Tapping a 7/16" X 20 TPI thread into 1/2" mild steel

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  1. Bill Crowell

    Bill Crowell FABO Gold Member FABO Gold Member

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    I'd like to drill and tap several of these holes. I've got my 25/64" drill bit for the tap, but I don't know what kind of tap to buy. Some of the types I've seen are "chip-free thread-forming", "spiral flute", "spiral point" and "straight flute".

    I was thinking of buying the "spiral flute" type because that kind seems to be recommended for tapping, but what is the deal with the "chip-free thread forming" type? Is it preferable to the spiral flute type for tapping threads like this?

    Can I do this threading with a hand tap, or am I going to have to buy a tapping head for my drill press?

    Thanks in advance for any replies and opinions.
     
  2. Demonracer

    Demonracer 71 Demon 00 Ram 16 Chrysler 300S 05 Caravan FABO Gold Member

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    It all depends on the holes, do they need threads all the way down, are they open on each end, tell us more about the endeavor.
     
    Last edited: Nov 11, 2020
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    • diymirage

      diymirage HP@idle > hondaHP@redline

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      new holes i always like to cut with spiral tapered taps

      straight tapered if you cant find spiral

      but definitely tapered, for the same reason most OEM bolts have no thread on the bottom 1/8 or so, keep it from going in crooked
       
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      • A Body 440 WHY NOT

        A Body 440 WHY NOT Well-Known Member

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        If you need threads close to the the bottom of a blind hole , do most of the thread cutting as mentioned , then finish with a ‘ bottoming ‘ tap. Threads will be very close to the bottom. NEVER bottom out a tap or bolt
         
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        • Treblig

          Treblig Well-Known Member

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          • Bill Crowell

            Bill Crowell FABO Gold Member FABO Gold Member

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            7/16" X 20 holes in a 1/2"-thick fixture and pulling plate, made from common "fish plate". I'm going to drill the proper size holes with a 25/64" bit, tap the hole all the way through the 1/2" plate, and then bolt things to it for straightening, pulling and repair. I'm sure it will also serve as a fireproof welding table.

            table.jpg
             
          • Demonracer

            Demonracer 71 Demon 00 Ram 16 Chrysler 300S 05 Caravan FABO Gold Member

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            Use PLENTY of thread cutting oil to get the best threads.
             
          • Bill Crowell

            Bill Crowell FABO Gold Member FABO Gold Member

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            Am I going to have to buy a tapping head for my drill press?

            What do you think of "chip-free thread forming" taps? McMaster-Carr and Grainger are pretty proud of them.

            tap.jpg
             
          • pishta

            pishta I know I'm right....

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            If you can get the corners under a drill press, use the press head to keep the bit vertical and turn the collet by hand to cut the threads. The splined collet will pull tap into the threads by itself. Not my favorite thing to do, tapping holes especially in brittle cast iron!
             
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            • dano

              dano Evil Handy Man

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              I've used this method a lot. It work great, especially with soft metals like aluminum, copper, brass and plastics where its easy to to start crooked and strip.

              Why 7/16 fine? It isn't overly common unless you have a really good hardware store or order the bolts and stuff you need.
               
            • Bill Crowell

              Bill Crowell FABO Gold Member FABO Gold Member

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              Thanks for all the advice.

              I think I will order one of the "chip-free thread forming" taps and tell you later how it works.

              What they call fish plate around here is made from mild steel.
               
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              • Bill Crowell

                Bill Crowell FABO Gold Member FABO Gold Member

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                Big enough to be strong, but not so big that it gets in the way, and lots of thread area for its size.
                 
              • el5dart

                el5dart Well-Known Member

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                been a machinist for 30 years. what always works best for me in any situation is a spiral cut bottoming tap.

                [​IMG]
                 
              • Treblig

                Treblig Well-Known Member

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                Don't forget to put a nice bevel (chamfer) on the entrance to the hole before tapping. Makes a big difference when starting the tap.

                aug2.jpg
                 
              • ch1ll

                ch1ll FABO Gold Member FABO Gold Member

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                Tool and die maker here and I’ll tell you that if you have just a few holes to tap, get a hand tap at the hardware store. First tap in trebig’s post. 3 or 4 flute. Spiral taps tend to break hand tapping if you’re not careful. I wouldn’t buy a tapping head unless you have lots of holes, just not cost effective. Thread forming taps displace the metal and no way can you drive it in by hand. Use paraffin as a lubricant/cutting fluid, I just use a candle by rubbing it across the flutes of the tap before each hole. Be aware that if you use a forming tap in a drill press, the hole you drill needs to be a little bigger than a hand tap drill. Off hand do not know what size that is. 7/16-20 is an odd size. Its fine thread. Coarse thread taps rule of thumb is 1.5 x diameter of tap of full threads so a 1/2-13 tap should have 1.5 inches of threads in the hole or a 1.5 inch thick plate. If you’re plate is not that thick then you would use a fine thread series tap.
                 
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                • ch1ll

                  ch1ll FABO Gold Member FABO Gold Member

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                  To show you the difference in tap drill sizes between a forming/chipless tap and a cutting tap, 1-8 cutting tap requires a 7/8 hole and a forming 1-8 tap requires a 15/16 hole.
                   
                • mopardude62

                  mopardude62 FABO Gold Member FABO Gold Member

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                  Treblig posted this video back in September. Some great information on taps.

                   
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                  • 69_340_GTS

                    69_340_GTS Well-Known Member

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                    No. This is used on aluminum or other soft metals.
                    Since your holes go all the way thru all you need is a hand taper tap ("starter tap") and just run it all the way down, making sure you have full threads all the way. You don't need any fancy spiral taps, those are for production work where you need to get the chips to come out (usually on blind holes). Just lube it up good with your choice of oil or grease and take it about 1/2 turn at a time, backing off a little each time.
                     
                  • Treblig

                    Treblig Well-Known Member

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                    "backing off a little each time." back off until you hear or feel the chip break. You'll feel it more than hear it but it takes a little practice. When you back up with the tap you'll feel a very slight "snap" as you break the chip free making it easier to continue.
                     
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                    • 64DartGTinAZ

                      64DartGTinAZ FABO Gold Member FABO Gold Member

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                      Grainger's pretty proud of most everything in their catalog...
                       
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