the reason why I don't think 12mm od won't be thick enough is that these are for rear carriers/grab rails.
with all the weight going up and down will no doubt put a lot of stress on the thin wall(2mm) which I think will eventually give way.
also with the 19.5mm tubing having 1.5/1.7mm wall the weld will need a wider area which is why 16mm OD will be more suitable.
must admit I'm not all that clued up on stress's etc its just my guesstimating thoughts if you know what I mean arry.
John if you are only holding the piece between the small contact areas of a drill vice then there will be a tendency for the job to tilt. You need to make up a small V block that can be put between the job and the moving jaw, assuming that the end of the work piece has been machined square !
Another way IF you drill table will rotate would be to clamp vice to table and rotate table 90 degrees then you will be holding the work piece along it's length.
I've thought about that.upon checking my little vice the that I have cut out isn't exactly square.
so more than likely that wont help either.
I have a friend who has a business who has a couple of lathes as well as vertical machines.
I'll ask him if he'll put a more squarer v in or better still 'Vee block.