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Excellent online article about dealing with chatter

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  • Excellent online article about dealing with chatter

    See here:

    http://www.mmsonline.com/articles/060101.html
    Free software for calculating bolt circles and similar: Click Here

  • #2
    Very interesting, esp. the varying RPM's part, I wonder how much (%) it varied per revoultion, I bet it was consistantly mismatched so as to not line up in the same degree's as the last cut.

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    • #3
      They seem to be dealing with equipment that does NOT have the issues of home shop stuff......

      We often have a huge 'forcing function" in the 120Hz torque ripple, plus loosey-goosey worn equipment.

      I found a huge difference going to 3 phase...... all the standard fixes suddenly worked. With single phase, nothing worked, slow, fast, all same-same.
      1601

      Keep eye on ball.
      Hashim Khan

      Comment


      • #4
        Originally posted by A.K. Boomer
        Very interesting, esp. the varying RPM's part, I wonder how much (%) it varied per revoultion, I bet it was consistantly mismatched so as to not line up in the same degree's as the last cut.
        You don't have to constantly vary the RPM, you're just avoiding the natural resonant frequency of the system, which would be the combined mass and vibration of the machine and the workpiece.

        The mass dampers that I mentioned here awhile back, often seen in high-end boring bars, do the same thing -- they have a mass held in tension with a spring, and you tune the boring bar with a set-screw to damp out the chatter.

        So in a home shop, you can do the same thing to change the natural resonant frequency of the chatter: change the RPM (up or down), increase the mass (put a lead weight on the toolpost), change the stickout, ...
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

        Comment


        • #5
          Old lead solder, no longer popular, works great when wrapped around or "sprung" inside a tubular turning. Same goes for "leading" used in old letterpress printing.

          On my Harrison M300, a small (20lb) bag of lead shot is amazing at dampening low frequency resonances that can be heard from the sheet metal (even though its a heavy gauge). Figuring out how to place and attach some of it is on the long term todo list
          Den
          Last edited by nheng; 08-17-2008, 01:18 PM.

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          • #6
            Using a hollow boring bar filled with lead slugs and tuned with a setscrew works to.You can play with speeds and feeds all you want,but when you reach resonance in that little boring bar shank nothing else will help.

            I noticed too recently the Chinese have entered the solid carbide,inserted boring bar market.
            I just need one more tool,just one!

            Comment


            • #7
              I find continuously variable speed on my lathe's DC motor is very helpful for chatter. I learned early on that faster works nearly as well as slower. When I hear chatter, I grab that knob. Don't have a VFD on the mill yet, so can't try it there.

              Evan, you need to whip up an el cheapo spectrum analyzer that plugs into the PC running Mach 3 so you can see the resonances. You could then turn the spindle override to minimize the harmonics. Somebody probably already has this, maybe even as a function of the software o-scopes that are available. The freq range is not very demanding in this application, so the electronics should be cheap.

              There are expensive systems that will do this for you that are outside the scope of the HSM. Usually they just analyze the harmonics and suggest an optimal spindle speed, but some are closed loop spindle controllers.

              Cheers,

              BW
              ---------------------------------------------------

              http://www.cnccookbook.com/index.htm
              Try G-Wizard Machinist's Calculator for free:
              http://www.cnccookbook.com/CCGWizard.html

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              • #8
                Evan, reading that MMSOnline made me go track down my favorite article on getting systematic about chatter:

                http://www.mmsonline.com/articles/100501.html

                Machining those sample blocks with steps gives you a very effective tool for understanding the performance envelope of your mill when using your favorite cutters.

                I am fortunate to have 2 identical Industrial Hobbies mills. One is currently being converted to CNC, and I've done an epoxy granite fill on the base and part of the column. I intend to use that step-block method to determine qualitatively whether the modifications actually made a difference or not.

                It seems to me a good way to get systematic about it.

                Cheers,

                BW
                ---------------------------------------------------

                http://www.cnccookbook.com/index.htm
                Try G-Wizard Machinist's Calculator for free:
                http://www.cnccookbook.com/CCGWizard.html

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                • #9
                  Bob, The bar chart at that link showing the optimal (stable) RPMs for their test setup is interesting.

