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Thread: Is It Possible to Slow Down a Grinder's Speed?

  1. #1
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    Is It Possible to Slow Down a Grinder's Speed?

    I am slowly learning to use hand tools and I am now considering a grinder for my chisels and plane irons. I have been looking at the 6" Jet grinder which is listed as 1/2 HP. Is it acceptable to slow down the speed (3450 rpm) with a router rheostat to help moderate the temperature of the steel? I have found a few variable speed grinders but the reviews have not been that stellar.

  2. #2
    Glenn, I think that the speed on the circumference of a high speed 6" wheel is almost the same as that on a half speed 8" wheel.

    Regards from Perth

    Derek

  3. #3
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    I like a 6" full speed grinder, I wouldn't slow it down. It won't take long to get used to it.
    Unleaded tastes a little tangy, supreme is kinda sour, and diesel tastes pretty good.

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    High speed grinders are just as good as slow speed. Just quench more often. I have never had a regular old style dry bench grinder that was slow speed. You can slow down a 1 phase motor with a variac unit,but they just cut the voltage down,and will ruin the motor. Must be 3 phase to use a VFD unit on it.

  5. #5
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    How about using a 6" wheel on an 8" grinder. Any advantages there?

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    I agree that a slow speed isn't really necessary. A few years ago, I bought a Woodcraft 8" slow speed that came with white wheels and now that I have it, I don't intend to buy a different one, but if I were doing it over, it would be a regular speed grinder and not even change the wheels.

  7. #7
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    Normal motor speed is 3450 or 1725 rpm. You can usually find a slow speed motor and rig up a slow speed grinder.
    Fred

  8. #8
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    Quote Originally Posted by Derek Cohen View Post
    Glenn, I think that the speed on the circumference of a high speed 6" wheel is almost the same as that on a half speed 8" wheel.

    Regards from Perth

    Derek
    6" at 3450 is 5419ft/m
    8" at 1725 is 3613ft/m

    That said, I have a slow speed 8", if I were buying again, I would have gone with high speed 6". But if you are afraid of burning the blade, might as well just buy slow speed grinder. It'll do anything high speed grinder will do, just slower.

  9. #9
    If the grinder comes with standard grey wheels, you could change one wheel from "grey" to white friable to reduce the chance of "burning" a tool.

  10. #10
    Quote Originally Posted by Sam Takeuchi View Post
    6" at 3450 is 5419ft/m
    8" at 1725 is 3613ft/m

    That said, I have a slow speed 8", if I were buying again, I would have gone with high speed 6". But if you are afraid of burning the blade, might as well just buy slow speed grinder. It'll do anything high speed grinder will do, just slower.
    That is greater than the numbers I have seen before. Did you calculate this? What is the ft/m speed for an 8" wheel at the circumference? This would give perspective to the 6" speed.

    Regards from Perth

    Derek

  11. #11
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    The router rheostats I've seen are designed for universal motors (motors with brushes is the shorthand way to think of it). I couldn't tell exactly which kind of motor the Jet grinder has, but I'm pretty sure it's not a universal motor. Split phase, probably.

  12. #12
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    Quote Originally Posted by Derek Cohen View Post
    That is greater than the numbers I have seen before. Did you calculate this? What is the ft/m speed for an 8" wheel at the circumference? This would give perspective to the 6" speed.

    Regards from Perth

    Derek
    That's already surface speed (hence circumference). From one of my trusty resources.
    Last edited by Sam Takeuchi; 01-23-2013 at 10:53 AM.

  13. #13
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    I have virtually no experience grinding woodworking tools, but am a bit familiar with industrial grinding and manufacturing.

    8" is the "weird" bench grinder since it is running too fast for many uses. 7" used to be popular where you wanted something more aggresive than 6". 12" is popular as a heavy duty grinder with the same grinding speed as a 6" since they are both 5500sfm.

    3450rpm is normal for 6-8"
    1725rpm is normal for 10-14"

    A 14" normal speed grinder is still running lower surface (grinding) speed than a normal 8".

  14. #14
    As Bill said, a grinder is almost certainly an induction motor and you'd need a VFD to slow that down. Those "router speed control" devices only work with universal motors.

    The speeds of 3450RPM and 1725RPM indicate that they are induction motors - a two pole motor for the 3450 and four pole for the 1725. A router speed control would not work on those. Universal motors tend to have much higher RPMs, often about 12,000RPM or higher.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  15. #15
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    Quote Originally Posted by Sam Takeuchi View Post
    6" at 3450 is 5419ft/m
    8" at 1725 is 3613ft/m

    That said, I have a slow speed 8", if I were buying again, I would have gone with high speed 6". But if you are afraid of burning the blade, might as well just buy slow speed grinder. It'll do anything high speed grinder will do, just slower.
    Quote Originally Posted by Derek Cohen View Post
    That is greater than the numbers I have seen before. Did you calculate this? What is the ft/m speed for an 8" wheel at the circumference? This would give perspective to the 6" speed.

    Regards from Perth

    Derek
    Well circumference of an 8 inch and 6 inch wheel is pretty easy to figure out, and RPM's make the rest pretty simple. A mile has 5240 feet, so if my math is remotely accurate, the 6 inch wheel is turning at around 62mph (or 100KMH approx)while the 8 inch wheel is turning at about 41.5mph (67KMH approx). That is a fairly substantial difference I would think but as to the effect it has on an edged tool, I am not sure.

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