Tapping odd metric threads.
To find the size of the tapping drill for
any ISO metric or BA thread deduct the thread pitch from the diameter of the stud.
ie M6, dia 6x1.00 pitch > drill size is 5mm
M8, dia 8x1.25 pitch > drill size is 6.75mm
M10 fine, dia 10x1.00 pitch > drill size is 9mm
etc. etc.,
Buying very odd taps can be a pain, but I have always found SKF Dormer very helpfull even with left hand threads. send them a letter telling them you are an engineering student on a degree course and you need the tap for a project (lies
).
In theory it is possible to convert a high tensile bolt or stud into a single use tap.
Use an angle grinder with a fine cutting disk or a dremmel (etc) to cut a couple of flutes along the length of the stud,
then use a pedistal grinder to put a slight taper on the start of the stud.
It is difficult but possible, if you get it right it works
for a couple of uses.
As Paul says the welding is not too much of a problem.
Try getting the tap first.
