When a new pump housing design landed on the desk, the team faced a familiar fork: cast it or mill it. The part needed 8,000 units a year, tight inside channels, and a thin wall that couldn’t leak under pressure. Casting promised a low per-piece cost but a long lead time for the die; CNC could start tomorrow, yet risked a higher unit price. The wrong call meant either sunk tooling or a budget blowout.
The Up-Front Cost vs. Unit Cost Tug-of-War
The real pinch was timing against volume. Die casting shines once the mold is paid for—parts drop out at pennies each—but that steel die can run five figures and take ten weeks to cut. A leak in the prototype mold would scrap the whole pump schedule. CNC, on the other hand, needs no mold; you load bar stock and go. The catch is that every minute on the spindle adds up when thousands are required.
It’s the classic standoff: gamble big on tooling for savings later, or pay steady machine rates and stay flexible.
How Each Process Played Out in the Field
They split the job. First 200 housings came off a 4-axis mill in aluminum 6061. No die, no wait—parts were in test fixtures within a week. Surface finish hit Ra 1.6 straight from the cutter, and the thin walls held 0.2 mm tolerance without sink marks. When the design was locked, a simple zinc die was cut in four weeks; the switch dropped the piece price from $12 to $1.80 after 3,000 units.
What tipped the scale:
- Zero Startup Lag: CNC delivered testable parts day one.
- No Porosity Worries: Machined walls stayed solid, pressure passed first try.
- Smooth Handoff: Final CAD fed the die shop; no redraws.
The hybrid route kept the project alive early and cheaply later. No rework, no panic.
When to Pick One Over the Other
Zinc or aluminum casting wins on straight runs over 5,000 identical pieces—tooling pays back fast. CNC rules for lower counts, complex internals, or when the drawing still breathes. I’ve seen bike pedals cast in bulk and medical clamps milled in dozens; both hit spec because the process matched the numbers.
For buyers weighing quotes, the math is simple: count the parts, count the changes, then pick the tool that fits. If you’re stuck between die and spindle, a quick volume check usually points the way. More on this at www.simituo.com.