I would recommend 1/2-13 because that's the most common size of mill table clamp sets. They can usually be picked up for a deal at most auctions. Really though you should pick you clamps before deciding on a thread size. 5/8-11 and 3/4-10 are both common at machine shops with bigger machines and are less desirable to the home machinist so they may be more economical to buy. I would also consider some threaded inserts that allow you to use a smaller bolt to hold something down if needed.
__________________
AL
|