As part of the day job, tomorrow I have to drill about 100 holes for
12mm bolts through 7" thickness of hardwood, with counterbores at each end.Wood bits until fairly recently came in imperial sizes, in fact some still do, but my local supplier's stocks of auger type are now metric. In the past I would have used a 1/2" bit for a 12mm bolt, holes in timber aren't usually clean enough for 0.5mm to really matter too much. Guess what size their metric * 1/2" * bits are now? Yes, they are THIRTEEN mm! A full, genuine 13mm, not 1/2" relabelled. That means a 12mm bolt will flop about and not do the job as intended
- plus it will probably fall out (they have to go in from underneath) before I can get the nut on. I can probably do something about it, maybe grinding the OD down a bit but that's going to be a real pain over 8" length.
12mm bits do exist, I could probably get one to order & I've got one as part of a Chinese set, but (a) it's not long enough, and (b) I was using the 16mm bit out of the same set, in the same timber, it was cutting beautifully until the screw tip pulled itself off the end of the bit, & left me with a hardened piece of steel at the bottom of a deep, unfinished, blind hole. I have no wish to repeat that!Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service