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1 1/4 NPT though Cast iron

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  • 1 1/4 NPT though Cast iron

    I have to drill a hole (only one) with a hand drill for a 1 1/4 for conduit though 1/2 inch of cast iron. Is that do able?

    If I can drill the hole can I use a tapered tap to cut threads though 1/2 of cast iron? or do I have to get a straight tap?

    Any advice? I never drilled or tap this large of a hole with a hand drill. The largest I've done 3/4 inch.

    I don't have a mag drill.

  • #2
    Originally posted by outlawspeeder View Post
    I have to drill a hole (only one) with a hand drill for a 1 1/4 for conduit though 1/2 inch of cast iron. Is that do able?

    If I can drill the hole can I use a tapered tap to cut threads though 1/2 of cast iron? or do I have to get a straight tap?

    Any advice? I never drilled or tap this large of a hole with a hand drill. The largest I've done 3/4 inch.

    I don't have a mag drill.
    Solly, I would offer you my mag drill. I just know where you are at cause I cant figure out the settings, my bust.

    Sharp (meaning brand new) and keep and eye for tilting the hand healed drill motor.

    I always like to do piolet holes with thick stuff. Nice chisel point bit bit will ride that piolet all the way through.

    In my trys. JR

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    • #3
      I would use an 1-1/2 hole saw. Take my time and turn it slow. Recommended speed for bi-metal HS in CI is 150 rpm. A hand drill won’t turn a tap that size. You’ll have to do that by hand.
      Sole proprietor of Acme Buggy Whips Ltd.
      Specialty products for beating dead horses.

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      • #4
        That's a big friggin hole by hand. what sort of drill are you proposing to use? I'd be thinking its not going to work out well without a core drill, i.e. rota-broach or hole saw. Its, a lot of torque to drill a hole that large with only body to stop it if it jams. Minimize that by minimize the width of cut as in annular cuter, hole saw etc.

        You could chain drill and file....not as crazy as it may seem if you've never done it as its only 1/2 an inch, and it is safe....but power would be easier
        in Toronto Ontario - where are you?

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        • #5
          I would have concerns that a hole saw or pretty much any hand drilling would wobble and make a oversize hole which would not form decent threads when tapped. If the finished hole is not to size and round, the threads tapped are quite likely going to be useless. Not know any detail on the overall size of the part, I would think of maybe boring the hole on the mill. A mag drill with annular cutter would also likely do a good job.

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          • #6
            "A mag drill with annular cutter would also likely do a good job." Yep, that would be a breeze. Sounds to me the OP wants to self torture though. I have an old Sears mega-drill that would cut that with a hole saw too. That or break both your arms if the bit snagged. It scares me to use it sometimes.

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            • #7
              Originally posted by CCWKen View Post
              "A mag drill with annular cutter would also likely do a good job." Yep, that would be a breeze. Sounds to me the OP wants to self torture though. :
              he said he didn't have a magdrill,

              I would have concerns that a hole saw or pretty much any hand drilling would wobble and make a oversize hole which would not form decent threads when tapped.
              I would be too worried given its a taper thread and for conduit
              in Toronto Ontario - where are you?

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              • #8
                Isn't there any Rental Shops in your neighborhood? You don't have to OWN a freekin' mag drill to use one.

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                • #9
                  thats a point, it would be the easiest way out. I've not used one on cast iron, you'd want to make it was flat enough to get a solid grip
                  in Toronto Ontario - where are you?

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                  • #10
                    Bi-metal holesaw is the way to go, and cast iron will drill much easier than steel which is a plus. By holesaw, we mean a good one , Lennox, Starrett etc. Tapping won't be too bad, if you are just threading it for conduit, you can probably go a little larger than the recommended tap drill for 1-1/4 NPT and be fine.
                    I just need one more tool,just one!

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                    • #11
                      Yep, I've used a 2 1/2" bi-metal hole saw in 1/2" A36. Not a breeze but sure beats drilling a bunch of holes in a circle and trying to tap that. (Do you see me rolling my eyes?)

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                      • #12

                        Agree that a holesaw is probably the best option. But for just one hole, sometimes you have to go with what you already have.

                        Got a piloted counterbore of about the right size? Back up the cast iron with something for the pilot - even a piece of wood - and it should do the job. If it’s too small, do you have an adjustable reamer to finish the hole?

                        Might even try one of the adjustable arm type hole cutters. The ones that look like a piloted fly cutter. I don’t think I’d try it on steel, but cast iron isn’t too grabby. Cut small, finish with a reamer if you have one.

                        Hole size is not critical. If you can get the tap to start in it, you’re fine. A tapered thread pipe tap acts somewhat as a reamer anyway, as it cuts the full depth of the hole the entire way.

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                        • #13
                          Hole saw and a fresh pipe tap will work fine. I've done stuff like that before, just take it slow and careful and don't cram the work.
                          25 miles north of Buffalo NY, USA

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                          • #14
                            What I got, rent a mag drill if there is a flat spot. Use a hole saw. Go slow 50 RPMs. Don't let it spin my arms off if it grabs. Tap will work, use big tap handle.

                            Time to go shopping.

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                            • #15
                              If you use the magdrill , you can put a center in it to start the tap.l

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