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Quality US made boring bar HSS bit

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  • #46
    Originally posted by Doozer View Post

    I have been reading about this in the forums for actually years.
    I understand elastic modulus. I have 2 pieces of paper to prove it.
    What I know about boring bars is this...
    Stress is how much force is put on something.
    Strain is how much it bends.
    I don't think hardening by itself affects the modulus all that much - not enough to be significant anyway.

    Generally bars that are hardened are made out of material than can be hardened. And usually the good bars are made out of tool steels. I think the alloying elements found in tool steels would affect the modulus much more than the hardness. Most cheap bars are made out of the lowest carbon steels available.

    A2 tool steel has an accepted modulus of 203 GPa. O1 (my favourite) has an accepted modulus of 214 GPa. Why is O1 higher? Tungsten. Tungsten is what gives carbide its high modulus.

    If you do a lot of material testing on plain carbon steels you will find that the modulus can be as low as 190 GPa or as high as 205 GPa. So if you are a very discerning person (and most machinists are) the potential 10% or so increase may make a difference in a cheap steel vs tool steel setup.

    If you think hardening changes the frequency response, there maybe something to that but I would probably suggest it has more to do with the way the part is clamped at the tool post than it does with actual damping of the material itself. And in terms of damping of the actual bar you want something like cast iron, which has very good damping properties. Except that from a deflection standpoint cast iron's modulus is way too low.
    Last edited by enginuity; 12-02-2019, 09:52 PM.


    • #47
      In one of the shops I worked, we had a outstanding lathe man named Jerry and he had various techniques to stop
      vibrations when deep boring on our dies. He had Lead wraps that clamped on his boring bars ( close fit ! )
      When the bar was short and stuck out to the rear, he clamped on the tail stock side , and when deep (extended) it was clamped near the tool bit.
      He also used sand bags, either tied onto the work, and to the carriage.
      When doing thin wall aluminum on a face plate, he used Plaster of Paris ( beware it rusts steel !) to support the work ( casted) and be chatter free.


      • #48
        in my opinion a hardened steel rod has a higher frequency because you're no longer hearing the fundamental frequency. the metal is hard and doesn't deform when struck with an object thus you're exciting harmonics with no dampening available at the impact of the hammer to dampen them. download a spectrum anallyzer app for your phone and see for yourself, or use a hard rubber hammer to strike the metal. it does make sense to me that you would see about a 1% increase in stiffness due to carbide content during hardening. but not the 50-200% increase in stiffness that would be required to correspond to how much "higher" the ringing sounds.
        Last edited by johansen; 12-02-2019, 11:47 PM.


        • #49
          Originally posted by PStechPaul View Post
          Perhaps it would be possible to make a stiffer boring bar by boring a hole through a steel bar and shrink fitting a tungsten carbide rod in it.

          That reminds me of an old method of making boring bars I read about: You bore a hole almost down to the tool holder end, thread the end of the bore, pour in lead, and once it cools you tighten a setscrew against the lead to put the steel in tension. The theory was that the tension makes it stiffer and the lead damps vibration.