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Firearms Legal Protection

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  1. #1
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    What Tap Drill For A 1/2-28 Tap??

    I can't find it listed anywhere??

    I cut a 1/4" off a A2 Flash Hider and need to drill it out and finish tapping.
    I have a Chicom tap in the mail, but can't find a tap drill size?

  2. #2
    MGO Member
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    15/32"

  3. #3
    MGO Member
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    You can figure that out and any weird thread by 1 by the thread pitch, to give you the depth of the thread then subtract it from the major dia.

    for instance 1/2-13 becomes 1 divided by 13 equals .0768, you subtract that from the major diameter, 1/2"=.500, which would give you .423" Pretty close to the standard .4219 or 27/64"

    In the 1/2-28, I get .036, so that gives me a minor dia of .464"

    Closest normally found drill I could find is a 11.8mm.

  4. #4
    MGO Member Roundballer's Avatar
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    I did it the easy way. I just looked in my Machinery's Handbook. AND, it lists the drill size as "special".

    Professorwiz is right on the money. You want a bit between 0.461~.467, and that is right there were Prof said.

    15/32 is too big at 0.4688 and 29/64 is too small at 0.4531 (but you could get away with that if you are careful).

    The 11.8mm is 0.465, but I have no idea where to find one.


    Life Member, NRA, Lapeer County Sportsmen's Club Disclaimer: I Am Not A Lawyer. Opinions expressed are not representative of any organization to which I may belong, and are solely mine. Any natural person or legal entity reading this post accepts all responsibility for any actions undertaken by that person or entity, based upon what they perceived was contained in this post, and shall hold harmless this poster, his antecedents, and descendants, in perpetuity.

  5. #5
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    MSC, www.mscdirect.com has them, I go for the Precision twist brand - Part #: 78512613,
    but they do also have the Hertel brand (junk but might do ).. click on the part number which will bring up the catalog page that they are all on.

    Had to do weird threads all the time at Roush for prototype work, had a set of the metric drill index to help life out.
    I've went small than figured out before, as long as I had a really good tap, but it usually made the threads very tight.

  6. #6
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    sub $8 Chicom tap is not quality

    I only have to go 1/4" of threads

    Guess I'll look through my drills and find one close.
    I used to have a adjustable boring tool for this. It's been 20 years and I'm unsure if that's the correct term.

  7. #7
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    This is to get a AR Pistol under 26" OAL however they want to measure. All the short flash hiders/ concussions devices are OOS

  8. #8
    I am a Forum User
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    Regular drills do NOT drill precision diameter holes. That's why precision machinists have reaming tools.

    Look here in the yellow box for twist drill accuracy. The average .25 to .5 dia drill gives a hole with a nominal tolerance of +.006/-.001


    https://www.engineersedge.com/manufa...tolerances.htm.

    If I need an accurately dimentioned hole I'll drill it a bit small, the use a carbide milling cutter to finish to size, or a reamer.

    Me? I'd use the 29/64 drill, a sharp tap (US made, not $8 from EBAY), plenty of the right cutting oil and know you need to go back and forth to cut well and clear the chips.
    Last edited by MI22Shooter; 02-24-2018 at 07:52 PM.

  9. #9
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    I've tapped a few threads in my day. I know about Chicom crap, I didn't look close at the ad , then the receipt came in Chinese

    I found a few reamers in my wood machinist box that hasn't been opened in years. the 123 and 25 50 100 blocks were not rusted

  10. #10

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