# Drill bit size for 10-24 & 10-32 screw



## bill39 (Sep 4, 2009)

Rather than contemplating the dust in my belly-button today I had the need to look at a tap & drill chart today.
Can some explain why a #25 bit is recommended for a 10-24 & a #21 bit is recommended for a 10-32? Isn't the bolt diameter the same?

Note: the same chart lists fractional bit sizes and suggests using a 5/32 bit for both.

Looking forward to your answers as I need to get back to my belly-button soon.


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## splatz (May 23, 2015)

bill39 said:


> Can some explain why a #25 bit is recommended for a 10-24 & a #21 bit is recommended for a 10-32? Isn't the bolt diameter the same?


They both have the same major diameter (outside diameter / clearance hole) but slightly different size for hole to tap with different threads. 

https://sizes.com/tools/thread_ncnf.htm


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## MechanicalDVR (Dec 29, 2007)

The finer thread removes less material some a slightly larger hole makes the tapping go a tad easier.

Also you will find it comes down to percentage of thread, those sizes are for 75% thread depth. 

The dies machinists use are adjustable for percentage of thread with taps the pilot hole determines this percentage.


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## FF301 (Jan 12, 2014)

Different decimal sizes.


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## bill39 (Sep 4, 2009)

Great info. Thanks guys.


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## telsa (May 22, 2015)

Get the Greenlee/Harbor Freight tap-drill set and STOP over thinking this stuff.


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## bill39 (Sep 4, 2009)

telsa said:


> Get the Greenlee/Harbor Freight tap-drill set and STOP over thinking this stuff.


Great advice Telsa. All of this technical stuff was making my head hurt.


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## telsa (May 22, 2015)

Trust me, you'll never find the magic drill to mate with your tap -- when the pressure is on.

I ought to know, I've got plenty laying around... somewhere.


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## macmikeman (Jan 23, 2007)

*These answers were all wrong.* The proper size bit to use is a button tek screw. For a pilot hole. Look down there in your pouch, there is thirty of them looking at you in the face... Then load a 10-32 tap into your cordless drill or use a Greenlee one that works for your impact drill. Macmikey is always faster..........


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## JoeSparky (Mar 25, 2010)

Close, but you're wrong too. Fastest way is 5/16 driver in your impact. Shoot a #10 tek screw in and back it out. Then shoot in a hardened thread cutting ground screw. 
While you are still chucking up the tap, I'm using my banking app to deposit the check for completing the job:thumbsup:


macmikeman said:


> *These answers were all wrong.* The proper size bit to use is a button tek screw. For a pilot hole. Look down there in your pouch, there is thirty of them looking at you in the face... Then load a 10-32 tap into your cordless drill or use a Greenlee one that works for your impact drill. Macmikey is always faster..........


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## MechanicalDVR (Dec 29, 2007)

JoeSparky said:


> Close, but you're wrong too. Fastest way is 5/16 driver in your impact. Shoot a #10 tek screw in and back it out. Then shoot in a hardened thread cutting ground screw.
> While you are still chucking up the tap, I'm using my banking app to deposit the check for completing the job:thumbsup:


However at the same time this and several other posts don't even come close to answering the OP's question.


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## John Valdes (May 17, 2007)

I hate when they give you a drill number instead of a actual drill bit size in fraction. I mean who really carries a whole set of indexed drill bits around?
I'm not a machinist. I'm an electrician.

I used to remember the common sizes for taps without looking it up.
Today I have to look it up unless its a tapcon blue screw.


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## MechanicalDVR (Dec 29, 2007)

John Valdes said:


> I hate when they give you a drill number instead of a actual drill bit size in fraction. I mean who really carries a whole set of indexed drill bits around?
> I'm not a machinist. I'm an electrician.
> 
> I used to remember the common sizes for taps without looking it up.
> Today I have to look it up unless its a tapcon blue screw.


I always recall the common ones:

#36--6-32

#29--8-32

#25--10-24

#16--12-24

#7--1/4-20

I've switched to the all in on drill taps a while back.


