Does anyone know of any open source lathe simulation software?

By open source do you mean user configurable or just free s/w?
If the latter, then there is http://www.cncsimulator.com/
Al.

By open source I mean software that distributes the source code.

Not sure if it would satisfy what you need but they do have a link for 'Plug-in interface for developers' using C++.
Al.

simulation rachat de credit (http://simulationrachatdecredit.org)
this interesting.. i think i'll stay around