# Drill size for a 1/2-28 hole?



## StrikerDown

I need to tap a 1/2"-28 TPI hole but I cant find the propper dril size on my chart. 

Does anyone know what the correct drill size is for this for this special thread (extra fine)?


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## Rothrandir

According to the drill/tap chart here, the correct drill size is 15/32.


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## NightTime

To determine tap drill size for special threads, use the following formula:

TAP DRILL SIZE = DIA - (.974 / PITCH)

EXAMPLE: 1/2-28
.500 - (.974 / 28) = .465
USE DRILL SIZE CLOSEST TO .465 --> 15/32

Ref: http://microadvances.com/drillchart.htm

Cheers!


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## StrikerDown

Wow! Thanks guys that was fast!


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## BoarHunter

Simpler yet, go metric !

M12 is 1.75 pitch so hole has to be 10.3mm and as any good shop has drill bit from 1 to 13 in 0.1 increment, no playing around with archaic letter, fractional, AWG bits ! Better yet throw them away so ou don't do any mistake ! 

Need a fine thread ? M12X1 common on bike by the way and cheap taps and dies available.


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## Anglepoise

BoarHunter said:


> Simpler yet, go metric !
> 
> M12 is 1.75 pitch so hole has to be 10.3mm and as any good shop has drill bit from 1 to 13 in 0.1 increment, no playing around with archaic letter, fractional, AWG bits ! Better yet throw them away so ou don't do any mistake !
> 
> Need a fine thread ? M12X1 common on bike by the way and cheap taps and dies available.



Welcome aboard BoarHunter.

I do not quite understand your post as it pertains to the original question, Quote: "I need to tap a 1/2"-28 TPI hole but I cant find the propper dril size on my chart" 
Like all the answers above, I would also use and recommend a 15/32 drill.

Nothing wrong with an M12 thread or number drills, but I don't think your answer will help the question from "StrikerDown" as 10.3mm is way too small a hole for a 1/2"-28 tap.


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## StrikerDown

Thanks again. 

According to the formula and the drill chart info above the 15/32 (one that I have) will work fine. 

Question: The drill chart also lists an 11.8mm (.4646) as being even closer to the formula answer, would this be a better choice for a tighter fit in alum or steel? 

Or would the extra effort in hand cutting the threads not be worth it?

I'm not sure if I am using steel or alum for this project, most likely I will use alum at least for a prototype.


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## Anglepoise

Many of the charts give you a range of drill sizes that will work and this range translates to the 'class of fit'. As most things in life, it's a compromise.

Only you can decide based on many factors. Strength required, effort to turn the tap etc. 

0.004" difference between 15/32" and 11.8mm drill is small in a 1/2" tap and 
in most cases would be decided as to the drill available to the hobbyist.

I would guess few of us would have both these sizes on hand.


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## precisionworks

> Many of the charts give you a range of drill sizes that will work and this range translates to the 'class of fit'.


Exactly.

Whenever a perfect thread is needed, forming taps are generally a better choice than cutting taps. Forming taps work by displacing, not removing, the material being tapped. Some people use them only in softer materials, but they work perfectly in hard metals *IF *you are able to provide the correct hole size. This often means buying a letter drill or a number drill, or using the surface grinder to grind a drill just for this purpose.

Everybody makes forming taps, and there are lots for sale on eBay at a fraction of new cost. My favorite is the Balax Thredfloer, which makes the most beautiful threads you can imagine, even under 10x magnification. Click on the link (only 1.25 Mb), and look on page 54 to see the hole sizes for 75% thread, 65% thread, and 55% thread. I normally use the 75% number.

http://www.balax.com/catalog.pdf


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## BoarHunter

Anglepoise said:


> Nothing wrong with an M12 thread or number drills, but I don't think your answer will help the question from "StrikerDown" as 10.3mm is way too small a hole for a 1/2"-28 tap.


 
Just that considering the forum and his request for an extra fine thread, I assume StrikerDown wants to thread tubes for a flashlight body. Therefore I suggested to make his life easier by sticking to standard diameters (and if possible also used in bikes and cars), pitches and tools, and at the same time get rid of all these funny charts using the simple formula; diam - pitch = pre drill hole; which is enough for this kind of job. 

For ex. M12 x1.25, M16x1.5 are very common fine thread in automobile and are suitable for tubing to house AAA and AA batteries.

Who in the world would design using size like 1/2-28 and fumble through charts and conversion factors ?


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## StrikerDown

Barry,

Forming taps sound like the perfect solution for what I want to do, but I can't find a 1/2-28.

It is an uncommon tap although 1/2-28 is very commonly used in what I am experimenting with.

Great info, Thank You, Gentlemen!


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## StrikerDown

BoarHunter said:


> Who in the world would design using size like 1/2-28 and fumble through charts and conversion factors ?


 
I am experimenting with different muzzle brake configurations and the end of the barrel I'm working with came threaded in that size. This is the standard for this caliber.

I could cut off and re-thread the barrel to a more commonly used thread but that brings with it a host of other unnecessary problems.


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## gadget_lover

BoarHunter said:


> For ex. M12 x1.25, M16x1.5 are very common fine thread in automobile and are suitable for tubing to house AAA and AA batteries.
> 
> Who in the world would design using size like 1/2-28 and fumble through charts and conversion factors ?



