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Tapping A Blind Hole...

Discussion in 'The Tool Shed' started by FinoCJ, Oct 5, 2018.

  1. Oct 5, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
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    Need to repair and tap a blind hole for a thread insert. I have some experience with the typical helicoil...but only in through hole. I realize I will need a different tap than my tapered one as its a shallow hole. I will need one with a square blunt end. Two questions...
    1) should I start the tap with my tapered one and then switch to the blunt nose...or should I just start with the blunt nose. Its in an awkward location and am worried about keeping the tap nice and square.
    2) is a typical helicoil ok for this application with the little tab I will need to break off on the bottom end?

    Thanks all
     
  2. Oct 5, 2018
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
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    What you're talking about is a bottoming tap, used in conjunction with a regular tap.

    How deep is the hole? You might get enough depth for the helicoil with just the regular tap.
     
  3. Oct 5, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
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    ok...seems like I should start the threads with a regular tapered tap and then move to the bottoming tap. I don't have a bottoming tap but I could wait and see if the regular tap would go deep enough. The hole is about 1/2" deep...thread is for 1/4"x20.
     
  4. Oct 5, 2018
    heavychevy

    heavychevy Sponsor

    Danielsville georgia
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    Be sure and use tap magic or a lubricant like wd40. Cut real slow and back your tap out to get shavings off. If you try to cut too much too quick the tap may break. You may already know this. If you do disregard. A tapered tap will start easier than a flat bottom, but you may have to finish up with your square bottom tap
     
  5. Oct 5, 2018
    tarry99

    tarry99 Member

    Northern California
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    Heli-Coil has it's own tap...........just need the proper hole size............and keep in mind if it's in a tight area.........you may only get one bite at that apple so take your time!
     
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  6. Oct 6, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
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    the trick may be finding the correct size heli-coil 'bottoming' tap as opposed to the tapered tap that comes with most kits. I am sure I can order one...but trying to find one local for time saving purposes....

    I believe its 17/64 drill bit...but will verify as I don't have the exact heli-coil in hand yet.

    yes it is....

    double yes

    Off to begin the search....If I can get all the correct tools today, then I can work on it slowly over the next few days....leaving town on Thurs so really trying to get this wrapped up before then.
     
  7. Oct 6, 2018
    tarry99

    tarry99 Member

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    You could also look beyond using a " Heli-coil " Brand as there are other thread inserts out there that I believe use a more conventional SAE tap.
     
  8. Oct 6, 2018
    47v6

    47v6 junk wrecker! 2023 Sponsor 2022 Sponsor

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    I have used helicoils. They are alright. Use the kit with the included drill bit and tap as per the recommendations. Don't try and do something else. They are marginal, but the answer for wrecked threads.
     
  9. Oct 6, 2018
    tarry99

    tarry99 Member

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    Although I'm a big fan of the Heli-coil brand and use them all the time..............

    These inserts work with conventional SAE taps............I've also used them before in certain applications.
    Solid Wall Threaded Inserts For Carbon Steel | E-Z LOK
     
  10. Oct 6, 2018
    Mcruff

    Mcruff Earlycj5 Machinist

    Albertville, AL
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    We use the above ones at work and also use Keensert inserts. If you can get a 2 of the tapered helicoil taps just grind one down to make a bottoming taps. We do that all the time at work. Just make sure to back taper relief on the bottom so that the chips don’t bind up and break the repurposed tap.
     
  11. Oct 6, 2018
    47v6

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    I have wanted to make bottoming taps out of regular taps. Please explain back taper relief and how to do it. Thanks!
     
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  12. Oct 6, 2018
    Mcruff

    Mcruff Earlycj5 Machinist

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    Take the tapered tap and grind the end off back to where the thread starts to get straight. Once you have done that remember the cutting edge is the forward edge of something turning clockwise when looking down from the top. Now that you know where the cutting edge is pick one of the flutes and make sure to grind the material away in a spiral manor on the end in the direction that leaves one flute as the primary cutting surface. You don’t need a lot of spiral maybe 5° or so. In other words the flutes will then cut in succession 1 after another. If you look at the end of a bottoming tap you will see this immediately. You don’t want all flutes cutting equally at the same time it puts so much load on the tap it breaks.
     
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  13. Oct 7, 2018
    nickmil

    nickmil In mothballs.

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    Good bit of info there, thanks McRuff!
    I've made tons of bottoming taps from taper taps but never done this. I go exceedingly slow though with lots of backing out to clean out the chips. I will try this.
     
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  14. Oct 8, 2018
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    I'd imagine you could start with a conventional tap, then sequentially grinding the tip off it and re-tapping until it performs as a bottoming tap.

    I've never tried it - just a crazy idea. Attempt at your own risk...
     
  15. Oct 8, 2018
    Norcal69

    Norcal69 Out of the box thinker 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    X2
    I did this just last week when I installed my new shift knob. It was the wrong thread 7/16 coarse and I needed 3/8-24. I drilled the knob out for 1/2-13 and started with a tapered tap, then cut a old tap down to make a bottom tap to get it fully threaded inside. From there I used a piece of 1/2-13 allthread center drilled on the lathe and tapped to 3/8-24 for my thread adapter.
     
  16. Oct 8, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
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    Thanks Mike and all - any tips to help ensure that I keep the end of the tap nice and square when I grind it down, or is it okay if its off just a bit. Based the tools available to me - guessing I would have to do this with a bench grinder or some sort of hand held cut-off whell. Maybe I put the tap in a piece of angle iron to help hold it steady and square to the grinder - of course I will have to get the height and alignment just right. Part of me is thinking I could put chuck a die-grinding abrasive wheel in my drill press and get something to hold the tap nice and vertical....or maybe it doesn't need to be that perfect and I can do it by hand?

    thanks Tarry - I like the idea of using conventional thread taps. I actually found some plug and bottoming taps for sale at a local hardware store.
     
  17. Oct 8, 2018
    Mcruff

    Mcruff Earlycj5 Machinist

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    You should be able to do it by hand, but if not sure and its on a flat surface, take a piece of steel and drill a hole thru it about .005-.008" bigger than the tap. Of course drill the hole with a drill press, not a hand drill. Now lay the flat piece of steel over the hole you need tapped and you have a guide for the tap to go thru.

    This is a machinist benchmakers block. The holes are guides for taps and punches.

    80924_R-1.jpg
     
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  18. Oct 8, 2018
    tarry99

    tarry99 Member

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    James , what are you working on?...............might help to understand , as there could be many other solutions.........the easiest may be to just go up in size to 5/16" or 3/8"...
     
  19. Oct 8, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

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    Its an oil pump mounting hole that goes into the timing cover and has stripped the threads. It may be possible to just step up the size hole and tap for 5/16 thread - but the hole in the pump housing would also have to be opened up and there isn't a lot of clearance for a bigger head. Might have to use a smaller clearance headed bolt. There is concern about opening the holes up too much in terms of oil plumbing passageways - but its probably not an issue at an additional 1/16. Right now, the biggest issue is finding the space to get a drill in place. I do have a right angle adapter for my drill, but forgot to take it over. The blind hole is deeper than the 1/2" I was thinking - more like 5/8 to 3/4". Tapered taps still look like they wont have enough full thread cut - but a plug tap would probably work - bottoming tap would be best.
     
  20. Oct 8, 2018
    tarry99

    tarry99 Member

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    One of the group that holds the pump and filter on?...........frame in the way?
     
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