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Annealing and re-hardening fasteners

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  • Annealing and re-hardening fasteners

    If possible, I would like to to anneal high grade fasteners, drill and tap a hole, and then re-harden. Is there any feasibility to this idea?

  • #2
    Hi gmm,
    I do a lot of tool hardening and tempering. The main thing is to know is, exactly what metal you are working with, have very precise control over hardening temperature and Quench medium temperature, have precise control over temper temperature.

    That being said, with experience, you can perform all of these tasks by feel and eye.

    With a bolt, be sure you understand the engineering aspects of what you are your or someone else's safety involved?

    Otherwise I would research by contacting the fastener mfg. and go from there.

    Short answer is if it is a heat treatable alloy you can Anneal and re harden a bolt or anything else made of that alloy.

    Ahh yes!
    As S&S has indicated...look out for liability!

    Also, by drilling and taping the fastener you have reduced it's x section and mass, thereby making it weaker!
    Do careful engineering and some testing!!!
    Last edited by PaulF; 05-23-2008, 01:56 AM.


    • #3
      GMM, I regularly through-drill and tap high grade and 316 SS fasteners, no annealing necessary.

      I won't weld fasteners for the same reason. If it's an outside job, you can never guarantee that they will be in original spec.
      Just got my head together
      now my body's falling apart


      • #4
        By annealing the bolt you will loose the grain structure that is running in a nice curve from the shaft to the head and loose strenght when rehardened.
        Rehardening in the right manner of course.
        The bolt is work hardened and the strenght of the material is not unidirectional.


        • #5
          I've often machined, drilled and tapped SHCS which are slightly better than Grade 8. (A piece of the threaded section drilled & tapped makes a good quick & dirty thread insert.) I believe they are in the 30's Rc which is the same as 4140Prehard.

          As Paul says watch for liability. It's amazing how when someone finds out I have a home shop, they want me to work on their car or gun. Yeah, I bought all these tools so I could carry out their half-ass plan and then get sued when they get hurt.
          Jon Bohlander
          My PM Blog


          • #6
            You can turn, cut, drill and tap Grade-8 bolts without anealing. I do it all the time. Well, not all the time but a lot.


            • #7
              MoldMonkey, you only get sued if you have liability insurance. Don't have insurance, don't get sued, its that simple. They will probably go after the manufacturer of your machines for selling to an uninsured person, or the drill manufacturer.

              Why do you think they go after gun manufacturers, not the [email protected]@hole that actually pointed the gun at a person and fired it?

              GMM22, listen to these guys, a "high" grade fastener is not the hardest thing in the world to machine. A grade 8 fastener made out of 4140 does land in the standard "prehard" range, not a big deal and no need to get fancy. The problem lies in the fact that graded bolts are based on strength, not material and heat treat, so it is a bit of crap shoot, just be conservative and you'll be fine.


              • #8
                Thanks for the replies. I am very surprised to learn that fasteners up to grade 8 could be drilled and tapped without annealing, but I must admit, I have never considered trying. There is no concern about liability as my reason for drilling fasteners is very innocuous.

                I am wondering if the size of tap could present an extra difficulty. I want to basically tap medium to high grade cap screw heads perpendicularly with a 4/40 tap, which is pretty small. I have snapped small hand taps before, but I guess I would put this down to inexperience. Of course the other trade off is one is inclined to buy inexpensive taps to soften the blow with breakage, and yet better quality taps are certainly apt to break less.


                • #9
                  GMM22, DO NOT SKIMP ON CUTTING TOOLS. Not a hard and fast rule. Expensive does not mean good, and cheap does not mean crappy. Buy quality.

                  Especially with small taps, I have seen the biggest difference in tap quality than any other tool. Buying 10 $4 taps may get you 20 holes and 10 scrapped out parts. A $15 dollar tap will get you 80 holes and no scrapped parts. On the bigger taps its not as bad. For best performance, and longevity a spiral point(gun tap) is hard to beat, even if you have to drill deeper in a blind hole. Stick to the name brands on the small stuff.

                  I like Greenfields(kennametal). OSGs, anything with a brand name will(should) do you good. A 4-40 is not the time to go cheap. "hand-taps" are not what you need, especially in that size.


                  • #10
                    All good advice

                    One point I should have made earlier, I always use flood suds on the lathe and tap-magic when (hand) tapping.
                    If a through hole (which mine usually are) use air to blow down swarf.
                    On stainless, especially, take it slow! A fraction of a turn, then back up. Stainless can be gummy.

                    Cheers, Lin
                    Just got my head together
                    now my body's falling apart


                    • #11
                      Don't know if they go that small,but the Greenfield EM series taps are tops in difficult materials.They have two different styles though that vary according to material hardness,the rc30 everyone has mentioned sounds about right.

                      When drilling hardenable material always use a sharp good quality drill set for the proper speed and a squirt can of coolant helps.
                      I just need one more tool,just one!


                      • #12
                        Expensive does not mean good, and cheap does not mean crappy.
                        When it comes to cutting tools I have found that Cheap always means Crappy and never more so than with taps and dies. High strength alloys depend on expensive metals such as cobalt to give them the required properties. The term "high quality cheap cutting tools" is an oxymoron.
                        Free software for calculating bolt circles and similar: Click Here