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Hole question for 1/18" tap

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  • Hole question for 1/18" tap

    I need to tap four holes in a cast iron housing, 1 1/8" 7 TPI. The "correct" drill for this is 63/64" which I don't have and can't get for a few days (not to mention the cost for a drill I may never need again) but I do have a 1" drill and was wondering how much strength I would use by going with that larger hole? I also realize that the 1" drill will almost certainly end up drilling a hole slightly larger than 1" so would the combined wrong drill size and tolerance error cause too much of a strength loss? Of course the tolerance error is going to be the same with both drills but since the 1" is going to be sloppy anyway it would matter more with that drill. I am trying to re-drill four stripped out 1" coarse thread holes to the 1 1/8"-7, Any other suggestions?

  • #2
    That's big enough to bore....

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    • #3
      Originally posted by Royldean View Post
      That's big enough to bore....

      Except that it's underneath a tractor and would not be practical to bore, anyway after posting this it occurred to me that since the holes already have 1" bolts the major diameter is already 1". So it looks like even with the proper 63/64" drill I would theoretically not even remove 100% of the old threads? Looks like my only option will be to finish these holes with that 1" drill or go to an even larger bolt, I have enough mass around the holes for the larger bolt but that would mean buying another tap ($$$$$$) and yet another drill, I already have a 1 1/8" tap and the four bolts so that will be my first option I suppose.

      I should have realized those holes are already technically too big before I posted this question, just wasn't thinking I guess.

      Comment


      • #4
        According to Machinery's Screw Thread Book, 63/64" gives 76% thread depth for a 1 1/8" 7TPI UNC. Don't suppose you have a 25mm drill, because that's the alternative and gives 76% thread depth as well. I don't know what these holes do, but the fact that they stripped at 1" dia suggests they are fairly heavily loaded, so it probably isn't a good idea to reduce the thread depth on the new ones. Seeing that 63/64" only gives 76%, have you got a 31/32" drill? That should give you around 85% thread depth, even if it drills a bit oversize, which is better, though you'll have to work harder tapping the threads.

        Richard
        'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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        • #5
          Looking at my chart here, for a 1-1/8" 7tpi thread, you drill 63/64" for a 75% thread or 1-1/32" for a 50% thread. Sounds to me that drilling 1" will work just fine.

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          • #6
            I am thinking if the old 1" threads are completely stripped out
            that the 1 1/8" tap will start and cut just fine.
            Did you try it?

            --Doozer
            DZER

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            • #7
              Machinery's Handbook lists tapping hole sizes based on the length of thread engagement as a function of the diameter. For your 1-1/8-7 example for 1B and 2B threads they recommend a hole size range of .977-.991 for an engagement of 1/3D to 2/3D; .984-.998 for engagement of 2/3D to 1-1/2D and .991-1.005 for engagements over 1-1/2D. If you're able to get some decent thread depth I think this confirms that a 1" drill will be fine...
              Keith
              __________________________
              Just one project too many--that's what finally got him...

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              • #8
                Many tap drill charts do not properly consider the rounding/flattening at the crest and root of the threads. This produces a larger tap drill than is actually needed for a particular percentage thread engagement. And a weaker thread. The standard thread form actually specifies a thread depth that is only 0.625 times the full depth of a sharp Vee.

                Both Machinery Handbook and my own tap drill calculator do take this factor in account. Both of them show the proper tap drill for a 1 1/8 - 7 tap is just about one inch. I get 0.999" and the tables in Machinery Handbook show a range, depending on the number of engaged threads, from 0.970" to 1.005" for class 1B and 2B threads. For class 3B threads the range is even larger.

                Use your 1" drill and tap it.
                Paul A.

                Make it fit.
                You can't win and there is a penalty for trying!

                Comment


                • #9
                  These are blind holes 1 3/4" deep and all I have is a plug tap so I won't be able to go the full depth. These bolts are stripped because they were tightened improperly using the wrong tool, the fellow tried to tighten them with an open end wrench from an awkward angle instead of using a socket and bar for proper leverage. The bolts worked loose and the side to side motion of the heavy frame bracket slightly wallowed out the holes. I think if I can get them re-threaded to the larger 1 1/8" bolt even to a depth of just a little under an inch and a half, which is what I should get using the plug tap, it should hold ok after proper torque is applied. That is it should hold if I can torque them tight enough without stripping them and that's why I was concerned about the hole size, what I have been told here is encouraging so I think I will give it a go and see what happens, Thanks everyone.

