Vacuum Casting vs CNC Machining for Covers and Housings
The question is not which is better. It is whether a face will be read at arm's length.
Short answer: machine the parts that carry load and have to be accurate. Cast the covers that get read by a person. In a humanoid programme, that is most of the volume.
| Factor | Vacuum casting | CNC machining |
|---|---|---|
| Cost per part | Drops across the batch after the mould | Flat, mostly labour and machine time |
| Start-up cost | Master pattern plus a silicone mould | Fixtures only |
| Cosmetic surface | Copies a polished pattern | Tool marks, then hand polishing |
| Tolerance | Cosmetic-grade overall geometry | 0.02 mm positional, 0.01 mm bores |
| Sweet spot | 10 to a few hundred | 1 to about 100 |
| When data changes | Re-cut the pattern, re-make the mould | Re-program and re-cut |
What the surface actually costs
A cover on a humanoid shell is read at arm's length, which makes its surface a product decision rather than a manufacturing one. Cast parts come out of a mould that copied a polished master pattern — the cheapest route to a clean cosmetic face at batch. Machined covers need hand polishing to hide the tool, and that time is not free, so it scales with part count.
Both routes are specified rather than described. Injection-moulded and cast faces are graded on the VDI 3400 scale, machined faces on ISO 1302 roughness grades, so a cosmetic requirement can be written on a drawing and inspected against. Photographs of a surface alone do not settle a dispute.
The break-even question
Break-even sits where the per-part saving from casting covers the one-off mould cost. Machining stops paying when the same part has to be set up again and again, because at that point fixturing and program time per part is what is being bought, not the metal. That point lands in the low hundreds for most aluminium and resin parts.
| Batch | Recommended route | Why |
|---|---|---|
| 1 to 5 | Machined or printed | No tooling to wear out |
| 10 to a few hundred | Vacuum casting | Mould amortises, colour matched |
| A few hundred to a few thousand | Rapid tooling and moulding | Per-part cost drops enough to pay for the tool |
What the base can hold
Both routes run inside the same plant, which is the point that matters for a procurement conversation. Machining centres hold 0.02 mm positional with 0.01 mm precision bores, standard travel is 2.0 × 1.2 × 0.9 m, and the Italian-imported five-axis gantry mill covers an 8.0 × 4.2 × 2.5 m envelope. Casting runs soft or hard resins with tunable colour from 10 parts to a few thousand. A pattern machined on Monday can be a finished cast shell on the inspection bench that week.
Why would I cast instead of machining a cover?
Because of the surface and the batch. A cast face copies a polished master pattern, which is the cheapest way to get a clean cosmetic surface across a whole batch. Machining one cover costs about the same each time; casting amortises the mould across every piece.
Can a cast cover hold the same tolerance as a machined part?
Not on the mating faces. Cast parts are good for cosmetic faces and overall geometry, less so for bearing bores and bolt-hole positions. The usual split is: cast the shell, machine the inserts, assemble at the base.
What quantity makes casting the sensible route?
Roughly 10 to a few hundred pieces with a cosmetic requirement. Below that, machining is cheaper because there is no mould cost. Above that, rapid tooling and injection moulding take over once the per-part cost matters.
Also read: vacuum casting versus injection moulding and the casting workshops.
Send the drawings, get a DFM view back
Rocky Chen replies with a process route, the tolerance risks we see in your data and a quotation — usually within one working day.