Is this enough time to make the connection?

Probably if everyone is on time. Is there a back up flight if you miss the Milan connects? At least if something goes wrong Delta will have to take care of you. But you would not want to spend an extra day in JFK. I would be more comfortable with an extra hour or two. Talk with Delta and see what they suggest.

I would be confident if it wasn't for the fact that you are changing planes in JFK. Look at the map of the airport in advance so you know exactly what you need to do when you get there and no time will be wasted. Good luck!

I would not do that. JFK (in fact, all NYC airports) are among the airports with the most delays. Spend a little time searching on the web and you'll confirm this. From the West we're more likely to hit delays if they happen because even the earliest flight east is arriving in the afternoon when any delays during the day are going to be piling up. And with the high loads on flights these days, if you don't get on your scheduled outgoing flight you may wait DAYS for the next confirmed departure.

Personally, I do one of 3 things when flying from the West Coast to Europe. 1) (far preferred) choose routes so my connection is in Europe; once I'm somewhere in Europe its far easier to get a later flight than it is to get another flight across the Atlantic Ocean 2) spend a night on the East Coast; more expensive, but then I figure I will be there to get the next flight. Go see a Broadway show! 3) give myself at least 4 hours if connecting domestic. This way if the flight is cancelled due to some problem I might be able to get a different flight leaving a little later and still get to the connecting airport.

Although I do not have experience with JFK, I took a flight from Amsterdam to LAX but had to switch planes in Munich Germany where I had 1 hour and 10 minutes. Because of weather problems, my plane arrived late and took off pretty late causing me to miss my connecting flight. We arrived in Munich where our plane was till on the ground but didn't let us board because the flight was already closed. I pressed the crew before we departed Amsterdam to ask the plane to wait for us to no avail. I begged some more when we landed in Munich and they would not hold our plane so I personally would never book another flight if the connection is less than two hours.

When changing terminals at JFK for a connection is it necessary to go through security screening at each terminal?

Delta's two terminals at JFK are connected post-security I think so the answer is no, you don't have to clear security again in JFK.