Query: Is it possible to slow down a bench grinder - Woodworking Talk - Woodworkers Forum
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post #1 of 44 Old 05-25-2020, 07:35 PM Thread Starter
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Query: Is it possible to slow down a bench grinder

I inherited an old but very good condition, 1/3hp Baldor Model 612, two-wheel bench grinder from my late father. The motor, wired for 125VAC, runs at 3600 RPM, which for my personal use is way too fast. Half that speed would be much better.

Can anyone please advise me: (1) Is it possible and practical to reduce the speed of the motor without damaging it?

(2) How might I go about it?

I'm not an electrician but have some knowledge of electrical circuitry, can read simple schematics, and have built small projects over the years.

Thanks
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Last edited by sonofSon; 05-25-2020 at 07:42 PM. Reason: more precise description
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post #2 of 44 Old 05-25-2020, 07:43 PM
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Google 120v speed control

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post #3 of 44 Old 05-25-2020, 08:27 PM
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Nope!

You can not change the speed of an induction motor using a "variable speed controller" as you would on a router which is an AC/DC brush type motor. Unless the motor has internal windings that allow for two speeds, you are out of luck. The standard RPMs for induction motors are 3450 and 1725 RPMs. However, bench grinders do come in both speeds. It is for this reason that I do not sharpen my plane blade or chisels on a 3450 RPM grinder, it gets the metal too hot too fast. I use a belt sander with a "belt drive" which I can change the speeds by moving the belt on a step pulley. A direct drive bench grinder has no such feature.

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The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #4 of 44 Old 05-25-2020, 09:48 PM
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Try to slow that one down, and you will burn it up. It has a centrifugal start switch that will switch on the start winding if you succeed in slowing it that much, and the start winding will quickly overheat and burn, since it isn't rated to be on more than a few seconds at a time. Induction motors are power line frequency dependent to govern their speed. You need to change the power frequency to change the speed, but again, look out for that start winding. They make power controllers for 3 phase induction motors to vary the frequency of the power going to them, and this will allow changing the speed, but slow the motor too much under load, and the motor won't be able to cool itself.

So, in short, don't try it. If you do, you will burn it up.

Nice grinder. Leave it alone and enjoy using it at it's intended speed. Buy another grinder designed to run slower, if you need that.

Charley
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post #5 of 44 Old 05-26-2020, 12:37 AM
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You could take it apart and disconnect the start winding, and then buy a VFD (variable Frequency drive) but that's lot of work and you may spend more on the VFD than a new grinder.


http://www.electricmotorwholesale.co...chList=,40~1/3
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post #6 of 44 Old 05-26-2020, 04:39 AM
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I agree, I paid $15 for my bench grinder which came with 80 and 120 grit wheels plus a buffing wheel.
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post #7 of 44 Old 05-26-2020, 11:38 AM Thread Starter
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Gentlemen: Thank you all very much for your helpful replies. You've saved me some grief, not to mention Dad's grinder. I can see this Website is going to be useful and enjoyable.

Although Baldor no longer makes the grinder in question, a Wyoming distributor of my acquaintance still had the factory documents on its Website so I was able to download the motor schematic last night. From that, I could see the motor is an induction unit with a capacitor start and a drop-out relay instead of a centrifugal switch.

After your timely warnings about what would happen if I reduced the voltage to the motor and the futility of altering the current frequency, I wanted to find out the grisly reasons so went to Quora and had it explained to me in grisly detail by a couple of E.E. types.

I'll still use the grinder for metal work but my chisel and plane blades will not go near it.

Stay well, all of you.
Bill
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post #8 of 44 Old 05-26-2020, 11:53 AM
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post #9 of 44 Old 05-26-2020, 01:11 PM
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That's an oversized light dimmer.

Quote:
The heart of the control is a triac fired in a manner to adjust phase and thereby vary speed.

Quote:
It is not designed for capacitor start motors.
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post #10 of 44 Old 05-27-2020, 06:23 PM Thread Starter
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JayArr is correct - my Baldor grinder is based on a capacitor start motor. Although I've taken apart, repaired and reassembled the Leeson Ph1 1.5hp TEFC motor Delta furnished with my tablesaw in 1986, I wasn't sure of the nature of this Baldor motor because it didn't have the usual external lump where a start capacitor would reside. It wasn't until I got hold of the motor schematic (attached) that I could see it was a capacitor start induction motor.

