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Boring bars Q .... round 2

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  • Boring bars Q .... round 2

    The otehr discussion of boring bars brought up an interesting question. I didn't want to hijack an old thread, so.....

    In older books on machining, I often see pictures of a boring bar in use. In almost every case the bar is sticking out a lot more than 4x diameter.

    However, in most every case, there is as much bar sticking out behind the holder as there is in front.

    Thinking about vibrations, that suggests that the rear extension will absorb some of the energy coming from the front part. After all its resonance should be around the same frequency, some energy should be transferred to it.

    The overall resonance would likely be a "bow" resonance with the middle coming "up" as the ends go "down", or vice-versa. The alternative is a "rocking" with the support as the pivot. Since I have also seen in pics some form of extemporized damper hung from the back, that is some confirmation of the "transfer effect" theory.

    Naturally, a common tapered "sheeps foot" boring bar has no such feature, they have no "tail". They also tend to chatter.

    So far I have not tried to confirm this, but it makes sense to me. Any thoughts?
    1601

    Keep eye on ball.
    Hashim Khan

  • #2
    A microphone, and a scope, and a small hammer......?
    Gene

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    • #3
      Make the boring bar out of pieces of a golden rectangle.
      Free software for calculating bolt circles and similar: Click Here

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      • #4
        The extra bar material doesn't absorb any energy. Where's the loss mechanism? All that happens is that you have twice as much steel vibrating. Energy does indeed transfer from the excited end to the other, but that doesn't mean that it leaves the excited end and stays away. Consider a tuning fork, but one striaghtended out so that it's no longer shaped like a fork, but shaped more like, well, a boring bar. It will still vibrate merrily whenever either end is excited.

        On the other hand, it is possible to remove energy at the tail end, but there has to be some loss mechanism to do it. A suitable loss mechanism might be a rubber pad and a substantial weight, the resonance frequency of the two tuned to be close to the resonance frequency of the bar. A metal spring won't work as a substitute for the rubber pad, as it's not lossy enough to convert stretching/compressing motion into heat. All that a metal spring would do is split the resonance peak into two slightly smaller peaks. Each is smaller but there are twice as many of them - no net improvement. The reason yuo want the damper pad and weight tuned to resonate is that the motion of the pad is maximized, which means that energy removal (conversion to heat) is also maximized. Another trick, following the same rationale, is to put the damper someplace where resonant response is large, such as out at the end of the bar.

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        • #5
          I don't think it's neccasary to "tune" out the resononance. If you where to just dampen it somehow the effects would go away.

          Turning brake drums, it is common to wrap a heavy rubber band around the drum to keep it from ringing. The boring tool itself did not vibrate, the drum did.

          If the length of the bar where causing it to vibrate wouldn't it stand to reason that simply dampening it with whatever would have a similar effect?

          I have seen long thin boring bars with a piece of inner tube slid and tied on to deaden them.

          Just tap on a bar with something and then listen to what it sounds like if you tap on it while touching it with your fingers.
          Gene

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          • #6
            Well, as far as energy leaving/staying away, or splitting the resonance peak..... that looks like it actually would have an effect....

            Seems that is a decent way of cutting the chatter ampltude in half....or at least to 0.707 of previous, on an energy basis. Energy going to the backside isn't present in the frontside, and the amplitude of vibration is related to the energy input.

            As far as damping, there would be damping of whatever amount the material in herently has in both ends. So yes I would expect there should be more losses, and so lowered vibration.

            Also possible that the dominant vibration mode lowers the forces transmitted to the holder.. Hold flexible stick at one end, twist wrist to shake opposite end up and down. Then hold a stick of double length in middle, and shake up and down to get similar amplitude. Second method takes a lot less wrist effort, so forces on the holder must be lowered, lessening "wind-up" due to chatter, and potentially damping better.

            Anyhow, there must be a reason why the old guys did that, and I don't think its because they were too cheap to cut the bar..... not when its 3 or 4 times longer than required....

            Anybody actually DONE that?

            What happened?

            Theories are one thing, but experience is another.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              "Hold flexible stick at one end, twist wrist to shake opposite end up and down. Then hold a stick of double length in middle, and shake up and down to get similar amplitude. Second method takes a lot less wrist effort"

              What you propose is not at all analagous to the situation with the boring bar. What might be a better analogy is holding a stick with a little man jumping up and down on the end. You'd still have to provide a moment to counter the impulse created by the little dude; the best you'd get is a little bit of counterweight from the backside of the stick, but that's very small compared to torque/moment necessary to support the jumper.


