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A micron sized carbide insert anti chatter boring bar, cheap.

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  • A micron sized carbide insert anti chatter boring bar, cheap.

    I bought some Valenite CPGT-val13780 inserts recently on sale for 6 bucks each. These are tiny little positive rake inserts that I wanted for a specific application, a small boring bar.



    The pictures are mostly self explanatory except for one thing, the anti-chatter feature. The boring bar is made from a grade 8 5/16" bolt with the head chopped off. It is drilled 1/8" about 1.5 inches deep. The hole goes past the shoulder by about .5 inch and accomodates a 1 inch slug of solid carbide from a broken tool. The remaining half inch of the hole is threaded to fit an 8-32 socket head cap screw. The cap screw is first drilled out slightly to deepen the socket to make room for the insert. The mouth of the bar is cut to a taper using a center drill and the seating face of the cap screw is cut to the same taper. It isn't critical as long as both tapers are somewhat similar.

    Then the screw is split with a hacksaw. The split is opened with a screwdriver blade enough to fit a flat needle file and the insides of the of the jaws filed to fit the shape of the insert so it is gripped by parallel flats top and bottom. This is easily accomplished by threading the insert holder into the bar until it just begins to grip the file.


    The outside of the screw head is filed with a pair of flats to allow a wrench to turn it and the sides of the jaws are filed to allow the insert to project slightly so it can cut.

    The anti chatter feature relies on two things: The step down in diameter and the carbide slug inside the bar. Tungsten carbide is much denser than steel and also much stiffer. By putting a slug inside it will disrupt any resonances inherent in the bar as well as stiffening it. To assist this action the hole is filled with oil and the carbide slug slowly pressed in as the oil escapes around it. I drilled the hole with the same size carbide drill bit as the broken piece so it produced a hole with a very slight amount of clearance. The oil acts to transmit forces to the carbide by making it act as a "squeeze film damper". When the bar bends the oil around the carbide rod must flow around the bar and absorbs energy when it does acting to damp the bar motion. The stiffness of the carbide insures that it doesn't bend right along with the bar.



    The question of course is how well does it work?

    Quite well it seems.
    The first image is of a gear made using powder metallurgy and then hardened. It was badly worn so I bored it out from 5/16 to 3/8".




    This next image is the finish attained on a piece of dead soft pure aluminum. That's the stuff that is normally very gummy and very difficult to machine with a good finish.


    Free software for calculating bolt circles and similar: Click Here

  • #2
    Originally posted by Evan
    To assist this action the hole is filled with oil and the carbide slug slowly pressed in as the oil escapes around it. I drilled the hole with the same size carbide drill bit as the broken piece so it produced a hole with a very slight amount of clearance. The oil acts to transmit forces to the carbide by making it act as a "squeeze film damper".
    That's pretty slick Evan -- that's a form of shear damper.

    You might try adding a spring or elastomer (i.e., rubber) pressing on or pulling the carbide slug -- that's how the commercial vibration damping bars work.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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    • #3
      I think I should make one...

      I have a few questions-
      1. do the remaining threads on the bolt serve any purpose? How do you mount this?

      2. It looks like there are ratchet teeth on the end of the conical cavity. Are these intentional and if so, how did you put them there and are they there to keep the insert tight, or for some other reason?

      3. What is the clearance between the carbide and the steel shank? Does the oil stay put by capillary action or some other means? Is the oil viscosity important?

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      • #4
        nice work evan. something i will be making very soon. add another project to the list!

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        • #5
          1. do the remaining threads on the bolt serve any purpose? How do you mount this?

          I left the threads on for the length and to show what it was made from. Other than that they serve no purpose. I mount it by clamping it in the turret tool post the same as other tooling.

          2. It looks like there are ratchet teeth on the end of the conical cavity. Are these intentional and if so, how did you put them there and are they there to keep the insert tight, or for some other reason?

          The "teeth" are purely a matter of lack of attention to detail. I believe I forgot to face the end of the bolt.

          3. What is the clearance between the carbide and the steel shank? Does the oil stay put by capillary action or some other means? Is the oil viscosity important?

          I would guess the clearance to be less than .001 but it really doesn't matter as long as it isn't rattling loose. It does require some clearance to function as a damper so using a regular drill bit the same size as the carbide will provide that. The oil was whatever I had handy, ISO 68 compressor oil I think. It won't run out because of capillary action. In fact it may not be possible to get the piece of carbide out either as it will be held in by suction.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Ignoring the anti-chatter feature for a moment, it's still a slick way of holding an insert. At first I skimmed past the taper on the mouth of the bar and the screw head...
            Do you really have to fine tune up the width of the slot on the screw socket (the "jaw" opening) so that it grips the insert just before the tapered screw head bottoms out on the taper of the mouth of bar?

            Thanks for another great home shop project writeup.

            -Matt

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            • #7
              Not much fine tuning is needed although filing the jaws to fit the insert is fiddly work. The taper is so steep that as long as the jaws are still spread slightly when the insert is in place it will screw into the taper and lock the jaws down tight. Also, replacement jaws are really cheap.
              Free software for calculating bolt circles and similar: Click Here

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