[defrost]

No matter the mass of the casing it will absorb and lose heat over some timescale and will expand and contract, changing dimensions.

The design is optimised so that the blade dimensions and the interior casing diameter closely match to a very high tolerance at the optimal operating tempreture.

The challenge is that the casing material and the blades are made of different materials and these materials will have different coefficients of expansion (although there is some room to 'dope' materials to change the CoE at the xpense of other material properties that can be more desirable).

The CoE means that the two expand at different rates as the tempreture changes - if they are "close" at optimal temp then they will drift away as the temp changes due to not expanding in lock step.

It's a tricky thing to get right.

( Also a challenge in multicoloured glass works as the differing colours must be batched to have similiar properties including matching CoE's so they don't pull away when anealing from hot glass work temps to room temp. )