Some points on metal tubemod venting:
1. It's likely that only small vents are needed for single-battery
devices, these may suffer meltdowns but as long as there is minimal venting then a catastrophic event is unlikely.
2. If two cells are used in series ('stacked') then things are different, and that situation is too complex to examine here.
3. Any vents need to be at the top of the tube as this is where any problem is likely to be.
batteries in meltdown can swell up and block the tube, and the escaping gas will vent from the top of the failing cell, not the bottom. The trouble with bottom-end vents is that they may look useful, but in some circumstances they may not work. Basically they are an easy cop-out, and the job is not done properly.
4. Two to four drill holes of about 3mm near the top of the tube should do the job. If you drill 4 holes, don't make the mistake of drilling 'through and through', in other words drilling twice to get 4 holes, as the second hole won't be accurately placed. Three holes is a good solution, it gives enough vent capacity and looks right.
5. As the holes are near the top of the tube you can get to the inside burr easily and remove it. You must do this. It's probably best not to drill any further down than about 15mm from the top of the tube (i.e. the top of the lower half of the tube, assuming it has a threaded connection).
6. There are all sorts of ways to drill holes into a tubular workpiece, it is probably better to google the ways of doing this than to go into it here. The issues are different from drilling a flat workpiece, so don't just go ahead without considering how you will mark the drill points accurately and how you will prevent the drill bit skating off.
7. Make sure to carefully deburr the inside and outside of the holes.
