Grinder question

B

Bill Bryant

Joined
Mar 19, 1999
Messages
54
When I finally get a good quality grinder one of the features I want is variable speed. It seems a lot of people want that feature. Why couldn't I just put a dimmer switch in-line with the power cord and mount it near me as I'm using the grinder (in an electrical box of course) so it's easily accessable? That way I can vary the speed at literally any speed between 0 and the max rpm rating of the machine. Does this damage or shorten the life of the motor? Has anyone tried this?

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Bill
"There's nothing friendlier than a wet dog"
"The more people I meet the more I like my dogs"
 
When I took a metalworking art class an as undergrad we used dremels that were plugged into dimmers and it worked really well. I'm no expert on electronics/electricity, but I think for some reason despite having the dimmer there is added stress on the electrical circuit due to the varying of the resistance. Maybe this simply means you would burn out the dimmers every now and then, but I thought it was tough on the motors themselves for some reason. Hopefully someone can enlighten us with real knowledge!

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http://www.geocities.com/CollegePark/Lab/1298/knifehome.html
Palmer College of Chiropractic
On Two Wheels
 
I know just enough to be dangerous.
Usually you can find a dimmer switch with enough amperage or wattage to work with a bigger motor. I've used them with small motors and burned them up. You can find veriable speed controllers in the grainger catalog that won't break the bank.

goshawk
 
Bill, take my advice,"don't do it". Your AC motor was designed to run at 60 cycle,using a dimmer switch or reostat lowers that rate and breaks down the windings causing overheat and lock-up. Think about changing your drive wheel or pulleys to go slower.
DK
 
Bill
The route that I have chosen is a DC motor with a commercial variable speed unit mainly because of reliability and fire safety.

The cheap way is a variable transformer from the surplus store, a rectifier bridge and a DC motor. With luck you could get away under $300. The hard to find item is the variable transformer.



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george
www.tichbourneknives.com
sales@tichbourneknives.com

 
When you do get your grinder, spend the extra
for a built in variable. They're worth it. A dimmer switch will work to a point, but is hard on the motor, & you will find that you loose torque along with the speed. I tried a simmilar thing with a buffing machine. Didn't work worth a darn...

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The size of a mans house does not reflect his wealth, but rather, his greed...BCK...Happy Knifemaking...
http://www.freeyellow.com/members6/cut/index.html
 
AC motors don't like low voltage, that's what you do with a variable transformer, reduce the voltage. Just a 25% decrease in voltage (speed) gives them an excuse to spit blue smoke and sparks in no time at all.
If you look in the surplus catalogs you can at times find 1-1.5 Hp DC motors with a 56C
frame (necessary if the motor mounts with 4 bolts thru a hole in the main plate) and a base or foot mount. Controllers can be found there also, but they need to be matched to the motor voltage and horsepower. I have built some this way with good results, but a word to the wise, any grinder with less than 2Hp is not much pumpkin when it comes to hogging off blade steel. DC variables of the 1-1.5 Hp will do ok if you take your time on the rough stuff. That slow speed sure is sweet for finish work, once you use one you wonder how you did without itso long.
Good grinding!!!!!

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old pete
 
Hi Bill,
I figured I would throw in my 2 cents worth.
Last year I spent the money on a new BIII bader but did not go for the variable speed motor on that one. Now later this year I plan to add another BIII with the variable speed motor. Changing the pullies is a pain but unless you need to switch often the standard motor is a bunch cheaper.

I will throw in a vote against the dimmer on the standard motor. You can get away with this on old motors that have brushes but it is a major problem on the newer capasitor type motors.

Jim
 
If you get a speed controller, make sure you get a variable speed motor. Leeson makes both. The are more expensive, but worth it in the long run. If you don't get them you'll probably wish you did. They are easier to learn on because you can set the speed to something you are comfortable with. Gringing steel can be intimidating at first when your fingers are so close to the wheel.
 
well i can see your question has been answered quite well....but will throw this in...although i think someone said something about it above.....ac motors fry when run slow....you need a dc motor with electronic variable speed controll....and the best part about it is the torque....the motor load up and stop with a lot of pressure at low speeds...i have a variable on my square wheel, my metal cutting bandsaw...my 9" disc sander (have a 12" one speed) and my south bend lathe....they are GREAT!!!!!!!!!! plan on spending between 500 to 600.... and if you can afford it get a big one...by the way...look at the max rpm of the motor... some are 2800 and some are 1725... get the slower one and you can run it at max voltage with less speed...you dont need the speed...
 
I just reread my post I made a slight error ok ok ok It was a big mistake. I put you can find when I ment to say you CAN'T find a dimmer heavy enough for bigger motors sorry about not checking my post before I submitted.
goshawk http://www.imt.net/~goshawk
 
I have a Burr King that I put a 2hp DC motor on. I put an heavy duty rheostat(sp) on the AC side of the rectifier. It does heat up, so at $4.00 a control, I bought a few extra at the surplus store just in case. It really works great. I have had the current one in use over a year without a problem.

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Ron Ruppé
www.ruppe.com/Knives/index.htm
 
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