Every mechanical part in your product has to be assigned to a process, and that assignment is called routing. Get it right and the part is cheap, repeatable, and fast to source. Get it wrong and you are machining a shape that wanted to be an extrusion.
Manufacturing processes are shape engines: each is very good at one family of geometries and bad at the rest. Classify your part by dominant geometry, then filter by volume and material, and the shortlist usually collapses to one or two candidates. This article gives you that classification.
Start with the dominant geometry
Before you think about volume at all, look at the part and ask what shape it fundamentally is. Almost every mechanical part falls into one of six buckets.
- Rotational. Shafts, pins, bushings, pulleys, threaded studs, knobs, nozzles. Anything you could make on a lathe by removing material from a spinning bar.
- Prismatic. Blocks, brackets, manifolds, housings with pockets and holes on multiple faces. Milled shapes.
- Sheet. Constant thickness, bent and cut — chassis, panels, covers, clips, brackets.
- Constant cross-section. Rails, heat sinks, frame members, trim. The profile is the same everywhere along the length.
- Complex solid. Organic or highly three-dimensional volumes: pump bodies, impellers, structural knuckles, gearbox housings.
- Small, intricate, and high-volume. Gears, latches, cams, small structural parts made by the million.
That single classification eliminates most of the process list before you have thought about anything else.
The routing table
| Geometry | Low volume (1–200) | Mid volume (200–20k) | High volume (20k+) |
|---|---|---|---|
| Rotational | CNC turning from bar | CNC turning, cold heading | Swiss screw machining, cold forming |
| Prismatic | CNC milling, 3D printing | CNC milling | Die casting or forging plus finish machining |
| Sheet | Laser cut and press brake | Laser or turret plus brake | Progressive die stamping |
| Constant section | Machined from plate | Extrusion, cut to length | Extrusion or roll forming |
| Complex solid | 3D printing, urethane casting | Investment or sand casting | Die casting, forging, MIM |
| Small and intricate | CNC or printing | MIM, fine blanking | Powder metallurgy, MIM, stamping |
Treat the volume bands as soft. The real break point is where tooling amortization drops below the machining cost per part, and that crossover moves with part size and material cost. The arithmetic behind it is laid out in choosing a manufacturing process by volume.
The subtractive family
Turning
A lathe spins the workpiece against a stationary tool. For anything cylindrical, turning is dramatically faster than milling the same shape — often by a factor of three — because the cutting is continuous. Swiss-type lathes with live tooling produce a finished small part, cross-holes and flats included, in one cycle. If a part is mostly round it should be turned; the tradeoffs are in when a lathe beats a mill.
Milling
Milling is the universal process: it can make nearly any prismatic shape from nearly any material with no tooling beyond standard cutters. That flexibility is exactly why it is expensive at volume — you pay machine time per part forever, with no amortization curve working in your favor. Milling is the right answer for prototypes, low volumes, large parts where tooling is prohibitive, and finishing operations on cast or forged blanks. Every pocket you make shallower and every face reachable without re-fixturing takes money off the quote.
Sheet cutting and bending
Laser cutting a flat blank and folding it on a press brake produces enclosures, brackets, and chassis with essentially zero tooling cost. It scales well from one part to a few thousand. Beyond that, stamping takes over. The design rules — minimum flange length, bend relief, hole-to-bend distance — are unforgiving and are collected in the sheet metal design guide.
The forming family
Stamping
A progressive die moves a strip of material through a sequence of stations, each performing one operation, producing a finished part with every stroke. Cycle times are measured in parts per minute and per-part cost can fall below a nickel. The catch is tooling: a progressive die runs tens of thousands of dollars and weeks to build, and design changes mean cutting steel again. The economics are worked through in metal stamping and progressive dies.
Forging
Forging deforms hot or cold metal into a die cavity. Because the grain flow follows the part shape rather than being cut through, forged parts are substantially stronger than machined or cast equivalents of the same alloy. Use it for load-bearing parts where failure is unacceptable — hooks, levers, connecting links, tool heads. Forgings are usually near-net shape and need finish machining on functional surfaces.
Extrusion
Push heated aluminum through a shaped die and you get an unlimited length of that profile. Die cost is remarkably low compared with other tooling, often a few thousand dollars, and the process is ideal for heat sinks, frames, rails, and enclosure bodies. A well-designed extrusion can integrate screw bosses, slots, and cooling fins that would each be a separate operation otherwise.
Cold forming and thread rolling
Fasteners and small rotational parts are not machined at volume, they are cold headed from wire and thread rolled. Material waste approaches zero and rates reach hundreds of parts per minute. If your part looks like a fastener, ask whether it can be cold formed before you quote machining.
The casting family
Casting produces complex solid shapes that no cutting process could reach economically. The variants differ mainly in tooling cost, surface finish, and achievable tolerance.
- Sand casting — cheapest tooling, roughest finish, good for large parts and low quantities.
- Investment casting — a wax pattern is coated in ceramic, melted out, and the cavity filled. Excellent detail and finish, moderate tooling, works in steels and superalloys. The comparison with machining is covered in investment casting vs CNC machining.
- Die casting — molten aluminum or zinc injected under high pressure into a steel die. Fast cycles, thin walls, tight tolerances, high tooling cost. The workhorse for volume aluminum housings, as described in aluminum die casting.
All castings need draft, uniform-ish wall thickness, and generous fillets, and all of them shrink. Assume a machining allowance on any surface that has to be accurate.
Powder and additive routes
Powder metallurgy compacts metal powder in a die and sinters it solid. It suits small, intricate, high-volume components — gears, cams, bearing races — with almost no material waste, at the cost of some porosity unless the part is infiltrated or forged after sintering. Metal injection molding is the related route for small complex parts: powder mixed with a binder, molded like plastic, then debound and sintered.
Additive manufacturing has moved into real production for a narrow but growing set of parts: complex geometry at low volume, internal channels that cannot be machined, consolidated assemblies, and spares. The criteria are in 3D printing for end-use parts.
Let material narrow the list
Material and process are not independent. Zinc and aluminum die cast well; steel does not. Steel forges and stamps beautifully; aluminum stamps with more spring-back. Titanium machines slowly but prints well. Work material and process together rather than in sequence — the process of choosing materials for a new product should include the question of who can actually make the shape you want in that alloy.
The one rule that saves the most money
Do not design the part and then look for a process. Choose the process early, then design within its rules. A bracket drawn as a machined block and later converted to a stamping is a redesign; one drawn as a stamping from day one costs a tenth as much.
Projects House routes and quotes mechanical parts across all of these processes through a global supplier network, and will tell you when your part is on the wrong one. Send a CAD file or even a sketch through our contact form for a routing review.