Hi Jonnyfive

Overall design concept looks OK, but some comments.

1) An extra 1" travel will be fine to allow the machine to go slightly over the home switch/prox and reverse back to settle at the home position. Gives a small amount of room before any physical hard stop.

2) When the Z is at maximum travel the collet needs to be close enough so that the cutting bit can level the board. I notice on your design that there is still plenty of Z travel left when nearly fully down, which is a bit of a waste. It also means when the Z axis is fully up/home it is still fairly close to the board which will limit the maximum height of the part you can cut. If you want to do vice work this gap looks too small. Most people work to 150 mm from a typical tool tip to the spoil board when fully raised. Yours looks considerably closer. Just check this is what you want the machine to do. You will see other machines on here use vertical risers on the bed, or vertical extensions of the side members for the base frame to get the main beam high enough that the gantry then runs on.

3) Never used R&P so can't comment. Others may be able to chip in on whether you can go up from 16 mm ballscrew to 20 mm ballscrew as an alternative step before going to R&P for the long axis, potentially tensioned at both ends. Wait for advice on that one . . .

4) I've seen racks shimmed in using a DTI mounted to the carriage of the rail, ensuring it runs parallel. Either use proper shim stock or aluminium foil if the gaps are not that big. One thing to consider is that the stiffness of that axis depends on the stiffness of the rack mounting so I wouldn't favour angle brackets and the like, rather a very solid shimmed mount back to the main gantry.

Best bit is when it is all built and moving you will be able to say JonnyFive is alive