[Update 1/2019: There is a new version added by request with a 56mm inner diameter. The original is a 52mm inner diameter. I have not test-printed the 56mm version, so if you print it, please leave a comment about if it works or not.]
This tool mount is designed for a spindle with a 52mm diameter body, such as the Inventables 300w Quiet Cut spindle, or a Chinese 300w DC spindle from AliExpress. The inside diameter is 52mm (no tolerance), so it is necessary to gently spread the rings to fit in the spindle.
Other mounts made me nervous how thin they are, so I over-built this one to hopefully outlast the spindle, with a thicker clamping ring, and four screws holding each ring to the tool mount. Unfortunately, this means it needs to be printed with supports.
Print two of them, oriented horizontally the way it is oriented in the STL file, and use supports. Do not stand it on end to print it. That would put the layer lines in critical areas where clamping pressure is greatest, and would make it much weaker.
I printed mine with 55% infill (the same as the rest of the MPCNC) with 3 top and bottom layers, and 5 perimeter layers for extra strength (probably overkill).
You can choose to mount these either at the top of the tool mount, leaving the bottom set of holes empty (as shown in my photos), or move them to the bottom of the tool mount and leave the top holes empty. In this way, you can move the mounts 25mm up or down.
== Hardware ==
1) The rings attach to the tool mount using the same #6-32 X.75 screws and lock nuts used in many other places on the machine (four per ring, eight total).
2) The tightening screws that holds the spindle in (one per ring, two total) are #8-32x1.5 screw and nut.