Looking at your pics, It looks like you are cutting dry. With al you need a coolant/oil otherwise it will gall. Use WD 40 if you have nothing else, but you can get a coolant mist system from www.littlemachineshop.com for pretty cheap.
It also looks like you are conventional cutting. To cut the outside of a feature you should go clockwise and on the inside go CCW. This will give a better finish. It's also best to leave .01" on the side and take a finish pass.
That said, a surface speed of 350/450 for a high speed steel end mill is a good starting point, for a 3/8 end mill that is from 3565 to 4583. 1/2 is 2673 to 3437. Depending on the rigidity of the mill (don't know much about Taig) the feedrate will be RPM *.002(+or_)* the number of flutes on the endmill. This can vary widely, but start at .002 and work up or down. For just cutting the outside of a part when the endmill is fully enveloped in the material, I would go no more than 1/3 than the diameter of the tool deep per pass. Depending on the rigidity of the Taig it could be more or less, probably more toward the lesser side.
Hope this helps. It would be a good idea to blow out the cutting area from time to time, but if all you have is a vacuum, then use that.
Let me know if you need more hints and good luck