Confused about screw & drill bits

[Era$erheads]

Hey folks, quick question. I've been staring at a tap and drill chart and got kinda confused.

Why does it say to use a #25 drill bit for a 10-24 screw but a #21 bit for a 10-32? The bolt diameter's the same, right? How come the drill sizes are different?

Would love some insight so I can get back to my project.

 

[10thingsihateaboutyou]

Hey folks, quick question. I've been staring at a tap and drill chart and got kinda confused.

Why does it say to use a #25 drill bit for a 10-24 screw but a #21 bit for a 10-32? The bolt diameter's the same, right? How come the drill sizes are different?

Would love some insight so I can get back to my project.

Good eye you got there. Even tho the bolt size's the same, the threads per inch are different. 10-24's got fewer, thicker threads so it needs a smaller hole (#25). The 10-32's finer threads mean a bigger hole (#21) to let it cut in clean. Take note of it so you won't forget. Or double check because I might have interchanged it AHAHAHA that's how confusing it is.

 

[Castello_V]

Ideally when you're tapping for 10-32, less material needs to be left in the hole to form those shallower threads, hence the larger #21 drill bit but the 10-24 needs deeper threads cut, so you use the slightly smaller #25 to leave more material.

 

[PGB1]

If you search tap & drill charts, you'll find different drill sizes based on desired thread engagement. The charts have columns for each. Even NF, NC and NEF (in the US) have different engagement percentages.

Typical, but not always, 75% is for soft stuff like aluminum and brass. Fifty percent is for steel, cast iron and stainless. Sixty or Sixty-five percent is the "compromise" size. Note that these are considered General Purpose specification. Certain projects require different percent of engagement, as called out on the prints.