Disassembly and Cleanup of a Sanford SG48 Surface Grinder

[ddickey]

Post some photos, why not?

 

[WobblyHand]

Can you post the schematic you worked with? Without that it's hard to tell if you followed it! Or the schematic you followed is incorrect. Can't decipher the puzzle without help from you

 

[ddickey]

Click download.

Sanford SG48 Surface Grinder Rewiring Diagram

This is a wiring diagram of full bridge rectifier for a Sanford Surface Grinder, Model SG48. It is designed to convert AC line voltage to DC voltage. There is a provision for motor on/off, electromagnetic chuck on/off/degauss. Included below is...

www.hobby-machinist.com

 

[WobblyHand]

Click download.

Sanford SG48 Surface Grinder Rewiring Diagram

This is a wiring diagram of full bridge rectifier for a Sanford Surface Grinder, Model SG48. It is designed to convert AC line voltage to DC voltage. There is a provision for motor on/off, electromagnetic chuck on/off/degauss. Included below is...

www.hobby-machinist.com

Oh. See it was posted earlier. My bad. Well, that's a simple schematic.

 

[WobblyHand]

So as I understand it you are still at round 3. So the fuse blows after turning on the mag chuck. But, before then, the motor worked, right? Sounds like it is the mag chuck circuit.

One thing I see is the "ground leg of the capacitor" is not grounded. This may or may not be an issue. So the DC voltage might be floating, ie. the low side could be a value that is not ground.

The other thing is the magnetic chuck. What is the rated input voltage? Is there a rated current? With J1 disconnected, from this circuit, can you measure the resistance of the chuck? Can you check if either lead of the chuck is connected to the chassis (machine) ground or anywhere on the chuck surface?

 

[vtcnc]

Here are recent photos of where I am at:

Left to right: mag switch(DPDT), spindle switch (DPST) and main switch (SPST)

I also checked the resistance of the chuck. No continuity. I also checked both connector legs against the various parts of the chuck. No continuity.

Not sure where to go with this.


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[WobblyHand]

I also checked both connector legs against the various parts of the chuck. No continuity.

I think this is good. A short to the case would be bad.

As for the resistance of the chuck, I don't know what it should be. However this link, https://www.magnalock.com/post/electromagnetic-chuck-tips-troubleshooting-your-chuck leads me to believe it should have some resistance. Do you have any information on what the chuck specifications are? This link https://www.practicalmachinist.com/vb/general/how-test-magnetic-chuck-176302/ seems to indicate a typical resistance of about 400-500 ohms. So an open circuit is probably a bad thing - as in, you will have to repair the chuck somehow.

The bridge rectifier. The mounting hole, I see has a metal ring? Is this ring isolated from the rest of the 4 terminals? I would think it should be. Is it? Make sure you Discharge the cap! Disconnect the wires from the bridge and measure it. Check each diode (one polarity across the diode should have low resistance, the other polarity of your test leads should have a much much higher resistance) and if the diodes are isolated from the chassis.

What was the DC voltage across the capacitor? What is your input AC voltage?

 

[ddickey]

Mine was ~800ohm. I can check it again after work.
@vtcnc did the mag cuck ever work?

 

[vtcnc]

I think this is good. A short to the case would be bad.

As for the resistance of the chuck, I don't know what it should be. However this link, https://www.magnalock.com/post/electromagnetic-chuck-tips-troubleshooting-your-chuck leads me to believe it should have some resistance. Do you have any information on what the chuck specifications are? This link https://www.practicalmachinist.com/vb/general/how-test-magnetic-chuck-176302/ seems to indicate a typical resistance of about 400-500 ohms. So an open circuit is probably a bad thing - as in, you will have to repair the chuck somehow.

The bridge rectifier. The mounting hole, I see has a metal ring? Is this ring isolated from the rest of the 4 terminals? I would think it should be. Is it? Make sure you Discharge the cap! Disconnect the wires from the bridge and measure it. Check each diode (one polarity across the diode should have low resistance, the other polarity of your test leads should have a much much higher resistance) and if the diodes are isolated from the chassis.

What was the DC voltage across the capacitor? What is your input AC voltage?

Thanks for the troubleshooting links. I’ll dig into these sometime today to gain a better understanding of the mag chuck and troubleshooting.


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[vtcnc]

Mine was ~800ohm. I can check it again after work.
@vtcnc did the mag cuck ever work?

I’m getting no resistance across the chuck. Good question on the chuck. It may be that it is cooked. I don’t know how that could be. Maybe it is in the connectors. I’ll do a little additional checking this weekend.


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