The mill and tumbler are very interesting. For a one man shop, the mill is much better than a seat and guide machine. With the right fixtures you can do seat and guide work, valve jobs, surfacing plus everything else a mill can do.
The mill will do the work of 3 machines. It may be slower than a S&G machine or dedicated surfacer, but a sitting machine doesn’t make money.
How big is the tumbler??? I can think of a million uses for a good sized one.
