what size drill bit for 1/8" NPT tap
RSS | Subscribe | Contact Us | Advertise | About Us
Results 1 to 25 of 25
  1. #1
    Join Date
    Nov 2006
    Posts
    94

    what size drill bit for 1/8" NPT tap

    just wondering what size drill bit I should be using tapping a hole with an 1/8" NPT tap,
    I only have a very small set of bits like 14 or so and none seem fit all that well,
    so I want to buy an individual drill bit that will work
    would like to know what size I should get

  2. #2
    Join Date
    Nov 2008
    Location
    south GA where its hot in summer and cold in winter
    Posts
    3,718

    Re: what size drill bit for 18" NPT tap

    Most good taps have that written on them. However since it may be small to read or nonexistent, my guess, without looking, would be 3/32
    Esab Migmaster 250
    Lincoln SA 200
    Lincoln Ranger 8
    Smith Oxy Fuel setup
    Everlast PowerPlasma 80
    Everlast Power iMIG 160
    Everlast Power iMIG 205
    Everlast Power iMIG 140E
    Everlast PowerARC 300
    Everlast PowerARC 140ST
    Everlast PowerTIG 255EXT

  3. #3
    Join Date
    May 2008
    Location
    Orange, TX
    Posts
    11,706

    Re: what size drill bit for 18" NPT tap

    'R' drill size actually (.3390"). 11/32" is closest fraction (.3437").

    http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html
    MM200 w/Spoolmatic 1
    Syncrowave 180SD
    Bobcat 225G Plus - LP/NG
    MUTT Suitcase Wirefeeder
    WC-1S/Spoolmatic 1
    HF-251D-1
    PakMaster 100XL
    SA-200 Blackface Code# 8638
    Star Jet 21-110

    Two term limit - one in office, one in jail.

  4. #4
    Join Date
    Jun 2008
    Location
    Central California
    Posts
    3,769

    Re: what size drill bit for 18" NPT tap

    Assuming you mean American National Standard Taper Pipe Thread, also known as Briggs Standard Pipe thread, the recommended drill size is 11/32". A tapered reamer used after will help give better threads.

  5. #5
    Join Date
    Aug 2004
    Location
    Minneapolis, Minnesota
    Posts
    102

    Re: what size drill bit for 18" NPT tap

    I just happen to get the 28th edition of the Machinery's Handbook today. This is the first thing I've researched that actually might be useful - everything else today has been browsing this tome ...

    1/8-27 tapered thread without a reamer - "Q" (0.332), with a reamer - 21/64, straight pipe thread - "R" (0.339).

    From smallest to biggest it is 21/64, then "Q", then 8.5mm, then "R", then 11/32.

    I think the common hardware store variety of pipe falls into the "tapered thread" category.

    But that is probably more than you wanted to know and the 11/32 that Duane suggested will probably do the trick.

  6. #6
    Join Date
    Nov 2008
    Location
    south GA where its hot in summer and cold in winter
    Posts
    3,718

    Re: what size drill bit for 18" NPT tap

    ohh,

    Sorry, bleary eyed. I skipped the NPT.
    Just thinking NC
    Esab Migmaster 250
    Lincoln SA 200
    Lincoln Ranger 8
    Smith Oxy Fuel setup
    Everlast PowerPlasma 80
    Everlast Power iMIG 160
    Everlast Power iMIG 205
    Everlast Power iMIG 140E
    Everlast PowerARC 300
    Everlast PowerARC 140ST
    Everlast PowerTIG 255EXT

  7. #7
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 18" NPT tap

    REAMER

    This is good!!! I never think about using a reamer. I do a lot of grease fitting tapping, and I just tap straight through with a non bottoming something or other tap. I forget what they call 'em. I'll have to go lool in the AM.

    Thanks Guys!!!!!!!!!!!!
    "Any day above ground is a good day"

  8. #8
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 18" NPT tap

    It's a tapering tap, just looked it up in an old Enco catalog. I guess I'm sorta safe
    "Any day above ground is a good day"

  9. #9
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 18" NPT tap

    Ok, here it is.

