When is it necessary to incorporate a tapped hole versus a drilled hole?

[EulersFormula]

If I need bolt holes drilled into a component and I have nuts available, is it ever necessary to have the machinist thread the holes if the system will not be exposed to vacuum conditions?

 

[tygerdawg]

Yes.
If your assembly will be disassembled regularly, I like to have threaded holes, not nuts+washers.
If you wish to reduce the part count of your assembly.
If the amount of material is not sufficient to support threads and bolt pull-out strength.
If the base material is soft (like aluminum) and would be subject to eventual thread damage.
I'm sure there are many other reasons.

 

[rorix_bw]

What significance is the vacuum conditions? Are you threading to create a seal, or is there some reason I do not understand, in how a nut and bolt hold, that requires vacuum?

 

[Sultan of Hel]

If parts are cheap and labor is expensive (as is usually the case), or if the force which the fasteners are withstanding is fairly high, I would suggest a bolt/washer/lockwasher/nut assembly. The washer distributes the force over a wider area on the surface (sparing the part) and the lockwasher keeps the nut from unthreading due to vibrations. There are two advantages to this setup:
1. If the thread strips on either the bolt or the nut, you can change those rather than drilling out the threaded hole and retapping it or resorting to expensive helical inserts.
2. You can use stronger fasteners than the base material, assuming relatively small shear forces or enough distance from the bolt to the next component or the edge of the plate (so that the plate can withstand the shear forces).

Where I work, we usually use threaded holes only where there is only enough space on the other side for a low-profile nut (which, with a threaded hole, can act as a locknut), or not enough space for anything at all.