I recently bought a harrison vertical mill that came with an Osborn tools collet chuck with a single collect inside it. It looks a bit like an ER collet but I believe it may be an OZ25 collet chuck. The taper appears slightly less than on an er collet and the length slightly longer. The collet appear to have an internal bore of 1", but the are no markings on the collet itself. Does anyone know what the clamping range of one of these collets is, is it likely to hold a 25mm dia end mill? ER collets have a stated range of 1mm but I cant find much info on these.
Oz25 collets are also known as DIN 6388. The clamping range is +0/-0.5mm from the marked diameter. Normally, they are etched on the narrow part of the front face.