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Cheap Variable speed grinder. 1725 rpm slow enough?

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by stingray4540, Jan 14, 2013.

  1. stingray4540


    Mar 26, 2007
    Well, my HF 1x30 bit the dust, so I'm in the market for a grinder. Funds are very limited though, would like to keep it ~$600 or less.

    Anyways, I found this variable speed bench grinder: http://www.sears.com/craftsman-professional-variable-speed-8inch-bench-grinder-21162/p-00921162000P

    And was thinking of using it as the motor for a coote.

    What do you think? Would that work? Are those RPMs in an acceptable range? I also want to use for final sharpening, so I want it to slow down enough.

    If that seems like a bad idea, what is the cheapest I could get into a variable speed grinder? Personally the variable speed is more important than the belt size, so if there are any variable 2x42, 1x42, 1x30s, I would be interested in hearing about those!
  2. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    Get a motorless grinder (Like a Coote), a good 1HP to 1.5HP motor, get a three step pulley, you have a VS grinder. It only takes seconds to change the speed ( Please, not while running).

    The other choice is a 1HP 3Ph motor and a VFD (plus the Coote). That will cost about $300 if you shop around a bit. California has lots of these drives and motors for sale in the industrial surplus shops.
  3. stingray4540


    Mar 26, 2007
    So where can I find these industrial surplus shops? I have no idea where to even begin to look for motors.
  4. stingray4540


    Mar 26, 2007
    Just thought of another possibility. What about using the motor from a band saw?

    There is a horizontal metal band saw with 3 step pully on craigslist for $120. I would have two tools in one! Although, that motor might be too slow?

    I am also picking up a wood bandsaw from my grandpa this weekend, maybe I could use the motor from that?!
  5. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    OK, lets talk some basics:

    Most motors turn at either 1750RPM or 3500RPM (basic numbers). The 1750RPM motors are better for running belt grinders............ That is what you should look for.
    The HP rating is what matters most. It needs to be a real rating, and bench grinders, vacuums, routers, skill saws, etc.....are notoriously rated at "Developed HP", which is a sales hype.
    A proper motor will have a plate mounted on it with all the motor info. That is where you will get the HP, speed, voltage, amps, phase, frame type, etc. If it doesn't have this, it may not be what you want.
    The amperage will be a good way to check the HP rating. For approximate comparison, 1HP = 7amps.

    Whatever the motor speed, the speed is reduced by one of two ways ( sometimes both together):
    1)The size of the pulleys on the motor and the drive shaft
    2) The size of the drive wheel.

    You can calculate the belt speed from these pieces of info. Belt speed should be figured in SFPM - surface feet per minute.

    A good speed range for a grinder is from around 500SFPM at slow to 5000SFPM at fast. If using pulleys, you will get three or four steps in this range. If using a VFD, you will get speed control from dead stop to screaming fast at the twist of a dial. This is far different than the "speed control" on a bench grinder like you showed in the link.

    Pulling a motor from an old tool is fine...if the motor wasn't the problem. The speed of the tool will not mater, as this was usually not determined by the motor, but by pulleys or a gear box. Most tools work great for what they were made to do, and poorly when modified to do a task they never were meant for. Sometimes it is money well spent to get the right parts and put them together.
    Last edited: Jan 14, 2013
  6. Patrickknives


    Dec 13, 2006
  7. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    The farm Duty HF motor would work.
    What you want for a grinder is a TEFC , 56 frame motor. 220VAC is preferred, but not necessary. 1HP minimum.
  8. fast14riot

    fast14riot Gold Member Gold Member

    Oct 27, 2010
    stingray, possibly member Carl_First_timer will stop by, he's in Redwood City and I believe runs a machine shop, he might be able to point you to a source for used motors in the area. I got my motors from a BART mechanic, but try talking to some HVAC guys, they might have some they pulled from equipmet, or know who sells them.

  9. stingray4540


    Mar 26, 2007
    Thanks guys. If all else fails, that HF looks like it would work. Great tip on the HVAC guys. I think my dad knows a few so I'll ask him to ask around.

    That would be great to hear from a local. I had actually considered starting a thread to find out who's in the area. It would be educational just to get a chance to watch someone work sometime.
  10. sintro


    Mar 5, 2012
    Ken Schwartz from knifeforum has an example of the variable speed grinder. Here's a video where he talks about the VFD on his coote grinder, http://www.youtube.com/watch?v=XSF3VF9aIp0 He starts talking about the VFD at about 2:50. And then he demonstrates the VFD at 10:00.
    Last edited: Jan 14, 2013

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