We just got a new Haas SL-20 (our first turning center) and are still trying to figure it out. Today we chucked up a 2" diameter brass bar about 2 feet long in an 8" three-jaw chuck, but the tail stock was pushing the bar through when the spindle turned on. We decreased the tail stock pressure to 150 psi, increased the clamping pressure to 400 psi, and scored the bar to get it to hold. The instructions say not to operate the machine if the clamping pressure is above 350 psi, but we don't want to decrease the tail stock pressure any more for fear of the workpiece being thrown. Why does the clamping pressure gauge go so much higher than 400 psi if the maximum operating pressure is so low (the max. pressure of all the chucks listed is 400 psi)? What is the recommended tail stock pressure for safe operation? The soft jaws are machined to hold almost the full diameter. Is there a more solid way to hold it? We are considering buying a 2" maximum collet-closing set-up, but it seems to me there is a better way (and I suspect our Haas rep. is trying to sell us more than we need).
Thank you for reading, and I would greatly appreciate any advice.

Two questions:

What rpm where you running?

Are the soft jaws you machined full size blanks?

With the chuck spinning centrifugal force tends to open the jaws and it can greatly reduce the grip when the rpm is above 2000 or so. The effect is increased when full size soft jaw blanks are machined and only the minimum amount of material is taken off; you finish up with a large amount of metal in the jaw spinning at a largish radius.

My policy with soft jaws is the bore them out as close as possible to the inner mounting bolt then chamfer them back and machine away an excess outside the outer mounting bolt. This reduces the amount of metal spinning around and reduces the centrifugal force tending to open the jaws.

This sort of thing is a continual headache for us in our shop, on manual lathes, using smooth hard jaws. There is just not enough friction on finished shafting, to resist push through when the tool is applied to the work for roughing operations.

Some sort of spindle stop is the best insurance, although using a rear chuck on the spindle to grip long bar stock also stops the slipping.

Perhaps add a stop to the back of one of the soft jaws (weld something on). Or make some kind of a cup with a lip to catch the front of the chuck, and deep enough to go back inside the spindle a sufficient distance. Leave the bottom of the cup thick enough to tap for a stop bolt if you envision using this for many different parts.

thanks for the help. I am not really the operator of this machine, I am just trying to help him out. I will let you know when and if he finds a solution that works.