View Full Version : Sanforg Mini Surface Grinder help needed!

12-03-2015, 06:33 PM
At the big buy my buddy found a great Sanford surface grinder ser#45 714 the young man didn't want to sell but after getting on my knees, hanging on his leg & begging he did. Came with the orig Sanford cart & tiny 3"x5" mag chuck which doesn't work. It has a transformer with no markings & a vacuum tube. Can anyone help explain how to convert the chuck to modern electronics? Thanks again!

12-03-2015, 06:40 PM
I don't know much about mag chucks, but if the electric ones are DC the tube is probably just a rectifier. Shouldn't be too hard to substitute solid state components. I used to be familiar with some of the tube numbers, but if you post it here someone will surely be able to say for certain. Meanwhile, does the tube 'glow'? Maybe the heater is burned out. The transformer should have a low-voltage tap for the tube heater, another thing you could check.

J Tiers
12-03-2015, 06:57 PM
A pic or two plus the tube number would help.

Basically, the chuck is a coil of wire wound around one or several "poles", which are magnetized when DC goes through the coil. There is insulation etc to make it work right.

The transformer supplies heater current to the tube, and may supply AC which the tube rectifies (maybe not, too).

If the coil in the chuck has continuity, and is not shorted to the metal of the chuck, then it may be worth working on.

What you will do is change the tube out for a pair of high voltage rectifiers, and maybe eliminate the transformer. One rectifier supplies the DC, the other with a resistor, absorbs the spike when power is cut off.

Pictures, (inside of the electronics box and outside) the tube number, and verification of a good coil and its resistance will allow moving forward on it.

12-03-2015, 07:12 PM
Tube is a GE 84, transformer is shorted out, magnetic coils(several) appear test fine. I'll have my buddy get pics. Wow, Thanks for the quick reply!

12-03-2015, 07:53 PM
The type 84 is a full wave rectifier capable of producing up to 350 volts DC.
It has a 6.3 VAC filament that draws 500 mA.
They are available on the Internet.
It should be easy to find a transformer or two to replace the one that is shorted.
You could use one for the output voltage and another for the filament.
Are you sure the transformer is shorted?
Has it burned up?
Does it blow a fuse?
The filament winding would be very low resistance.
Take the tube out to measure the transformer, it could be the tube.

12-03-2015, 08:03 PM
It's also possible that the line voltage, 115VAC is rectified by one half of the type 84 tube and the other half is used as a "snubber" as J Tiers suggested.
The tube filament could still be shorted so you should remove the tube to check the transformer.
If the 6.3 VAC transformer is bad it's easy to find a replacement.

12-03-2015, 08:10 PM
Here is a transformer for $6.54 that would work.

J Tiers
12-03-2015, 08:14 PM
It could also be full wave rectified, and snubbing provided by a small capacitor. I have seen that on mag drills. Pics may provide that information, if they are decently close up and show wiring.

I should have emphasized that.... Pics should be close up and inside ones should show wiring clearly.

As for the shorted transformer, the heater coil will measure shorted, since it is low voltage and relatively high current. If there is no other winding, then it almost certainly is as Seastar suggests, which is also what I would expect.

Jim Williams
12-03-2015, 08:15 PM
When you find out what voltage your original parts provide, I will be interested in comparing your values to the output of my Sanford power supply. Mine has been converted with a solid state rectifier, and I think that the magnetic chuck is weak. These little Sanford grinders look like toys, but they are capable of serious work on small parts.


12-03-2015, 09:49 PM
Here is a posting that has a lot of info on these grinders.
About half way down page two there is a posting with an attachment that has a diagram for making a solid state power supply for the chuck


12-03-2015, 09:55 PM
Ron I don't see the posting.

Jim, my buddy found on the internet it was 400V.

J Tiers
12-03-2015, 10:01 PM
In that case, it sounds like either there IS a coil output winding on that transformer, OR it is a double diode because it is a voltage doubler. That would provide the rectifier, and the damper diode all at once. In that case there should be at least one, and maybe two electrolytic capacitors in the thing. It would also use the input full-wave.

If it is a doubler, it would convert to something like the following.

Diodes should be high voltage 1 or 2 amp parts, 1N4007 are easily available. Capacitors will depend on the current needed by the chuck. Output volts will be nearly 400V, depending on line voltage. DC points go to coil of chuck.

http://img.photobucket.com/albums/0803/jstanley/doubler_zpsf9ef0i46.jpg (http://smg.photobucket.com/user/jstanley/media/doubler_zpsf9ef0i46.jpg.html)

12-03-2015, 10:22 PM
Sorry I forgot to add the attachment, it's there now.

12-04-2015, 08:21 AM
Ron, I still don't see it?

12-04-2015, 01:24 PM
Third times the charm - this time I checked it after editing and it's there.

12-04-2015, 01:34 PM
It seems like there should be some kind of current limiting in the circuit if you go with silicon diodes. The vacuum tube naturally limits current. I might be tempted to run it through a lamp dimmer type controller or a Variac during testing just to be safe.

J Tiers
12-04-2015, 02:55 PM
A resistor would work, but in many cases C1 is enough impedance. The assumption is that any protection is outside the rectifier portion. The vacuum tube has more need for protection, as its current limit is lower, and a surge can strip the cathode.

In most circuits, the vacuum tube impedance is more a nuisance than a benefit. Sometimes, as with tube guitar amplifiers, it has some potential useful impact on sound.

That link is present, but it fails to load..... the page or something in address is snafu.

12-04-2015, 03:42 PM
My 12"x24" walker didnt have a power supply so I made one. I knew the working current of the chuck was one amp due to a tag on it. So I used an auto-transformer rated at 0-140AC. Used a automobile rectifier. A couple switches and a fuse holder. I have opened the one amp fuse a few times before I decided to mark the face of the electrical box for max position of the dial. It works great. Cant move my vise when the chuck is active. JR

http://i1183.photobucket.com/albums/x461/_GLE_/HSM/control%20guts1_zps7jmnktm3.jpg (http://s1183.photobucket.com/user/_GLE_/media/HSM/control%20guts1_zps7jmnktm3.jpg.html)

http://i1183.photobucket.com/albums/x461/_GLE_/HSM/control%20guts2_zpsmdtn0hxs.jpg (http://s1183.photobucket.com/user/_GLE_/media/HSM/control%20guts2_zpsmdtn0hxs.jpg.html)

http://i1183.photobucket.com/albums/x461/_GLE_/HSM/control_zpspzg1pnb7.jpg (http://s1183.photobucket.com/user/_GLE_/media/HSM/control_zpspzg1pnb7.jpg.html)

http://i1183.photobucket.com/albums/x461/_GLE_/HSM/walker_zpseckv8sv1.jpg (http://s1183.photobucket.com/user/_GLE_/media/HSM/walker_zpseckv8sv1.jpg.html)