Router/Spindle flywheel to quiet chatter


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Thread: Router/Spindle flywheel to quiet chatter

  1. #1

    Default Router/Spindle flywheel to quiet chatter

    I am considering building a CNC router. I am in the wrap-my-head-around-it stage. Ideally it would work for wood and light metals.
    Has anyone seen the use of a flywheel added to a router/spindle to improve the cutting? I think there would be little impact on energy use. It's possible that some favorable damping could be obtained through flywheel construction, I haven't really thought about that. Are such flywheels built into any routers now, that you know of?

    The idea is the added inertia would even-out the load experienced by the motor. Obviously the motor rotor has rotational inertia: the question here is could such an added flywheel allow for a given motor to improve it's cut. My assumption is that the flywheel does not need to be very large or heavy, though I haven't made any attempt at evaluating that.

    This is somewhat related to the flywheel action of a large-diameter bit, but in that case the cutters' relative distance from center mean that the router bit body alone can't comprise sufficient rotational inertia. This explains their sometimes-difficult behavior.

    The major con is the added inertia with speed changes. If that's not a big problem, a smaller motor might be coaxed to cut like a bigger one.
    It could counteract vibration more directly, reducing damping capacity expectations on the rest of the gantry, etc.

    While fine-tuning might be necessary (size & weight), there might be wide tolerances for acceptable performance.
    The ideal place to mount it might be the other end of a double-shaft motor. A safety shroud could be a good idea in some cases. In that location it won't interfere with normal machine operation.

    I started off posting this in a wood router forum, but it occurs to me the idea would apply as readily to metal machining, so I begin to wonder if machinists had thoughts or experiences with it.

    If anyone experiments with this, I'd love to hear about it! Eventually, I probably will, but it will be months or years based on my current project list...

    Thanks!
    Keith



  2. #2
    Member awerby's Avatar
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    Default Re: Router/Spindle flywheel to quiet chatter

    A very small flywheel won't do much if anything. A large one would need to be very carefully balanced not to make things worse. The faster a flywheel spins, the more critical it becomes to balance it, and the better the bearings would need to be. Most CNC router spindles go at upwards of 18,000 RPMs. While it's possible to run pulleys off a flywheel and spin them slower than the tool, that introduces a lot of mechanical complication. Also the weight of a flywheel would increase inertia, and make it harder for the machine to change direction. I think these reasons may have something to do with why you never see flywheels on machines like this. If you're noticing chatter in your cuts, increasing the rigidity of your machine would be a more promising remedy.

    [FONT=Verdana]Andrew Werby[/FONT]
    [URL="http://www.computersculpture.com/"]Website[/URL]


  3. #3

    Default Re: Router/Spindle flywheel to quiet chatter

    Hi Andrew,
    Yes, it would need to be well-balanced, although there might be dynamic balancing options analogous to Dynabeads.
    And yes, it will be spinning fast: that's the point. In fact, because it's spinning quickly, it doesn't have to be big. The question here gets to the nature of chatter.
    Generally, the torque that is exerted by the motor on the bit is counteracted by the rest of the machine.
    The loosy-goosyness of the entire arrangement then allows things to bounce around when forces are too great (i.e. the bit bounces off the piece: a small movement, but very big forces) for the particular dynamics of the machine to counteract (it may have different stiffness in different dimensions). I wonder if chatter is essentially resonant feedback? Or maybe only sometimes. I wonder if it's visible in very small bit speed changes...
    By moving the power transfer to a flywheel on the same shaft, the machine is actually left out of the loop. The flywheel isn't intended to power the bit for any length of time, only the fraction of a second at a time that it takes to start or maintain chatter. It provides an instantaneous "umph" similar to the constant "umph" of building a stiffer machine. At a fraction of the cost.
    Anyway, this is all theoretical until someone points me to work that's been done on it, or someone does some experimenting.
    Here's a simple version of an experiment: if you take a bit/material/speed that develops chatter in a certain context, and switched to a bigger spindle motor, everything else unchanged, would that change the chatter problem? This would be very much like adding a flywheel.
    Regards,
    Keith



  4. #4
    Member mactec54's Avatar
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    Default Re: Router/Spindle flywheel to quiet chatter

    Quote Originally Posted by NervesofSteel View Post
    I am considering building a CNC router. I am in the wrap-my-head-around-it stage. Ideally it would work for wood and light metals.
    Has anyone seen the use of a flywheel added to a router/spindle to improve the cutting? I think there would be little impact on energy use. It's possible that some favorable damping could be obtained through flywheel construction, I haven't really thought about that. Are such flywheels built into any routers now, that you know of?

    The idea is the added inertia would even-out the load experienced by the motor. Obviously the motor rotor has rotational inertia: the question here is could such an added flywheel allow for a given motor to improve it's cut. My assumption is that the flywheel does not need to be very large or heavy, though I haven't made any attempt at evaluating that.

    This is somewhat related to the flywheel action of a large-diameter bit, but in that case the cutters' relative distance from center mean that the router bit body alone can't comprise sufficient rotational inertia. This explains their sometimes-difficult behavior.

    The major con is the added inertia with speed changes. If that's not a big problem, a smaller motor might be coaxed to cut like a bigger one.
    It could counteract vibration more directly, reducing damping capacity expectations on the rest of the gantry, etc.

    While fine-tuning might be necessary (size & weight), there might be wide tolerances for acceptable performance.
    The ideal place to mount it might be the other end of a double-shaft motor. A safety shroud could be a good idea in some cases. In that location it won't interfere with normal machine operation.

    I started off posting this in a wood router forum, but it occurs to me the idea would apply as readily to metal machining, so I begin to wonder if machinists had thoughts or experiences with it.

    If anyone experiments with this, I'd love to hear about it! Eventually, I probably will, but it will be months or years based on my current project list...

    Thanks!
    Keith
    No, no one would normally even consider this, if you want to run a bigger cutter then you need Hp to run that size cutter, adding a flywheel does not give you more Hp.

    Starting it would be the problem with any of these spindles, and if the spindle loaded up it would be all over quite quick, as it would not be able to overcome the extra inertia, the extra current draw it would need you will see lots of smoke

    Speed has nothing to do with chatter by itself, if you use the correct speed and feed to overcome chatter, if the machine is not ridged enough and can flex while cutting then this to will create chatter also.

    Mactec54


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    Member peteeng's Avatar
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    Default Re: Router/Spindle flywheel to quiet chatter

    Hello Keith,
    A flywheel is used to capture energy and then release it to smooth the machine speed if its producing power in spurts eg a flywheel on an IC engine. You may notice that electric vehicle motors (and cnc spindles) don't use flywheels as they produce power smoothly. Machine chatter is not a function of the spindle usually unless its bearings are dying. Its a function of the overall machine compliance (or lack of rigidity) and its damping character plus the run out of the spindle. The better the spindle the less the runout. In my view a flywheel would not improve the cutting performance or reduce chatter. You could consider that the spindle rotor is a flywheel and its inertia is published so you could look at that and see how big or small you would need to go to change the spindle inertia... unfortunately you can't cheat physics and use a small motor to do a big job. If you need 10HP to do a job you will be disappointed using a 5HP motor (sans flywheel or with flywheel)... Peter



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Router/Spindle flywheel to quiet chatter

Router/Spindle flywheel to quiet chatter