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 Post subject: What size drill bit for 1/2-28 tap?
PostPosted: Tue Nov 18, 2008 9:54 pm 
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Silent But Deadly

Joined: Wed Mar 19, 2008 10:37 pm
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Location: Berea, Ky
Been trying different drill sizes and cant seem to find the right one for 1/2"-28 tap. Thanks


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 Post subject:
PostPosted: Tue Nov 18, 2008 10:11 pm 
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29/64"


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 Post subject:
PostPosted: Tue Nov 18, 2008 10:16 pm 
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Silent But Deadly

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Location: Berea, Ky
Tried that one but doesnt give me full thread engagement. Seems like its too big. Tried 7/16 also and that seems too small. Maybe need to try different drill bit. Any other sizes that works


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PostPosted: Tue Nov 18, 2008 10:47 pm 
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Silent But Deadly
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Location: Crackerville, Northern Nevada
http://reference.kennametal.com/Calcula ... lSize.aspx

Above link includes bore size calculator for any threaded fastener. Class of fit etc.

Hint: Do not exceed 80% thread in non precision thread generation. A better fit for your gun barrel application would be 70%. :)

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 Post subject:
PostPosted: Tue Nov 18, 2008 11:02 pm 
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Silent But Deadly

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Location: Berea, Ky
featherblue wrote:
http://reference.kennametal.com/Calculators/Dual/TapDrillSize.aspx

Above link includes bore size calculator for any threaded fastener. Class of fit etc.

Hint: Do not exceed 80% thread in non precision thread generation. A better fit for your gun barrel application would be 70%. :)


Thanks, so the major dia would be .500 TPI=28 thread height 70% gives me 29/64? Am I doing this right? thanks


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 Post subject:
PostPosted: Tue Nov 18, 2008 11:02 pm 
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Well, that is what we've used for years with no problems.


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 Post subject:
PostPosted: Tue Nov 18, 2008 11:07 pm 
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Silent But Deadly

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Location: Berea, Ky
Loves2Shoot wrote:
Well, that is what we've used for years with no problems.


Ok thanks, might be my drill bit cutting too much material.


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 Post subject:
PostPosted: Tue Nov 18, 2008 11:10 pm 
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Silent But Deadly

Joined: Fri Aug 08, 2008 6:38 pm
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Location: Massachusetts
Always drill a pilot hole slightly undersize first...step drill it up to the tap dril size. Surf internet for "Tap Drill Chart"...that should help.


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PostPosted: Wed Nov 19, 2008 1:03 am 
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Silent But Deadly

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Location: South Sioux City, NE
I always use 15/32 and have never noticed a problem.

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 Post subject:
PostPosted: Thu Nov 20, 2008 12:07 am 
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Silent But Deadly
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Location: Crackerville, Northern Nevada
Handy formula:

To calculate the tap drill size by hand, use this formula:

TD = MD - (1/N)

where TD is the tap drill size, MD is the major diameter of the tap (e.g., 3/8 inch for a 3/8"-16 tap), and N is the number of threads per inch (16 in the case of a 3/8"-16 tap). For a 3/8"-16 tap, the above formula would produce 5/16 as a result, which is the correct tap drill diameter for a 3/8"-16 tap. The result produces a tap drill size that results in an approximate 75 percent thread.

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 Post subject:
PostPosted: Fri Nov 28, 2008 4:03 pm 
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Silent But Deadly
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I used 29/64. Didn't have a problem. Fits 3 different guns.

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 Post subject:
PostPosted: Fri Nov 28, 2008 7:29 pm 
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Silent But Deadly

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Location: Berea, Ky
I used 29/64 on a stainless muzzle brake I'm making and it worked great. For some reason it didnt do good on aluminum.


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 Post subject:
PostPosted: Fri Nov 28, 2008 7:42 pm 
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Is this for a can to attach to a standard AR15 thread?

I only ask as I've always been under the understanding that it is best to use a lathe to cut the threads rather than use a tap. I could be wrong though.

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 Post subject:
PostPosted: Sat Nov 29, 2008 1:10 pm 
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Silent But Deadly
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bakerjw wrote:
Is this for a can to attach to a standard AR15 thread?

I only ask as I've always been under the understanding that it is best to use a lathe to cut the threads rather than use a tap. I could be wrong though.


There's really nothing wrong with tap cutting, it's just that the way to tap a straight hole is to use a lathe... Usually people are resorting to taps because they don't have a lathe, so the holes come out egged, diagonal, or otherwise imperfect.

GsT

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 Post subject:
PostPosted: Sat Nov 29, 2008 2:21 pm 
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Silent But Deadly
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My thread work was done on a mill.

The threads are neat and tight. The rough spot you see in the pic of the threading is just a piece of fuzz or something from where I used a rag to apply the cold blue paste.

I wasn't happy with the cold blue, so I ended up case hardening it before tigging it and then applied moly over everything after it was welded to the tube.

I guess what I'm trying to say is there's more than one way to do it.

Image

Image

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 Post subject:
PostPosted: Sat Nov 29, 2008 5:03 pm 
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15/32 for 75% thread.


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 Post subject:
PostPosted: Sat Nov 29, 2008 7:31 pm 
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You can use the tailstock on a lathe to line up the tap. I always get the ones with the countersunk hole so you can use a dead center for alignment. It works well with a mill or would even help on a drillpress if you use one.


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 Post subject:
PostPosted: Sat Nov 29, 2008 10:41 pm 
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if i'm doing my math right a 29/64 drill would give you 101% thread


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 Post subject:
PostPosted: Sun Nov 30, 2008 2:27 am 
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Kennametal says that 0.468 (15/32) is 69% in a 1/2 x 28 and
.4652 (29/64) is 75%, but I guess they could be wrong.

I just know it works if you do it right. I don't think most folk can even notice 5-6% and you can get that if you don't have good setup anyway.


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