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I thought the original question was about collets for the lathe Jo
It was you that said they were good for tool holding
ER collets will only hold a part true if the part fits for the whole length of collet length. When I hold short parts in an ER collet, I use a short piece that is the same diameter I am holding, that supports the back of the collet so it can't collapse.5C and R8 collets will hold a short piece quite well , as long as it is within a very small amount of the collet nominal dimension.There is a really big difference in quality and concentricity with all collets, and unless you buy the precision series of collets, the rest are all a gamble.