Bench Grinder Conversion

Hey guys i thought i would share with you my way of turning my standard bench grinder into a variable speed Bench grinder. Things your going to need are.: 1 A Decent Dimmer switch, 2 A Outlet to pug into, 3 A double outlet box to keep it all in and safe, 3 A Old 3prong extention cord.
Ive now mounted this to my wood Bench Grinder stand. Over time you will more than likely have to replace the Dimmer switch (Because it wasn't Meant to take that kinda load). With this your 3500rpm can be turned down to a 1500rpm Grinder with ease. And saves you the cash of paying for one of the over priced Variable Bench Grinders.

Here is a pick of it

I've always been under the impression that doing something like that on a load bearing motor would burn it out. Let us know how, and how long, it works!

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Originally Posted by Utopian

I've always been under the impression that doing something like that on a load bearing motor would burn it out. Let us know how, and how long, it works!

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This is the same way a variable bench grinder works. The only thing that's going to get damaged is the dimmer switch over time. I thought the same thing but I ran it by the motor shop I use.

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Originally Posted by Utopian

I've always been under the impression that doing something like that on a load bearing motor would burn it out. Let us know how, and how long, it works!

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I had heard the same thing, but I am no electrical engineer, and don't really understand what's involved.

I am curious to know how it works out.

If the grinder motor is a universal motor, it can be controlled by a triac circuit. What device commonly uses a triac circuit ? A light dimmer !!!

A lot of the import grinders do not use universal motors so the dimmer thing will burn out the motor. Think about it like this. If this was really no harm on the motor or not a fire hazard to the switch. Why do even the discount lathe, sander and drill press makers use DC variable speed motors? Why not just slap on their regular motor and a $4.50 dimmer switch. Makes you think huh?

Trillium,

Keep us posted. I for one am interested in your findings. I'm hoping to assemble a bit of a rig at the cottage this summer, and I'd love to avoid spending oodles of cash on a variable speed!

I have a variable speed grinder I picked up in Lowes on sale a while ago. It wasn't that expensive, 50.00, maybe....it works great. I've since taken all the guards off....It's good enough for what I do....I thought about a dimmer switch, or Rheostat myself, and saw this, It was easier....

Keep experimenting and sharing! Best of luck and don't forget to wear eye protection. Grinding aluminum on a wheel is also a no-no as the wheel will heat up and explode.

This -could- be how a variable speed grinder works, though the dimmer they use is likely heavier duty than this. And yes, you risk burning out the engine if you are not careful. Basically the lower you go in rpm, the less load you should be using.

Other variable speed setups would either use a discrete number of speeds (which can be implemented using switches by shorting / combining coils in a certain way) or best case a frequency drive. The frequency drive is the best, and also more expensive solution.

But for light duty low cots, this works like a charm.

There was a discussion about this on "another website" recently. I think it boils down to if the motor is the wrong type (which most buffer motors are) it wont go more slowly, but you'll lose all your torque, so very little pressure on the wheel will slow and stop it. If you still have plenty of torque with the motor slowed you should be ok.

Went and double checked it. If its a brushless or synchronous motor it shouldn't work, at least not with a normal rheostat that just reduces power. I have no idea if that light dimmer switch is different somehow, though.

Ok so I've been using this for a little over a week now and it works well. It's not perfect, I have to start it a full rpm and then turn it down. I can only turn it down to about 60% speed because any lower and you tend to lose torque. I'm having no issues with the switch warming up ( I didn't really think I would it's a 600 watt switch) the motor is not getting warm either. I do stall it out when loading up the greaseless compound, but I think I would do that with any of these small 6" bench grinders that are only 3.4 amp.
So my conclusion is that it works and it's a cheap way of doing it. And always you get what you pay for.
$35 grinder
$3 switch
$2 out let
$2 box
Cord I had (burnt out the one end plugging in my diesel van)

Any update available or would else like to comment on the safety/effectiveness of this solution?

I just picked up a 6" 3600 rpm Ryobi (Chinese import) bench grinder and I'm considering trying this method this to slow it down.

Í wouldn't know about the motor, or different types of motor, though putting your hand on it and seeing whether it get hotter than it used to might (just might) give you an early warning. The Prime Minister asked Michael Faraday just what this new electricity of his was, and Faraday answered "I don't know, sir, but I'd wager you'll tax it." Who am I to claim to know better?

I'm rather surer, however, that overloading any switch is not a good idea. The grinder is probably safer than most things though, since you would have no reason to leave it running and unattended.

I picked up a 6" variable speed Porter Cable bench grinder for $70 at Lowes. Haven't had any issues with it, Just need to take the guards off and away we go.

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Originally Posted by whavens

I picked up a 6" variable speed Porter Cable bench grinder for $70 at Lowes. Haven't had any issues with it, Just need to take the guards off and away we go.

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What's the lowest RPM rating on that machine?

Research leads me to believe that something under 1800 would be relatively safer for use with blades.

