In designing a carriage lock for my mini lathe, my main considerations were simplicity, unobtrusiveness and minimizing any tilting of the carriage when it’s locked down. Since the left side of the carriage is forced down into the lathe bed during cutting, I thought it would be best to make a lock that attempts to counteract that by clamping the right side of the carriage to the bed.
On my lathe, the carriage is shaped like the letter H, so there’s an open space between the front and back surfaces of the carriage where one can put the carriage lock. Unfortunately, the cross slide gib adjustment screws pass over that part of the carriage, so I couldn’t just lay a thick chunk of metal across the opening, because it would interfere with those screws. Instead, I milled a small ledge into the right side carriage opening, then I milled a block of mild steel to rest on the ledge and lie flush with the top surface of the carriage. A countersunk M6 socket head cap screw goes through this steel block to another piece of steel shaped like a flattened T-nut that rides along the underside of the lathe bed. To lock the carriage, you just tighten the cap screw a bit less than a quarter turn. Once I confirmed that the carriage lock worked well, I glued the top steel block into place with epoxy.