I am nobody's expert on electic motors and I was really fascinated to see how the start circuitry was designed, using a relay that is sized and calibrated to energize the start circuitry when the main winding draws peak current on startup, and then drops the start circuit out as the current draw falls when the motor approaches running speed. The centrifugal switch on my Leeson is a nice piece of work and I'd guess expensive and difficult to scale down below a certain mass. At a guess.
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post #11 of 44 Old 05-27-2020, 07:19 PM
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Umm... I don't see a capacitor in that schematic.


The parallel lines are relay contacts not a capacitor. (google "3cr current relay")


I still wouldn't run it on the dimmer control but I thought I'd point out to you that there is no capacitor.
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post #12 of 44 Old 05-28-2020, 12:31 PM Thread Starter
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You could have fooled me - I thought I was looking at a non-polarized capacitor symbol. Live and learn. I'll Google the link you mention and run it by you once I think I've figured out how this start circuit works.

Thanks,
Bill
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post #13 of 44 Old 05-28-2020, 01:34 PM Thread Starter
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Well, I think I've made a start on understanding how it works. Let's see. Please check me on this: This is a split phase motor, the auxiliary winding consisting of relatively thin, high resistance, low inductive reactance wire, the obverse of the main winding. The two windings are initially in parallel. I don't understand what a "traveling wave" is but the phenomenon apparently serves to create enough torque to start the rotor moving, albeit the torque created is much less than what a capacitor-based start circuit can create. As the rotor approaches running speed and current draw diminishes, the auxiliary winding is dropped out of the circuit by either a centrifugal switch or a current-reading relay. From what I have read and think I understand, split phase motors would be used in low-torque applications like my grinder or a fan, say.

Pass? Barely Pass? Fail? Clueless?

Thanks,
Bill
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post #14 of 44 Old 05-28-2020, 08:28 PM
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It's only 1/3rd horsepower, if you slow it down it'll be useless, it'll bog down on a feather!
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post #15 of 44 Old 05-28-2020, 10:03 PM
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The Baldor 612 was the bottom of the Baldor line. Even then, they were sweet little grinders. Very smooth and quiet. Much better than the Asian made stuff offered for cheap today.
If you're having burning problems using a 3450 rpm grinder, you need better instructions on using a bench grinder. One can burn on the slow speed grinders as easily as a high speed grinder.
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post #16 of 44 Old 05-28-2020, 10:42 PM Thread Starter
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Bob,

Probably it's my age - I'm slower these days to deal with things that happen fast. Other than for dressing cut machine screws and smoothing edges of sheet stock, I had thought to use this grinder for squaring up a worn chisel or plane blade before putting a new edge on it by hand with stones. I've done that with slow grinders in the past and was able to keep the blade sufficiently cool that it didn't draw the temper. With the help of other members I've now come to understand Dad's grinder is a one-speed pony, which is o.k.

I'd be happy to learn from any wisdom you'd care to impart. With only a few exceptions, I'm almost completely self-taught and that is what makes this forum especially useful and enjoyable.

Bill

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post #17 of 44 Old 05-29-2020, 08:09 AM
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The Baldor 612 is still listed with MSC for $456.39 so that's the potential quality level grinder you're putting together. I've bought many over the years for $20.00 from sellers that thought the 612 was the same as an Asian-made cheapie.
https://www.mscdirect.com/browse/tnp...SABEgJuPfD_BwE
The problem with the 612 isn't the grinder, but the tool rests. They are too light and flimsy for such a good grinder. I'll make my own out of thicker steel. Heavier tool rests make all the difference in the world. Add a few Kipp handles, and you've got a lifetime of convenience.

An 1800 rpm 6" grinder is incredibly slow in removing metal. I hate them. The trick is light quick passes. I can make 100 light passes on a plane iron or chisel in about 60 seconds. Keeping the stone clean and sharp is another key detail. Cheap stones aren't worth the trouble. They clog and heat too easily. By holding your fingers close to the cutting edge, you can feel the heat building so you know when to back off. The little rocks in the wheel only stick out so much so that's about all you can remove. To quantify with numbers, a light quick pass will remove about .001" to .0005" of steel. 100 light quick passes removes a lot of steel. Once the edge is formed, the steel goes right to the sharpening stones for sharpening.

