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  • Thread forming vs cutting taps

    What are the disadvantages of a thread forming tap over a thread cutting tap or are there any?
    John

  • #2
    In soft materials the form taps produce a stronger thread. But in tougher materials I wouldn't even think about them. The tap drill sizes are quite abit different as in a standard tap the material is cut while the form tap actually displaces the metal into the thread form. Consequently they require larger holes to start with. As for a tap drill chart for them I don't know if it a standard size for all materials or if if varies by alloy . Plus I wouldn't even think about using them dry even for "straightening" out a damaged thread
    Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

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    • #3
      In my shop threads done with dies look much better and you ain't as likely to get cut on any jagged edges! I expect there are places you can go single point that you have a hard time with a die. Certainly on size. Some of the big stuff gets real expensive real fast.
      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
      Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

      It is true, there is nothing free about freedom, don't be so quick to give it away.

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      • #4
        One more question, softer material being aluminum and brass or cold rolled steel?
        John

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        • #5
          Aluminum yes, CRS can be done. Brass. I don't think I would try it. But then it would depend on the material. Anything that is half-hard, say Alloy 360 in its standard as sold condition IMO does not form well unless it is anealed. Try bending a piece of flat stock and see what happens. It usually is not pretty
          Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

          Comment


          • #6
            Disadvantages of thread forming taps......?

            The tap drill hole size is much more critical in thread forming, a few thou' difference makes a great deal of difference in percent of thread.

            Forming taps take more torque and don't work as well for hand tapping.

            In some materials the required torque can be so high that conventional workholding may not be enough. Like forming 1/2-13 in 5/8" SS304 round bar, a collet may not have enough grip to do the job, even a 3 jaw chuck is iffy.

            The cost per tap is greater, but that should be paid back easily with the number of holes that can be done with the tap.

            Takes a bit more experience to fully understand the usage than a cutting tap.



            On the other hand the advantages.....

            Lots and lots.
            Last edited by DR; 11-21-2006, 03:45 PM.

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            • #7
              I have a small set of cold form taps. Popular SAE sizes. A fellow at work swore by them for softer stuff. So I bought a set or two, Greenfield brand. They do work nice.

              Here is the tap drill recomendations that came with them. If I'm not mistaken, there is a listing for the tap drills in the Machinist handbook as well. I think that these were close to that listing.

              -tap---------75%-------65%
              6-32-------ط0.1221---ط0.1243
              8-32-------ط0.1481---ط0.1503
              10-24------ط0.1688---ط0.1717
              10-32------ط0.1741---ط0.1763
              1/4-20-----ط0.2245---ط0.2280
              1/4-28-----ط0.2318---ط0.2343
              5/16-18----ط0.2842---ط0.2879
              5/16-24----ط0.2912---ط0.2941
              3/8-16-----ط0.3431---ط0.3474
              3/8-24-----ط0.3537---ط0.3566
              7/16-14----ط0.4011---ط0.4059
              7/16-20---ط0.4120---ط0.4154
              1/2-13----ط0.4608---ط0.4660
              1/2-20----ط0.4745---ط0.4779

              rock-
              Civil engineers build targets, Mechanical engineers build weapons.

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              • #8
                Originally posted by Spin Doctor
                Aluminum yes, CRS can be done. Brass. I don't think I would try it. But then it would depend on the material. Anything that is half-hard, say Alloy 360 in its standard as sold condition IMO does not form well unless it is anealed. Try bending a piece of flat stock and see what happens. It usually is not pretty
                Spin, according to the clerk at Metal Supermarket, aluminum sheet and alumium round stock are two differant animals. Flat stock will crack, workharden or fail a bit sooner than aluminum round stock. Can anyone else confirm this?

                jpr, I've theaded brass rounds stock with no trouble using dies with no problems.
                - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                It is true, there is nothing free about freedom, don't be so quick to give it away.

                Comment


                • #9
                  Originally posted by Your Old Dog
                  In my shop threads done with dies look much better and you ain't as likely to get cut on any jagged edges! I expect there are places you can go single point that you have a hard time with a die. Certainly on size. Some of the big stuff gets real expensive real fast.
                  Practice makes perfect Dog. With a little practice you can make a thread with a single point tool look as good or better than any done with a threading die. Most threads I've seen cut with dies that weren't perfectly sharp looked worse than any cut with a single point tool. Also one single point tool is a lot cheaper than a bunch of dies. The biggest cause of rough threads is running the work to slow while cutting the thread and no lubricant. At work we use thread forming inserts on the lathe that actually cut the top of the thread behind the one being cut normally which makes a very smooth thread on the top. The only problem with this is you must have an insert for each thread pitch which is way to expensive to use in the home shop.
                  Jonathan P.

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                  • #10
                    YOD, A lot depends on the alloy and the temper/heat treat. For example 6061-T6 in round or flat should have the same properties. The aluminum alloys I have worked with most are 2024, 6061 and one in the 7000 series in various temper states. The 2024 is tougher than the 6061 but the 6061 machines better. We ordered (me actually) both materials in round and flat. We also used some hollow bar and hex.
                    Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

                    Comment


                    • #11
                      Never had any thread forming taps, but I have used the process in aluminum. Just today I had a video tape deck with a stripped out hole in aluminum sheet for a M3 screw. After consulting the table of sizes, I took a 6-32 SS screw and just forced it in. Tooling was a #2 Phillips screwdriver and the screw itself. Perfect fit and it tightened up perfectly. I didn't check the percentage of thread but I suspect it was at least 70%. I have used this trick before many times. It seems to work best when using an English size to replace a metric or a metric to replace an English.
                      Paul A.

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

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                      • #12
                        A forming tap is stronger than a cutting tap.
                        Think about it a cutting 2 or more flutes to weaken
                        the tap where as a forming tap has just a small
                        grove to let lubricant down and to prevent
                        hydraulic locking. also not round as they appear
                        if you roll one between your fingers you will know
                        what I am talking about.
                        As for just soft material, rubbish
                        at work we have a part made of cast 316 stainless
                        that gets 6-32, 10-32 and 1/4-20 thread. we do those with forming
                        taps, we lube with Tap heavy. we also use a CNC to do this.

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                        • #13
                          Thanks for your patience with my stupidity guys! I miss read the question. (Gawd, if old what's his face was still here I'd be sliced and diced by now )

                          But since I've already bared my stupidity let me ask this. Wouldn't a thread that was formed as opposed to cut be a stronger product with a smoother finish? Wouldn't a formed die act a bit like forging a product for increased strength?
                          Last edited by Your Old Dog; 11-22-2006, 07:40 AM.
                          - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                          Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                          It is true, there is nothing free about freedom, don't be so quick to give it away.

                          Comment


                          • #14
                            Dog I'm pretty sure that the rolled threads are stronger than cut ones also just like axle splines that have been rolled are stronger than cut ones. If you are talking about a regular thread die they cut metal as they are run up on the part so the threads are still probably no stronger than if done with a single point tool in the lathe.
                            Jonathan P.

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                            • #15
                              A nice trick for making threads in sheet metal where the material thickness isn't enough for normal threads is to use a form tap with a tap drill smaller than recommended. Since the hole is small the excess material will extrude out the back side when tapping to create a longer length of thread engagement.

                              This method takes a few tries to determine the best undersize tap drill.

                              Be cautious with tough sheet metal like some of stainless alloys, they may not extrude well and you could break a tap. For soft aluminum this works well.

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