For the shrink that fits - Alice's adventures in tolerance land

More thoughts about the group body casting.

Having stared at the casting after looking directly at sun for a few minutes I am now pretty sure that it is an assembly of two parts: the bulk plus a small internal sleeve that holds the dispersion screen. Putting these two parts together could be done in a number of ways, but to avoid the possible consequences of heating the casting to soldering or brazing temperatures, I think that a shrink fit is probably the best bet.

From the engineeringtoolbox:

Shrink-fits are assembled by heating them to temperatures where the expansion exceeds the interference. Required temperature heating can be calculated as

dt = δ / α d_i         (1)

where

dt = temperature heating (^oC, ^oF)

δ = diametric interference (mm, in)

α = coefficient of linear expansion (m/m^oK, in/in^oF)

d_i = initial diameter of hole before expansion (mm, in)

Diametric interference can be calculated as

δ = dt α d_i         (2)

So assuming the sleeve is at room temperature and the body is heated in boiling water:

Nominal hole diameter d_{i =} 58mm
Temperature heating dt = 80^oC
Coefficient of linear expansion α for C23000 brass = 18.7x10^-6 (°C)^-1
Coefficient of linear expansion α for C87850 brass = 18.5x10-6 PER °C (20-100 C)

Therefore:

for C23000
δ (80^oC) = 80^oC x 0.0000187 (°C)^-1 x 58mm = 0.086768mm

for C87850
δ = 0.08584mm

...or more or less the same.


So over-sizing the sleeve diameter by 0.08mm (which is pretty close to 0.003") will allow it to be shrink fit into place with minimal force.

And after drinking the potion mark "drink me" Alice can now go down the rabbit hole to tolerance land. 

Interference H7/s6 S7/h6  - Medium drive fit for ordinary steel parts or shrink fits on light sections, the tightest fit usable with cast iron. 

The tolerance for the sleeve and the hole diameters is important as the two add together. So a symmetrical tolerance of +/- t will possibly result in a total tolerance of 2t.

So allowing for a minimum of 0.05 interference which should still provide a 'medium drive' shrink fit, the diameter of the hole should be 58 +0.015/-0 and the sleeve diameter should be 58.08 +0/-0.015

So in the worst case scenario, the hole is 58.015 and the sleeve is 58.065 which still results in 0.05 interference.