Hydraulic chuck question: Is it normal to get a bit of jaw deflection?

[MadMax]

I'm running a 6" jaw on a Daewoo CNC lathe with bored soft jaws holding bar stock.

The chuck chuck and actuator are supposed to be able to handle up to 40bar of pressure, but I'm noticing a small amount of lift between the soft jaw and the moving block on the chuck at the serration. Even at very low pressure (15bar) I can see a bit of lift when I shine a flashlight thru the serrations. I've tried cranking the screws harder to no avail. I'm sure that the screws aren't too long. They should be holding the jaws down on the serrations.

I'm worried that I may be stretching the screw closest to the centre of the bore and inducing a fatigue load that may bust a screw. Is it normal to have a little bit of lift under the innermost end of a softjaw?

[JimPAC]

What is the dia. your turning and how wide are your jaws (Surface Hold)?

Are these old and not flat?

Turn your pressure down until you can turn the stock by hand under pressure!

Add a little until you can't turn the stock by hand and check for SEAT to the chuck and stock!

Are all three the same is it one two or three giving problems?

If your Soft jaws have plenty of beef left it sounds like to much pressure!

Start over ~ Re bore lighten up on pressure and try again!

[JimPAC]

You know the most common problem is bottom out screws!

[Geof]

If the soft jaws are lifting off the bases then either the screws are too long or not tight enough. But are you sure this is what you are seeing? Even when the jaws are firmly bolted down it is possible to see light through at the top and bottom of the serrations because there is a small amount of clearance there.

When the chuck clamps you do get a very small amount of lift from the clearance between the bases and the chuck body. This is why it is a good idea to machine the soft jaws with the jaws clamped against a boring ring so the jaws are lifted in the same manner they will be when in use. A boring ring is just a ring that is clamped on the OD using dowels in the counterbores for the bolts.

[MadMax]

I'm pretty sure the screws aren't too long. I measured their bottom out height without jaws and the tops of the screws are higher than that in the jaws.

I'm chucking 1.5" solid aluminum bar with jaws turned with a boring ring. I tried bluing a bar and found I"m getting even contact.

How does one pick a chuck pressure? I'm running at 300psi which is a little under half the actuator capacity and well under the chuck capacity. I'm doing some turning at 3500rpm and don't want to risk too much jaw centripetal pull.

[Geof]

On bar stock I generally run at maximum, on tube I will reduce the pressure to reduce distortion.

[Glacern]

The top jaws should not lift from the base jaws. It's normal for the base jaws to lift slightly, especially if you're using tall topjaws and are clamping just a small part of the bar.

300psi or about 2MPa is pretty decent pressure. The max recommended pressure on our Mori's 6" subspindle chuck is 350 psi. For 1.5" aluminum bar, you're probably fine where you currently are.