[trademark]

for cnc lathe with hydraulic chuck.
Is there a rule of thumb for turning parts without a center.

I would think this would be based material,diameter,chuck pressure,
how much of the part is in the chuck, how big a cut is being taken.

I am thinking a 6" Dia. part by 6" long held in the chuck by 1/2
is not a issue.
but a 3" Dia. x 6" long held in the chuck by 1/2" may be a problem.

since throwing a part out of the chuck can be detrimental to a lathe and operator.
i would like some info on this subject

[txcncman]

It is not just a factor of chuck pressure. In fact, too much pressure can cause the chuck to flex giving less contact area on the part material. I have counteracted this flexing by cutting the jaws with a taper on them. It is also a factor of material hardness and flex and type of cutting tool and tool geometry. I have successfully turned parts with 3.0" material diameter to 2.625" diameter with 14"+ of stick out, including the facing process on heat treated 4140, 28-34 RC, chucking less than 2.05 deep. That is almost 5X diameter of stick out.

[holbieone]

when it comes to machining nothing is etched in stone, just a general guideline on how to do things.

the size and the rigidity of the machine , tooling , material , geometry all come into play

you will have to develop a feel for it ,

reading what you wrote sounds like you have a good idea of whats involved

[neilw20]

You can micro rough the jaws too.
12v battery, a piece of carbide, and spark all over the gripping surface.
Leave microscopic bits of carbide behind.
Won't make much difference holding soft materials, but it can help stop hard material walking out of the chuck.
If the jaws bow out tightening them them it is a losing battle.
The extra gripping of using a 4 jaw chuck often helps.
Bit more setting up, with a DTI, but you get really good at it with a bit of practice.
1-2 minutes work to better than a thou. -- hahaha but not the first time you try to do it.

[dertsap]

3 to 5 x dia
most other factors don't matter as much as leverage does if the part gets bent and starts thrashing everything in its path


.

[fizzissist]

Use soft jaws, clamp on a small dia bar end and clamp down with the desired pressure. Bore the jaws in front of the bar end to the dia of the blank so the jaws are dead true. This compensates for the splay of the jaws as you put the pressure on, and conforms perfectly to the blank.

1/2" on a 3"dia x 6" long part doesn't leave me with a comfy feeling. But... for added comfort, turn down that 1/2" section to just clean up and be round, measure it, then bore the jaws to that exact size (AND... face the bottom of the bore for the end of the blank to register up against for added security!!) . ... Then, proceed at a relaxed pace, and you should be ok. I wouldn't hog with that kind of set up.

If you get excellent conformation between the diameters of the blank and the jaws, you have good jaw width (assuming 1.5" wide?), AND the back seats well against the inner face of the jaws, you'll get some good clamping forces and security. Think about it,,,,, the part just doesn't have anywhere to go.

[fizzissist]

for cnc lathe with hydraulic chuck
since throwing a part out of the chuck can be detrimental to a lathe and operator.
i would like some info on this subject

Btw....
this is why you NEVER leave the door open unless ABSOLUTELY necessary.

I had a 5/8"dia x 6" long SS part flip back into the chuck at 1800rpm during cutoff.... it hit a jaw, was flung into the safety glass, and BROKE it.
$580 later and a change of undies, I was back in business. And alive.

[dertsap]

I missed the holding 6"x6" with only 1/2" .
I'd be going pretty serious with a dead blow hammer on that part to make sure that doesn't move before attempting to spin that thing , even then I would come up with an alternative

edit
eh now that i thought about it , its not the picture i had painted in my head , I'd still whack it with a hammer first

.

[fizzissist]

the dead blow is now instinctive to me, didn't think to even mention it since it's automatic....

(don't forget to relieve the inside corners of the jaws, and chamfer/radius the blank so there's no interference!)

SMACK!! .... yup. that sounds good.

[metlcutr55]

not really to the ops question but an alternative to Neils anti walk solution, a thin wipe of clover compound has stopped hard parts from moving in my hard mill jaws.