[deadlykitten]

i just had an idea, that also involves a bolt, only that the bolt is not secured

it may be possible that your parts are being pulled because of elastic deformation, thus, considering that your chuck is in 100% condition, the thin walls may deformate, and contact area is reduced; the more force you use on your chuck, the higher the deformation; only that, after you remove the part, it simply shows no deformation, because everything was elastic

a technique to clamp parts with thin walls, is to put inside them a rod, then clamp the od; for example, if the id is 70H7, and you check it with a go gouge, then the rod diameter should be just a few 0.01s under the go gauge; like this, part won't deformate, and a higher grip force will be available, because nothing is lost/transformed into elastic deformation, or, more precise, the elastic deformation can not go beyond the play between the id of your part, and the od of the rod / kindly

[IsakAndersson]

a technique to clamp parts with thin walls,

But we are not actually clamping on the thin walls. As I said before we are not allowed to do that because we found out the hard way what happens when we do. The part that we clamp on is the "hat" and it's mostly solid all the way through. These parts have been hardened also so if anything is deforming it is the soft jaws. Of course, we could use harder jaws but then we risk marring or scratching the parts instead.

[deadlykitten]

but in the image from post 3, the jaws are on the small od, not on the big od .... please, am i missing something, or those are actually clamping the big dia ?

[IsakAndersson]

but in the image from post 3, the jaws are on the small od, not on the big od

Bit of an optical illusion there. It's actually clamping on the hat about 4 mm deep.

[deadlykitten]

ooookey if your part is pulling out when clamped like that, then it will definetly lose grip if you use the inner mandrel

the chuck delivers much more force at the bigger diameter, while the collet will deliver less force at the inner diameter

perhaps there is also a shoulder on your jaws, while the collet has none, so ...

is it possible to increase those 4 mm ? like at least 15, or even more ? i suppose that there is an interference, but it should be possible to avoid it by using custom shaped jaws; such custom jaws are cheaper to do in house, and they should deliver

[deadlykitten]

if custom jaws can avoid interference, then customize them even more, by adding a shoulder in front, in order to avoid parts pulling out during machining; like this, when you close the chuck, don't push your parts towards the chuck, but pull them instead