              One interesting and slightly relevant thing is that hard drive cases are built of "damped sheet metal." This is actually two layers of sheet metal with a viscous gel sandwiched between them. The viscous gel acts as a damper to absorb any vibrational energy imparted from the spinning disk drive. The difference between a solid case and the damped one is like night and day. If you drop the solid one on a desk it makes a very typical "clang" sound. The damped one simply makes a quiet thud. It would be interesting to see if this could be applied to boring bars.

              -Justin

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              • #8
                <font face="Verdana, Arial" size="2">Originally posted by J Tiers:
                Theories are one thing, but experience is another.</font>
                Hey, I'm not getting this stuff from a ouija board or, perhaps worse, Google. About a dozen of the courses I aced in my MIT days were in shock and vibation control. They were scattered through ME, EE, and Aero & Astro - same physics, different math. They weren't on the required course lists but they were there if you bothered to hunt them down.

                The vibration dampers I designed a few years later at GCA worked, and worked spectacularly well, even if the damned electrical engineers didn't understand how (no wires going in, so obviously they couldn't do anything, right?). GCA made wafer steppers. Those make dies (integrated circuits on silicon warers) for microprocessors and large memory chips. We were using geometries/line widths of about one micron in those days, about the best you can do with visible light in a production environment. Not surprisingly, vibrations on the order of a micron in amplitude were fatal to the machine's operation. Those are what I eliminated with tuned dampers. Theory and practice, an unbeatable combination.

                Don't believe it? Suit yourself.

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                • #9
                  I single wrap heavy gauge lead solder around the bar if it tends to chatter. It changes the resonant frequency but it also dampens the bar because it is slightly loose. As it vibrates the soft solder is slightly deformed. Lead is pretty lossy stuff, not much rebound.
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                  • #10
                    Nobody has actual experience? Hmmm.... Seems like some old-time machinist type must have done this and know something.....

                    <font face="Verdana, Arial" size="2">Originally posted by sauer38h:

                    Hey, I'm not getting this stuff from a ouija board or, perhaps worse, Google. About a dozen of the courses I aced in my MIT days were in shock and vibation control..........

                    Don't believe it? Suit yourself.

                    </font>

                    Don't get all in a twitter.....

                    I took some similar courses too.... just not as many as the ME's.

                    But I have noticed that most questions can be looked at several ways. Some ways are wrong, some are true but actually irrelevant, and some are the real key.

                    I am well aware of tuned dampers. EE's use them too....

                    In this case, tuning may be somewhat irrelevant, if only because it isn't clear what to tune to... I typically hear a number of distinct "pitches" of chatter.... its not all one tone. And, the structure isn't relatively fixed and known, nor is the driving signal.

                    Since the use of the double length bar is common to a great number of different people in different places, there is a strong suggestion that it may be helpful. The question is whether it is, and if so, how.

                    We have theories, right or wrong, but those who use it have experience.... and experience usually trumps an incomplete or wrong theory..... just consider the fact that the bumblebee can fly.....

                    You asked a question, which I answered....

                    The loss mechanism is the material inherent damping, whatever that is... And whatever it is, there are two of the vibrating bodies with losses, instead of just one.

                    If the same energy comes in the front arm, it is split in some proportion between the front and back, and logically, the amplitude might be smaller at the front end as a result. If not, the addition of the rear portion must increase the energy input in some way that isn't clear to me.

                    If there are two frequencies, that also may be helpful since each should be lower in amplitude. You must have used that feature in mechanical work at some time, EE's do it.

                    And, your tuning fork analogy may actually be better than you think..... consider it again....

                    First, the tuning fork as a "fork" has symmetrical vibration vs its axial center line... The handle is along the neutral line for side-to-side vibration, and is itself symmetrical. Any energy going there is largely due to non-symmetrical arms. Theoretically, they would be balanced.

                    Some small amount of energy is in an axial component of movement due to the arms vibrating in an arc, not straight. That is why you can put the tuning fork end against a surface at right angles, and get a sound. Not very much sound laying the knob on sideways though... Keep that thought....

                    Second, not all materials will make a good tuning fork.... quite a lot of them don't. The bad materials tend to be non-hardened, soft, materials, not unlike HRS, or even CRS. HRS, for instance, makes a crummy bell, it will "tink" but usually will not ring well compared to bell metal. That shows damping internal to the material.

                    Now, when you straighten out the tuning fork, its NOT a fork, and the vibration modes are entirely different. No longer is the handle a neutral point. The previously small lengthwise component is made a major component, and the symmetrical component is tiny.

                    You can have a rocking mode vibration, with front going up and back going down. Similar to a balance wheel in a clock.

