Re: Model brake component size comparison to prototype


Paul Woods <paul@...>
 

Tony

I am greatly insulted by your tone and it appears that you have not put any thought into your comment.  Seriously?  You are trying to tell us that ALL the metal in a casting cools to freezing point at exactly the same moment?  You might know a lot about metallurgy but I know a bit about building things from metal.  I hadn't bothered mentioning that the brass caster's tale matched my own experience gained building steel ships because it didn't seem important, but fine: the metal might eventually shrink equally, but not all at the same time because cooling happens from the outside in, and this can be used to our advantage.  I have dealt with plenty of  castings, such as A-frames for supporting ships' propeller shafts, with significant variations in section size.  The orientation of the casting as it is poured is quite important, with the thicker part intentionally placed above the thinner part so that it acts as a reservoir keeping the thin section filled as it cools....as a metallurgist you should be aware of the high density of metals such as steel and brass, and thus the pressure that will be exerted that can keep pushing semi-molten metal down towards the bottom of a mould when it is cooled slowly enough - that is the key, achieving a slow enough rate of cooling.  You can't tell me it doesn't work because I've already done it; yeah sure, the metal shrinks but if it's done right it is possible to control WHERE the shrinkage occurs.  The column that lost-wax brass parts are stuck to when they are cast serves as one heck of a big reservoir of molten metal as the 'branches' are freezing so I don't have any reason to doubt what the caster said.

Paul Woods
retired mechanical engineer

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