1. Quick lookup by process or part
Enter a process name (laser cutting / die casting / die forging / wire EDM / extrusion) or a part type (gear / housing / shaft / sheet-metal bracket / frame / valve body / spring) to get accuracy capability, economic batch size, tooling investment and the recommended process chain.
2. Order of decisions: stock first, then forming, then finishing
Step 1: fix the stock form Plate / bar / extrusion / casting / forging / powder blank. This step sets material utilisation and every later operation, and it drives most of the cost.
Step 2: fix the forming method Cutting (turning, milling, drilling) to remove material / plastic forming (stamping, bending, drawing, cold heading) / joining (welding) / additive (3D printing).
Step 3: fix how precision is achieved Conventional machining → finishing (grinding, honing, lapping, slow-wire EDM) → special processes (die-sinking EDM, wire EDM, laser). Each step up in precision steps up the cost.
Step 4: sequence heat treatment and surface finishing Stress relief after roughing, semi-finishing before hardening, grinding only after hardening, surface finishing last. Get the order wrong and the accuracy is gone.
Five questions that settle the process: ① How many per year? ② How big and how heavy (machine limits)? ③ What material (cast iron / carbon steel / stainless / aluminium / titanium)? ④ Is the shape a body of revolution, a constant section, thin-walled, or a closed cavity? ⑤ Is the key requirement accuracy, stiffness, fatigue life, sealing or appearance? And one more: can lead time and surface finishing keep up?
In one line: the shape decides the process
| Feature | First choice | Why |
|---|---|---|
| Bodies of revolution (shafts, sleeves, discs, flanges) | Turning (+ grinding) | Turning is the cheapest way to make a round part; use a sliding-head lathe for small-diameter bar and automatic lathes for high volume |
| Long constant-section parts (frames, rails, heat sinks, window profiles) | Aluminium extrusion | Saw to length, drill and mill after extruding; the more complex the section, the more extrusion saves over machining |
| Flat sheet parts (brackets, panels, covers) | Laser cutting (+ bending) | No tooling needed, works from one piece; above 12 mm switch to plasma or flame cutting |
| Complex 3D solids (housings, shells, cast brackets) | Casting (sand / die / investment) | The most shape freedom; volume decides between sand and die casting |
| Load-bearing parts (connecting rods, crankshafts, hooks, gear blanks) | Die forging | Forging keeps the grain flow continuous, so fatigue strength is clearly higher than a casting or a part cut from solid |
| Closed cavities and passages (valve bodies, manifolds, impellers) | Casting; 3D printing for one-offs | Machining cannot produce a closed passage, and welding one up leaves seams and distortion |
| Narrow slots, sharp corners and profiled holes in hardened parts | Slow-wire EDM / die-sinking EDM | After hardening you cannot mill it — only special processes will do |
| Very high volume small parts (fasteners, terminals, small gears) | Cold heading / stamping / powder metallurgy | Unit cost can fall to a fifth or a tenth of machining, but the tooling must be spread over enough parts |
3. Capability comparison (accuracy / roughness / batch / tooling)
"Economic batch" is the order of magnitude at which a process starts to pay, not a hard limit; the tooling figures are typical of small and mid-size Chinese shops and serve only as a comparison guide.
4. Process by part function (including what not to do)
5. Choosing by volume: where the costs turn
| Volume | Recommended process | What not to do | Cost structure |
|---|---|---|---|
| 1–10 pcs Prototypes and repair parts | CNC machining, laser cutting, sand casting (wood pattern), 3D printing, welded fabrication | Cutting a steel or die-casting tool (the tool costs far more than the parts) | Almost all labour and material, with no tooling to amortise |
| 50–500 pcs Small batch | CNC with simple fixtures, investment (lost-wax) casting, lost-foam casting, bending plus welding | Progressive-die stamping (the tool spread over so few parts is far too expensive) | Mostly labour; tooling appears but stays small |
| 1,000–10,000 pcs Medium batch | Die casting (over 3,000), simple stamping dies, die forging, cold heading, powder metallurgy (over 3,000) | Staying with one-off CNC machining (the unit price never comes down) | Tooling starts to show clearly and unit labour falls |
| Over 50,000 pcs High volume | Progressive stamping, multi-station cold heading, automatic turning, die casting with automated deburring | Manual work on general-purpose machines (poor consistency, high labour share) | Tooling is negligible; material and cycle time decide the cost |
When to cut a tool (shop rule of thumb): cutting a tool is safe when tooling cost ≤ saving per part × quantity × 0.3; in other words, tooling within 10–20% of the order value is acceptable. If it approaches the order value, start with CNC or a soft (temporary) tool and cut the steel tool once the volume is there.
