Delta 6" variable speed grinder potentiometer...

[john brady]

Hello All!

This forum (General Woodworking & Power Tools), seemed to be the closest one for my question. I have a Delta GR250 6" variable speed grinder that I purchased a little over 10 years ago. I only use it for sharpening my lathe tools, so it hasn't had a lot of heavy use over the years-- just short bursts of on-sharpen-off cycles. Since I only use it for sharpening tools, I always had the speed set to the lowest setting and it always worked fine.

But then about two months ago, I noticed the grinder would vibrate when I'd turn it on. I assumed the wheel needed balance, but then realized it was because the speed had increase dramatically, despite the dial being turned all the way down (to the lowest speed). When I mentioned this to my son he said it was the speed dial, which is technically called a "potentiometer" (or "pot" for short) and that was typically how they act when they go bad.

I removed the access panel so I could see what the pot looked like, and see if it had any markings that I could reference to get a new one, but it doesn't have any markings on it whatsoever, so I'm kind of stuck. I found plenty of them for sale (the biggest selection seemed to be at a place called ExpressParts.com that sells parts for guitars and amps), but before I can order one, I need some help to determine what size I need to order.

Sooooo, I was hoping someone here may have had the same issue and replace theirs, or some of you guys might be familiar with them and would be able to tell me what size I need to order.

The grinder has since settled down just as mysteriously as it it began, but before it acts up again, I'd like to have a new one on hand. The grinder specs are: 110-volts, 2-AMP's with a speed range of minimum 2,000 RPM's to maximum 3450 RPM's. Hopefully this is enough information to help anyone knowledgeable about them to identify what I would need. But if not, here is a link to an actual photo of the pot AND a snapshot of the label from the rear of the grinder: http://www.jebswebsite.com/misc_pics/grinder_potentiometer_group.jpg.

Any help or suggestions anyone could offer would be appreciated.

Cordially...John

John E. Brady
JEB's PENs
www.jebspens.com (http://www.jebspens.com)
Hand-turned Pens / Pencils
& Custom Fountain Pens

[Rod Sheridan]

Hi, you could measure the resistance between the the two outer terminals ( with the grinder unplugged).

That will give us an idea of what type of potentiometer you need.

Regards, Rod.

[john brady]

Hi Rod,

Yes, I did do that. My son suggested I take them, but I didn't know what to do with them once I had them. I also wasn't sure the reading would be accurate since the pot was defective (I only know enough about electrical to be dangerous!). So if it helps, below are the various readings I got using a (brand new) Mastech 5826B multimeter. As I said, I don't know much so I took several reading with the meter leads on different posts and with the post turned on both high (all way clockwise) and low (all way counter-clockwise).

Dial on HIGH with RED meter lead on Wiper (center post):
Black lead on black pot post = 20.0k
Black lead on white pot post = fluctuated between 00.0 and 00.2.

Dial on LOW with RED meter lead on Wiper (center post):
Black lead on black pot post = 3.89.
Black lead on white pot post = .2.

Other readings with meter leads at both ends:
Black lead on black pot post, Red lead on white pot post = 5.0.
Red lead on black pot post, Black lead on white pot post = 3.543.

Thanks again for your help. I hope these are helpful!

Cordially...John

[John McClanahan]

It sounds like the pot had some dust in it. If you ever had an old radio that would scratch real bad when changing the volume, same thing. Adjusting the speed dial from minimum to maximum, back and forth numerous times (while turned off) will wipe the dust away so the internal contacts make good connection.


John

[Keith Westfall]

If you can access the inner part, (small hole, open area, etc) spray some electrical contact cleaner in it and twist it back and forth a few times. Might be good again for a while...

[Dave Cullen]

Hi Rod,

Yes, I did do that. My son suggested I take them, but I didn't know what to do with them once I had them. I also wasn't sure the reading would be accurate since the pot was defective (I only know enough about electrical to be dangerous!). So if it helps, below are the various readings I got using a (brand new) Mastech 5826B multimeter. As I said, I don't know much so I took several reading with the meter leads on different posts and with the post turned on both high (all way clockwise) and low (all way counter-clockwise).

Dial on HIGH with RED meter lead on Wiper (center post):
Black lead on black pot post = 20.0k
Black lead on white pot post = fluctuated between 00.0 and 00.2.

Dial on LOW with RED meter lead on Wiper (center post):
Black lead on black pot post = 3.89.
Black lead on white pot post = .2.

Other readings with meter leads at both ends:
Black lead on black pot post, Red lead on white pot post = 5.0.
Red lead on black pot post, Black lead on white pot post = 3.543.

Thanks again for your help. I hope these are helpful!

Cordially...John

OK. First, the value of a potentiometer is the resistance between the 2 outer terminals. Secondly, you'd need to remove it from the machine (at least 2 wires) before measuring because the circuit that it's attached to will affect the reading.

Once you have the resistance value you also need to know the wattage rating, which is a function of physical size and construction materials. Wire wound pots can handle more power than resistive materials for any given size. But by the pictures you've linked to I'd say a standard 2 watt size would be fine.

Lastly, there are 2 standard "tapers", which is the way resistance changes as the pot is rotated. The choices are either linear (what you want) or audio (a logarithmic relationship used for audio volume controls).

Parts Express is a good source. You may have to adapt your application for a different shaft style & knob. Or, if you're feeling adventurous, go to Mouser Electronics or DigiKey - they have more choices.

All that being said, the most common failure for a potentiometer is that the wiper loses connection with the resistive element, especially if it's been in the same position for a long time. The first line of attack is to get some cleaner spray from Radio Shack and spray it into the control while rotating back & forth -

http://www.radioshack.com/radioshack-control-contact-cleaner-and-lubricant/6400148.html#.VL6ao9LF_Vo

Good luck.

[john brady]

Hi Guys,

Thanks everyone for taking the time to respond. Dave-- thanks for the details-- it was VERY helpful. I'll get some contact cleaner and give that a try. Since the grinder isn't used much, maybe the cleaner, and moving the speed up a little (to move to a different contact position) might give me a few more years on it.

Thanks again...John

[Myk Rian]

The resistance value is often stamped or printed on the shell. There is no mystery involved with them.