When you look at it, even though for many years people would always use their lathe for milling, it isn't the ideal way to go nowadays. It is OK for the very occasional job, but for large project work such as this, not really the way to go.
By the time you have bought all the bits to do things on the lathe, and made all the special fixtures for doing the job, it is in fact a lot more cost effective saving your pennies up a bit longer, and sourcing either a second hand or small mill.
There must be thousands of vertical slides sitting under benches, never to see the light of day again. By the time you have bought a decent versatile vertical slide, and purchased a vice to fit it, you would most probably be half way to the cost of a mini mill.
John
