One used to have a laminated booklet called the Zeus Table in the workshop with the clearance and cutting sizes listed, fortunately the internet has everything now:
table.
I see that 1.55 is a bit inconvenient, but it should take an M1.8 thread.
Tap (Thread) Size Drill Size(mm)
M1 x 0.25 0.75
M1.1 x 0.25 0.85
M1.2 x 0.25 0.95
M1.4 x 0.3 1.10
M1.6 x 0.35 1.25
M1.8 x 0.35 1.45
M2 x 0.4 1.60
M2.2 x 0.45 1.75
The "x 0.35" means that the distance between the threads is 0.35mm. You need to make sure that your tap and die (for the screw) have the same pitch - not hard for this one as I think there's only one standard coarse pitch at this size.
As James says, buying just what you need is best, especially as small taps and dies are very expensive. A cheapo set from Machine Mart (
040219154) will set you back £38 - this does have an M1.8 as it happens.
Now the really difficult thing is using them!
Taps come in pairs and you should use the pointier one first.
As James says, they need lubricating. I find oil best as waxes tend to glue the swarf into the threads. Rotate the tap or die slowly up to a quarter turn then rotate it back that same distance. This cuts the swarf into the grooves and helps spread the lubricant onto the next piece. Make up the quarter turn you lost and progress a further quarter turn before backing it off... and so on. This is slow and tedious but always works.
Sometimes, with the right clearances and lubricant you can skip all the reverses... but then one day the tap breaks off and you glumly promise yourself you won't do that again as you try to get the broken bits out...
After using the pointy one, remove it and run the less pointy one through - you won't need to do the backing off this time with such small sizes. This sets the size correctly where the pointy one is sized for making the first cut.
Threading the screw piece is basically just the same. You don't often get adjustable dies at this size so that's less to worry about. If you have two dies supplied as a pair, use the one with the bigger hole or indentation first... but I think it's pretty unusual to make pairs at this size. Again progress slowly and reverse at least 1/4 every turn.
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