Blows a breaker, what the heck??

M

Mike8623

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OK guys I bought this old 50's dumore tom thumb tool post grinder. Put it on my lathe and plugged it in and it propped a breaker. I reset the breaker took it off the lathe, plugged it in and it ran just fine. Every time I put it on the lathe and plug it in it blows the breaker. As long as I plug it in and don;t touch metal it runs just fine.........but if I touch it to metal it blows the breaker. Anyone have a solution or know what is wrong.
 
Sounds like you may have a short to ground situation when mounted on the machine. Could be a cracked electrical cord insulation the leaks into the lathe like some sort of heat sink or maybe the way it's mounted is putting pressure on an area that is causing the short inside or around that point.

Turn the lights off and try powering it in the pitch dark while you look for a flash.

I wasnt sure how those grinders were setup so i did a quick google search......if its similar to the ones i just seen then I would look very closely at the cord where it comes out of the grinder housing.
 
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Does the grinder have a 3 pronged plug? If so, check to make sure the ground in connected.

Sounds like you have a short inside so you'll have to do some continuity checks and see what's going on. Chances are, if you were to touch something grounded and was holding on to the grinder without it attached to a machine, you would most likely get a shock!

I'd check things out before running it.

Ted
 
Agree with all above- the fault lies with either the grinder OR the lathe. One of them has 120 volt leakage to the chassis and/or a poor ground situation.
Unplug the Dumore and test resistance of both prongs of the power cord to the metal housing using an ohmmeter- normally should read very high resistance, 500,000 ohms or more- likely this is where you will find a problem, if you have not been getting shocks from the lathe
Mark
 
the dumore has a cord that looks like the original one 60-70 years old and it looks it. I have not gotten any shocks from my lathe and I have been using it a lot lately. I did see a spark jump from the tool grinder to the lathe when I had the grinder plugged in and put it on the lathe and then as the grinder touched the lathe a spark and the circuit breaker popped.
 
If you have a scrap piece of rubber hose (tape or other cheap but appropriate material for this purpose) that you could slice open long wise to wrap around the cord to insulate it for testing purposes you will be able to verify the cord being the problem or not.
With that said the most likely problem area is probably gonna be right where the cord exits the housing.
 
Stop and check with volt meter.

To be safe do the following.

Get a cheap extension cord that is 2 prong not grounded.

Find a flat spot to work. Place newspaper on flat spot as insulator and plug grinder into cord while cord not plugged in.

Get a voltmeter.

Plug in cord and do NOT touch the grinder.

Measure voltage from grinder housing to the safety ground on the outlet.

Should be zero if not be careful, unplug extension cord from outlet and look at which prong of the cord is in the smaller slot as this is the hot side.

Flip plug over and plug it back in and repeat measurements.

Old equipment was not grounded and normal wiring is that the return connects to ground.

If there is a short from cord to case flipping the cord over stops the fuse blowing.

Stop now and open up the motor where the cord goes in.

Follow the wire that resulted in the voltage measured above and you may find it touching the case somewhere.

Once you figure out where the problem is then repair but that means new cord with a ground.

Good luck..



Sent from my SAMSUNG-SM-G930A using Tapatalk
 
It could be a bad cord or it could be a short in the motor windings somewhere- sometimes hard to find- be careful
Mark
 
After 60+ years rubber or plastic on cords gets brittle and break. If you see cracks on the cord especially near where it goes into the motor or plug, replace. On the 2 prong plug, were both prongs the same size (not good)? If different, the bigger prong is neutral (wire should be white). When you replace the cord make sure that the wire (most cases black) is connected to the smaller prong side goes to the switch. That is the hot side. Best use a three wire and connect the green to the motor case. Black wire will still go to the switch.
The other thing is when you take the motor apart, check to see that there are no shorts from the motor wiring itself. Since it is a tool post grinder, there could be some metal filings or dust. Brush off with a fine brush and spray with a electrical cleaner such as CRC.
 
For some reason, back in the days when they made tools and appliances with two-prong plugs, they made them with both prongs the same size. They then used the neutral wire as a ground, connecting it to the chassis. This meant you had a 50/50 chance of plugging it in with the chassis hot. Not a real problem until you touched the tool (or fridge, or whatever) and touched a properly grounded surface (like a stove or sink). Or, as in this case, the tool was actually touching the grounded surface (the lathe).

If that's the case, if you pull out the plug and turn it over before plugging it back in, then reset the breaker, it should work normally. If it does, unplug it and replace the old cord with a new, three-wire cord with the chassis connected to the real ground wire and the neutral NOT connected to the chassis.
 
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