    Apparently I've been messing up. I shouldn't tap thru. It destroys the taper, and doesn't give the NPT a good bite. Idiot

    I'm only grabbin' good thread near the top of the hole. No taper
    Attached Images Attached Images  
    "Any day above ground is a good day"

  10. #10
    Join Date
    Jun 2008
    Location
    Central California
    Posts
    3,769

    Re: what size drill bit for 18" NPT tap

    This is good!!! I never think about using a reamer. I do a lot of grease fitting tapping, and I just tap straight through with a non bottoming something or other tap. I forget what they call 'em. I'll have to go look in the AM.
    Is that the size you use, Samm? I've seen grease fittings in a variety of sizes, from #12-24 and/or #12-28 to 1/4-28 to 1/8" pipe to huge stuff on dozers and such, IIRC. Those first few are straight thread.

  11. #11
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 18" NPT tap

    Oh no, not the size. The way I've been using the tap wrong. Everything I have is 1/4-28. And that's getting hard to find here. They're all going to metric at the store.
    "Any day above ground is a good day"

  12. #12
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 18" NPT tap

    I think the drill size is 7/32
    "Any day above ground is a good day"

  13. #13
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 18" NPT tap

    I need to quit running the tap all the way thru to the straight part of the tap.

    Well, that added another 3 posts
    "Any day above ground is a good day"

  14. #14
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 18" NPT tap

    Hell, the damn tap says 1/4-28, that's all I know So does the package of nipples
    "Any day above ground is a good day"

  15. #15
    Join Date
    Sep 2006
    Location
    NJ, USA
    Posts
    3,387

    Re: what size drill bit for 1/8" NPT tap

    Samm,

    1/4-28 is a 'regular' thread, a UNF thread (Unified National Fine thread). The hole and tapped threads for that are straight, not tapered.

    Then what/where is that "taper chamfer tap" thing doing, you ask?

    To make it easier to tap a hole and turn it into an internal (female) thread, the tap often has a taper at the tip in order to help the person line up the tap with the hole and to 'ease into' the actual cutting of the threads.

    That taper chamfer tap is just fine (and the generally preferred tap to use) when you are tapping a through hole (a hole that goes all the way through the material, as opposed to a "blind hole" where the hole stops in the material and has a closed bottom to it). The taper of the tap helps you line it all up (hopefully square with the hole, or you end up breaking the tap!), and then you start turning the tap and it cuts the threads and you run the tap all the way into the hole until you are past the tapered section of the tap and the threads are then nicely cut with the straight part of the tap. Done.

    For an NPT (National Pipe Thread) item, the actual threaded portion is tapered. And because they are based on pipes and pie sizes, the NPT numbers don't directly correspond with the size in inches. So, a 1/8 NPT thread is not 0.125 inch in diameter but is based on the thread that would be on the end of a piece of 1/8 inch pipe and is thus about 0.405 inch in 'big' diameter. The threads (and pipe end and the hole as well) are tapered to make them leak less when you put them together and if you put some thread tape/dope on the NPT items and tighten them up enough, they will usually not leak (much ).

    The OP's question has already been answered. Q or R or 11/32. Tapered reamer optional, as the tapered pipe tap pretty much has the taper built in.
    The best laid schemes ... Gang oft agley ...

  16. #16
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 1/8" NPT tap

    Hey, thanks guys. I've been doing it wrong all this time Time to get with the program.

    No wonder the damn things don't snug up until the nipple is clear into the hole
    "Any day above ground is a good day"

  17. #17
    Join Date
    Jan 2008
    Location
    Arkansas
    Posts
    1,202

    Re: what size drill bit for 1/8" NPT tap

    arvidj,
    Did you get the book or the disk?

    If disk, what format is it and can you search the whole voluum?
    A butterfly without wings,
    is just an ugly bug


  18. #18
    Join Date
    Aug 2004
    Location
    Minneapolis, Minnesota
    Posts
    102

    Re: what size drill bit for 1/8" NPT tap

    Quote Originally Posted by Magnetic Mechanic View Post
    arvidj,
    Did you get the book or the disk?

    If disk, what format is it and can you search the whole voluum?
    I just got the book - the large type edition

    I have an old 27th edtion CD around someplace but didn't use it very much because "the need" and "the computer" were never in the same place at the same time. I will try reinstalling it this weekend and evaluate the search capabilities.

