ER32 collet question

[fizzog]

I've just acquired a pile of used ER32 collets and was sorting through them in the hope that the complete size range was present. Unfortunately there wasn't, as one size was missing, however several sizes were duplicated.
On closer inspection I found that there were two types of collet. The majority have the same bore diameter right the way through, but where the sizes are duplicated, one of them has a large diameter counterbore approximately 14mm deep in the non-business end, so that all the gripping is done at the front end of the collet.

Please forgive what may be a stupid question, but would I be right in thinking that these are simply designed for holding shorter items than the normal minimum length (seem to remember reading somewhere that 20mm is the shortest you should hold in an ER32 although I can't find the reference now), or would they be intended for some more obscure purpose?

[mmaidnz]

The odd ones were probably blanks,modified to suit a particular job.

[44767]

The smaller sizes would be the ones with relief at the back. For ease of manufacture and because any more than 2 x diameter is unnecessary for holding, the back would be machined out so that grinding all the way through was not required. Having said that , I think that they recommend gripping by at least 2/3 the length of the collett to stop it collapsing at the back.

Cheers, Mike

[fizzog]

Thanks for the replies gents. As you suggest it is the smaller sizes that are made this way. Makes sense to remove any unnecessary material in bulk before the precision grinding takes place.