I always use impoerial drill bits with the exception of the One diameter pen (10mm), cigar (10mm), sculpted pen (8mm), slimlines (7mm) (there might be others, this isnt comprehensive!) All others are designed around imperial dimensions, the engineering world still manufacture to these dims, especially in the states and this element of the world is still in the imperial past. In softer woods you can get away with a larger drill bit but the denser woods such as rosewoods, ebonies and the like, the crrect drill bit is the best. If you prefer to use PU glue (which I hate because of its tendency to shift the tubes whilst curing and the fact that it is the messiest adhesive around), then you again can get away with it as PU expands. You will get a much poorer adhesion with CA despite the fact that it is only a small increase. One small air pocket is enough to allow the veneer to fracture whilat turning.
The JR Gent tube dims accept a 15/32 drill bit and a 25/64.
There are other suppliers already posted and more not, I use www.ukdrills.co.uk and find the cost good and the service excellent.
There are many drill bit conversion charts published on the net, this is one of the better ones Drill Size Conversion Table.
