Hey, Graham and Rob, thanks for the links, very informative.
Bob, I have a few concerns with the 5C collets. First, they are very limited on the range you can clamp in them. My set only goes down to 32nds. I would have to double it to go down to 64ths. Second, I have trouble getting the 5C collet closer to clamp the work tight enough. I probably need to insert a thrust bearing under the hand wheel I use to tighten the collets. Finally, and this is related to the first concern, as you close the collet, the outboard end closes faster and clamps harder then the the portion closer to the spindle. Not usually a problem except when the work is several thou smaller than the collet.
The thing I find attractive about the er collets, is that they close uniformly along the axis and clamp the work uniformly.
Having said all that, the er collet holder would be a lot of work to make (accurately) and the cost of a full set of er40 collets would not be insignificant.
Chuck