Good question and something I have spent a while looking at. A Shapeoko ATC is a very cool idea but really needs to be well thought out. As standard GRBL doesn’t support any kind of ATC function, there are no dedicated outputs, no stored tool offsets, no function around tool carriages. It also requires a ATC spindle of some variety, a bunch of inputs/outputs for safety and likely air or another kind of pneumatic device to drive the tool changer. All in there is a significant gap in hardware and software which needs to be closed.
I have installed a couple of ATC units now and the average cost for these is around $3000 per unit unit when you consider a decent ATC unit, VFD, tooling/collects, air requirements, possible hardware around a tool holder and the driver hardware. Even if we could do it for half the cost, would be a chunky step up. I’m sure there is appeal but it might be limited to a select group of users.
With the recent-ish introduction of BitSetter it seems that what might be a little more appealing is the ability to quickly change tool, re-probe the tool and continue the job?