So far I've seen two automatic tool changers for the X2:
A) The Z-Bot changer, as sold by littlemachineshop.com
B) Dan's hossmachine changer
The Z-Bot mounts to the left side of the table. No tools overhang the table, but it has to have enough space over the workpiece to swing the longest tool into position under the spindle. The tools are mounted in E-clips mounted on a chain drive. The headstock positions itself to the correct Z height for the E-clips to match the slots in the toolholders, and then the table scoots over to snap the E-clip onto the tool. The headstock then rises up off the toolholder and the chain indexes around to the next desired tool.
The Z-Bot is very expensive, but after looking at the manuals on the littlemachineshop.com site, it does a whole lot - there's a whole separate QC controller, a power drawbar is included, Mach3 screens included, and a whole array of interlocks and safety devices. It appears to be a well-engineered system, but $1500 is completely out of my price range.
Dan's hossmachine changer is much simpler. Tools sit in a rack on the left side of the table, occupying part of the table's travel area. The table simply positions itself under the headstock, which then runs up and down to grab the toolholders. You have to provide a power drawbar to operate it; any power drawbar will work; Dan provides plans for those, too. You can build Dan's design for peanuts, but the more rows of tools you mount, the more X workspace you lose. Dan gets around that with his "X2 Freak" mod to increase table travel.
I lust for an ATC, but the Z-Bot isn't in my price range. I could build Dan's design easily enough, but I don't want to lose the table space (or go down the X2 Freak trail; I'd like to get this thing finished someday!).
Over the weekend I've been sketching another method. Basically, it's a carrousel (like one of those rotary CD changer tables) mounted on swinging arm attached to the side of the column. It would require a stepper to rotate the table and an air cylinder to move it from "park" to under the spindle.
Operation:
1) ATC mode runs headstock to top of column
2) air cylinder swings tool carrousel under spindle (indexed to an empty space)
3) power drawbar opens, dropping tool into counterbored slot in carrousel
4) stepper indexes tool carrousel so correct tool is in position
5) headstock lowers onto tool. (The swinging arm holding the carrousel has some compliance via a spring in the hinge)
6) drawbar closes
7) headstock lifts tool out of pocket in carrousel
8) air cylinder swings tool carrousel back to park position
This all assumes the carrousel is counterbored to hold the toolholders and slotted so the tools can be removed after being lifted enough to clear the counterbore. You could use big E-clips if you can figure out how to make them.
It's a lot simpler than it sounds; it's just a swinging tray that spins to present the right tool to the spindle.
Pros:
All toolchanges occur near maximum Z. You don't have to move the table unless a tool would hit the workpiece.
No loss of table workspace.
Tools stored high and off to the side would collect fewer chips that might cause trouble.
Cons:
Requires a stepper output and a relay output for the air cylinder.
Tools not positively retained; they could dance around if the column vibrates.