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  • Tapping blind holes in steel

    (From my thread on shop class):

    I had to tap a 1/4-20 hole that I drilled with the specified #7 drill (0.201") to about 3/4" deep, and after a few turns the tap it became very hard to turn, in spite of liberal use of Tap-Magic and backing out 1/2 turn occasionally. It is a full taper 3 flute alloy steel tap from a HF set, but seems good quality and I just used it with no problem to tap 12 holes in my parting tool post. That was easy-to-machine 1215 steel, however, and were through holes in 0.3" to 0.4" material, while the manifold is 1018. I was able to tap it deeper by using a 4 flute carbon steel tap I reground to an almost bottom or plug tap, but each time I could only get about 1/2 more turn before switching.

    I had a similar problem with the two mounting holes on the bottom, which I drilled to a depth of 1" with a slightly larger 13/64" (0.203") drill. The drills seemed to be 0.195" to 0.199" diameter, but they usually drill a few thousandths over. My buddy in the shop class broke a tap when he tried to tap the bottom holes, so the common element is the steel, although I don't know for certain if he took the same precautions as I did. I was able to get usable threads to a depth of 0.5", which should be enough, but the drawing calls for 0.6 min.

    A search of the forum turned up some threads from 2010 and earlier, and of course the advice is to buy high quality taps, especially with spiral points and spiral flutes. But I want to find out why I had so much trouble. I will see if I can use a spiral tap at school tomorrow, and maybe it will work just fine and tell me that my taps are the problem.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

  • #2
    My reference table shows a #1 (0.228") pilot for 1/2" thick steel.
    Kevin

    More tools than sense.

    Comment


    • #3
      1. It is a blind hole, so the chips are likely building up in it. You have to remove them. One way is a cotton swab: stick it in and rotate it as you retract it. Another way is a can of compressed air with a straw. Insert the straw. PROTECT YOUR EYES and blast away.

      2. The chips can also build up in the flutes of the tap. I find that after 1 or 2 turns in a blind hole that I must withdraw the tap and clean it. I use a toothbrush.

      3. Harbor freight taps are not the best quality. You can get good ones from McMaster (any that they sell) or look for name brands from other suppliers. That being said, if the tap did not break, it is probably not your problem.

      If you are going to to to tap to the bottom of a blind hole, you will need a set of three taps: taper, plug, and bottom styles. However, if you have the room, it is usually better to drill the hole deeper and not tap it all the way down. That also allows some room for the chips to accumulate, making clean-out less necessary.
      Paul A.
      SE Texas

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

      Comment


      • #4
        Yes - tapping a blind hole in steel, a quality spiral-flute tap is the bee's knees. Honestly, spiral flute taps are great all around if you just want one type of tap.

        At the shop I work at, we regularly tap any and all materials using the normal 75% thread tap drill - mild steel, A2, 304 stainless, etc.. We use normal OSG black oxide HSS taps, typically spiral point, for some reason. I believe it really all comes down to the tap...

        Comment


        • #5
          My first suggestion is get a known good quality tap and see if you have the same problem. With that being said even good taps don't last forever, they will dull to the point that you will have trouble tapping some material before it breaks.

          Like the others said, make sure you are taking care of the chip accumulation issue as well.

          Comment


          • #6
            One machine shop I used to work for had Tapping Wax. They were sticks of a soft yellow wax that you would stick down a blind hole and then run the tap to the bottom. The wax pushed the chips up the flutes and provided some lubrication. And the wax would stick to the tip of the tap (always used a spiral point tap) and the hole would be clean when you finished. Have not seen tapping wax since but it was a great way to tap a blind hole.

            P.S. As far as I am concerned, small 3 or 4 flute taps are only good for chasing threads, A 2 flute spiral point is much stronger and gives better, faster results.

            Comment


            • #7
              Already mentioned, but get rid of the HF taps. When you use a good quality tap from McMaster or elsewhere, you will FEEL the difference. If you cannot drill deeper and still have a blind hole, you best get a quality tap. Bob.

              Comment


              • #8
                Originally posted by KJ1I View Post
                My reference table shows a #1 (0.228") pilot for 1/2" thick steel.
                If that's the case then you either have a really crappy chart or are looking at the wrong pitch (threads per inch.) That drill size is for a 1/4-28 UNF, and only f you want a scant 50% of thread.

                http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

                This is the type of tap which should be used in a blind hole, a spiral flute tap. The one on the left would be for short-chipping materials such as brass, cast iron, bronze. The one on the right can do steels and stainless steels.

                The only ones if this style that I've ever broken were the cheap crap. Never use anything less than a name brand HSS, HSS-E or HSS-Co tap. Threading is most often the last step in a workpiece, so why take a chance that it'll be the start of a whole 'nuther job of getting the broken POS tap out, cleaning the hole and trying to get it done with a good one.



                For through holes, a spiral point tap is the choice. Here's an example of a top quality one, in this case one for austenitic stainless steels.

                Last edited by PixMan; 03-11-2015, 07:44 PM.