    Arvid

  19. #19
    Join Date
    Aug 2004
    Location
    Minneapolis, Minnesota
    Posts
    102

    Re: what size drill bit for 1/8" NPT tap

    Quote Originally Posted by Magnetic Mechanic View Post
    arvidj,
    Did you get the book or the disk?

    If disk, what format is it and can you search the whole voluum?
    Ok, I managed to find the CD and install it and then spent a few days working with Industrial Press trying to get the access code to work. They were great to work with and all if working now.

    In the end, the book is simply a secure Adobe .pdf file so everything that you can and can not do with an Adobe reader is available - table of contents, searching, printing a page, etc.

    Plus there is a 'Additional only on the CD' section with ...

    Mathematics
    Cement, concrete, lutes, adhesives and sealants
    Surface treatments for metals
    Manufacturing
    Symbols for drafting
    Forge shop equipment
    Silent or inverted tooth chain
    Gears and gearing

    Note that this is the 27th edition CD so some of the extra stuff may have made it into the 28th edition hard bound.

    And I see that I can upgrade to the 28th edition of the CD for about two thirds the cost of a new CD - something you can't do with the book - i.e. send the old on back and get a new one for two thirds the price. But then again lugging the laptop around the workshop doesn't seem to be to apealing either so I guess I need both.

    Arvid

  20. #20
    Join Date
    Jan 2008
    Location
    Arkansas
    Posts
    1,202

    Re: what size drill bit for 1/8" NPT tap

    Thanks.
    I think I need to get that disk.
    Have the book but cant hardly see the small print.
    A butterfly without wings,
    is just an ugly bug


  21. #21
    Join Date
    Aug 2004
    Location
    Minneapolis, Minnesota
    Posts
    102

    Re: what size drill bit for 1/8" NPT tap

    Ok, hold the phone - stop the presses ...

    I just noticed a nuance in the electronic version. Though it is a .pdf it is MULTIPLE pdf's. Each chapter is a .pdf file. And what this means is that you need to enable the "full reader search" capabilities of Adobe and point it at the Machinery's Handbook directory to search the entire book. The default Adobe setting is to just search the "current document", and given that each chapter is a disparate document it will only search one chapter. But be forewarned that searching it all means that you are declaring it either Miller Time or coffee break time - what ever makes you boat float - because it is going to take some time.

    Note that the multiple pdf's is also why getting the 28th edition to work was reported to be such a hassle. Each of the pdf's - i.e. each of the chapters - had to be unlocked separately. Note that they may have fixed that issue. I guess I will find out when I get the upgrade.

    Arvid

  22. #22
    Join Date
    Jan 2008
    Location
    Arkansas
    Posts
    1,202

    Re: what size drill bit for 1/8" NPT tap

    Thanks - tis as i thought.
    A butterfly without wings,
    is just an ugly bug


  23. #23
    Join Date
    Aug 2008
    Posts
    9,266

    Re: what size drill bit for 1/8" NPT tap

    Quote Originally Posted by Magnetic Mechanic View Post
    Thanks - tis as i thought.


    Are ya sure it tis, maybe it twasn't, could be it tweren't, or maybe it ain't

    "Any day above ground is a good day"

  24. #24
    Join Date
    Jan 2008
    Location
    Arkansas
    Posts
    1,202

    Re: what size drill bit for 1/8" NPT tap

    Twas!
    A butterfly without wings,
    is just an ugly bug


  25. #25
    Join Date
    Dec 2008
    Location
    Texas
    Posts
    136

    Re: what size drill bit for 18" NPT tap

    Quote Originally Posted by duaneb55 View Post
    'R' drill size actually (.3390"). 11/32" is closest fraction (.3437").

    http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

    every shop I have ever been in has a big poster chart like this some where usually next to the bins/drawers were they keep the drills and taps. Would be nice to have this as a sticky some where around here maybe in the same thread as the sheet metal thickness chart?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
RSS | Home | Penton Media | Contact Us | Subscribe | For Advertisers | Terms of Use | Privacy Statement