On 2008-12-16, Randy wrote:
Anyone out there ever work on or service a mag-chuck? I just won a
chuck on ebay, 8 x 24 for my grinder? seller states it plugs right
into the wall. Every one I've seen uses a control box. Can I build
my own? Anyone have a schematic?
Anyone know what happened to Hitachi? All my seaches come up with
nothing as far as a chuck goes.
Hitachi model # HU-824
Well ... the one for my Sanford grinder runs (poorly) on AC, and
very nicely on DC rectified and filtered from the power line.
The original circuit in the base of the grinder was like this:
DEMAG Switch SPDT
+------------------------------------o
| OFF /______o-----------+
| resistor selenium \ |
120 VAC o----+-----WWWWWWW----------||-----+-----o 3 Mag
20 Ohms rectifier | HOLD 3 Chuck
10 Watts ___ 3 Coil
___ 8 uF 250 VDC Electo 3
| -lytic |
Neutral o-----------------------------------+-----o-------------------+
The capacitor was dead, and I did not trust the selenium rectifier
so I redesined it -- using a silicon bridge rectifier, and a 2000 uF
250 VDC capacitor. I replaced the SPDT switch with a DPDT switch, so I
switched both sides of the coil so the mag chuck was fully disconnected
when not turned on.
To hold, you switch to "HOLD".
To demagnetize (after holding) you switch to "DEMAG"
Then you switch to "OFF".
It does not do a complete job of demagnetizing the workpiece,
but enough so that it is easy to pull off the chuck. If you want better
demagnetizing, keep the switch on DEMAG while you slowly move the
workpiece away from the chuck, and only switch it fully off when you are
about six inches away.
I knew that my mag chuck could take line voltage and rectified
line voltage by looking at the circuit in the base of the grinder. I
just re-designed it to be a better system using more modern components.
(I kept the wirewound resistor, and (of course) the mag chuck. :-)
I actually just finished yesterday replacing it with a Permanent
Magnet chuck, because the manual for the grinder warned (quite
reasonably) to *not* use coolant with the electro-magnetic chuck. If
you wanted to do that, you would need an isolating transformer. The PM
chuck is also just barely smaller than the travel of the wheel in both
axes, so it is a better fit than the electromagnetic chuck.
Be warned -- if you use the circuit shown above, *don't* use
coolant.
You will need to make sure that the chuck is intended to run on
117 VAC in degauss (DEMAG) mode, and 170 VDC in "HOLD" mode. If not,
you may need to change the voltage -- either step it down for a lower
voltage one (in which case you have an isolation transformer anyway), or
step it up for a higher voltage one. The same applies there, except
that you will need to change the voltage ratings on the rectifier diode
or bridge, and on the capacitor.
The resistor is there only to keep the surge current to a
minimum when the system is plugged in -- or when the switch is set to
"HOLD" if you re-wire it so the switch is before the rectifier, which
would be a good idea.
Good Luck,
DoN.
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