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ideas for limiting grinder speed?

nctacoma | Posted in Tools for Home Building on September 16, 2007 04:07am

Greetings,

I am being cheap here so……..
I need to grind, then polish a concrete countertop in a few days. The diamond pads I bought (grits 50, 120, 220, 400, 800, 1200, 3000) are recommended for a maximum speed of 4000rpm.
I don’t really want to have to buy a wet variable speed polisher because they are fricking expensive to use once and then never use again, seems like a minimum of $250 and up.
Any ideas on limiting the speed of my regular grinder. It has one speed, 10000 rpms. I was thinking of wiring it up to a dimmer switch to a gfi and hoping that would allow me to control the rpms. Think this would work? Any other ideas (besides buying the right tool for the job!)

Thanks

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Replies

  1. BillBrennen | Sep 16, 2007 04:34am | #1

    Rent the right tool for the job?

    edited to add: Running the grinder on a light dimmer switch will probably kill it dead. Don't do that.

    Bill



    Edited 9/15/2007 9:36 pm by BillBrennen

  2. Huntdoctor | Sep 16, 2007 04:46am | #2

    Running your grinder thru a heavy duty fan reostat should not hurt your grinder any. Use at least 6 foot of 12ga wire with a male plug on one end and a female on the other and the stat in the center. I have used one many time and never hurt  tool.

    Huntdoctor

  3. User avater
    SamT | Sep 16, 2007 04:59am | #3

    Hold yer thumb on the wheel. The harder you hold, the slower it goes.

    SamT

    1. dovetail97128 | Sep 16, 2007 05:03am | #4

      SamT, Now that is getting down to the "nitty gritty" of the question."Poor is not the person who has too little, but the person who craves more."...Seneca

  4. mikeroop | Sep 16, 2007 05:19am | #5

    i seen this thing once that you plugged a router into and it had a dial on it to control the speed not sure what it was called but some one will know

    1. joeh | Sep 16, 2007 05:46am | #7

      Radiac.

      I have one somewhere, definitely not a $15 thingee though.

      Joe H

  5. alwaysoverbudget | Sep 16, 2007 05:22am | #6

    check harbor frieght for a router speed control,think it's about 15.00. i used one on my grinder with diamond pads for granite,it worked out well.    lowes did have a cheapo off brand 4.5 grinder that was vairable speed for less than 50. but i don't know if they still carry it. larry

    hand me the chainsaw, i need to trim the casing just a hair.

  6. RedfordHenry | Sep 16, 2007 05:46am | #8

    I'm no electrical engineer, but I do know that some electric motors can, and some can't, be controlled with rheostat type switches.  I seem to remember reading an article in FWW some years back on the basics of electric motors for shop tools and wished that I had it handy when I tried to use a variable speed switch on my shop bench grinder.  After plugging in my grinder through the rheostat, it turned over slowly once or twice then it started smoking.  It was toast in short time.  I don't know whether or not you can do what you want to do with your grinder, but recommend that you be certain before trying.  

  7. bobtim | Sep 16, 2007 05:47am | #9

    Bought the 40 buck varible speed polisher from Harbor Freight.  Added a water feed with some soft copper and a hose valve. ALWAYS use a GFCI when wet.

    It's a cheap grinder, but what can go wrong with a grinder. Not much precison needed. When mine gets too gunked up and dies, I will chuck with no regrets.

    Lots of work grinding, but very interesting. I made a 9'  half ellipse for an island. All thats left is installing it. Weighs 400 lbs..... lots of friend & lots of beer.

  8. User avater
    Dinosaur | Sep 16, 2007 06:17am | #10

    You want to get a motor controller which is completely different from a 'dimmer' or rheostat. This is a device specifically designed to control motor speeds, and it will work on any motor which does not have (a) its own internal speed controller (your grinder doesn't) or (b) a 'soft-start' switch (highly unlikely on a grider).

    The least expensive type of motor controller is made for ceiling fans and will fit in a standard Gem box; there are continuously-variable types or three-speed types. You should be able to find this in any big box's electrical department. Figure it'll cost about $15-20. Just make sure it can handle the wattage of your grinder (it should be able to).

    Dinosaur

    How now, Mighty Sauron, that thou art not brought
    low by this? For thine evil pales before that which
    foolish men call Justice....

    1. BillBrennen | Sep 24, 2007 06:05am | #12

      Just an addendum:Motor controller will work on universal motors, such as found on portable power tools. For induction motors you will need a frequency drive controller, much more money, and not advised for full loads.Bill

      1. User avater
        Dinosaur | Sep 24, 2007 06:55am | #13

        Thanks for that; you are right to point out the difference.

        Dinosaur

        How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

  9. Karl | Sep 16, 2007 08:11am | #11

    Most stone fabricators have a makita 9218sb in their collection of most used tools. It looks like a big 1/2 drill motor with a 5/8-11 spindle instead of a chuck. It turns 4000 rpm and gets used for a lot of polishing operations.

    If you took a high speed drill and chucked a 5/8-11 bolt into it you would have a poor mans equivalent to the makita. The only problems are that you won't find a high speed drill with a 5/8 chuck and the balance of the drill may not be optimal for polishing operations.

    Most buffers for automotive finishes turn slow speeds and have a 5/8-11 spindle. They shouldn't be that hard to rent, borrow or buy cheap.

    Good luck,
    karl

  10. DanH | Sep 24, 2007 02:14pm | #14

    Used to be you could buy a speed control intended for use with power tools. Rarer now, but a few are still available: http://www.houseoftools.com/product.htm?pid=15530&catid=1885

    If your view never changes you're following the wrong leader
  11. junkhound | Sep 24, 2007 04:31pm | #15

    I'll assume you know a little about elec. safety, wiring, etc. 

    I'd just put a 10A diode in series in the line. Cuts the rms voltage to about 80 Vrms.

    If that is not enough, a half empty spool of 14 AWG also added in series with a 1/2 drive socket extender partially thru the middle will adjust the speed to just what you want.

    Lot'sa ways to reduce universal motor speed. 

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