The spring loaded live center is a great idea.

Why not put a spring behind the shank, in a holder, and leave the set screw
loose?
Most of the shank/holder combo's I encounter have a good fit, and a stiff
spring in the holder would likely give you the tension you desire. This
should work in cnc/pneumatic tailstocks as well.

Here's a little trick I use to get the benefits of spring loading without
the $$, in manual machines, which may give you some ideas.

Lathe tailstock:
The hand wheel is usually pretty free-wheeling, so what I do is bump the
work w/ the tailstock/center, turn the handle to the 2 o'clock position
(facing the chuck from the bed end), lock the tailstock down, and load the
handle w/ cored brass/bronze weights. This puts a constant tension on the
live center, and the weights usually stay in the 2 to 4-o'clock positions,
for good torque.
You could also rig a spring from the handle, hooked under the bed someplace.
Works really, really nice.

Milling/table tailstocik:
These have tougher, smaller wheels, not ideally suited for what I just
described.
But, by clamping a suff'ly long lever arm to said wheel, you might be able
to achieve the same effect.
Altho I think here, a simple stiff spring in the holder behind the shank
would be more convenient.

I haven't used the spring-behind-shank method, cuz I don't currently have a
straight-shank live center. But this sounds like a good reason to get one!
--
Mr. P.V.'d
formerly Droll Troll
"Protagonist" wrote in message
. ..
Any one can point me to, where can I buy economical straight shank (1/2")
live centers, if it's spring loaded even better.
The one I found in the MSC or Shoptools catalog are expensive $250-$300.

Julius