A drill stop loosely based on an ER20 collet. Select the appropriately sized collet for your drill bit, insert it in to the nuts and tighten down over your drill bit. It clamps firmly over a large area which means a low probability of it coming loose or moving. In addition collets are more concentric with the axis of drill rotation so if you're drilling wood, you have a much lower likelihood of marring the surface. It does a much better job than those metal-rings-with-set-screw that are common. Nuts are knurled for easy hand tightening/removal. I recommend putting a bit of PTFE lube or vaseline (which is what I did) on the threads and taper (which are M28x2). The collets themselves should be +/- 1mm of flex; if you put the drill in and tighten down all the way and you can still move it, you should be using a smaller collet.