Ah good point. With a substantial lathe spindle, there's probably enough mass to dampen those oscillating milling forces. My current machine just has a Dunham 5c headstock, nice beefy iron chunk but not much mass in the rotating parts. The next machine I plan to build might have a larger main spindle, but not sure yet. What id really like to do is use my Dunham 5c headstock as the sub on my new machine. Already has the pneumatic closer, then use a larger spindle for main, big enough for 16c or 3j through capacity for feeding bigger bars through. Sub doesn't need big through capacity, I can use 5c step chucks on that end for part transfers with bigger diameters.

Some might scoff at this, but I've actually been looking on aliexpress at some decent lathe spindles and bt30 cartridges. Looks like some of it is decent stuff to me. About 1200 to 1500 for either one. They have specific ratings for balance, rpm, runout, etc. Also some reviews from people that bought them. There's a lot of cool machine building stuff on there. Ground ballscrews, genuine hiwin rails, big harmonic drives, big cross roller bearings, etc. There's obviously a lot of cheap crap on there, but some of it looks good unless they are just flat out lying about grades and ratings.