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  1. #1
    PhillipM is offline Cast Iron
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    Default Drill size for form taps

    Never used a form tap before, but I've got a job that would benefit from them so have ordered a dozen OSG taps, as my existing suppliers don't stock forming taps, 1/2-20 UNF, what size drill should I be using with these, as the initial hole needs to be larger IIRC, and I don't have an OSG catalogue.

    Cheers.

  2. #2
    Ox's Avatar
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    Default From the OSG poster on the subject

    #1


    ------------------

    I am Ox and I approve this here post!

  3. #3
    allez is offline Plastic
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    Default

    Quote Originally Posted by Ox View Post
    #1


    ------------------

    I am Ox and I approve this here post!

    #1 is for a 1/4-20

    1/2-20= 75% thread .475 65% = .477 55% = .480.

  4. #4
    g-coder05's Avatar
    g-coder05 is offline Stainless
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    Default

    go to WWW.balax.com and download there thread former pdf file. the sizes are also in your machinist bible.

  5. #5
    PhillipM is offline Cast Iron
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    Default

    What machinists bible?

    12.1mm looks spot on for the thread engagment I'm after, and easy to get hold of over here, thanks for the help.

  6. #6
    Ox's Avatar
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    Default

    Quote Originally Posted by allez View Post
    #1 is for a 1/4-20

    1/2-20= 75% thread .475 65% = .477 55% = .480.



    Yes it is!

    That's what I thought I read.

    It's still Monday Morn here....

    For 1/2-20 their chart reads 31/64ths.

    But just making the hole to the pitch D or slightly larger is really the name of the game enyway... And a decimal reamer may be a good idea to make good and repeatable threads...

    -------------

    Sorry for that.
    Ox

  7. #7
    g-coder05's Avatar
    g-coder05 is offline Stainless
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    Default

    Machinist bible = Machinist hand book

  8. #8
    PhillipM is offline Cast Iron
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    Default

    Yep, like I said, don't have one!

    Normally I just calculate what size I need but I wasn't sure how to do it for a form tap.

  9. #9
    ietech's Avatar
    ietech is offline Hot Rolled
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    Default Machinery's Handbook

    My book says 0.4531 (29/64") or (11.501mm) for a 75% of full thread. Of course my handbook is a 1943 12th edition -- maybe things are different now.

    Assuming 75% is what you want then I would follow the book.
    Last edited by ietech; 10-06-2008 at 01:31 PM. Reason: Corrected transposed numbers.

  10. #10
    PhillipM is offline Cast Iron
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    Default

    Hmm, sounds far too loose for a rolled thread?

  11. #11
    ietech's Avatar
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    Default I'm not sure

    Quote Originally Posted by PhillipM View Post
    Hmm, sounds far too loose for a rolled thread?
    I have never used a (roll) form tap and have been on Google now for a while and really don't find anything but advertising for tap dancing and also some pictures of roll form taps.

    There is an obvious difference so I will continue to find more info -- the process, speeds and feeds, and size requirements may be of significant importance. I just not sure.

  12. #12
    ietech's Avatar
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    Default Old handbook again

    MHB says the diameter of the blank for 1/4" thru 1/2" should be 0.002" to 0.003" less than the pitch diameter.

    Major dia for 1/2"-20= 0.5000
    Pitch dia. = 0.4675"
    Minor diameter = 0.4350"

    So 0.4675-0.003 = 0.4645 Slightly larger than the 29/64" I said earlier. So the options based on this are: 29/64" (0.4531), or 15/32" (0.4688), Maybe reaming the 29/64" to size would work.

    I'm gonna have to do this at least once just to say I tried it. Not much call for it in my world tho.

  13. #13
    PhillipM is offline Cast Iron
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    Default

    I'll let you know after I've broken half the taps

  14. #14
    ietech's Avatar
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    Default Let us know

    Quote Originally Posted by PhillipM View Post
    I'll let you know after I've broken half the taps

    Please let us know what you come up with I am very interested in learning about something I have never done.

  15. #15
    Ox's Avatar
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    Quote Originally Posted by ietech View Post
    MHB says the diameter of the blank for 1/4" thru 1/2" should be 0.002" to 0.003" less than the pitch diameter.

    Major dia for 1/2"-20= 0.5000
    Pitch dia. = 0.4675"
    Minor diameter = 0.4350"

    So 0.4675-0.003 = 0.4645 Slightly larger than the 29/64" I said earlier. So the options based on this are: 29/64" (0.4531), or 15/32" (0.4688), Maybe reaming the 29/64" to size would work.

    I'm gonna have to do this at least once just to say I tried it. Not much call for it in my world tho.


    I think your gunna have a very bad first time experioence running those numbers.

    I like to generally run my holes a bit tighter than recommended, But I don't think I ever end up with a start hole SMALLER than PD. I recommend that you re-think that.


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    I am Ox and I approve this post!

  16. #16
    apestate is offline Stainless
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    Default

    Machinery's Handbook recommends working to 50, 55, and 60% full thread for form taps. Just like this chart:

    http://www.championscrew.com/form_tap_drill.htm

  17. #17
    ietech's Avatar
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    Default I will be trying

    Quote Originally Posted by Ox View Post
    I think your gunna have a very bad first time experioence running those numbers.

    I like to generally run my holes a bit tighter than recommended, But I don't think I ever end up with a start hole SMALLER than PD. I recommend that you re-think that.
    -------------

    I am Ox and I approve this post!
    I'm gonna try it as soon as I can and be assured I will follow your advise -- I respect the fact that you have sucessfully done this before so I will start with your numbers and work back to my calculations just for fun -- Thanks this is an interesting thread.

  18. #18
    ietech's Avatar
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    Default Good Link

    Quote Originally Posted by apestate View Post
    Machinery's Handbook recommends working to 50, 55, and 60% full thread for form taps. Just like this chart:

    http://www.championscrew.com/form_tap_drill.htm

    Thanks for the link to the chart I will be saving it in my favorites.

  19. #19
    allez is offline Plastic
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    Default

    Quote Originally Posted by allez View Post
    #1 is for a 1/4-20

    1/2-20= 75% thread .475 65% = .477 55% = .480.

    This info i posted is from the Balax book.

  20. #20
    MikeJB is offline Aluminum
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    Default

    What material are you tapping; it makes a difference as there is some 'springback' when using form taps. Aluminium is quite soft so it forms quite well. Stainless OTOH will spring back a little so a larger drill is needed for tougher materials. Remember that you can only form tap ductile materials so cast iron is a no-no.

    Form taps are actually made to a 6HX tolerance which is larger than a 6H; this is to factor in the springback.

    You had best do some testing and use the largest drill that will still give you a good thread; these taps use a lot more power than a cutting tap and you should run them faster as well.
    I usually check with a go/no go gauge to allow me to use the largest drill possible while staying within tolerance. You will break less taps and have longer tool life the bigger you drill.

    I am also an advocate of ER tapping collets; they have a square in the bore to hold the tap securely They are expensive but since starting to use them we have never broken a tap. Just a few degrees of slippage in a collet will put enough strain on the tap to break it. WNT have them.

    I know you were just asking for drill sizes but I hope the above info. is of some use.

    I assume that you are rigid tapping in a VMC...

    Regards,

    Mike.

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