Stress
We hate to break it to you, potential machinist, but yeah, you'll be pretty stressed out in this job. You work under tight deadlines, for low pay, in order to create a very particular product. There is no such thing as "good enough" in the machinist world.
There is only "perfect" and the occasional "no way am I paying you for making that thing." And it's not just the final product that needs to be perfect. Standard machinist practice demands cutting precision to at least one thousandth of an inch, and a good machinist will try to cut to one ten-thousands of an inch: 0.0001" (source).
Additionally, if you're stressed out by deadlines, don't go to your workplace looking for relief. You're not working in an office with a wellness guru who comes in once a week for lunchtime happiness sessions, nor do you have ergonomic exercise balls for chairs.
Around 20% of machinists work in a machine shop. Another 20% work in what's called "machinery manufacturing," or basically the same exact thing but louder and bigger (source).
All this is to say that there isn't really a, ahem, calm aspect to this career. It's loud, it's dirty, and it's dangerous. If you aren't comfortable with one or any of those factors being present in your day-to-day life, you may want to rethink a job in this field.