drill size for 4-40 screws

Paul Baker

I was trying to find the diameter of standard 4-40 machine screws to be used in mounting a PCB, from what I can find I think they are 0.093" or 0.095". Would a 0.1" drill be sufficiently large, or do they not account for the threads when giving the diameter?

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Bean

Paul,
0.100 would work but I usually use 0.125 to allow for some tolerance on the placement of the mating part.
Bean.

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Paul Baker

Thanks Bean.

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StarMan

From Machinery's Handbook, the major diameter of a 4-40 thread is .112.· The recommended clearance drill for a close fit is .116 (#32 drill size), free fit diameter is .1285 (#30 drill size).

Chris I.

LoopyByteloose

Starting out with an undersized bit never hurts.

Usually a smaller bit [noparse][[/noparse]if it isn't so small as to break] will cut faster for a first-time hole.· And it usually allows you more control for accuracy.

After that, you can go up to an appropriate size.

I learned this when I had to drill 5/8" holes in 1/2" steel plate.· If I started with a 1/16" or 1/8" and then did the 5/8" hole, it took a few minutes with a·3/8" chuck electric·drill.

On the other hand, even with a sharp 5/8" bit it seem to take hours without the pilot hole.· And, the drill bit wanted to skip away from the center.

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······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Jeff Dege

For precise placement on a lathe, people start their holes with a center bit.

You can't drill a through-hole with a center bit, but you can start it where you intend it to be.

I've never seen anyone use a centerbit on a drill press, but I expect it would work.

(Personally, I've found that a center punch provides enough of a starting hole to prevent drift.)

Paul Baker

Thanks for the advice on drilling, but this is a board to be fabbed, and hence professionally drilled. I just need to know the size hole that I can be assured of a free fit using a 4-40 screw, it seems I should go with .1285". (If·I were drilling it myself, I would start small and work up until I found a large enough size, but I need to get it right the first time in my gerber).·Thanks for everyone's advice.

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