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## splatz (May 23, 2015)

telsa said:


> Trust me, you'll never find the magic drill to mate with your tap -- when the pressure is on.
> 
> I ought to know, I've got plenty laying around... somewhere.


When I buy the drill and tap, I always buy at least two of each, which is an example of using Murphy's law in reverse. The Irwins at Ace or whatever are cheap. 

When you're done, masking tape them together so the sharp parts are under the tape but not touching. Write the size on the tape. Store them in a toolbox drawer with the tap wrenches. 

Maybe 1/3 of the time you'll remember to bring them to the job when you need them. Other times, it's another trip to Ace.


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## macmikeman (Jan 23, 2007)

splatz said:


> When I buy the drill and tap, I always buy at least two of each, which is an example of using Murphy's law in reverse. The Irwins at Ace or whatever are cheap.
> 
> When you're done, masking tape them together so the sharp parts are under the tape but not touching. Write the size on the tape. Store them in a toolbox drawer with the tap wrenches.
> 
> Maybe 1/3 of the time you'll remember to bring them to the job when you need them. Other times, it's another trip to Ace.



I bought a twenty five year supply of straight 10-32 taps. (around 200 or so I think it was) and still have around thirty of them . I keep one in my apron and the rest in the drawer's that come with American Van Shelving kits on my Express vans. And tek screws in apron also. Always.


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## John Valdes (May 17, 2007)

MechanicalDVR said:


> I always recall the common ones:
> 
> #36--6-32
> 
> ...


How about posting the actual fractional size of the drill instead of the number? This is what I hate. Using a number instead of the fraction.
Do they even put the number on the drill?
Don't you need a drill index to know what number it is?
And who can read the drill after a few uses anyway.



splatz said:


> When you're done, masking tape them together so the sharp parts are under the tape but not touching. Write the size on the tape. Store them in a toolbox drawer with the tap wrenches.


That's how I have been doing it for years. Drill and tap taped together.

I was taught that drill bits should be called drills and the actual tool that drives them the drill motor.


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## MechanicalDVR (Dec 29, 2007)

John Valdes said:


> How about posting the actual fractional size of the drill instead of the number? This is what I hate. Using a number instead of the fraction.
> Do they even put the number on the drill?
> Don't you need a drill index to know what number it is?
> And who can read the drill after a few uses anyway.


I don't use the fractional size, never have.

If they put the number on the bit it would be too small to read anyway.

I don't keep them in a drill index.

When I buy a tap I buy several of the corresponding bits with them and put them in a prescription bottle with the tap size written on the top. 

With my eye sight at this stage of the game I'd need a magnifying glass to read anything on a drill bit smaller than maybe 3/8".


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## telsa (May 22, 2015)

John Valdes said:


> How about posting the actual fractional size of the drill instead of the number? This is what I hate. Using a number instead of the fraction.
> Do they even put the number on the drill?
> Don't you need a drill index to know what number it is?
> And who can read the drill after a few uses anyway.
> ...



Machinists went over to numbers generations ago.

The difference in sizes requires a micrometer to spot.

BTW, the differing numbers do not increase in a systemic way. 

They are MATED to the taps that are mated to the fasteners. 

Everything was backed into this way.

For the longest time, these sizes were trade secrets.

This or that engineer would figure them out by trial and error -- and would stay quiet.

It took the pressure for interchangeable parts (military spending) to force the players to open up and standardize.

It, this transition in attitude, was a revolution in the 19th Century.


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## John Valdes (May 17, 2007)

telsa said:


> Machinists went over to numbers generations ago.
> 
> The difference in sizes requires a micrometer to spot.
> 
> ...


I am at this moment looking at a drill gauge.
Each hole is marked with with a fraction and a millimeter size. No number.

I do agree a number would be much easier to remember, But I was taught to use the fraction which is hard to remember if you are not using them regularly.
Kinda glad this came up. I found the drill gauge in my desk drawer.

