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Thread: 1.75" female threads on blind hole?

  1. #1

    Post 1.75" female threads on blind hole?

    Ok, so I am going to take a block of 7075 aluminum and bore 2 holes about 6" apart. Then the openings need to be female threaded to accept threaded male gland nuts to hold stuff in.

    How does one make such large threads into tehe openings of holes> I know I can use a four jaw chuck and offset each hole till it indicates on center and then thread it internally with a boring bar with a thread insert, but is there a way to do it on a vertical mill?

    It seems UNC or UNF taps that size are next to impossible to come by and make be really expensive if they did make them.

  2. #2
    Join Date
    May 2001


    Big...2 choices: 1-the lathe method, which you already know; or 2-make a tap. Thread some steel, cut some lengthwise grooves. Since you're cutting into aluminum the tap won't need hardening unless you're going to use it a lot.

  3. #3
    Join Date
    Jun 2001


    You are on right track. You could use toolmakers buttons to establish hole locations. I cheat, stick on mill, use DRO, and then make some kind of accurate hole I can indicate in on when I move to 4 jaw chuck.

    Some times I spot drill, then bore-ream with endmill. Example 7/16 drill followed with a 1/2 endmill, not reamer, the endmill is stiff, holds location, and makes a good enough surface to run an indicator on.

    Good luck

  4. #4
    Ron LaDow Guest


    That's not cheating; that's using what you have...

  5. #5
    Join Date
    Jul 2001


    I hate cutting threads in a blind hole. My method is to find someone in the shop and bet him ten bucks he can't do it. It's worth the money.

  6. #6
    Join Date
    Apr 2001


    Set up a dial indicator on the lathe carriage so you know when to disengage the halfnuts. If you run in slowest backgear, you can stop the carriage within a thou or two of the same place every time, even with a fairly coarse thread.

    Or run the lathe in reverse and thread it coming out. You'll have to put in enough of a runout groove in the bottom of the hole to start the threading tool in.
    Try to make a living, not a killing. -- Utah Phillips
    Don't believe everything you know. -- Bumper sticker
    Everybody is ignorant, only on different subjects. -- Will Rogers
    There are lots of people who mistake their imagination for their memory. - Josh Billings
    Law of Logical Argument - Anything is possible if you don't know what you are talking about.

  7. #7


    Yes, it can be done in a vertical mill if it is CNC and has spiral interptation for milling threads (works great). Call around to some new machinery dealers that sell Haas or Okuma CNC mills and ask for a "demo". If you are nice to them, they may do it for you for free - but I would not count on it.

    If you have to do it the hard way, you might be able to get a tap from KBC. Making your own tap for two holes is not a bad idea but use lots of lube when you tap it. A-9 works well. If you can chuck it safely and thread it in the lathe do it.

    Have fun
    Dave Smith

  8. #8


    There has to be somebody that makes taps larger than 1.5".....I am not into making everything though...

    I have seen helical interpolation prgrams for CNC, but it is a regular vertical mill, and I am the numerical control,

    I just got both the lathe and mill delivered today. The knuckleheads put the aluminum angle extrusion upside down so the too short end only covers half of the encoder rod for the carriage feed readout axis.

    On the mill they extended the table encoder rod so far that one cannot screw in the male taper pipe thread x hose barb fitting drain tap for the coolant hoses.
    Oh well, I guess they will have a lot of correcting to do....

  9. #9


    Special order is a wonderful thing. I've seen as big as 2 7/8" pipe taps (probably bigger available if you want!!). On a mill a boring head is what you would use for threading but these are not cheap either. Mind you with even a bottoming tap the bottom 2 threads or so aren't much use so for a shallow connection you may have an interesting time. You may wish to look at a banjo style connection. This is used on a lot of European machinery. I am not sure on details but I can give you what I know if you are interested.

  10. #10
    Ron LaDow Guest


    C. Tate,
    Very good idea, combining machining with psychology...

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