Hi Netanel,
If at all possible, i would clamp the rail to the edge of the table in the position you want it and use the exact size drill bit that matches the hole size in the rail to mark the holes. From there, get rid of the rail and clamps and as others have said, start with a pilot hole. Something like a 3/16" bit would be a good size to start with since it won't flex too much. From there, i'd just go straight to a 9mm if that's the size you need. My bestest buddy growing up was a pattern maker, and he always said to machine cast iron dry. He said the cast iron is self lubricating (or something to that affect. It was after all probably at least 40 years ago when he told me that. :)) I took his advice, and unless i'm trying to tap a hole in an akward spot, never use a cutting fluid.?
Take your time with setting up an marking the holes, as that's where you need to be exact, the actual drilling and tapping will be the easy part. :)
Regards,
Bryce
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Bryce Comer