The sping idea will work, or there are two other options. Cantalever it, with a pull and weight, to balance the head (with no ball screw / exact weight or really close, or you can use a pnematic cylinder with a pressure valve with the length you need and balance it that way. I like the air cylinder as it adds minimal weight (vs cantaliever), and is linear vs the sping. All you need is a little portable air tank, and a small compressor, or just your average compressor (10 gal or so). I dont know about your servo motors, but as long as they can resist pull down if you are taking a heavy cut, with an up spiral, then you will be fine with any of the above options. (BTW the air cylinders, were used to balance our big 5 zxis routers at my old shop, and worked well, and the heads weighed upwards of 600lbs.).... I have some pictures I could upload (not great) if you would like.