What a base actually does
The base carries the machine's static and dynamic loads, decides how quickly cutting vibration dies out, and sets how the machine moves as shop temperature drifts. Surface finish, accuracy, and tool life are largely won or lost there, which is why the material matters more in the base than almost anywhere else in the machine.
The three candidates, in numbers
| Property | Gray cast iron | Mineral cast | Steel |
|---|---|---|---|
| Density | ~7.2 g/cm³ | ~2.3 to 2.4 g/cm³ | ~7.85 g/cm³ |
| Young's modulus | ~100 to 130 GPa | ~30 to 45 GPa | ~200 GPa |
| Vibration damping | good | best | poor |
| Thermal stability | moderate | high | low |
Damping is where the three separate. Steel is stiff but rings. Gray cast iron damps roughly 10 times better than steel, because its graphite flakes absorb vibration internally. Mineral cast damps roughly 6 to 10 times better again than cast iron. Published figures vary widely with resin, aggregate, and test method (the mineral-cast advantage over cast iron is quoted anywhere from about 3x to 30x), so treat the multipliers as order-of-magnitude, but the ranking itself is not in dispute.
Thermal behavior compounds the advantage: mineral cast conducts heat poorly, so the base drifts slowly and evenly instead of following every draft and sun patch in the hall.
What mineral casting costs you
Stiffness. With a quarter to a third of cast iron's Young's modulus, a mineral-cast base needs substantially more cross-section to hit the same rigidity target. The base gets bulkier, not lighter than the equivalent iron base, even though the material itself is about a third of the density. You buy damping and thermal calm with volume, and the machine's architecture has to be designed around that from the start.
Where cast iron still wins
Machinability and stiffness per volume. Complex structural parts with tight machined interfaces, bolted joints, and changing cross-sections favor gray cast iron, which is why the Unicorn 5-200 Modular uses both: mineral casting for the base, gray cast iron for the large structural castings above it. Choosing one material for everything makes a brochure simpler; choosing per function makes a machine better.
Why this matters at 60,000 rpm
The Unicorn 5-200 Modular runs an HSK-E32 spindle to 60,000 rpm on small tools where surface finish is the product. At those speeds, chatter is the limit on everything: finish, tool life, and how hard you can push. A base that swallows vibration instead of storing it is the reason the accuracy targets on the datasheet are achievable at all.2
2 Design target, to be verified at factory acceptance testing, scheduled mid-August 2026.