                  It makes you wonder if you really need fully variable speed or VFD when there may only be a few "sweet spots" in the speed range that give quiet, stable cutting with an excellent finish. Of course, I guess you need the control in order to find them if the mfgr. hasn't already done that for you (by gear or pulley ranges).

                  There's a cheap and dirty spectrum / vibration analyzer which we all have. Just rest your coffee cup (beer mug) or cup of water on the machine table, compound rest, etc. The pattern in the water will show you the pitch you're hearing and also a rough indication of the intensity of it.

                  Den
                  Last edited by nheng; 08-17-2008, 04:26 PM.

                  Comment


                  • #10
                    Originally posted by nheng
                    Bob, The bar chart at that link showing the optimal (stable) RPMs for their test setup is interesting.

                    It makes you wonder if you really need fully variable speed or VFD when there may only be a few "sweet spots" in the speed range that give quiet, stable cutting with an excellent finish.
                    There will be one optimal RPM for each machine setup, but you'll never have one RPM that's optimal for all setups. Like I said, you're tuning the resonant frequency of the system, and that frequency changes depending on the workpiece mass, the cutter mass/rigidity, etc etc...

                    That's why the tunable boring bars are - ahem, tunable They also come in sets of several boring bars, because even a single tunable mass-damping boring bar is not enough to cover the full spectrum of resonant frequencies you'll find on a machine tool.
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                    Comment


                    • #11
                      Originally posted by nheng
                      Bob, The bar chart at that link showing the optimal (stable) RPMs for their test setup is interesting.

                      It makes you wonder if you really need fully variable speed or VFD when there may only be a few "sweet spots" in the speed range that give quiet, stable cutting with an excellent finish. Of course, I guess you need the control in order to find them if the mfgr. hasn't already done that for you (by gear or pulley ranges).

                      There's a cheap and dirty spectrum / vibration analyzer which we all have. Just rest your coffee cup (beer mug) or cup of water on the machine table, compound rest, etc. The pattern in the water will show you the pitch you're hearing and also a rough indication of the intensity of it.

                      Den
                      Oh ho, Den, you amazing stud!

                      I love the coffee cup analyzer.

                      Your intuition that there are particular "optimal ranges" is spot on. There are optimal ranges for spindle rpm, feedrate, and DOC.

                      Here is a fascinating mathematical model that tries to predict those ranges:

                      http://highspeedmachining.mae.ufl.ed...selection.html

                      If you wade through all those parameters you get a set of sawtooth graphs that show you the optimal ranges. For the sake of seeing what the shape of the curves are, just click the button at bottom without changing any parameters and you'll see those optimal ranges. It gives you an idea how chatter moves in and out based on bands.

                      Of course these models assume you have the pedal to the metal on every parameter, so if you have a very light depth of cut + slow feeds you may not see very pronounced bands around spindle speed.

                      Cheers,

                      BW
                      ---------------------------------------------------

                      http://www.cnccookbook.com/index.htm
                      Try G-Wizard Machinist's Calculator for free:
                      http://www.cnccookbook.com/CCGWizard.html

                      Comment


                      • #12
                        There may BE no "sweet spot" if you have a strong forcing input such as, but not limited to, 120Hz 100% torque ripple.

                        Once the drive is not creating the problem, i.e. if you have a VS drive of some sort, then you almost certainly will find good spots.
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          You sound like a reformed smoker Jerry since you converted to three phase.

                          My mill very definitely exhibits "sweet" rpm ranges when taking very deep cuts. It doesn't much matter what the material is either, it's the frequency that matters. Usually I find that I am cutting too slow, especially when cutting steel.
                          Free software for calculating bolt circles and similar: Click Here

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                          • #14
                            Originally posted by J Tiers
                            I found a huge difference going to 3 phase...... all the standard fixes suddenly worked. With single phase, nothing worked, slow, fast, all same-same.
                            I should have gone to 3 phase 8 years ago...It was going to cost $10 000 just for the power company to install it to the meter box...run one wire 1500 feet and a transformer..

                            I would hate to think what they would charge now...
                            Precision takes time.

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                            • #15
                              You can always get a PhasePerfect. Better quality 3 phase than line they say.

                              Way back when I had a 7x12 and I had some issues with chatter i found varying the speed help stop resonance.

                              On a mill get variable flute end mills. The uneven spacing of the flutes stops resonance.

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