather than being continuous material. That means the same part can be strong or fragile depending on which way it was built. Orientation is therefore a design decision: place the part so the main load runs along the layers, and treat the layer direction as a deliberate choice.

If you print carbon fibre, glass fibre or metal filled filaments, yes. Those abrasives wear a standard brass nozzle out within a spool or two, and the symptom is a gradually enlarging bore that quietly degrades dimensional accuracy before it becomes obvious. Hardened steel and ruby-tipped nozzles cost more and conduct heat differently, so expect to retune temperature when changing to one.

Good enough for fixturing, gauging within reason and most functional work, but well short of machined tolerances. Accuracy depends on the machine, the material's shrinkage, the part geometry and how the part is oriented and supported. Where a printed part must locate to a machined feature, design in a clearance and a machined or reamed detail, rather than expecting the printed dimension to hold a close fit.

Support removal, which is the main task, and it varies from trivial to laborious depending on geometry. Dissolvable supports on a dual-material machine remove that work at the cost of a second material and longer prints. Beyond that: cleaning up the first layer, drilling or reaming holes to size, and optionally annealing some materials for strength. Surfaces can be smoothed, though for shop fixtures it is rarely worth the time.

Dry. Most engineering filaments, and nylon in particular, absorb moisture from the air, and wet filament prints with bubbling, stringing, poor layer bonding and a rough surface that is often blamed on the machine. Keep spools in sealed containers with desiccant and dry them before use where the material calls for it. Dry storage is the cheapest single improvement to print quality in most shops.