I have an idea there was some discussion touching on this a while ago (Andrew?), but can't find it by looking back.
I've just successfully modified a BT30/ER32 collet chuck to be close enough to QC30 to function without a problem in my Bridgeport. There's a bit of a chamfer on the lower face of the shoulder, before it reaches the proper thickness, but that doesn't seem to matter in the slightest. The hardness of this particular item was right at the limits of what I could turn with carbide inserts, and I should really have ground the last few thou off so as to have proper control over the final dimension, but it seems OK at a few thou undersize.
BTW I've got a couple of these spare after buying a batch off ebay, so if anyone is looking for an ER32 chuck, either 'as is' BT30 or modified to QC30, with no collet nut, please get in touch. Of course they don't have the extended spigot of the QC30 spec, but that doesn't matter for my machine. They can also be used in an ordinary 30int machine, without modification, with an extended M12 drawbar.
Cheers Tim
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