Every time I have a need like this and can go to my lathe I'm Happy.
I have a PC 2.5 hp router on my CNC table. It has collets for 1/2" and 1/4
cutter shanks. A while back I needed to hold 1/8" bits so I made a small collet to fit inside the 1/4 " collet. I know you can buy them but it cost nothing to make and I needed it now. Ok....I came across a couple dozen
solid carbide 3 flute end mills for a few dollars. I went to use one yesterday
grabbing my 1/8" collet and the bit just fell through. Its shank is .1165".
So I made a collet for that size. I had a numbered bit thats .116" out of my set of 115. I don't see how I ever made it before I owned a lathe. Here are some pics. The larger bit is .072" at the cutter end and the other is .031".
Nick