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                  • #10
                    Originally posted by radkins View Post
                    Except that it's underneath a tractor and would not be practical to bore, anyway after posting this it occurred to me that since the holes already have 1" bolts the major diameter is already 1". So it looks like even with the proper 63/64" drill I would theoretically not even remove 100% of the old threads? Looks like my only option will be to finish these holes with that 1" drill or go to an even larger bolt, I have enough mass around the holes for the larger bolt but that would mean buying another tap ($$$$$$) and yet another drill, I already have a 1 1/8" tap and the four bolts so that will be my first option I suppose.

                    I should have realized those holes are already technically too big before I posted this question, just wasn't thinking I guess.
                    Hmm, were you planning on using a hand held drill motor to ream these holes out with a standard twist/SD drill bit?

                    If you can, I will be the first to admit that you are a much better man than I.




                    Dave

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                    • #11
                      Hear is another thought if you are concerned about these bolts staying in place.
                      Instead of using bolts, make up some studs and taper off the end threads to correspond to the taper on your plug tap.
                      Then you can screw the studs in until they lock in the taper. We used what was called taper lock studs in applications on heavy equipment where they were under very heavy loads.
                      You can then use nuts and crank them down to hold it all good and snug.
                      Then go back out and retorque the nuts after the machine has been in use for a while.
                      We had a D9 that had pulled the upper studs that hold the ripper frame to the the back of the main frame.
                      We had to use step studs because there was not room to go larger through the ripper frame.
                      After it was all together we retorqued them about once a week for a while then put very small weld tacks between the nuts and the ripper frame.
                      After that all that was necessary was to inspect the weld tacks once in while to see if they had cracked, which would show that it was moving, then retorque them again if they were cracked.
                      Larry

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                      • #12
                        Another thing to consider if you still want to use bolts is, after you have run your plug tap in as far as it will go, you can cut the tapered end off the tap and run it in further. I never throw away a broken plug tap because I can clean up the end and make it into a bottoming tap.

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                        • #13
                          @Radkins

                          It seems that the larger drill size wouldn't be an issue, but in reference to your original quest for a 63/64" drill bit I had a thought about reducing the 1" dill bit to 63/64", a little over .016" difference by etching the 'extra' material.

                          My test for this idea was a size 'U' drill bit, an extra one with a chipped lip, which was cleaned and degreased, then suspended in the etchant. After 25 minutes, the diameter went from .368" to .362", about .00024" per minute.

                          The etchant idea was borrowed from an artists' web site that addressed etching iron or steel. When I first heard of this etchant, I didn't think that there was a chance that this stuff could work. I was wrong.

                          The solution is comprised of copper sulfate and sodium chloride. I mix two separate solutions that are saturated brines of each salt. They are mixed for use at 50/50 proportions.

                          The etchant leaves a sooty deposit, residual carbon, and a light copper flash that wipes off. Chemical milling at it's simplest. My copper sulfate came from a chemical supply house, but I believe that you can find it at garden centers as 'root killer'. I used softener salt.... no iodine. Don't know if that would matter, but who needs extra variables...

                          Don't breath the dust or drink the pretty blue liquid. The solution, which is blue as mixed, will turn lighter and lighter as it's usefulness ends.

                          paul
                          paul
                          ARS W9PCS

                          Esto Vigilans

                          Remember, just because you can doesn't mean you should...
                          but you may have to

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                          • #14
                            Something to try that won't cost a mint might be a piece of tool steel turned to the proper diameter for the tap you want to use, ground to a cannon drill shape (D Bit) and hardened and tempered at the business end only. You may even be able to use a larger grade 8 bolt turned to size and formed to become a "D" bit. With patience, lots of lube and frequent chip cleaning I believe it would make a nice, properly sized hole all the way to the bottom of your original stripped hole. It could be driven either with an electric variable speed drill or a tap wrench if a square were put on the opposite end. Depending upon the location you may even be able to make a guide plate for it with a hole the proper size bore in a 1/2" thick or so plate on the lathe and bolted to the machine so it aligns with the stripped hole.

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                            • #15
                              I wouldn't fancy drilling that size hole, id be thinking a rotobrach cutter with mag base, i can imagine drilling it and stalling the drill on a creeper! Funny but dangerous!, i do have an old wolf drill that can drill a one inch hole, its about 400 rpm and 3hp, the biggest i have used was a 3/4" and yes it stalled on breakthrough, it did snap like a carrot and elastic bandage was required for some time for a badly sprained wrist, never will i hold on to a drill that big with that much torque again, stupid.
                              A 23 mm cutter is a common thing in the welding suppliers around here, though i still dont know why annular cutters don't come in tapping sizes?
                              Mark

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