Disassembly and Cleanup of a Sanford SG48 Surface Grinder

WobblyHand said:
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?
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Arrggh. I read through the PM post on "How to Test Magnetic Chuck" and they indicate zero or near zero ohms means its probably toast. Mine is reading zero.

Another problem I discovered is that the connectors on either end of the cable are reversed. They are "Jones" style connectors, if my memory serves me well. There are two spades per end. One spade larger than the other. When I checked for continuity on the cable, the large-to-large read zero. Large-to-small checked out with continuity. I'm wondering if the previous owner replaced the cable and polarity was inadvertantly reversed and that is what cooked the whole system to begin with???

This project may be dead in the water. I do have a mechanical magnetic chuck and can still use the machine, but it sure would have been nice to get the original electro-magnetic chuck working properly.

If I can disassemble it and see what might be wrong, I'll post pictures.

Bryan
 
vtcnc said:
Arrggh. I read through the PM post on "How to Test Magnetic Chuck" and they indicate zero or near zero ohms means its probably toast. Mine is reading zero.

Another problem I discovered is that the connectors on either end of the cable are reversed. They are "Jones" style connectors, if my memory serves me well. There are two spades per end. One spade larger than the other. When I checked for continuity on the cable, the large-to-large read zero. Large-to-small checked out with continuity. I'm wondering if the previous owner replaced the cable and polarity was inadvertantly reversed and that is what cooked the whole system to begin with???

This project may be dead in the water. I do have a mechanical magnetic chuck and can still use the machine, but it sure would have been nice to get the original electro-magnetic chuck working properly.

If I can disassemble it and see what might be wrong, I'll post pictures.

Bryan
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Yeah, I didn't want to say that... My background is electrical engineering, but I have never used a mag chuck before. However, I wouldn't think that reversing polarity would be harmful. Basically, the chuck is an electromagnet, which is just a coil, and is passive. Shouldn't care about polarity, unless there was a fault in it or some diodes or other active elements. If there is a diode, then maybe it has faulted short. You may be able to take it apart and figure it out. If you are lucky you will see some discoloration that may give you a clue. If there's no active circuitry, then it could simply be that the insulation got shorted out in one or more places. Maybe it was taken apart once and the cover was not replaced carefully and some wires were pinched.
 
@vtcnc have you had a chance to look at your chuck? Just wondering if you had cracked it open or not.
 
WobblyHand said:
@vtcnc have you had a chance to look at your chuck? Just wondering if you had cracked it open or not.
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I did. Disconnected magnet wire. Damn if I can't get the enamel stripped and soldered. Anyone have a recommended temperature for soldering fine magnet wire?
 
That should be good news about the wire! You found the disconnect.

The "good enamel" resists melting off under heat. I have found some magnet wire that uses a heat sensitive insulation that you can just solder without doing anything special. However, most magnet wire is coated with a tough insulation, the "good enamel". Unless someone else knows of something better, I have often had to resort to mechanical means to remove the enamel insulation. I have often scraped off enamel with a razor blade, or xacto knife, but you have to be very careful not to knick the wire.

I am presuming the wire is copper, so any 60/40 or 63/37 lead tin solder would work. If you want or need to go lead free, there's plenty of choices as well. Probably want to use some heat shrink or kapton tape to keep it from shorting to case.
 
I usually have to scrape the enamel off magnet wire too.

I am sure I will probably be corrected here but when my father gave me a surplus 6 x 18 magnetic chuck he told me that you put the part on then magnetize the chuck with DC current for a half minute or so. Something like 60v or 80v dc I think. Then when you are done grinding you demagnetize the chuck with AC current. Basically magnetize with the rectifier in the circuit then demagnetize by removing the rectifier from the circuit using the same step down transformer.

I still have the chuck, I just need the rest of the grinder to go with it. I remember using a VARIC, a rectifier and a momentary DPDT switch to wire it up and it did work. I might have just been using the VARIAC to minimize the chance of the chuck smoking when I was testing it... I don't remember. Like WobbyHand said there is no polarity in my magnetic chuck.

I figured that the magnetic chuck would be energized during the entire grinding operation but my father was usually right on things like this.

P.S. I have cauterized a mess of server hard drives with my magnetic chuck, it does a great job at that.

Edit: There wasn't an Internet when he gave me the magnetic chuck. A quick Google search says that I have an Electro-Permanent magnet chuck and it does appear to work as I described. It also appears that there are Electric magnetic chucks that will not hold a part without being energized.
 
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With the right parts you can set up a chuck to demag with a capacitor. That's how my home built Magnabend works. Choosing the value is a but tricky, though.

Sent from my SM-G715A using Tapatalk
 
I wish someone made a mag chuck to fit the Sanford SG's. They are all 4" x 7" which is to wide and can't be ground. Those that do offer one are more expensive than the grinder itself.
 
wow long project... but so nice.

I tried for literally decades to find a miniature surface grinder to no avail. Now my brother and I have an 8x24 monster
 
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