Below are some shots of the 612s I've done over the years showing the tool rest components that I've remade. I hope these will offer some inspiration. Sharp tools are a joy to use and a properly rigged Baldor 612 grinder will offer a faster and easier way to get there.
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post #18 of 44 Old 05-29-2020, 10:15 AM
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Quote:
Originally Posted by DrRobert View Post

"It is not designed for capacitor start motors." You missed reading this statement in your link.

This speed controller will work fine on universal motors, like those in an electric drill, but they will not properly vary the speed of an induction motor whose speed is controlled by the power line frequency. Also, if you reduce the voltage of an induction motor to the point where it is no longer following the power line frequency, you will have lost most of the torque of the motor. Slowing it will also cause the centrifugal start switch to engage the start winding and capacitor circuit, which is not designed to operate for more than a few seconds at motor start-up. This will result in the start winding and capacitor being destroyed (you will let the magic smoke out).

Charley
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post #19 of 44 Old 05-29-2020, 11:04 AM Thread Starter
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Bob, Thanks for taking your time to help me along. I can see you've put some serious beef in those tool rests. I've noted that what I have assumed to be the original rests are pretty light and awkward and don't want to stay in adjustment. I particularly appreciate the photos that illustrate the mods you made. I'm not much of a metalworker but I think I might be able to replicate what you've done.

I can't identify for certain what wheels are on Dad's grinder except to say one is coarse and one is less so. I note that yours is set up with a wire wheel and a single stone. Do you use this setup for a particular purpose? Also, if you'd care to do so, recognizing that the primary purpose of my machine would be to sharpen edge tools, what brand and grade of wheels would you recommend?

Also, almost forgot: Do you find it advisable to dip the tool in water as you grind or just use your fingers to feel for excessive heat?

Thanks,
Bill

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post #20 of 44 Old 05-30-2020, 09:23 AM
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Bill, I think those inside wheel guards on your grinder are cast iron. Iím envious. The later versions were cast aluminum.

I always keep a water pot and wiping rag near by when grinding. Grinding is a hand activity and errors in pressure and pace can and do occur. About 100 passes in 60 seconds is a good pace Iíve found. Light and quick. Your fingers will tell you when the heat is rising in the blade. Sticking a wet blade into the wheel introduces contaminates that gum up the surface of the wheel and makes the wheel heat more than it should. Adages referring to the tedium of grinding are very true, but the results are worth it. The grinder shapes. Sharpening is done on stones.

That setup on the 612 with the wire wheel was something I fixed up for the basement shop area when I needed a wire wheel. I got the grinder without wheel guards, a $15.00 buy at a flea market. I had an extra set of wheel guards in stock. Iíve got well over a dozen types of bench grinders Iíve accumulated over the years. I grind my own shaper steel profiles for slip knives. A handy cut-off wheel for your grinder is a Norton A60OBNA2. No other wheel is as good or as safe. For your amusement, Iíll link a thread below.
http://www.owwm.org/viewtopic.php?t=56115


Try using the wheels youíve got for the immediate future. Those $100.00 CBN wheels wonít be much different right now. The kind of wheel can make a little difference, but what makes a BIG difference is the condition of the abrasive on the surface of the wheel. You need two things; a diamond (1/4 carat) wheel dresser and a star (Huntington #0) wheel dresser. These are long term investments that will last decades. The diamond dresser scrapes all of that mess off the surface of the wheel and trues up the wheel for very smooth running. It also makes a mess so wear a dust mask. While the diamond cleans and trues the wheel surface, it also dulls the exposed abrasive granules. That new, clean surface will burn like crazy. The next job is to beat the dulled abrasives off the surface. Take the star dresser and press into the wheelís surface while moving side to side. The pressure should be hard enough to almost push the grinder around on the bench. Again, more mess. The duration of the star dresser use should be about five seconds and thatís it. Youíll be amazed at the difference. Once the diamond has been used, try grinding something and observe the few sparks generated. Try the same piece after the star dresser and youíll be amazed at the amount of sparks generated and how much better the wheel cuts. You need both tools to get a good job. The diamond cleans and trues the wheel and the star dresser restores the sharpness. When the wheelís abrasive gets dull, a few seconds with the star dresser cleans things up. Youíll use the star dresser much more often than the diamond. If you read on the internet that a diamond is all you need, disregard that source of information.


The below photos are from a 2004 catalog so the prices will be a little different but not a whole lot. Thereís no need to spend a pile on the diamond dresser.
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