# Which Drill bit



## jamiedolan (Sep 2, 2008)

HI
Can anyone tell me what the proper drill bit size would be to use for a 10-32 machine screw - drilling into metal? (I assume I would use a regular metal bit there are no special bits that kind of thread for a machine screw are there?)

Thanks
Jamie


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## Sammy (Mar 11, 2007)

5/32" bit but you will need to tap the hole first unless you have a cutting screw.


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## buletbob (May 9, 2008)

19/32" should be used for the 10-32 tap die size hole


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## Sammy (Mar 11, 2007)

BB..... 19/32" drill bit for a 10/32" tap?? 


Wont have any threads on it but you'll have enough room to climb thru the hole and put a nut on the back side....


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## jamiedolan (Sep 2, 2008)

Sammy said:


> BB..... 19/32" drill bit for a 10/32" tap??
> 
> 
> Wont have any threads on it but you'll have enough room to climb thru the hole and put a nut on the back side....


Sorry, I should have said what these are for, they are just small grounding screws, about 3/8 or 1/2" long. No nut is used. Most boxes only have one hole to put the ground screw in, sometimes it is hard to get to, sometimes I want to put in a second screw. The other holes in the box are just a little small and I wanted to be able to drill them out if necessary, maybe I just need a 10-32 tap bit?
thank you.
Jamie


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## Sammy (Mar 11, 2007)

If there is an exisiting hole thats undersized, then yes, you can overbore it with a 5/32" drill bit and tap it. Or pick up a few self tappers at the hardware store. 

[I knew you werent using nuts.]


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## buletbob (May 9, 2008)

Sammy said:


> BB..... 19/32" drill bit for a 10/32" tap??
> 
> 
> Wont have any threads on it but you'll have enough room to climb thru the hole and put a nut on the back side....


Now you got me thinking!!! I Googled it and that was what came up in a tap chart. Sorry for the miss information, And Thanks Sammy for the correction. BOB 
I will go back and see what the HE// I saw.


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## ponch37300 (Nov 27, 2007)

a 10-32 screw isn't 10/32" in diameter. The 10 is the size of the screw and the 32 is the number of threads per inch. The correct drill bit is a number 21 or 5/32". They do make these which are a drill bit and tap combo http://www.grainger.com/Grainger/items/4GB12


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## Nestor_Kelebay (Jun 17, 2008)

Jamie: The advice to use a 5/32 inch drill bit was right on. This will give you an actual (theoretical) diameter of 0.15625 inches, which is gosh darn close to the recommended drill bit size of 0.1570 inches. The tap will just remove a tiny bit more metal to enlarge the hold a hair, that's all.

No need to read the rest.

My Cleveland Twist Drill Canada chart gives a tap hole size for a #10 X 32 tpi screw as a #22 drill bit, which will have an actual diameter of 0.1570 inches. When you drill a hole, the drill bit itself will bend a bit with the compressive force put on it, and that means it will drill a slightly larger hole than it's actual OD. In this case a #22 drill bit has an actual diameter of 0.1570 inches, but the hole it drills will typically be 0.0032 inches oversize, giving you a 0.1602 inch hole size. With that ACTUAL size hole, then 73 percent of the thread depth will be grabbing metal; the rest will be grabbing air.

You CAN use a #21 drill bit, that has an actual diameter of 0.1590 inches, and with the same amount of overbore, the hole you get is 0.1622 inches in diameter, and the larger hole will mean that only 68 percent of the thread depth will be grabbing, which isn't really a concern for something like a grounding screw. It doesn't need to be torqued down hard.

http://neme-s.org/Shaper%20Books/Odds%20and%20ends/Cleveland%20Twist%20Drill%20Ready%20Reference.pdf

The tap drill size info for a 10X32 screw is given near the top of the right side of page 7 of the above PDF file. You can read the chart better if you use the enlarge feature to view the PDF at 200% of actual size.


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## iMisspell (Jun 2, 2007)

32 pitch is some-what a fine thread, specially for a small diamitor hole, you would have to be careful (even if you have tapped small fine threads by hand before) or your gonna snap the tap (even though the box is soft metal, its easy to twist or get cock-cock-eyed). For a grounding screw i would go with a self-taper like suggested above, it will be much better, less of a chance to break the tap or strip out the hole and will be less time consuming. In the shop (i work in a machine shop) tapping 10-24's in a blink-hole can be a pain depending on the metal, 32 pitch is worse. For a grounding screw, i would not think twice, self-tapping screws :yes:

_


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## Nestor_Kelebay (Jun 17, 2008)

I'd just put a #10X32 tap in a cordless drill and it'll tap the hole lickety split. Then just reverse out.


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