tapping very short blind hole

Start with a bottoming tap, and finish off with a second bottoming tap that you have ground the end off to get full threads all the way. It will need a bit of care to get the first tap started plumb - best done in a tapping stand or drill press without power
Andrew

Good advice, also keep a blast of air over the hole to keep any swarf from getting under the bottom of the tap.

I might go as far as using a spiral flute tap for both operations, but that might be overkill since you're only looking at five turns!
Mark Rand

Before you can use the M5 tap you must make a blind flat bottomed hole either 4.2 mmm diameter for metric course or 4.5mm for fine

So once the "body" of the drill goes down 4mm the "point" will have broken through the remaining 1mm! How have you drilled this hole? did you use a 4.2 or 4.5mm slot drill or centre cutting end mill or did you grind off the point of a standard drill?
Alan
Alan

Thanks all, I decided to "solder" a threaded bit on instead. I am using Sven's aluminium soldering/brazing wire, and so far it works really well.
Sven is a guy who sells wire at model engineering shows - on that subject, anyone else going to Bristol tomorrow?
Sven has a showman's act I suppose you'd call it, soldering bits of ali sheet, making holes in them then filling them again, selling by the "Sven meter" which is two outstretched arms worth.
I don't know the name of the product he sells. but it is light years better than the usual aluminium brazes, ie it actually works!
I have had it for ages, bought 2m and gave one as a christmas present, but this is the first time I have used it, wire brushed and "tinned" the surfaces before joining - well, as I said,it actually works, at least so far.
I's quite a bit more expensive - like 5x the price - than the usual aluminium brazing type rods, but the difference in performance is - wow, best thing since sliced bread!
Of course it will all probably fall apart tomorrow .. but for today, yippee!
-- Peter Fairbrother

Our showmen solder the sides of aluminium drink cans.