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pcarpenter
10-05-2006, 11:30 AM
I was poking around looking to buy a magnetic chuck for my recently acquired surface grinder and a web search turned up this article (which has little to do with my search for a mag chuck for my grinder):

http://www.manufacturingcenter.com/tooling/archives/1202/1202magnet.asp

The conventional logic is that magnetic chucks are to be used for low-force operations like surface grinding, but these magnetic chucks are clearly in a different class....perhaps more on the order of the big magnets used on cranes for handling scrap and other materials.

How many of you who work as "real" machinists use such a chuck in a milling environment? The concept sure could make work holding with complex parts easier.

I would almost tend to think that they would be unsafe (ie, sucking loose tooling etc from the workbench near the machine etc, but perhaps that is not realistic. I believe gaussian force drops off with the square of the distance, so perhaps it's not that big of a deal.

Paul

Evan
10-05-2006, 11:48 AM
I believe gaussian force drops off with the square of the distance, so perhaps it's not that big of a deal.

Fortunately it's the cube of the distance which is why we don't see cars stuck to the side of the hospital MRI suite walls.

b2u44
10-05-2006, 11:50 AM
That's pretty interesting. I never figured that a magnetic chuck would be useful in a high-force setting like on a milling machine. I would guess that it is like the double-sided-tape method of holding down small workpieces -- it works, but you need to be careful with it. It must be a pain to clear chips off of it.

Rusty Marlin
10-05-2006, 12:30 PM
I've used these on a VMC. Impressive is an understatment. The one we used had 2" square poles flush with the surounding matrix. I made a bunch of 2" sqr blocks that were then bolted to the tops of the poles and then machined to match the workpeice creating a nesting corner. The part was lowered with a crane, settled into position and then the magnets were "charged". The part was a large L shaped weldment about 50-55 inches long and 6" on a side made from 1" flat stock.The magnetic chuck held on to the the part by one edge of the weldment so that a face of the 6" width was up. A corner about 48" long and 1" on a side was used to hold the part. I thought for sure it would pop out of that corner and trash my mill cutter, dent the tabel and anything else bad that could happen. So the first thing I did was climb up on the part and jump on the overhung edge (all 230# of my fat a$$ 6" from the holding surface) and the part didn't even twitch.

The magnet held for 2, 3" passes x .1dp to clean up the face. Then we drilled a mess of holes in the face, flipped the part and repeated. Chip clean up was easy. The magnetic force is well balanced and only penatrates about 1.5 inches into the part. After the magnet is discharged it goes through an auto de-guass cylce and the chips just brush off.

JeffKranz
10-05-2006, 01:06 PM
I work for an injection molding company and our new presses have magnetic platens to hold the mold. Now you just close the press with the mold in there and turn on the magnets. Open the mold and the molds stay in place. Our heaviest mold weighs around 50,000 lbs and magnets are holding it. You can't turn off the magnetic until the mold returns to its home position. They even work it the power is out. It was a little scary to see the die maker jump inside the press to make some changes with these heavy blocks of steel be suspended without any clamps.

wierdscience
10-05-2006, 07:36 PM
I remember seeing a picture of a mag chuck holding a piece of plate maybe 3x12" about 24" long with a 4" insert face mill peeling chips from a cut that looked to be .200 Doc,that was impressive.

Thing to remember is the part must have significant mass and be reasonably flat for the chuck to hold.I remember a helper who tried to use a mag base drill to run some 4" conduit holes in a electrical enclosure.He started drilling,the drill started spinning, until the cord came out the wall:D No,they don't grip 14ga sheetmetal to good:D

madman
10-05-2006, 07:39 PM
Bought a huge one for the 5 axis OKUMA MILL. So many chips stick to it even after its turned off (residual magnetism) what a poiece of ****. The operators dont even use it. So much for the old school clamp er down technologie. Mike

nheng
10-05-2006, 07:44 PM
Madman, The sticking chips are simply a problem waiting for a fix. It's possible to have a control box that applies ac power to the chuck and gradually tapers it off to nothing. It would actually be a giant demagnetizer at that point. Feel free to offer this idea to the chuck manufacturer ... and have them send me a check ;)

added - complexity and cost wise, it would be simpler and cheaper than a VFD except that the field strength would be ramped down by reducing voltage/current instead of frequency like the VFD does

Den

nheng
10-06-2006, 08:31 PM
Doh ... didn't realize that they were either permanent mag systems or "switched" permanent mag, activated by a pulse. Either way, my suggestion is a dud unless it's a powered magnet.