View Full Version : Mystery magnetic chuck

Herb Helbig
09-14-2002, 07:25 PM
I just picked up a small magnetic chuck marked
"Brown & Sharp 761 Patent Pending"
from an antique shop. It's about 1.75 x 2 x 3 inches with an on-off toggle in the
small end. 3/16 " above the base (2 x 3) on each long side are 1/4" wide by 1/4"
deep grooves that appear to be for clamping the chuck to a table. One pole piece
pair is1/2" above one clamping groove. The other pole piece pair is on top
(opposite the base) on a 9/16" wide plateau running the 3" length and over the
other clamping groove.

I wasn't able to find anything about it on the B&S web site. I'd be grateful for any
information about what this might have been used for. It seemed like a good way
to hold steel parts for (gentle) milling when a vise would clutter things up, but was
disappointed with how easy it was to twist a flat piece of steel held by the chuck.

I can e-mail a jpeg photo to anyone interested.


Al Messer
09-14-2002, 09:58 PM
I would guess that it is used for holding a test dial indicator.

09-15-2002, 01:23 AM
The 761 is the shorter version of the 760 - these are called Permanent Magnet Blocks. B&S and Suburban still sell these style blocks - they are in the J & L catalog. They were always very expensive tools and were usually thrown out when some numbnuts put it on the degausser after grinding. There is a 750-4 v-block, the 255 and 760 are both flat top blocks - their difference being the 255 is a fine pole model for smaller, thinner items. The only other magnetic block B&S had is a toolmakers knee 745-46-1 square to .oooo5"/inch - around $2,000US.

A nice find - tools are so much fun! http://bbs.homeshopmachinist.net//biggrin.gif

Herb Helbig
09-15-2002, 11:50 AM
Many thanks Al and Dave! What minds out there!


Peter S
09-15-2002, 09:00 PM
There may be a safe way to dismantle these chucks, but I have found (the hard way) that when you dissasemble this type of permanent magnet...they lose their magnetism. I did this years ago to a DTI base, had to get it re-magnetised. If your unit is weak, this could well be the problem. If you find someone with a remagnetiser, it should be a simple job to fix. Try an auto electrician, preferably an old-established outfit that could repair magnetos.
I have a few different types of permanent mag. chucks, made by 'Eclipse' (UK), eg magnetic V block, positioning V blocks.

[This message has been edited by Peter S (edited 09-15-2002).]

Herb Helbig
09-16-2002, 01:36 AM
Peter -

Thanks for reading my mind and for valuable instruction! I was considering taking
this magnetic block apart, but more from curiosity than from the notion that it
might have been weakened and might be rejuvenated. I see now that Thrud really
made that suggestion in his note, though I didn't pick it up. You guys are really
worth studying!

I wonder whether the 761 is new enough to make use of rare earth magnets.

I have no experience with switchable magnets of this sort except for DTI
holders. From experimenting with my 761, I'd guess that the torque exerted by an
end mill might twist a workpiece even with a light cut.

Is there a way to check that the holding power of a magnetic chuck is in spec? I
could measure the force necessary to slide or twist a workpiece, but have no
standard of comparison. I've been wondering if they are meant primarily for
grinding operations where the cutting forces are probably very small.

I downloaded the Eclipse magnetic workholding catalog - they show nothing as
small as the 761, nor do they mention holding forces except for their electro-
magnetic chucks. Neat catalog! They invented the magnetic chuck the year I
was born!

Are there tricks to increase the friction between the chuck and work? For example,
I noticed in a recent posting about drilling balls, that paper between vise jaws and
the work can increase holding power. I also know that the magnetic force falls
off rapidly with separation so a trade-off is, as usual, inevitable.


09-16-2002, 06:39 PM
I have had mixed results in repairing the mag blocks. Most of them just have a alnico V magnet that are fairly lame to begin with. I repaired one by purchasing Rare Earth Magnets and drilling them with a diamond core drill. Two of the magnets literally exploded when internal stresses were release - got some bad cuts (exploding R.E.Mags happens way to often with me!). Holding power was well over 100Lbs - the bad part is they are so strong the push-on/push-off type you cannot turn off!. I might have to try much smaller magnets. The core drills get covered in magnetic particles that are impossible to remove. A twist type knob like yours would be easier. I get my magnets at Lee Valley Tools and out of dead hard drives.