On Wed, 23 Nov 2016 22:43:09 +0000, Mike Perkins wrote:

I bought some 6mm burrs on a 3mm shank.

I also have a Silverline rotary tool:

http://www.silverlinetools.com/en-GB...i-Tools/249765

I find the burr walks out and I can't seem to tighten up the collets
sufficiently to make it stay in place for a decent length of time.

The link says "3.2 mm collet" -- lokks like you have a 3 mm shank in an 1/8"
collet.


Thomas Prufer