I'm not sure what part of the world BoarHunter is from, but he seems somewhat biased towards metric for some reason.

The beauty of threads is that you can use many different sizes and configurations, depending on your need.

In the OP's case, he's matching an existing thread. That's a very good reason to "fumble through charts".

On the other hand, your suggestion of M16x1.5 requires a thicker tube than would be needed for a 28 tpi thread. The 1.5 mm pitch uses a deeper thread.

Since I live and work in a world that is part metric and part imperial, I have tools to do both. Since I'm not going to rely on my poor old memory for that formula, I'll be using a chart no matter what thread I happen to use. That makes it a wash. Either way I look things up.

In the US one seldom sees a complete set of metric drills, but a full complement of reasonable imperial sizes is available at a moderate price. 

It's funny, but the chart that came with my metric tap set includes a cross reference of what imperial drill to use.

Daniel


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## BoarHunter

StrikerDown said:


> I am experimenting with different muzzle brake configurations and the end of the barrel I'm working with came threaded in that size. This is the standard for this caliber.
> 
> I could cut off and re-thread the barrel to a more commonly used thread but that brings with it a host of other unnecessary problems.



OK, I see, for caliber 5.56 , and for historic reasons, US barrels are indeed threaded 1/2X28 (or 1/2X20) for muzzle brake or silencer.

Note HK prefers M14X1. 

Not really related to flashlight.

BTW, the real issue is not the threading but perfect alignement of the barrel axis and the device you screw on it and you can't rely on a thread for alignement !

Anyway this is a flashlight forum so ...


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## precisionworks

Balax doesn't show 1/2-28 in a former:shakehead

EBay has a cut thread tap for under $20:

http://cgi.ebay.com/1-2-28-GH3-4-FL...QrdZ1QQcmdZViewItemQQ_trksidZp1713.m153.l1262


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## StrikerDown

Thanks Barry, I looked around several places but can't find anything in the way of special thread forming taps. I have a couple 1/2-28 cutting taps, but the forming tap, that I had not heard of, looks like the ideal solution, or should I say that anything that cuts better threads would be... well... better!

I guess it would depend on the availability of an internal threading bar small enough, but can a hole that size be threaded on a lathe? If so I might give that a try when my lathe comes in.


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## precisionworks

> can a hole that size be threaded on a lathe?


It can, but you'll need either a solid carbide bar with ground tip, or a solid carbide bar with an indexable threading insert. Circle Machine makes both - their indexable insert bar, .187" diameter, will work inside a bore of only .272".

http://metalworking.mscdirect.com/CGI/NNSRIT?PMPXNO=1965873

http://www.kennametal.com/images/pd...f;jsessionid=FBNLVAOWVACEPLAUCYPCFEVMCQFBYIV0


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## brickbat

precisionworks said:


> It can, but you'll need either a solid carbide bar with ground tip, or a solid carbide bar with an indexable threading insert.



..or, since this is likely a one-off project, a $1.00 1/4 inch HSS tool bit and 10 minutes on a pedestal grinder to make a small inside threading tool. Works for me...

http://www.wttool.com/category-exec/category_id/15882


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## precisionworks

> forming tap ... looks like the ideal solution


They're pretty cool. No chips, so blind hole threading is easy. Because the threads are formed by metal displacement, the resulting surface is burnished - smooth as glass. Good info here:

http://www.precisiontwistdrill.com/techhelp/pdf/thread_forming.pdf


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## StrikerDown

precisionworks said:


> It can, but you'll need either a solid carbide bar with ground tip, or a solid carbide bar with an indexable threading insert. Circle Machine makes both - their indexable insert bar, .187" diameter, will work inside a bore of only .272".
> 
> http://metalworking.mscdirect.com/CGI/NNSRIT?PMPXNO=1965873
> 
> http://www.kennametal.com/images/pd...f;jsessionid=FBNLVAOWVACEPLAUCYPCFEVMCQFBYIV0


 
Looks great, something else to add to the eBay shopping list.



precisionworks said:


> They're pretty cool. No chips, so blind hole threading is easy. Because the threads are formed by metal displacement, the resulting surface is burnished - smooth as glass. Good info here:
> 
> http://www.precisiontwistdrill.com/techhelp/pdf/thread_forming.pdf


 
Wow, as far as taps go those walk on water! I have broken too many taps over the years mostly from chip bind and impatience! These are some cool tools!

I can't tell you how much I appreciate all the info you pour into this forum!:thumbsup:


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## precisionworks

> I have broken too many taps over the years mostly from chip bind and impatience!


Forming taps are stronger than cutting taps because of their solid cylindrical shape - the flutes on a cutting tap are the source of flex & breakage. BUT, forming taps require hole sizing to be dead on, which means a correctly sharpened twist drill. Also, because they are compressing & displacing metal, their torque will be anywhere from 50% to 100% more than the same size cutting tap. The increased torque surprises everyone the first time they drive one by hand.

Mine almost always are driven by the mill, in a tapping head, which lets the tap stop at hole bottom while the spindle continues to turn. Two other brands, besides Tapmatic & Ettco, are Procunier & Yuasa (Japanese).


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## NightTime

Forming taps also make stronger threads because the material is compressed.


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