What about the term drill bit vs drill?


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## MechanicalDVR (Dec 29, 2007)

John Valdes said:


> I am at this moment looking at a drill gauge.
> Each hole is marked with with a fraction and a millimeter size. No number.
> 
> I do agree a number would be much easier to remember, But I was taught to use the fraction which is hard to remember if you are not using them regularly.
> ...


Coming up in the ranks of family businesses (plumbing, mechanical, electrical, carpentry) and drill bit referred to an auger for wood and drill was a twist drill for metal. 

The driving force was a brace, egg beater for small 'drills', or a drill motor.

My Uncle Frank even had a footlocker of old breast drills in the shop from days gone by.


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## bill39 (Sep 4, 2009)

Wow, 19 responses and counting to this simple question. I just love our trade.

On a serious note, there's a time and place for both Tek screws and a machine screw.

Also, my handy-dandy Craftsman tap & drill set lists the correct drill size by number not fractional size.

Thanks again.


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## MechanicalDVR (Dec 29, 2007)

bill39 said:


> Wow, 19 responses and counting to this simple question. I just love our trade.
> 
> On a serious note, there's a time and place for both Tek screws and a machine screw.
> 
> ...


How many responses are actually answers to the original query?


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## bill39 (Sep 4, 2009)

MechanicalDVR said:


> Coming up in the ranks of family businesses (plumbing, mechanical, electrical, carpentry) and drill bit referred to an auger for wood and drill was a twist drill for metal.
> 
> The driving force was a brace, egg beater for small 'drills', or a drill motor.
> 
> My Uncle Frank even had a footlocker of old breast drills in the shop from days gone by.


As apprentice innocently said 'hand me the drill' once and was instructed in no uncertain terms (bullied in today's parlance) that there were drill bits, drill motors, drill presses, and auger bits. Just saying drill was a no, no.


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## bill39 (Sep 4, 2009)

MechanicalDVR said:


> How many responses are actually answers to the original query?


Probably only 2 or 3. I just enjoyed the back and forth on the rest.


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## Jhellwig (Jun 18, 2014)

John Valdes said:


> How about posting the actual fractional size of the drill instead of the number? This is what I hate. Using a number instead of the fraction.
> Do they even put the number on the drill?
> Don't you need a drill index to know what number it is?
> And who can read the drill after a few uses anyway.
> ...


There aren't many number or letter sizes that cross to an actual fractional size drill bit. The numbered ones are the ones in between the fractional sizes of standard drill bits. I usually end up going to the next fractional size up from the numbered one. The number system is a pain and sometimes hard to find the bits.


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## MechanicalDVR (Dec 29, 2007)

Jhellwig said:


> There aren't many number or letter sizes that cross to an actual fractional size drill bit. The numbered ones are the ones in between the fractional sizes of standard drill bits. I usually end up going to the next fractional size up from the numbered one. *The number system is a pain and sometimes hard to find the bits.*


1. McMaster-Carr

2. Grainger

Always bought bags of 10 in tap sizes.


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## splatz (May 23, 2015)

This chart shows you where the nearest fractional fall...

http://www.lincolnmachine.com/tap_drill_chart.html


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## MechanicalDVR (Dec 29, 2007)

Any of you guys buy from Sexauer?

They are like the Snap-on guys of the repair parts set!


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## scotch (Oct 17, 2013)

MechanicalDVR said:


> Any of you guys buy from Sexauer?
> 
> They are like the Snap-on guys of the repair parts set!


Even up here in Canada ; used to buy from Sexauer...."better than a coffee break ! "....but the local rep go smacked with dump truck and it all fell apart .


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## MechanicalDVR (Dec 29, 2007)

scotch said:


> Even up here in Canada ; used to buy from Sexauer...."better than a coffee break ! "....but the local rep go smacked with dump truck and it all fell apart .



:thumbsup:

They used to come to two of the family businesses and I bought from them for years.


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