Balanced motor needed for surface grinder?
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  1. #1
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    I've heard that surface grinders require balanced motors. Is this true or are new motors sold today made well enough that it is no longer necessary?

    I looked over the Leeson and Baldor web sites and see no mention of balancing as a spec in other than HVAC applications.

    Mike

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    Mike, I have an old Walker-Turner surface grinder. The manual states that the motor/pulley were balanced at the factory. It also states to send them back for balancing if the pulley is ever moved.

    A few years ago somebody removed the pulley. It does leave slight chatter marks now.
    Kind of tough to send it back to the W/T factory now.

    Les

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    Depends. Sometimes the the guy who balances armatures that go into general purpose induction motors has a good good day and you're the guy lucky enough to buy it. Then again...

    Every little thing wrong with a grinder is telegraphed onto the finished surface. This includes motor balance, tortional vibration, lumpy belts, out of balance pulleys, bum bearings, and vibrations transmitted to the machine from the floor.

    Thus a balanced motor and near perfect pulleys and belts are pretty much of a necessity. You can remove the armature and take it and the pulley you use with it to a balancing shop and pay maybe $60 to $120 for a spin in the Schenk balancer and a correction to whatever they're calling a "precision balance" these days of ISO 9000 umpty-ump. The balance is so critical you have to be sure the pulley key and set-screw are installed exactly in the same place and the same way as when the motor was balanced.

    Most general purpose single phase motors are not suited for grinder spindle service because of their tendancy to hunt magnetically or "cog". Therefore if you're looking to run the machine from single phase don't change out the factory three phase motor if it's so equipped. Run it from a phase converter or a FVD. You'll be glad youy did.

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    Forrest,
    I've got a 5" X 7" Sanford surface grinder with a 1/6 hp Robbins & Myers single phase motor. If I read you right, this motor is not your standard run of the mill motor in terms of balancing? I was thinking of putting on a standard 56 frame 1/4 or 1/3 hp ODP motor, as the original motor is on the way out. I was hoping to make a new mounting plate and use a standard motor. What's ya thoughts?
    Best regards,
    Bob

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    Thanks for the responses.

    For Forrest - the motor is a GE 1-HP 3-phase and appears to be a replacement for the original as it seems a lot newer than the grinder. It's run from an Hitachi L-100 VFD and I should probably look into the parameters as they are currently set pretty much at the default. There might be bearing noise from the grinder, though that seems to have dissipated with a few hours of running time.

    The question was prompted because I'm getting some shallow scallops (~ 0.0001" deep) and trying to determine the cause and see if it can be or is worth fixing.

    Here's what's been recommended to me so far in no particular order:

    1) Replace motor with a balanced one
    2) Replace table gear rack (rack and pinion table drive)
    3) Replace motor pulley belts (which seem OK to me)
    4) Replace or rebuild spindle, which has ~ 0.00075" runout and about 0.0015" deflection with pressure up or down. KO Lee specs 0.0001" on their current surface grinders.

    The motor seems like a possible candidate for priority 1 - hence the question on balanced motors.

    The rack is $200, about what a machine tool motor runs from Leeson or Baldor. A spindle will run me $1500. KO Lee recommends that these be returned to the factory (no surprise there) for rebuilding but someone else reported success doing it themselves with a similar KO Lee grinder. Belts would be cheap by comparison, but I doubt that they are the problem.

    I've documented most of what I've checked here, in case anyone is interested:

    http://member.newsguy.com/~mphenry/s...&_bearings.htm

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    Forgot to say: the grinding wheel has to be balanced too. Remove the wheel and start the grinder spindle. A coffee cup of water set someplace on the wheel head should register a flat calm in the liquid. Then remove the wheel and hub. Same deal.

    Usually the better quality surface grinders have a optional balancing mandrel to take the factory wheel hubs. Then you give the wheel a static roll on a knife edge static balancing rig. And as the wheel wears sometimes you balance it more than once.

    http://www.sme.org/cgi-bin/get-mag.p...076&ARTME&SME&

    As for Bob-O's 1/6 HP motor I don't know what to say. I'd go for a three phase motor and a VFD or a DC motor and PWM (NOT half-wave SCR) drive before I'd try any single phase motor. Many of these small single phase motors used on small grinders are shaded pole that run fairly smoothly. This is an expensive way but three phase (and DC motors on good DC) run smoother on the synthecized three phase from a VFD than they do on lumpy line power.

  7. #7
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    Forrest - the cup of water was most educational. Standing waves were visible with the belts disconnected and only the motor running. They got a bit worse when the belts were attached to the spindle pulley.

    That experiment plus the slightly worrisome noise the the motor makes on startup have me looking for a replacement motor. Guess I'll call Baldor & Leeson tomorrow and see what they recommend.

    Thanks for the tip!

    Mike

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    Hi all,

    I need your expertise
    I bought a second hand TATUNG surface grinder model TSG350 " as is where is" from a Tooling company that closed.. After few months, the spindle became so noisy.. To get rid of extra cost, i was planning to replace the bearing by myself..

    Any suggestions/comments please? Is the bearing so special that it could not be easily found from bearing suppliers?

    thanks,

    Dany Velasco


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