6. Drawing feature → process trap
| On the drawing | The process problem | What to do |
|---|---|---|
| Sharp internal corner R < 0.5 (bottom of a blind pocket) | Milling is limited by the cutter radius, so in practice R = cutter radius; a sharp corner in a casting or forging hot-cracks | Note "unnoted radii R0.5–1", or state "cutter radius R0.5 permitted" |
| Deep pocket, depth-to-width ratio over 4 | Long tool overhang, deflection and chatter — neither accuracy nor finish holds | Switch to a casting or printed blank, or split the part into an assembly |
| Deep hole, length-to-diameter ratio over 5 | The drill wanders, chips clear badly and tools break | Use a gun drill or deep-hole drill, or drill from both ends and blend |
| Thin wall under 1 mm (metal) | Clamping distorts it and it chatters in machining; welding burns through | Move to stamping, die casting or drawing; if it must be machined, leave temporary ribs and use a vacuum chuck |
| IT6 tolerance plus hardening | Hardening distorts 0.1–0.5% and the part cannot be cut afterwards | The route must read: rough machine → heat treat → grind or wire EDM, with "grind after heat treatment" on the drawing |
| Large face with 0.02 flatness | Milling struggles to hold this, especially on a cast blank | Grind or scrape; age the casting first and split machining into rough and finish |
| Threads under M6 called out straight in a casting | Shrinkage porosity, torn threads and stripped threads — assembly torque will not hold | Use press-in nuts or wire thread inserts, or add a boss (wall ≥ 1.5d) |
| Mating holes or mounting faces on a welded assembly | Welding distorts it and the hole positions and flatness go out | Stress relieve and machine after welding, and mark "machine after welding" |
| Ra 0.4 with grinding not allowed | Turning or milling will not hold it; only lapping or polishing will | Relax to Ra 0.8 or allow grinding or honing; set sealing faces by what they actually need |
| 304 stainless specified at HRC 50 | Austenitic stainless cannot be hardened by quenching — physically impossible | Use 420/440C/17-4PH, or resort to nitriding or work hardening |
| ±0.05 on 3 mm sheet | Neither laser cutting nor shearing reaches it (±0.1–0.2) | Mill the edges or use fine blanking; for ordinary sheet metal call out GB/T 1804-m |
| Large parts with no parting line or ejector marks | Sand casting always leaves a parting plane and die casting always leaves ejector marks | Switch to lost foam, investment casting or 3D printing, or accept the parting line somewhere (on a cosmetic face) |
| Castings and forgings with no draft or machining allowance | Every supplier does it their own way and wall thickness varies after assembly | Note "machining allowance X mm, draft 1°–3°", or cite GB/T 6414 |
7. Typical process chains
- Casting: tooling (wood or aluminium pattern) → moulding → melting and pouring → shakeout and cleaning → stress-relief annealing → rough machining → (UT/MT inspection) → finish machining → surface treatment
- Forging: cut stock → heat → die forge → trim and straighten → normalize or quench and temper → rough machine → finish machine → (carburize or induction harden) → grind
- Sheet metal: laser or plasma cut → deburr → bend → weld → stress relieve → machine the interface faces → surface finish (powder coat or zinc plate)
- Precision shaft: bar → rough turn → quench and temper → semi-finish turn → harden (or carburize) → grind → (nitride) → superfinish or polish
- Die casting: die → cast → degate and shot blast → deburr → machine mating faces and holes → (T5 ageing) → paint
- Welded frame: cut sections and plate → fit up and tack → weld → stress relieve (550–650 °C) → machine the mounting faces in one set-up → powder coat
- Aluminium extrusion: extrude → saw → age (T5/T6) → mill slots and drill → anodise → assemble
The heart of a process chain is never losing the datum: after every heat treatment and surface finish the old datum face moves. Machine the finishing datum last, or use temporary bosses or centre holes as a single datum; the datum called out on the drawing must be a face the process can actually produce.
8. Common selection mistakes (pits we have fallen into)
- Using a casting where a forging is needed. Shrinkage and gas porosity start fatigue cracks, and the bending fatigue life of a casting can be a third of a forging's. If forging is impossible, at least move to ductile iron QT600 or inspect 100%.
- Cutting a die-casting tool for a small batch. A die costs 100,000–500,000 CNY, and under 3,000 parts the unit cost explodes; bridge with sand casting, investment casting or CNC.
- Tapping small threads straight into a casting. Threads under M6 tear out easily in castings — use press-in nuts or wire thread inserts.
- Deciding to drill after hardening. Above HRC 45 you can only grind, wire EDM or spark erode; every hole, slot and thread must be made before heat treatment.
- Machining a thin wall to size before heat treatment. Distortion after heat treatment cannot be corrected; leave stock and finish machine afterwards.
- Welded parts with no "machine after welding / stress relieve" note. The holes do not line up at assembly, and rework costs far more than the machining.
- CNC machining a long constant-section part. A 10 m rail or frame beam should be extruded or roll formed — the cost difference is more than five times.
- Treating a 3D printed part as finished. SLM surfaces run Ra 6.3–12.5 with steps and support marks, so mating faces must be machined; expect dimensional accuracy of about ±0.2.
- Calling out machining tolerances on a stamped part. IT7 on sheet metal means fine blanking or even machining, and doubles the cost; call out GB/T 13914 or GB/T 1804-m instead.
- Forgetting the stock allowance. Castings need 3–5 mm per side and forgings 2–4 mm; designing to net size leaves thin walls and patches of as-cast skin after machining.
- Treating surface finishing as a last-minute coat of paint. Plating, anodising and powder coating change dimensions by microns to tens of microns, so mating faces and threads need allowance or masking.
- Comparing unit price instead of total cost. Machining has a high unit price but changes easily; die casting is cheap per part but a tool change costs tens of thousands. For low volume with many changes, CNC works out cheaper.