Tools - Magnetic Sine Chuck (Rescue)
[12-8-2018]
I bought this 6" x 8" magnetic sine chuck at an online auction, with nothing to go on but a picture. I won the auction for $45, but when I picked up the chuck I fund it was not permanent magnet chuck as I had thought, but was instead an electromagnetic chuck. Of course, there was no power supply with it, and the power cord was cut off about an inch from the chuck.
The good news is that the chuck was otherwise in pretty good shape, apart from need a good cleaning. That being the case, I decided to see if I could connect a power supply to it and get it working.
On the bottom of the chuck there was a small (about 1-1-4" diameter) access plate.
Removing the access plate revealed the wires for the magnetic coils. Fortunately, although the wires were very dirty, I was able to remove the old stub of power cord so I could wire in a new cord.
An access hole in the side of the chuck leads to the access plate cavity. The photo to the left shows the new power cord. Note the plastic bushing where the cord enters the chuck - I made this to help protect the cord from wearing, and to provide some strain relief.
The insulation on the coil wires was brittle and cracked, so I reinforced it with heat shrink tubing. After connecting the pwer cord wires to the coil wires, I insulated with additional heat shrink tubing.
The magnetic chuck power cord was wired to a DPDT switch (latched - off - momentary) wired up as a polarity reversing switch. The switch is latched on to power the chuck for operation. To remove items from the chuck, the switch it turned off, and then to the momentary-on (reversed polarity) position to demagnetize the work piece.
The wiring for the reversing switch is straightforward, but I have included it here for those who need it.
The switch is an "20A 125V ON-OFF-(ON) DPDT 6 Terminal Toggle Switch Momentary With Boot"; note that the parenthesis indicate the position is momentary. As wired, the top position is latched ON (stays on), the middle position is OFF, ad the bottom position (for demagnetizing) is momentary ON.
Here are some pictures of the switch:
Switch with boot
Side View
Bottom view.
The switch is further wired to banana plugs to connect to a DC power supply I purchased to use as a magnetic chuck power supply, as well as for other uses I have in mind. The power supply will provide DC power up to 120V, although I find I get good holding power at only 50 V. This is just the results of an initial test; I'll need to do further testing once I actually put the chuck to use.
I was happy to see that the reversing switch worked well - just switching to the momentary-on position for a fraction of a second does a good job of releasing the work piece.
Note: I also have a 24V DC power supply. Since I already had this on hand, I initially tried it with the magnetic chuck. The 24V only gave weak magnetism - enough to tell me that the coils were intact and working, but not enough for actual use.
Further Experiments
I made some additional tests to look at voltage, amperage, and holding power and made the following measurements:
At 100V the holding power was very strong. I tried the reversing switch to release a test piece, but the residual magnetism was still quite strong. I then tried giving brief reversed voltage pulses at 85V, 70V, 60V, and 50V and this worked a bit better.
As a final test, I set the voltage at 100V to clamp the test piece, switched to the off position, reset the voltage to 50V, and then gave a reverse voltage pulse - this seemed to work much better. Repeating the test, and making sure that the reverse pulse was very brief worked even better.