You should be able to get an MT2 posilock chuck that will take your collets.

You may also be able to just get a new arbor for the existing chuck.

Neil

Not always, but some milling chucks come with separate arbors, more usual for boring heads.

But your collets should fit any posilock/osborn titanic meant for the same range of sizes. You might find it cheaper to buy a whole set and then sell yours complete.

My Posilock chuck has an integral R8 arbor.

Agree with Neil that you are best off selling your set complete and buying a new set on MT 2 arbor.

Posilocks were generally sold to the amateur market as complete sets in nice boxes. About the only readily available spares are extra collets to make metric or imperial sets dual standard. Amateurs pretty much never wear out sets so they don't get broken for parts.

Clive.

ER collets are always secure if you tighten them up to the specified torques. Which are scary and certainly not achievable with the little pressed steel spanner you get in the set. One firm recommends 42 ft lb for ER 16, 59 for ER 20, 77 for ER 25 and 100 for ER 32. I'd not care to be hauling that hard on a spindle only restrained by the usual lock pin. Ball bearing nuts help a lot but you still need things tight.

For all sorts of reasons an ER may hold fine at much lower torque but its only guaranteed to hit its retention and TIR specifications if tightened to specifications. With their long compression range ERs are wonderful beasties but there are some inherent engineering compromises. They do have to be very well made to hit peak performance. I pretty much never use my set. Posilock or direct in the spindle R8 type usually.

Clive.

Pozilocks, Clarkson et al dont really squeeze significantly on the shank anyway during installation. Initial collet action is more for getting things concentric than outright grip. Which is why you hand tighten the outer ring onto its stop and draw the collet out against the seat by screwing the cutter in. Bumping the ring up a nats doobie with the spanner gives a bit of grip but thats mostly to keep things under control when cutting starts. The real holding comes from the cutter screwing in further under cutting loads. Unless you actually manage to find cutting loads that will actually start unscrewing the cutter under the screw thread ensures that the only way the cutter is pulling out will be to bring both collet and outer ring too. Which will be an impressive smash up.

Clarkson, Posilock et al balance the loads as the cutter screws out, pushing the collet forward until it has enough grip to stop the cutter turning. So the exact extension isn't known. Which is why they are out of favour for CNC purposes. Only a thou or three variation but thats enough to mess things up. If you try to set things up like a conventional collet by tightening the outer ring to make it grip the balance will be upset, movement become unpredictable and there is a good chance of shattering the centred end of smaller cutters. I have a pair of 4 mm with ends shattered that way due to trying to use the system like a normal collet.

Clive