One thing I forgot to add - if you must use the 4-40 screw size for some
reason, you can save a lot of grief by using a slightly larger bit than
the standard tap drill. You're not really too concerned with optimal fit
and strength here - it's more about being able to make a whole lot of
usable screw holes without losing too many bits and taps, or reworking.
You can experiment and see what size makes the best tradeoffs.
It's amazing sometimes how a little more root clearance can makes things
so much nicer - a shallower cut, so less torque on that tiny tap, more
room for chip clearing, and better lubing. You can also get deeper
threads to compensate for less meat in the thread root. Even the drill
bit will have a better chance of survival, since it will be slightly
bigger and stronger.
Of course, if you go too far, there won't be enough thread - but then
you'll have reason to jump to the next size up.
Ed
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