Depth setting ring, depth collar, tool distance ring, whatever you want to call it, these things are awesome for small router bits like the 1/8" endmills and drills you use in a CNC for things like PCB milling and drilling. They are, however, not commonly found on the cheaper bits I've accumulated, so I designed a tool for setting the bit depth appropriately. And, naturally, a ring I can use with my bits.

This design involves a base piece which has a pedestal the height of which the bit should project from the chuck (which includes the ring thickness) and a press which is calibrated such that the point of the bit is precisely at the bottom of the press. The press should be tapped to take a bolt of some sort (I'm using an M5x18 bolt, but YMMV) and the bolt should be long enough that a jam nut can be added to lock it in.

To use, the ring is placed in the hole of the base, the bit is inserted into the press with the tip out, and then the press is placed over the base so the tip goes through the ring hole and once everything is lined up, the press is (lightly) hammered down (hitting the bolt) until it bottoms out on the base.

If the print tolerances on the ID of the ring are good, the result will be a firmly set ring that you won't be able to budge with just your fingers. If the tolerances aren't right, results range from a loose ring (which is usable in some applications, but not recommended for an ATC) to a cracked ring.

The base is customized for the desired bit projection of the machine in question (i.e. my Bungard is designed for 21mm), and the press is customized for the specific length of bits it's intended for (I'm currently using 36mm and 38mm long bits). Or you can use whatever you prefer, or vary the application to the different bits (i.e. v-bits might have a shorter projection).

It's not going to be perfectly precise, but it should be good enough if you're using a tool length offset probe. If you're using expensive micro-engraving bits you should probably get your source to set the depth rings.