I am building a 4 axis wood mill/lathe driven by Mach 3. The part I am interested in is at the bottom of the page at
http://turningaround.org/4_axis_mill.htm . It is a red oak newel post 5.75" square by 60".

The area of concern is the transition from 5.75" square to 5.5" round in .5" on Y axis and preventing tear out of the wood. Here are my asumptions
1 use up cut or straight cut .5" router bits cutting on the side of the bit so the cut is with the grain.
2. rotation speed ( A axis) at the trancition needs to be much slower than the rest of the piece.

Assuming you're starting with square stock, I'd try this. When standing in front of the lathe, with the work spinning toward you, as with a conventional lathe, plunge the router straight down from the top of the piece, on the RIGHT side of thefinished square section, like a parting tool. Plunge very slowly to prevent tearout. Having the tool slightly off center toward the front or rear of the piece might help if your seeing tearout. Trial and error will be your friend.

To do the left side of the square section, reverse the rotation of the spindle so that the cutting action is the same.

Increase speeds until you see tearout, to find the limits. I'd also try with opposite rotation and a climb cut to see which works better.

I think coming in from the side might cause more tearout, because the depth of cut would be greater, and you'd be cutting endgrain in light passes, which almost always tears out, especially with oak.

One last thing. Always use very sharp tools.

I think that you're worrying about the wrong thing....if you're running a 1/2inch straight cut bit....how are you going to cut the rest of the design....you should be using a 60 or 90 degree v-cut bit mounted on the spindle.

I assumed that the cuts would be from end to end taking max cut as defined by G code and that the area of transition would have to be at a slower speed. Which brings up the question of how I define a slower A axis speed without manually editing the G code for the area of transition?
The .5" cutter should finish about 95% of the piece and I will either do a tool change or finish it on my manual lathe.

I see, it's your webpage. Nice looking machine you've built.

I've just bought a very basic 4 speed wood hand lathe, so now I'm looking around this section for the first time.

Suggest that you go to American Association of Woodturners (AAW as google search) and find a local club. Actually my next machine be a MechMate modified with 4th axis. www.mechmate.com Much better design with free plans and active forum. Most hard to make parts are avaliable at a very fair price. Beware turning can be very addictive!