                Comment


                • #9
                  Say what? That second picture is NOT a spiral point tap! That's a starter or maybe a plug tap. It doesn't have the tapered lead in that cause the chip to spiral forward of the tap and out the back side of the hole instead of traveling up the flutes.

                  Comment


                  • #10
                    Something most folks do not realize is that 55-65 percent threads are only a few percent less strong than 75% threads and they are oh so much easier to tap due to dramatically less torque requirement.
                    Tooling lasts longer and tap breakage is virtually eliminated. The crests of the thread do not add significantly to the threads strength.
                    For all but the most demanding applications I usually aim for a 60-65% percent thread rather than what most tap/drill charts recommend. Quite often lately I've even used 55% threads and have been amazed both by their strength and the ease of tapping the hole.
                    Below some info that may make your day less stressful.

                    http://tapmatic.com/tapping-questions/tapping-torque-vs




                    http://blog.cnccookbook.com/2012/06/...e-broken-taps/



                    -
                    OSG suggests 55-65% threads for most applications. The force required to drive a tap is way less than for 75%. This is the range I use most of the time unless I’m really worried about strength. If the length of the threaded hole is more than about 3x the diameter of the fastener, 55-65% threads are very likely going to be stronger than the fastener itself anyway.

                    Last edited by Willy; 03-11-2015, 10:32 PM. Reason: grammar
                    Home, down in the valley behind the Red Angus
                    Bad Decisions Make Good Stories​

                    Location: British Columbia

                    Comment


                    • #11
                      Originally posted by garyhlucas View Post
                      Say what? That second picture is NOT a spiral point tap! That's a starter or maybe a plug tap. It doesn't have the tapered lead in that cause the chip to spiral forward of the tap and out the back side of the hole instead of traveling up the flutes.
                      The lighting in PixMan's pic may not be the best as I think the details are hidden in the shadows but it shares the same profile as the spiral point tap below.

                      Home, down in the valley behind the Red Angus
                      Bad Decisions Make Good Stories​

                      Location: British Columbia

                      Comment


                      • #12
                        Originally posted by PixMan View Post
                        For through holes, a spiral point tap is the choice. Here's an example of a top quality one, in this case one for austenitic stainless steels.
                        IMHO the word spiral was typed when the word gun was meant. Gun taps shoot the chips forward.

                        Comment


                        • #13
                          I frequently have the pleasure of tapping 1/2" armor plating with the cheap TiN taps. new guys break ten taps before noon, and spend hours grinding broken taps flush and trying to tap two more holes somewhere other than where the customer specified.

                          meanwhile the GHOGs ( grey haired old geezers ) have the other door tapped and the accelerometers mounted before noon. the young guys ask the old guys what they are doing wrong. a 60 year old high school graduate who was doing a thing before a college guy was born can't tell that college boy a thing until college boy asks.

                          the gray haired old geezers figured out a long time ago:

                          Its' called a tap for a reason.

                          It isn't called a "put that puppy in a drill and spin it up to 1,000 RPM. puh-DUKSHUN, son, that's what it's all about!"
                          It isn't called a "put a cheater bar on that sumbatch and teach it the meaning of torque!"
                          and it isn't called a Timex because you wind it like a watch.

                          if you apply the tiniest bit of reason, you can see that the only part of the tap that removes material is the one to 5 tapered threads at the front. all the rest of the threads guide, support, and get jammed up with the crud the one to 5 tapered threads at the front break loose. do not apply torque at the tip of a brittle steel rod by twisting.

                          you operate a tap by tapping. when you remove the tap to blow the crud out with compressed air, you back it out until it binds, then you tap until it turns. don't torque, don't twist, tap.

                          there are pockets of even harder steel in armor plated steel. you can feel them when your tap hits them. when you get to the point where you feel one at the edge of your tap, you don't break taps.

                          side note: drilling armor plated steel: Norseman jobber AG 190s. keep plenty of spares on hand. when someone working for another contractor tries drilling armor plated steel with HSS bits, hand them an AG 190 with a drill stop made of duct tape and tell them not to punch through the steel and hit the 30,000 PSI concrete inside, because the drill is instantly ruined. they give you a goofy look, stare in amazement as the drill goes through the armor plate like it's brass, and tell you they broke all the bits you loaned them and where can I get these?

                          Comment


                          • #14
                            There are spiral point taps (sometimes called gun taps)
                            and there are spiral flute taps. Some noobs think all
                            spiral taps have to look like a twisted roller coaster.
                            Spiral points often are miscategorized or go unnoticed.
                            Don't be a noob, learn your taps.
                            Also, a 2 flute gun tap is the strongest (against breakage)
                            when tapping tough material. This is because less meat is
                            ground away to form the flutes.
                            --Doozer
                            DZER

                            Comment


                            • #15
                              Originally posted by kf2qd View Post
                              P.S. As far as I am concerned, small 3 or 4 flute taps are only good for chasing threads, A 2 flute spiral point is much stronger and gives better, faster results.
                              I have to agree with this. On the smaller taps I do much better with a 2 flute.

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