                    You can have a "flapping" resonance, with back and front going up, and center the opposite. The latter is the modified regular tuning fork resonance, the axial component exaggerated.

                    A rocking mode vibration cannot be sustained unless the "handle" or mounting point is rigid enough , otherwise there is no restoring force.

                    The "flapping" mode is possible as a simple suspended bar. Reference xlyophone.

                    But the action at the center support is entirely different for symmetrical "flapping" of a bar vs one-ended "flapping". A xylophone must NOT have contact in the middle, it damps the sound. Support must be at the two node points.

                    In one case there is a couple around the middle, or mounting, point (also present in "rocking" mode) and in the other there is not.

                    For teh rocking mode....
                    With one end only, there is only the mass of the one end involved, while with two ends, there is the mass of the opposite end also.

                    If the bar has any rigidity, and the mounting has any flexibility, the opposite end of the bar must move if the cutting end does. And the mounting presumably has some flexibility (everything is made of jell-o).

                    For the "flapping" mode, it seems that the center point must exert a large restriction on that. Generally, a toolpost is far tighter against that vertical movement than it is against rocking. The xylophone is damped.

                    Either way, double the mass, with a somewhat lossy (we don't know HOW lossy" coupling between them. Possible conversion of an unsymmetrical force to a more symmetrical force. Seems logical that an amplitude reduction would result.

                    NO, I'm not ready to toss the idea that there is a benefit, just based on what you said.

                    I am not sure you have correctly understood nor characterized the action... Of course I may not have either....

                    I'd still like to hear from someone who has done it.



                    [This message has been edited by J Tiers (edited 01-19-2006).]
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      I do it all the time. Up until very recently I have always used a lantern style holder on my SB. I have an Armstrong boring bar holder as seen in the pic. You can see from the marks on the bar that it is always held near the midpoint of the bar.

                      I have never given it much thought but I have very few problems with chatter. If I do I wrap solder on the bar as I said, either on the left or right side of the bar. Sometimes I will clamp a small block of lead to the right side.

                      I think what matters here is the the overhang on the right is NOT the same resonant frequency as the left. The Q of the system is fairly high I would think so for a resonance between the two halves to occur the lengths and/or masses would have to be very close. If they aren't matched in frequency then it will tend to discourage resonances at either end since it will not likely be held at a node.





                      [This message has been edited by Evan (edited 01-20-2006).]
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                      • #12
                        Burghardt"s Machine Tool Operation, First Edition, 1919 shows the Armstrong type boring bar holder with the bar in about the 1/2 position.

                        He says "...the position of the cutting edge may be adjusted, will usually give better satisfaction than the forged (non adjustable) tool."

                        He addresses chatter farther on stating "When the shank is long, the spring of the tool is excessive, and the tendency to chatter is increased...... It may be avoided, usually, by having a boring bar of sufficient cross section, not too long, and held rigidly."

                        No mention of set up or leaving a protruding bar. I would guess the length is merely for convenience, and is often seen protruding as that is the best setup, with the shortest extension possible.

                        There may be a happy coincidence with vibrations canceling each other out, but it is just that, not a design factor. If it did cancel out chatter in a certain location, would there not be an equal possibility of chatter being induced if it were setup in another position?

                        Often it is necessary to "break" the rules when machining, i.e., boring a blind hole that is deeper than the optimum setup. When this situation arises, the extra length will be needed.

                        There is some interesting information on grinding a cutter to reduce spring into the work. He also stresses minimal length in other sections.

                        [This message has been edited by JCHannum (edited 01-20-2006).]
                        Jim H.

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                        • #13
                          That's interesting, and typical of the pics I referred to, although your total length is somewhat shorter than I have seen. You actually are not far from the 4:1 ratio, counting from the actual cutter location.

                          1601

                          Keep eye on ball.
                          Hashim Khan

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                          • #14
                            J,I think it all has to do with speeds and feeds with some bearing on cutter grind.

                            I think the key is,the examples in the books and the one given by Evan both have something in common,lathes that don't spin up more than 500-600rpm and have plain bearings.

                            I have noticed that the old conehead machines I have used have much less problems with boring bar chatter than modern gearheads do.I think it's partly the fact that they use journal bearings and most don't spin up very fast,or at least fast enough to generate resonance.
                            I just need one more tool,just one!

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                            • #15
                              take this along with a grain of salt but.....


                              I thought when shooters wanted to tune the vibrations out of a barrel they rested it approximately 1/3d back from the end on a nylon button. Why is a boring bar not held in the same manner to avoid harmonic relationships instead of enhancing them? If it's a dumb question forget it came from me
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