Tap And Die Question.....

Discussion in 'Bad Dog Cafe' started by boneyguy, May 25, 2019.

  1. boneyguy

    boneyguy Doctor of Teleocity

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    I broke a 1/4" - 20 NC tap and am trying to replace it. I've located an inexpensive one (my set is inexpensive as I don't really require much) and it's described as 1/4" - 20 NC for 7/32"..also it's described as for "re-threading".

    So, two things.

    1) I'm tapping a freshly drilled hole in soft steel...would this tap be appropriate as it seems to be designed for re-threading a previously threaded hole, no? Is it going to survive cutting new threads in soft steel?

    2) I'm also confused by the for 7/32" part of the description. On my chart for tapping that came with my set it says to drill a 13/64" pilot hole for the 1/4" - 20 NC tap. That's the size of hole my tap broke off in....(it had bottomed out but I kept turning...doh!)

    What I'm doing is drilling into the side of my table saw miter slide bar in several spots and I'm inserting nylon threaded set screws in order to take up the slack in the miter slot.

    Thanks for any help.
     
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  2. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

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    Buy a good tap so it doesn"t happen again.
    7/32 is too big for coarse thread. MAYBE for fine.
    Use cutting oil.
    Go in 1 turn at a time, back off, and go on 2 turns.
    If you need threads right to the bottom of a blind hole, get several thteads cut, then grind the taper off the end of the tap.
    Grind slowly and cool/quench often in water so you dont alter the hardness of the tap.
    You don't want to see blue on the tap when grinding.
     
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  3. schmee

    schmee Poster Extraordinaire

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    7/32 is the tap drill size. If it says for rethreading it's probably not good hard steel... It might work once or twice. Hard to say. But that tap isn't expensive is a good on is it?
     
  4. Sean Mac

    Sean Mac Tele-Meister Ad Free Member

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    HSS or "high speed steel" is the ideal material for a tap or die.

    It air hardens unlike plain carbon steel.

    A very slightly larger than recommended tapping size hole loses very little in strength.

    It greatly reduces the chances of the tap breaking off in the hole.

    "Trefolex" or a similar "cutting lube" is essential.

    Getting a broken tap "spark eroded" out of a workpiece is too expensive to chance I think.
     
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  5. Brad Pittiful

    Brad Pittiful Doctor of Teleocity Ad Free Member

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    the right is a tap and the left is a thread chaser...which do you have...my guess is the one on the right

    [​IMG]
     
  6. boneyguy

    boneyguy Doctor of Teleocity

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    The one I broke is like the one on the right.....the replacement I'm looking at is below.....I've only got 4 holes to tap in what seems to be soft steel.....I had no problem drilling into it using run of the mill bits.....if I were to use this new bit I'd probably re-drill the pilot holes to 7/32"....they're currently drilled to 13/64"....the threads don't need to be too rugged because I'm only inserting nylon grub screws and they aren't really putting any load on the threads. So I guess I'm asking if the tap in the pic below will do the job I've described. I can get a better tap later....I just want this done so I can get onto the project that needs doing!!

    The pic shows the style of tap....it's obviously not the size I need ....it's from the store's website and it displays this pic no matter which size you select.

    download (1).png
     
    Last edited: May 25, 2019
  7. boneyguy

    boneyguy Doctor of Teleocity

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    Okay.....looks like I found what I need at Lowe's...(I don't normally shop there so it didn't occur to me to look)...they have one left and they've set it aside for me.....gotta run, they're closing soon!!

    462074_02694804_001.jpg
     
  8. trev333

    trev333 Telefied Ad Free Member

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    Trefolex and a generous pilot hole.. heard that before....:lol:

    yeah, I broke lots of small taps as a metalwork apprentice building truck bodies, etc.. ..:rolleyes:

    I haven't heard that word for decades now, trefolex.... the old Polish blacksmith I was paired with in my first year would send me off to the store to get "the grease" when we were cutting threads in jobs... I'm glad the storeman knew what "the grease" was, the green tin....... I think the smell of it is still imprinted in my memory... not bad, just so familiar... good times...:)
     
  9. aerhed

    aerhed Friend of Leo's

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    You're showing a tapered pipe tap. You want a regular old 14/20 NC and I'd try the 7/32 drill on a hole first. If the threads seem a little loose then go to the next number size larger bit for the next hole. Taps come in different classes. You're looking for low class.
     
  10. richiek65

    richiek65 Tele-Afflicted

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    Ah yes I can still smell the Trefolex from my fitting and machining apprenticeship days!
     
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  11. trev333

    trev333 Telefied Ad Free Member

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    and the milky soluable oil mix we used as well for drilling/machining... mmmmm :)
     
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  12. trev333

    trev333 Telefied Ad Free Member

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    I remember chassis steel being much harder than mild steel.... a tap breaker...:D

    I learned not to be too cute about pilot holes... getting out broken taps is a baitch...:twisted:
     
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