Interesting thread. Here is a picture of a Maximum variable speed grinder from Canadian Tire that retails for $139.00
If you look at the variable speed control to the right of the on/off switch, it sure looks like a dimmer switch to me.

http://i300.photobucket.com/albums/n...pspbkpj2jn.jpg

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Originally Posted by TrilliumLT

Ok so I've been using this for a little over a week now and it works well. It's not perfect, I have to start it a full rpm and then turn it down. I can only turn it down to about 60% speed because any lower and you tend to lose torque. I'm having no issues with the switch warming up ( I didn't really think I would it's a 600 watt switch) the motor is not getting warm either. I do stall it out when loading up the greaseless compound, but I think I would do that with any of these small 6" bench grinders that are only 3.4 amp.
So my conclusion is that it works and it's a cheap way of doing it. And always you get what you pay for.
$35 grinder
$3 switch
$2 out let
$2 box
Cord I had (burnt out the one end plugging in my diesel van)

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TrilliumLT, I think this is a darn fine idea! I don't know why I didn't think of this. Thirty-eight years of being a licensed, Amateur Radio operator and building countless electrical circuits including power supplies and amplifier stages and components in the last 4 decades. I should have tumbled to this earlier. There are several 6" grinders for sale here in my neck of the woods around the $40.00 mark.
I'm gonna scoop your idea and put it to work.

Harbor Freight makes a variable speed controller for a router tool. However that being said; A dimmer switch, potientiometers, router type speed controls only work with universal, brush type motors. A grinder, if memory serves, use an induction motor.
So, yes these will work; but at your or my risk.

The best is a variable frequency drive but by then you might as well just purchase the variable speed grinder/polisher.

https://www.youtube.com/watch?v=-6E1uI8yOy0

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Originally Posted by shikano53

Harbor Freight makes a variable speed controller for a router tool. However that being said; A dimmer switch, potientiometers, router type speed controls only work with universal, brush type motors. A grinder, if memory serves, use an induction motor.
So, yes these will work; but at your or my risk.

The best is a variable frequency drive but by then you might as well just purchase the variable speed grinder/polisher.

https://www.youtube.com/watch?v=-6E1uI8yOy0

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The HF bench grinder and buffer are both induction motors and the variable speed control for routers does not work with them.

I've tried it. Save yourself the trouble, it's not worth the experiment.

Voidmonster, thank you. I was just trying (unsuccessfully) to point out that you can use a potientiometer, which is basically what a dimmer switch and the Harbor Freight example above are. All a dimmer switch do is restrict, the voltage, not regulate the voltage. So the voltage that we restrict using a dimmer switch produces heat. Which as other forum members have pointed out, will overheat the grinder motor. A variac, or VFD (variable frequency drive) regulates the voltage and does not produce excess heat as a by-product.
Good call from you!

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Originally Posted by shikano53

Voidmonster, thank you. I was just trying (unsuccessfully) to point out that you can use a potientiometer, which is basically what a dimmer switch and the Harbor Freight example above are. All a dimmer switch do is restrict, the voltage, not regulate the voltage. So the voltage that we restrict using a dimmer switch produces heat. Which as other forum members have pointed out, will overheat the grinder motor. A variac, or VFD (variable frequency drive) regulates the voltage and does not produce excess heat as a by-product.
Good call from you!

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I'm always glad to pass along my mistakes so others don't have to make them!

It didn't burn anything out, it just didn't work well at all.

Interesting thread, I was toying with doing somthing like this but didn't want to burn down my workshop...

Quote:

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Originally Posted by shikano53

Harbor Freight makes a variable speed controller for a router tool. However that being said; A dimmer switch, potientiometers, router type speed controls only work with universal, brush type motors. A grinder, if memory serves, use an induction motor.
So, yes these will work; but at your or my risk.

The best is a variable frequency drive but by then you might as well just purchase the variable speed grinder/polisher.

https://www.youtube.com/watch?v=-6E1uI8yOy0

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correct, while there are workarounds such as a variac- would work for speed- with all of these you are suffering loss of power with loss of RPM. What you would do best with is a tri phase, DC perm mag, even an old treadmill motor and controls. So you get constant power under torque- I confess I didn't read what were building, but I saw a grinder and if you need the power of a grinder motor- you need to pick the right motor first for speed control to not limit your power.

Trying to make a buffer or a belt grinder? You can also use belts and wheels, calculate ratio for size to speed and if a single slower speed is what you want- you'll have more torque at lower rpm.

Thanks folks. I'm not trying to thread jack and my apologies to the OP. I'm on a budget and my original thought on reading the thread was to give it a go. After thinking more about it I decided to look for a variable speed machine. I am in the process of reading through the numerous and awesome threads in the workshop section. I read where someone stated, "Do your homework" and then go shopping so you don't waste money." So my quest at the moment is to find a grinder/polisher to work an estate razor through the restoration process.
Any suggestions would be great.
Again, my apologies to the OP. I didn't mean to hijack your thread.
On the bright side. I managed to buy a used drill-press and band saw on Kijiji and saved a couple of hundred bucks in the process.